Geology

Earth Sciences

2009-09-07T11:37:00.000-07:00

Earth science (also known as geoscience, the geosciences or the Earth sciences), is an all-embracing term for the sciences related to the planet Earth. It is arguably a special case in planetary science, the Earth being the only known life-bearing planet. There are both reductionist and holistic approaches to Earth science.

There are four major disciplines in earth sciences, namely geography, geology, geophysics and geodesy. These major disciplines use physics, chemistry, biology, chronology and mathematics to build a quantitative understanding of the principal areas or spheres of the Earth system.

The following fields of science are generally categorized within the geosciences:

- Geology describes the rocky parts of the Earth's crust (or lithosphere) and its historic development. Major subdisciplines are mineralogy and petrology, geochemistry, geomorphology, paleontology, stratigraphy, structural geology, engineering geology and sedimentology.

- Geophysics and Geodesy investigate the figure of the Earth, its reaction to forces and its magnetic and gravity fields. Geophysicists explore the Earth's core and mantle as well as the tectonic and seismic activity of the lithosphere.

- Soil science covers the outermost layer of the Earth's crust that is subject to soil formation processes (or pedosphere). Major subdisciplines include edaphology and pedology.

- Oceanography and hydrology (includes limnology) describe the marine and freshwater domains of the watery parts of the Earth (or hydrosphere). Major subdisciplines include hydrogeology and physical, chemical, and biological oceanography.

- Glaciology covers the icy parts of the Earth (or cryosphere).

- Atmospheric sciences cover the gaseous parts of the Earth (or atmosphere) between the surface and the exosphere (about 1000 km). Major subdisciplines are meteorology, climatology, atmospheric chemistry and atmospheric physics.

- A very important linking sphere is the biosphere, the study of which is biology. The biosphere consists of all forms of life, from single-celled organisms to pine trees to people. The interactions of Earth's other spheres - lithosphere/geosphere, hydrosphere, atmosphere and/or cryosphere and pedosphere - create the conditions that can support life.

[source]

A

2007-12-31T23:59:00.000-08:00

▪ accretion (veins) ▪ accretion (tectonic) ▪ accretionary prism ▪ allochthonous nappe ▪ angular unconformity ▪ anatexis ▪ anticline ▪ aphanitic texture ▪ aplite, aplite dike (◙ aplite) ▪ argillaceous, argillic ▪ assimilation ▪ assymetric fold ▪ assymetric vein ▪ attitude of structures ▪ autochthonous ▪ axial plane ▪ axis (fold) ▪ azimuth ▪

Ancient continents ▫ Baltica

accretionary prism

2007-12-22T21:08:00.000-08:00

An accretionary prism, accretionary wedge, or accreted mélange is a wedge-shaped body of faulted and folded material accreted (added) to a continental margin in a subaqueous thrust zone.

A mélange is a body of mappable-sized blocks of different rocks jumbled together with little continuity of contacts. The origins of mélanges are either tectonic, submarine sliding (olistrosomes), or diapirism.

Olistostromes are mélanges formed by accumulation of submarine, gravitational flow as semi-fluid bodies, so are stratigraphic units that lack true bedding, yet are intercalated between normal sedimentary bedding sequences.

Accretionary prisms can arise by:
▪ Having been scraped off subducting oceanic crust and accreted to an island arc or continental margin at a subduction zone.
▪ Accumulation, at greater depth, of a mass of deformed trench sediments and ocean floor sediments on the underside of the tectonic plate that lies above the subducting plate (tectonic underplating). Trenches infilled with thick turbidite sediments typically have listric thrust faults that flatten into a decollement horizon that is often close to the top of the pelagic sediments.
▪ Some accretionary prisms with few reflectors on seismic profiles may contain chaotically deformed sediments (broken formation or mélange) similar to those known from ancient orogens.

Accretionary prisms and accreted terranes are not equivalent to tectonic plates, but rather are associated with tectonic plates and accrete as a result of tectonic collision. Materials incorporated in accretionary prisms include:
▪ Ocean-floor basalts – typically seamounts scraped off the subducting plate
▪ Pelagic sediments – typically immediately overlying oceanic crust of the subducting plate
▪ Trench sediments – typically turbidites that may be derived from:
--- ▪ Oceanic, volcanic island arc
--- ▪ Continental volcanic arc and cordilleran orogen
--- ▪ Adjacent continental masses located along strike (such as Barbados).
--- ▪ Material transported into the trench by gravity sliding and debris flow from the forearc --------ridge (olistostrome)
--- ▪ Piggy-back basins, which are small basins located in surface depression on the accretionary -----prism.
--- ▪ Material exposed in the forearc ridge may include fragments of oceanic crust or high------------pressure metamorphic rocks thrust from deeper in the subduction zone.

◙ subduction zone magmas ◙

[links: images: formations: accretion prisms: Jurassic and Cretaceous Franciscan Formation - an accreted melange or accretionary prism that formed in a trench along a subduction zone; low-grade metamorphosed accretionary complex with greenschist facies metamorphism, shear deformation, crenulation cleavage, kink band, Otaki Group;
diagrams: accretionary prism and olistostrome associated with Santiago Peak Volcanics and associated intrusives; structural zones and lithologic zones and metamorphic zones associated with subduction zones and orogeny; webpages: Geology of the Point Reyes Area, California - accretionary complex of a subduction zone, transform plate boundary, granitic basement of a continental magmatic arc; webpages: trenches and mélanges]

anatexis

2007-12-11T23:04:00.000-08:00

Anatexis (loss of texture) is partial melting of rock under extreme conditions of temperature and pressure.

Anatexis more often results from tectonism (regional metamorphism) than from magmatism (contact metamorphism) because magmas typically carry too little excess heat to melt significant quantities of surrounding rock while themselves continuing to remain molten.

Anatexis provides a common form of magmatic mixing in areas of active magmatism, wherby adjacent magma bodies can develop transient subsurface communications before their eruption or final subsurface emplacement. Anatexis-related magmatic mixing involves the secondary melting of mid- to lower crustal rocks upon contact with much hotter, rising mafic melts of mantle origin, and produces felsic (feldspar- and quartz-rich) magmas in arc and continental rift settings. Such melts may reach high crustal levels carrying both mantle heat and mantle material.

anticline

2007-12-11T04:19:00.000-08:00

Anticlines are upwardly convex folded structures that form arches with the youngest (last deposited) strata at the hinge.

Anticlines are A-shaped structures that develop during crustal deformation as the result of compression that accompanies orogenic mountain building.

(images at left - click to enlarge - top, schematic of an anticline; botttom, road cut exposure of anticlinal fold with syncline to right)

Anticlines are of particular interest to oil-exploration geologists because the Earth's largest oilfields occur in large, gentle anticlines in thick sedimentary rock sequences.

Click here for an image that shows an anticline with a hinge that has a flat top and two steep limbs, creating a box-like shape. Folds with this shape are called box folds. Both synclines and anticlines can be box folds.

[links: images: formations: anticline; anticline and syncline near Calico Ghost Town, Yermo, California; anticline; Teton anticline; anticline; satellite: anticlines near Paradox, UT, and anticline crossed by transverse stream, and Uncompahgre uplift; plunging anticlines and synclines north of Moab, UT; plunging anticlines and synclines, Dinosaur National Monument, UT]

argillic

2007-12-07T05:18:00.000-08:00

Argillic or argillaceous refers to clay.

Argillic alteration of rocks involves conversion of certain minerals to minerals of the clay group, such as kaolinite (below right) and montmorillonite (bottom right).

Clays are hydrous aluminium phyllosilicates, typically less than 2 μm (micrometres) in diameter, and are distinguished from other small soil particles, such as silt. Clays may be residual or transported, and generally result from:
▪ the chemical weathering of aluminosilicate-bearing rocks (such as granite, containing feldspars),
▪ solution of rocks containing clayey impurities, such as limestone,
▪ disintegration and solution of shales,
▪ hydrothermal alteration.

Clays exhibit the smallest size of soil particles, flake or layered shape, affinity for water, and a tendency toward high plasticity.

In soils, argillic horizons are diagnostic clay accumulations, often designated as Bt (B horizon dominated by deposited clay, "t").

For example, "alfisols have an argillic, a kandic, or a natric horizon and a base saturation of 35% or greater. They typically have an ochric epipedon, but may have an umbric epipedon. They may also have a petrocalcic horizon, a fragipan or a duripan." Glossary, Map, Soil Orders.

links: Micromorphology of argillic horizons / Soil Formation and Classification, What is Soil?, Soil Science Glossary, USDA gallery of soil profiles, soil facts, soil education

assimilation

2007-12-06T05:16:00.000-08:00

Assimilation is that process of magmatic differentiation whereby ascending magmas evolve chemically by recruiting easily melted or dissolved components (fusibles) from the walls of their conduits.

Under the agency of heat and magmatic fluids, ascending magmas pick up volatiles, silica, trace elements, and occassionally fragments of wall rock. The heat that the melt gains by leaving behind quick-freezing refractories (an exothermic process) is typically sufficient to compensate for heat lost in the endothermic reactions required for the assimilation (melting) of country rock components. This trade-off ensures that assimilation can proceed without causing the melt to freeze (solidify).

Any wall rock fragments that survive more or less intact, without completely melting or dissolving into the magma, are called xenoliths. Surviving wall rock crystals are called xenocrysts. Together, xenoliths and xenocrysts provide invaluable information about rarely exposed lower crust and mantle levels by carrying these materials up within the ascending magma.

▪ Bowen's Reaction Series

B

2007-11-30T23:59:00.000-08:00

▪ back-arc basin ▪ Baltica ▪ banded (ribbon) vein ▪ basalt ▪ basaltic breccia ▪ batholith ▪ blended unconformity ▪ boudin (vein) ▪ boudin, boudinage ▪ breccia ▪ bysmalith ▪

back-arc forearc

2007-11-24T21:13:00.000-08:00

Back-arc basins are associated with tensional forces caused by asymmetric seafloor spreading and oceanic trench rollback at some convergent plate boundaries.

Back-arc basins develop where island arcs are split longitudinally, roughly along the line of the magmatic axis, forming a rift that matures to the point of seafloor spreading, thus allowing a new magmatic arc to form on the trenchward side of the basin. This division strands a remnant arc on the side of the basin away from the trench and subduction zone, and the remnant arc shifts away from the arc axis as it reforms.

Most of the sediment that reaches back-arc basins originates in the active magmatic arc. Back-arc basins usually spread for a few tens of millions of years, then spreading ceases, converting the spreading back-arc basin to a fossil back-arc basin or marginal basin.

The Okinawa Trough is a backarc basin lying between Japan and Taiwan, created by extension within the continental lithosphere behind the Ryukyu trench-arc system. The Okinawa Trough is at an early stage of evolution from arc type to backarc activity [s].

Forearc basins are sea floor depressions located between subduction zones and their associated volcanic arc.

Forearc basins receive sediments from the adjacent landmass, the island arc system, and trapped oceanic crustal material. Oceanic crustal fragments may be obducted onto the continent as ophiolites complexes during terrane accretion.

The Central Valley of California developed as a forearc during the late Cretaceous and early Paleogene.

[links: images: maps: swUS in the Late Triassic (215 Ma) as the Cordilleran arc developed, a new back arc basin formed behind the McCloud arc (wp, contrasting hypothesis), by early Cretaceous, the Cordilleran arc had converted to a classic continental (Andean-style) arc comprising a fore arc-trench system, fore arc basin, and Andean arc. (The fore arc-trench was site of famous Franciscan mélange). The Great Valley sequence was deposited in the fore arc basin and the Sierra Nevada batholith complex formed within the magmatic arc; Okinawa trench; diagrams: simple diagram of features of an intra-oceanic island arc subduction zone; the Taigonos segment of the Uda-Murgol Island Arc, and Pekulney segment of the island arc, Russia (wp); hypothetical x-c of Great Slave Craton (wp); webpages: Geological History of the western US: swUS: Precambrian, Cambrian, Ordovician, Silurian, Devonian, Mississippian, Pennsylvanian, Permian, Triassic, Jurassic, Cretaceous, Tertiary]

Baltica

2007-11-24T12:04:00.000-08:00

The continent of Baltica (green) existed from the Late Proterozoic to the Early Palaeozoic and included what is now the East European craton of Northwestern Eurasia. Baltica was created as an entity not earlier than 1.8 Ga. Before this time, the three segments/continents that now make up the East European craton were in different places on the globe.

Laurentia is shown in red, Siberia in peach, embryonic Gondwana is yellow, Australia-East Antarctica grey.

The Baltic Shield (Fennoscandian Shield) now forms the continental core of Europe. The shield is composed of the oldest Precambrian crystalline rock in Europe (Archaean and Proterozoic gneisses and greenstone deformed by rounds of ancient tectonic activity.)

The tectonically stable shield region was unaffected by the Caledonian, Hercynian, and Alpine orogenies, though mountains rose at shield margins. The Baltic Shield is exposed in Finland, Sweden, and Norway as a result of scouring by continental ice sheets during the Pleistocene epoch.

[ more on Baltic Shield ]

basalt

2007-11-24T06:58:00.000-08:00

Basalt is a hard gray or black, mafic igneous volcanic rock that is usually fine-grained due to rapid cooling of lava, though it contain larger crystals in a fine matrix (porphyritic), be vesicular, or be a frothy scoria.

Basalt magmas form by decompression melting of peridotite in the mantle. The crustal portions of oceanic tectonic plates comprised predominantly basalt, derived from upwelling peridotite in the mantle below ocean ridges. The basalt shield volcanoes of the Hawaiian island chain sit above a mantle plume, or 'hot spot'. (left - click to enlarge - aa flows over ropey pahoehoe in Hawaii - image courtesy of USGS.)

Basalt is TAS classified according to the relationships between the combined alkali content and the silica content. Basalt typically containts a preponderance of calcic plagioclase feldspar and pyroxene; olivine can also be a significant constituent. Accessory minerals include iron oxides and iron-titanium oxides, providing basalt with a paleomagnetic signature.

Phaneritic, shallow intrusive igneous rocks with a basaltic composition are generally referred to as dolerite (also called diabase) or gabbro.

(image left - click to enlarge - courtesy USGS - top-down: basaltic lava; lava field; flow-lines in basalt formation; close-up of vesicular basalt with olivine crystals; surface of basalt hand specimen; basalt columns.)

[images - roll-over link for preview (where available); large images (well worth a visit) show only as a corner on preview : water-sculpted basalt at Fossil Falls in Yosemite : Basalt Fall unterhalb des Hengifoss, basalt columns, Dverghamrar basaltic columns in Iceland, 2 : cliff of basalt columns : Columbia River basalts, Catherine Creek arch in Miocene columnar basalts : flowing curves of basalt entablature in Yellowstone : basalt columns Armenia : basalt field : basalt and sandstone : 3.7 Ga moon-rock basalt : hand-specimen : hand-specimen vesicular basalt, vesicular basalt with olivine phenocrysts, 2 : hand-specimen diabase : hand-specimen diabase porphyry : hand-specimen diorite : hand-specimen gabbro : hand-specimen scoria : thin-section basalt, 2, 3; thin-sections moon basalts ]

batholith

2007-11-24T04:09:00.000-08:00

Batholiths are complex intrusive bodies composed of plutonic igneous rocks, usually of felsic or intermediate rock-types, such as granite, quartz monzonite, or diorite. Batholiths are also called granite domes.

The exposed surface area is defined (by geographers) as more than 100 square kilometers, though large geological batholiths may have less exposure. Areas smaller than 100 square kilometers are called stocks. The entire batholithic emplacement is typically so large that the bases are rarely exposed, and batholiths have steeply inclined walls that form prominent dome structures when exposed by erosive removal of initially overlying rocks.

Sometimes batholiths arise through several smaller diapiric intrusions (plutons) and have a complex history of magmatic intrusion and crystallization at depths of 5 to 30 kilometers. Batholith formation is commonly associated with lithospheric plate boundaries, where tectonic interactions between plates are associated with large scale melting of crustal rocks and the formation of deep magma chambers. As erosion uncovers the crystalline rock that formed at great depth, crystal structures respond to the decrease in load and expand, rendering the plutonic rocks susceptible to exfoliative weathering.

Spheroidal weathering produces boulder fields, and sheet-like exfoliative weathering, which is accelerated by frost wedging, creates smooth structures like Half Dome in the 40,000 sq km Sierra Nevada Batholith in Yosemite (upper left, right - click to enlarge). The Coastal Plutonic Complex of British Columbia and Alaska is even larger, extending for 1,800 kilometers and covering 182,500 sq km. Other North American batholiths include the following: The Idaho Batholith is a composite mass of granitic plutons covering approximately 15,400 square miles in central Idaho [map, Castle Peak, Contact Batholith]. Guichon batholith in British Columbia contains several large, low-grade copper deposits. The South Mountain Batholith (SMB) of southwestern Nova Scotia is the largest granite batholith in the Appalachian Orogen with an approximate area of 7300 sq km.

The Cornubian Batholith was intruded into south-west England at the close of the Variscan Orogeny (late Carboniferous - Permian).

[images: photos: Baja Batholith spheroidal weathering, Boulder Batholith & spheroidal weathering, Conrnubian batholith Cligga Head, Enchanted Rock batholith & tree at summit, Golden Horn Batholith, Rocky Mtn. Nat Park, Yosemite batholith, batholith, outcrop Granite Mountain; websites: Avalonia, Halifax Pluton, Boulder Batholith, Cornubian Batholith, Peninsular Ranges batholith, Sierra Nevada Batholith & lithography; lithography: conness contact, Enchanted Rock granite, 2, Julianehåb batholith granite, gallery, mingling of microdioritic and granodioritic magmas in the calc-alkaline Pataz Batholith of the northern Peruvian Cordillera Oriental, schlieren banding in the Sausfjellet pluton, rock gallery including Vioolsdrif Batholith; maps: Boulder Creek Batholith, Cornubian Batholith, Enchanted Rock Batholith, Halifax Pluton, Idaho batholith ecoregions, Verkhisetsk batholith, batholiths in Wisconsin; diagram: plutons and volcanic landforms

boudinage

2007-11-10T02:43:00.000-08:00

Boudinage refers to structures deformed by extension in ductile shear zones. Boudinage structures contain a rigid tabular body that has been stretched and deformed where embedded within more deformable (less competent) rocks.

Competent tabular bodies that are susceptible to boudinage include veins and strata such as sandstones. Where conditions favor brittle fracture rather than ductile deformation, imbricate (overlapping) fracturing occurs.

In boudinage, the competent bed break ups into sausage-shaped boudins – forming structures such as ribbon-like boudins or chocolate-tablet boudins (depending upon the axis and isotropy of extension).

[links: images: Amphibolite boudins in gneisses; formations: Zoroaster Veining, boudins composed of quartz and plagioclase, boudin of metagabbro (HP mafic granulite) in tonalitic gneiss]

Bowen's Reaction Series

2007-11-10T02:35:00.000-08:00

N. L. Bowen, experimenting on the order in which minerals crystallized from a melt, determined a reaction series for minerals:

discontinuous -------- continuous -------most refractory / least stable at surface
-----\------------------------/
---olivine -------------Ca-rich feldspars------------------mafic
-------\--------------------/
-----pyroxenes -.-------/
---------\------------ plagioclase
-------amphiboles -.--/
-----------\------------/
------------\------ Na-rich feldspars ------------------intermediate
-------------\--------/
---------------biotite
------------------↓
-------------orthoclase
------------------↓
-------------muscovite
------------------↓
-- ------------quartz---------------------------------------felsic
--------------------------------------------most fusible / most stable at surface

The feldspars crystallize in a continuous series with the calcium rich endmembers cystallizing out of a melt earliest, the sodium rich members crystallizing later, and the orthoclase feldspar endmember cystallizing last. In the discontinuous branch, the nesosilicate olivine constructed of simple [SiO4]^4- tetrahedra crystallizes first, followed in sequence by single-chain pyroxenes, amphiboles, sheet micas, and 3D-framework quartz.

Minerals, such as refractory olivine, which crystallize out of melts at depth are least stable and most prone to weathering, at Earth's surface. Conversely, highly fusible quartz is very stable at the surface.

See magmatic differentiation.

breccia

2007-11-07T19:49:00.000-08:00

Breccia is a are clastic, sedimentary rocks comprising angular fragments from a previous rock structure that have been cemented in a matrix.

Classification of breccias relates to their constituents, mode of occurrence, constituent fragment size, types of clasts, and source of clasts.

image: basalt breccia with epidote groundmass, courtesy of Siim Sepp

bysmalith

2007-11-02T05:11:00.000-07:00

Bysmaliths are more or less vertical and cylindrical bodies that crosscut (discordant) adjacent sediments and are bounded by steep faults. Bymaliths are commonly associated with the mountain-building (orogenic) processes, and they are typically composed of granites or granodiorites.

Bysmaliths are considered to be conical or cylindrical laccoliths. They develop when highly viscous magma is injected into strata. Because lateral spreading along the bedding is limited by viscosity, the magma moves upward to form the cylindrical shape. Overlying rock layers are fractured. The walls of bysmaliths slope away from each other with depth, which makes their diameter increasingly large at greater depths.

[image Black Mesa bysmalith (Henry Mountains, Colorado Plateau, Utah), close-up of Wolf porphyry, Mixes Baldy-Anderson Peak Bysmalith close to the Blankenship Divide; ref, ref 2, gallery, Henry Mountains Wilderness]

C

2007-10-31T11:59:00.000-07:00

▪ chonoliths ▪ clays ▪ crack-seal filling (veins) ▪ craton ▪ concordant ▪ conformable ▪ cryoseism ▪ cumulates ▪

Cratons ▫ Laurentian Craton

chonoliths

2007-10-17T10:09:00.000-07:00

The term chonolith is used to describe intrusive igneous bodies with a nonspecific, irregular shape that does not fit into other categories of plutonic structure (such as dike, sill, or laccolith).

A cactolith is a quasi-horizontal chonolith composed of anastomosing ductoliths, whose distal ends curl like a harpolith, thin out like a sphenolith, or bulge discordantly like an akmolith or ethmolith.

Ductoliths are horizontal plugs of teardrop cross section, or a headed dike.

Harpoliths are large, sickle-shaped intrusions injected into previously deformed strata, intruding horizontally in the direction of maximum orogenic displacement.

A sphenolith us a partly concordant, partly crosscutting injected body of igneous rock, in which country rocks are overturned in some sections.

Akmoliths are intrusive bodies injected along a decollement, which send numerous tongues into the overlying folded rocks.

Ethmoliths are cross-cutting bodies of plutonic rock that narrow downward and are thus funnel-shaped.

concordant

2007-10-10T22:47:00.000-07:00

Concordant or conformable, when referring to plutonic bodies, indicates that the intruding magma of sills and laccoliths lies parallel to rather than cutting across country strata, as do discordant structures such as veins, dikes, bysmoliths, and batholiths.

A concordant coastline comprises bands of different rock types that run parallel to the shore. The rock types are typically of alternating resistance, so that the coastline forms distinctive landforms, such as coves. A discordant coastline comprises rock types of alternating resistance that run perpendicular to the shore, creating distinctive landforms when the rocks are eroded by ocean waves. Less resistant rocks erode faster, creating inlets or bays; more resistant rocks erode more slowly, remaining as headlands or outcroppings.

Concordant flows at different points in a river system have the same recurrence interval, or the same frequency of occurrence. The term is most often applied to floodflows.

craton

2007-10-07T23:04:00.000-07:00

Cratons, or continental platforms are the ancient, stable geological provinces at the core of continents (orange in image at left). Cratons have peristed for more than 500 million years (some over 2 billion years).

A shield is defined as a craton in which basement rocks have been exposed by erosion at the surface; and such shields typically comprise Precambrian basement rocks.

Cratons characteristically consist of ancient felsic igneous rocks in the crystalline basement at the center of continents, as distinguished from the linear belts of geosynclinal troughs at the margins of continents. Although the oldest rocks and proto-continents date from the Hadean, the earliest large cratonic landmasses formed during the Archean eon when radioactive decay maintained heat flow at three times current levels.

Cratons have a thick continental crust and deep roots that extend into the mantle beneath to depths of 200 km. Cratons sit on anomalously cold mantle that is more than twice the approximately 100 km thickness of mature oceanic or noncratonic continental lithosphere.

Slave Craton

cryoseism

2007-10-07T01:09:00.000-07:00

Cryoseisms, frost quakes, or ice quakes are non-tectonic seismic events caused by the expansion of water that has frozen in rock cracks or soils. The expansion of water on freezing generates stresses in the surrounding rock, and this stress may be released explosively in a cryoseism. By a different mechanism, cryoseims can accompany glacial surges when ice at the base of a glacier, which had been frozen to bedrock moves suddenly when released by a thin layer of lubricating meltwater.

D

2007-09-30T23:59:00.000-07:00

▪ décollement folds ▪ deformation ▪ diapir ▪ diatreme ▪ dikes, dikelets (aplite dikes, pegmatite dikes) ▪ dip ▪ dip direction ▪ direction (dip, plunge, strike) ▪ disconformity ▪ discordant ▪ duplex ▪ dykes ▪

décollement folds

2007-09-20T21:09:00.000-07:00

Décollement (detachment) folds develop during folding, secondary to separation of a (more competent) layer from an underlying (less competent) layer as deformation proceeds.

The most spectacular décollement folds develop when a homogeneous layer of uniform thickness overlies a less viscous layer. The incompetent layer can comprise evaporates, shales, or heated lower crust (viscosity inversion with depth).

Décollement folds involve a mismatch between horizontal dimensions of layer above versus below the detachment zone, with the upper layer larger in at least one dimension. Such a dimensional mismatch can result when:
▪ the upper layer is stretched as in diapiric folds,
▪ the upper layer has broken away and slid off an uplift, or
▪ the lower layer has been shortened by thrusting or subduction (thrusting at the margins - Sichuan basin; thin-skinned décollement driven forward along a thrust plane in low-viscosity salt and gypsum deposits - Jura Mtns; subduction - Appalachians, Anti Atlas; continental telescoping and thickening shortened the lower layer in the Western Overthrust Belt of the central Rocky Mountains.)

The Sichuan basin of central China includes some of the best samples of décollement deformation.

◙ subduction zone magmas ◙

[links: images: formations: view perpendicular to thrust movement along Early Tertiary décollement folds in Tertiary strata, décollement at base of black shales (Bravaisberget Formation, Midterhukfjellet, Bellsund, Spitsbergen; décollement fold, Reed Wash area, western San Rafael Swell; folds created by "thin-skinned" thrusting over a décollement in underlying rocks, Mt. Kidd, Kananaskis Mountains, Alberta, Canada; extensive outcrops of Upper Jurassic gypsum-anhydrite décollement layer and siliciclastic strata, Sierra Madre Oriental (SMO) salient, near Galeana; décollement, Montagne Noir; close-up: décollement folds in Khosh Yeilagh fm. (N Iran); webpages: gallery of geological phenomena]

deformation

2007-09-20T18:09:00.000-07:00

Sediments and rock structures are subject to deformation under the influence of imposed stresses.

Stress is defined as a force applied over an area, F/A.
Stress may be uniform and equal from all directions:

▪ confining pressure of overburden

▪ release from confining pressure due to exposure by erosion (diagenesis and retrograde metamorphism).

Alternatively, stress may be unequal from different directions (differential):

▪ compression

▪ extension (tensional)

▪ shear stress (applied obliquely)

Causes of stress include

▪ uniform confining, lithostatic stress due to overburden (burial)

▪ tectonic stress

▪ expansion of water that has frozen in rock cracks or soils (cryoseism)

When rocks deform in response to imposed stress they exhibit strain, which is the differential change in size, shape, or volume of a material. Materials differ in their responses to stress, depending upon composition, conditions of temperature and confining pressure, and strain rate. However, regardless of intrinsic degrees of brittle or ductile qualities, all strained materials pass through 3 successive stages of deformation: elastic, ductile, and fracture (failure, or brittle deformation). Provided that the strain rate is sufficiently slow to allow minerals to accommodate structurally, minerals can adjust to applied stresses by a variety of mechanisms.

Forms of deformation include:
unconsolidated sediments

▪ slumping

▪ folding (fold anatomy)

▪ mass wasting and landslides

▪ faulting (fault attributes)

consolidated rock

▪ brittle, ductile, or elastic deformation due to lithostatic or tectonic stresses

▪ --earthquakes

▪ --faulting

▪ --folding

▪ --cataclism, milling, and brecciation

▪ --orogenesis

diapir

2007-09-16T23:08:00.000-07:00

A diapir or piecement structure results from the upward intrusion of a more buoyant material into/through overlying strata. Diapirs are most commonly composed of evaporitic salt deposits (salt domes) or gas charged muds, but may be igneous.

As ancient seas evaporated they left salt deposits that were buried by sediment. Because the salt deposits were less dense than overlying rock the buoyant mass of salt ballooned upward, intruding into the overlying rocks through weak spots. The intruding “salt bubble” is called a salt diaper, and in most environments, salt diapirs erode rapidly on reaching the surface, leaving craters such as the ones shown below left (Salt Dome & Craters on Melville Island). In arid regions, salt domes may persist (below right - click to enlarge - Salt Dome in the Zagros Mountains, Iran ).

If the rising plug of salt (called a salt diapir) breaches the surface, it can become a flowing salt glacier (bottom right - click to enlarge image - Iran's salt glaciers).

images Earth Observatory : Subscribe to the Earth Observatory :

[links: images: formations: gabbroic anorthosite diapir, west side of Wagers Peak in LZb (postulated mechanisms of formation are 1. that this is a diapir of liquid or mush that pushed aside and broke the overlying layers and then caused turbulence in the magma that caused deposition of the disturbed bed, or 2. that this body is actually a surface deposit of relatively hydrous, plagioclase-rich residual liquid that percolated upward through the crystal mush floor cumulates.)

diatreme

2007-09-16T23:04:00.000-07:00

Diatremes are breccia-filled volcanic pipes formed by gaseous explosions.

Diatremes may
● breach the surface, producing a tuff cone of consolidated volcanic ash
● form filled relatively shallow craters known as a maars
● form other volcanic pipes

The term diatreme sometimes refers to any concave body of broken rock or tuff-breccia, generally formed by explosive or hydrostatic forces, whether or not the structure is related to volcanism. Some diatreme, phreatic explosions result from the interaction of hot magma with relatively shallow groundwater.

dike

2007-09-16T13:39:00.000-07:00

Dikes or dykes are discordant tabular or sheet-like bodies of magma that cut vertically or almost vertically through and across strata, though some dikes are steeply inclined.

Hundreds of dikes can invade the cone and inner core of a volcano. Dikes may occur in swarms of parallel dikes, particularly where there has been crustal extension. In regions of crustal extension, fracturing may open the route for filling by magma from a deep source, or intrusive magma may promote the fracturing and extension of the crust. Outcrops of dikes can range from a few metres to many kilometres in length, and can spread lateral distances from a few centimetres wide to over 100 m. Very thin dikes or dikelets are sometimes called veins. The Great Dyke of Zimbabwe is a gabbroic mass nearly 500 km long and about 8 km wide [sat. image, 2, 3].

Because dikes intrude relatively cool country rocks, they frequently display a chilled margin, with grain size becoming coarser towards the centre where the rate of cooling has been slower. If the dike cooled very slowly at great depth, the large crystals of pegmatite dikes have had time to form.

Pegmatite dikes represent crystallization from a residual melt fraction, but pegmatites are formed from a water-rich fluid, so are very coarse grained. Most pegmatites contain quartz, alkali feldspar, micas, and tourmaline. However, some pegmatites contain minerals such as tourmaline, garnets, apatite, beryl, topaz, spodumene, magnetite, sphene (titanite), and zircon, and various other rare minerals. This occurrence of rare minerals results from progressive concentration of trace elements into the last fraction of melt because these elements have not been removed by earlier crystallization during the solidification of the bulk of the magma. [image Pegmatite vein in granite boulder, Glenwood Canyon; Tanco is perhaps the most fractionated igneous body on earth, and a super giant among chemically complex pegmatites]

Aplite dykes are commonly found in granitic bodies. Aplites are light coloured, fine to medium grained, and equigranular. Aplites formed from the ultimate residual melt after most of the crystallization of the granitoid was completed, so aplites are rich in quartz and alkali feldspar and sometimes muscovite. [image links of aplite rock and formations]

duplex

2007-09-04T08:12:00.000-07:00

In structural geology, a duplex is a system of imbricate (overlapping) thrusts that branch off from a single fault below and merge with a thrust fault above. Duplexes form stacks of thrust-bounded rock bodies, which are bounded by roof and floor thrusts. The rock body that is bounded by faults above and below is called a horse.

Duplexes are formed through continued thrusting along a floor thrust with successive collapse of thrust ramps. Antiformal stacks are defined as systems of totally overlapping thrust horses that are characterized by a coincident trailing branch line. Antiformal stacks result when the forward motion of a forward-breaking thrust sequence is interrupted or completely blocked from regular forward development of a foreland-vergent duplex system. Antiformal stacks commonly occur in the cores of mountain chains, mainly in continent-continent or arc-continent collision zones, where the subducted plate acts as an obstacle, forcing faulting upward.

The Lewis thrust forms a 450 km long fault with thrusting of Precambrian limestone over the top of Cretaceous shale (taking place 160-145 Ma). This Alberta-Montana thrust sheet has a duplex structure that exhibits the geometries of both a hinterland-dipping duplex and an antiformal stack, and that contains inclined and stacked thrust horses that are bounded by the main fault traces.

The Lewis thrust surface is a low-angle thrust fault with ramp-flat geometry, indicating that the thrust moved horizontally, stepping upwards through stratigraphic layers. The higher thrust faults in the Lewis duplex are folded over lower faults ramps and their associated horses, indicating that slip on the higher thrusts occurred first, as the formation of thrusts progressed downward and toward the foreland.

◙ subduction zone magmas ◙

[link: images: animations: animation of duplex formation; panoramas: Tarndale, New Zealand, Wairau Quicktime panorama (along the fault zone); Death Valley Quicktime Panorama; Interactive 360 degree view of the San Andreas Fault at Wallace Creek; Earth Revealed, courtesy of Anneberg Media, requires Windows media Player; horse: horse wedged between two faults; formations: remarkable internal imbrication or "duplexing" of a single layer (turbidite sequence somewhere in Middle East, from AAPG Bulletin); extensional duplex; duplex in late Paleozoic limestone, Crows Nest Pass, southern Canadian Cordillera; viewing a duplex reverse fault structure, Redwall Fault in Sinclair Canyon, Kootenays, Rockies; beautifully developed duplex structure in glacial lakebeds at Skardu, Karakoram-Himalaya; outcrop scale extensional duplex, Triassic rocks, coast of Chile; thrust fault duplex, near Albany, NY, within the Hudson Valley fold and thrust belt; isoclinal upright anticline (lower part of the Lønstrup Klint Formation in the frontal part of the Stortorn Section) with right limb that constitutes an imbricate duplex formed by connecting thrust-fault splays (white dot-and-dash lines). The fold is interpreted as a hanging-wall anticline developed during fault propagation and successive imbricate stacking; complex duplexing at least three imbricates have been sheared off from and shoved under the continuation of the layer above, in sandstone layer in Tertiary turbidites of Olympic accretionary prism, Washington; imbricated slump structures, imbricated slump structures in sandstone beds and closer view;duplex in sedimentary sequence deformed under partially consolidated conditions, Pliocene tuffaceous mudstones and sandstones, south of Tokyo Japan; duplex in Albany limestone, VT; geomorphic disruption in response to the Hope Fault oblique strike-slip duplex in the Lottery-Mt Lyford area, North Canterbury, NZ; Valley and Ridge, duplex, Pellissippi Parkway; diagrams: fault propagation fold duplex; footwall imbrication; duplex-unit model for fault-bend folding of duplex segments, wp; duplex thrust interpretation of the effect of the downdip step in the plate interface, Hikurangi Subduction Interface, NZ; Gunnison thrust, duplex structure of fault-bend-fold, central Utah; webpages: Thrust Faults; DeGray Spillway, I, II, III]

image of imbricate thrusting in the Himalayas courtesy of USGS

E

2007-08-31T23:59:00.000-07:00

▪ enclaves ▪ en echelon ▪ epithermal deposits ▪ exchange of volatiles ▪

en echelon

2007-08-22T04:38:00.000-07:00

The term 'en echelon' refers to closely-spaced, parallel or subparallel, overlapping or step-like minor structural features in rock (faults, tension fractures), which lie oblique to the overall structural trend. Conjugate deformation structures are related in deformational origin.

Extensional stresses create fractures that can infill with calcite. When rocks deform in a brittle manner, the fracture pore can subsequently infill with some form of cement, such as calcite. Typically, crystals will nucleate on the fracture wall and grow into the opening. (Sometimes apparent fracture are completely reduced by a prismatic or fibrous mineral that is oriented long axis normal to the wall. In this case, the force of crystallization of the ‘filling’ material may be the actual cause of the opening of the fracture.)

[links: images: small scale: calcite in fault, calcite filled extension fractures in limestone; en echelon dikes, Woollen Mills, NY; en echelon extension fractures in quartz, 2; conjugate pair of en echelon extension fractures; en echelon fractures, San Andreas fault; dolerite dyke filling an en echelon fracture set, Hoedjies Punt, Saldanha Bay; en-echelon dyke geometry, Paternoster; close-up: en echelon faults in Tertiary sedimentary rocks, Blacks Beach in La Jolla, California; quartz epidote pod with en echelon fracture, and close-up; close-up of dolerite dyke filling an en echelon fracture set, Hoedjies Punt, Saldanha Bay; stresses involved in formation of en echelon veins; un système de fentes en échelon matérialise une faille potentielle; large scale: en echelon folds of Raplee Ridge anticlinal Monument Upwarp in Utah; en echelon volcanic fissures, Kilauea, Hawaii; webpages: Les Failles et Microstructures associées (Associated Faults and Microstructures, translated poorly, by google, from French)]

enclaves

2007-08-11T21:12:00.000-07:00

Magmatic enclaves are volumes of rock surrounded by emplaced host rock of related but distinct composition and of separated genesis (incomplete magmatic mixing).

Enclaves are distinguished from xenoliths, which are fragments of metamorphically altered older country rock that fell into magma or lava and became enveloped within igneous rock. Enclaves are also distinguished from schlieren, which are concentrations of mafic material that have crystallized out of a single magma.

Microgranitic (mafic) enclaves are common components of plutonic bodies and may represent the result of mingling of mafic and felsic magmas. These enclaves are often ellipsoidal or elongate in shape.

Studies on mafic enclave-host granite pairs indicate that enclaves and their host granites share compositional characteristics indicating their close relationship, but that the two groups of rocks are not cogenetic. The enclave-host relationship was probably acquired through pervasive mechanical and chemical interaction (especially differential interdiffusion) between two originally independent magmas. Microgranitoid enclaves typically show microstructural
evidence that suggests that prior to incorporation of parent magma globules into the host magma (during magma mingling), the enclaves underwent magma mixing, in a setting where the mafic magma was more abundant than the felsic magma.

The most mafic enclaves are generally the most stable enclaves with respect to disruption by entraining flow, and proto-enclaves with silica contents close to those of the host granite are highly unstable. Enclaves are often interpreted as strain markers. However, most deformation of microgranitic enclaves probably occurs at relatively high temperatures (950 -1050 °C), so the enclaves record magmatic strain of the host over only a limited temperature-time range in the host’s cooling history. Observation of apparent deformation of enclaves in a liquid regime implies that magmatic flow velocities are likely to be below 10 m/yr in enclave-bearing plutonic systems. [r, 2, 3]

[links: images: Mafic microgranular enclave (MME) dense zone in the upper part ofthe Kurobegawa Granitic Pluton, Japan, and mafic microgranular enclaves (MME ) mingled with filsic crystal mush mainly consisting of porphyritic plagioclase and quartz; Rock dragon enclave; magmatic enclave rich in biotite and relatively fine-grained; magmatic enclave at Elephant Rocks; mafic enclave dike; mafic magmatic enclave (Nikia lavas) with quench rims; mafic enclaves in tonalite with leucocratic halos around the mafic enclave, and magmatic fabric in tonalite indicated by prefered orientation of long axis of mafic enclave; diorite enclaves in Ross of Mull Granite show rounded and lobate forms and more angular forms. This varitety suggests that the diorite and granite magmas coexisted as liquids, although it is possible that the granite was partially crystallized when the diorite magma was intruded. Some show complex veining structures, and many have conspicuous K-feldspar megacrysts not present in the main diorite. These textures show that the two magmas have interacted extensively in some cases.; Silvermines Granite at Tiemann Shut-Ins with a variety of enclaves shapes, from rounded / diffuse to sharp / angular; melanocratic enclave in a leucocratic granite; enclave from ERSNA and the ERB, feldspars within this enclave show both rapakivi (left) and anti-rapakivi (right); rock, 2; mafic enclave, Brazil; mafic enclaves in tonalite of Lake Mary, mafic dike in tonalite of Lake Mary containing mafic enclaves similar in composition of that of the mafic dike, 2, mafic dike with dark, chilled margin in tonalite of Lake Mary, concentration of mafic enclaves in tonalite of Lake Mary, mafic enclaves with plagioclase xenocrysts inherited from the tonalite of Lake Mary host, differential weathering of mafic enclaves in tonalite of Lake Mary, aplite dike cutting cluster of rounded mafic enclaves; close-up: spheroidal mafic enclave in granodiorite of Summit Lake; Desolation Valley granodiorite with mafic enclave, surrounding granodiorite has anhedral interstitial K-feldspar and rim myrmekite, but the mafic enclave is not penetrated and replaced by K-feldspar; change in colour related to minute changes in mineralogical composition that may indicate the presence of a magmatic banding; thin-sections: portion of mafic enclave from Desolation Valley granodiorite which contains hornblende (tan), biotite (reddish), albite-twinned and zoned plagioclase (black and white), and quartz (cream); webpages: Igneous Processes in the Moruya Batholith, Igneous and Metamorphic Geology Field Trip.]

epithermal deposits

2007-08-09T15:04:00.000-07:00

Most epithermal ("shallow heat") deposits occur in veins, irregular branching fissures, stockworks, or breccia pipes. Colloform and replacement textures are sometimes recognized, but the majority of epithermal deposits are characterized by open space filling textures (crustification, comb structures, symmetrical banding).

Many epithermal deposits appear to be spatially associated with hot springs and geysers, and these hydrothermal systems may be considered the surface expression of epithermal systems. Alteration of wall rock is predominantly argillic and is accompanied by silicification. Epithelial deposits are commonly associated with large gossans, which are intensely oxidized, weathered, or decomposed rocks that usually form the exposed, upper portions of ore deposits or mineral veins. Common gangue (waste) minerals in the tailings include: quartz, calcite, fluorite, barite, chalcedony, rhodochrosite and dolomite.

Epithermal deposits include a wide variety of ores: Au, Au-Ag, Ag, Pb, Zn, Cu, Sn, Sb, U and Hg.

F

2007-07-31T23:59:00.000-07:00

▪ fault attributes ▪ faulting ▪ fault (shear) zones ▪ fold anatomy ▪ folding (attitude of fold structures, anticline, monoclines, overturned and recumbent folds, ptygmatic folds in migmatites, slumps, syncline) ▪ foliated texture ▪ forearc basin ▪ fractional crystallization ▪ fragmental texture ▪ frost quake ▪ fusibles and refractories ▪

fault attributes

2007-07-24T04:11:00.000-07:00

Faults can move horizontally to the left or right and/or vertically, and the rake of a fault results from a combination of vertical (dip) and horizontal vectors. Rake = angular slip direction of hanging wall block measured in the fault plane and from the dip vector.

Net-slip comprises dip vector component and horizontal vector components.
a. net-slip = total slip of fault.
b. dip-slip = dip-parallel slip component.
c. strike-slip = strike-parallel slip component.
d. vertical-throw = vertical component of net-slip.
e. horizontal-throw = horizontal component of net-slip.
f. heave = stratigraphic heave = apparent horizontal component of the net-slip.

Vertical throw, which is the vertical component of net-slip, is different for dipping strata than stratigraphic throw, which is the vertical offset of faulted strata.

If the hanging wall, which lies above the fault, moves downward relative to the footwall, then the fault (as in diagram) is a normal fault. A detachment fault is a regional, low-angle, listric normal fault formed during crustal extension. Listric faults are curved normal faults in which the fault surface in concave upwards because the main detachment fracture following a curved path rather than a planar path. Slumps are listric faults.

Crustal extension stresses create sunken graben blocks bounded by parallel normal faults and lying between horst blocks that lie higher than the sunken graben blocks. Compressional forces during orogenies can elevate horst blocks.

If the hanging wall is thrust upward above the footwall, then the fault is a reverse fault. A thrust fault is the special case of low dip-angle faults that formed during regional compressional deformation.

If the predominant movement is in the horizontal direction, then the fault is a strike-slip fault (transform fault or wrench fault). The term tear fault can be used to indicate a steeply-dipping wrench fault that bounds or cuts the hanging wall of a thrust or normal fault (also used for mode III faults).

faulting

2007-07-24T03:12:00.000-07:00

Complex deformation with failure of strata and accompanying movement of one rock body relative to another creates geologic faults, fault lines, or simply faults. The fault zone is that area of complex deformation that is associated with the fault plane.

Whether in a normal or a reverse fault, the hanging wall is defined as the rock body above the line of the fault, while the footwall lies below the fault.

Shear stresses that cause faulting in shear zones may be associated with tectonic compression at plate boundaries, with tectonic extension, with impact compression, or with compression by overburden.

Stresses build in rocks where friction prevents simple slippage. Stresses initially cause deformation of rock structures, and only when accumulated potential energies exceed the strain threshold will rock bodies fail and relative motion occur across the fault.

Initially, rock failure may occur on a small scale (microfracturing, microseismicity). However, continued compression can cause the large-scale slippage associated with seismic events (tremors and earthquakes).

Relative movement (slip) determines the type of fault that occurs:

▪ strike-slip or transform faults, such as the San Andreas Fault
▪ normal
▪ reverse (thrust) faults such as the hugh Lewis Thrust

[link: images: animation: model of earthquake centered on Hayward Fault; small-scale faulting: small fault in Delmar formation; small fault in Mosaic Canyon, Death Valley, and recumbent folding; fault; low-angle thrust fault in moraine; moderate-scale faulting: normal fault; fault; growth fault with well developed clay smear exposed in cliff face near Albuquerque New Mexico; fault in rocks (left slid down relative to right), Mosaic Canyon, Death Valley, and limestone/marble fault breccia; Early Carboniferous quartz-cemented, mineralised fault breccia in a fault strand of the Billefjorden Fault Zone, Gråkammen, Austfjorden, Spitsbergen; breccia; fault breccia, Haughton impact structure; normal fault, 2, fault, 2, Birmingham Shale, Bakerstown Station, PA; Blue Anchor, Somerset, 2; labelled fault; Surpise Cliff Fault in Ricardo Group sediments, close-up, and aerial view of fault; fault, Black Hills; Internal Oriskany Hanging Wall Deformation; Devil's Elbow fault separates red Tertiary megabreccia (hanging wall) from strongly deformed Proterozoic gneiss in (footwall), another footwall block in right foreground, and close-up; San Gabriel Fault (where the light rock meets the dark rock); fault in complex zone of oblique thrusting on the Alpine Fault (mylonite thrust up to 2 km westwards over Quaternary gravels, NZ); fault; detachment fault; thrust fault, Melabakkar; Early Tertiary folded and thrust Permian and Carboniferous strata with Cretaceous dolerite sills; view subparallel with thrust movement, and isoclinally folded dolerite sill between two thrust faults, Midterhuken, Bellsund, Spitsbergen; early Tertiary thrust ramp (left) and box folds (right) in Triassic strata, typical foreland structures of marginal fold-thrust belts, Engadinerberget, Wedel Jarlsberg Land, Spitsbergen; synsedimentary faulting: extensional duplex or synsedimentary growth faults in the Triassic Botneheia Formation, Eistradalen, Agardhdalen, Eastern Spitsbergen; collapse of a sandstone plateau or delta front. Festnigen sandstone, base of Helvetiafjellet Formation (Barrême), and (closer) Festnigen sandstone, base of Helvetiafjellet Formation (Barrême), KvalhovdenArea, Kvalvågen, Eastern Spitsbergen; large-scale faulting: San Andreas Fault, 2, from Shuttle, topographic map, diagram, folding; Keraf Suture in Sudan Collision Zone; Lone Pine, CA; Fault line in Southern California; Alaska's Denali Fault, map, 2, Denali Fault Earthquake Fact Sheet; Fault Scarp, Rock Creek, and closer view, 2; Furnace Creek Fault; Transform Strike-Slip Faults across Atlantic Mid-Ocean Ridge; Faults and Fault Zones in Ross Sea; webpages: picture gallery of Svalbard, Jan Mayen, and Dronning Maud Land, Antarctica]

fold anatomy

2007-07-10T00:44:00.000-07:00

Folding buckles strata away from the linear, planar, and horizontal.

Folds may be symmetrical or asymmetrical. In a symmetrical fold (image at left), the axial plane is vertical and the limbs (sides) dip symmetrically from the axis. In an asymmetrical fold, the axial plane is tilted from the vertical with one limb dipping more steeply than the other.

▪ the axial plane of a fold is an imaginary plane surface that divides a fold as symmetrically as possible.

▪ the fold axis is a line drawn along the points of maximum curvature of a layer of a fold (parallel to the hinge in anticlines and synclines).

▪ the plunge of a fold is the angle between the horizontal plane and the fold axis (when this is not not horizontal).

▪ the plunge direction is the geographic quadrant towards which the plunge is directed

By convention, plunge direction is expressed as a three-digit number, and the plunge by a two-digit number. Thus, a line plunging 35º toward the azimuth 35º will be noted: 45º->035

Monoclinal folding drapes strata as though over a ledge, whereas hinge folding wraps the limbs of a fold as though hinged around the fold axis.

Because the eroded surface of a plain could fail to indicate the sub-surface folding relationships of ridges of resistant strata, folding is most easily elucidated by examining structures eroded through folds (side view of diagram at left).

However, the apparent shape of a fold may be distorted by the angle of erosional exposure relative to the fold axis (image at left).

(Try this with a stick of celery.)

├ .. On geological maps, the stike and dip of bedding, rather than axes and folds, are indicated.

▪ faulting ▪ fault attributes – net-slip, dip-slip, dip vector, strike-slip, vertical-throw, horizontal-throw, horizontal vector, heave, rake

folding

2007-07-10T00:43:00.000-07:00

Geological folding involves the plastic deformation (bending, buckling) of a single or multiple (stack) strata, such as sediments and rocks, which were originally planar horizontal surfaces. Although even brittle rocks may undergo plastic deformation when stresses are applied over considerable periods of time (low strain rate). Beyond plastic deformation, rocks fail structurally and faulting occurs.

Folds may be isolated or may occur in extensive fold trains, and folding may range from the microscopic scale to mountain-sized folds in orogenic belts. Folds are classified according to size, fold shape, tightness, and dip of the axial plane.

Folding can occur by flexural slip, buckling, or mass displacement under under varying conditions of stress, hydrostatic pressure, pore pressure, and temperature.

Slumping of material before deformation causes synsedimentary folds. Folding orientation can produce:

▪ anticlines

▪ synclines

▪ domes

▪ basins

▪ décollement folds

▪ monoclines

▪ overturned and recumbent folds

▪ slumps

▪ ptygmatic folds

links: images: plunging anticlines and synclines north of Moab, UT; Folding Satellite Images gallery; Capitol Reef, UT, satellite image and false color image of a monocline and syncline crossed by transverse stream; Capitol Reef Fold, Capitol Reef from ground, 3D anaglyphic images (needs 3D glasses), wp; anticline, 2; folds, 2; fold axes; fold and unconformity; kink fold; kink fold in mountains; refolded; overturned fold; synclinal; close folding and lineations, White Mountains, NH; complex folds in shale and sandstone, Cumana, Venezuela; ductile thrust faulting and large-scale fold nappes, 2, and complex fold interference patterns produced during formation of the fold nappes; Lower Palaeozoic Gondwanan sediments (ca. 500 My) metamorphosed and folded during Variscan Orogenesis (ca 350 My), Aiguilles Rouges, Lac d'Emosson; folding in schist; chevron fold, road cut Kingston-Rhinecliff Bridge, NY; fold, 2, road cut, Catskill, NY; California fold; sheath fold with strongly curved hinge line; glaciotectonic fold, Melabakkar, and close up, 2, 3; ptygmatic folding in quartz vein in Archean metasediments; folded Cambrian limestones and shales, Bay of Islands, Newfoundland ; large folds (Osceola) indicate laterally directed strain away from 4 km distant transient crater (Weaubeau structure of southwestern Missouri); folds in the Barranquin Formation, northeastern Venezuela; webpages: Geology Gallery : Folds : Geological Structures : Secondary Rock Structures : Mouser

fractional crystallization

2007-07-07T23:45:00.000-07:00

Fractional crystallization (fractionation) is that process of magmatic differentiation that accompanies the failure of early-forming crystals to react to the melt that remains. The process of fractional crystallization is responsible for the bulk of differentiation that is occurs in igneous rocks.

As ascending melts cool and react with country rock, those minerals in the melt that have the highest melting points or the lowest solubilities (quick-freezing refractories, like olivine and pyroxene) crystallize out first, leaving minerals with the lowest melting points or solubilities (quick-melting fusibles, like silica) behind in the melt to freeze out last.

Latent heat associated with phase change is released by the crystallization of refractories, replacing heat lost by conduction to the surrounding country rocks, lost to melting of country rock, and lost to the assimilation of fusibles in the country rock. Fusibles enter and refractories leave the melt at characteristic temperatures and pressures, and these exchanges tend to occur at specific depths along the ascent. The remaining melt loses volume as it rises, rendering its fusibles increasingly concentrated. Thus, exchanges within ascending magma leave behind a trail of solid refractories and country rock alterations.

Gravitative differentiation is the commonest form of fractionation, and results from the phenomenon that most solid minerals are denser than their parent melts. As denser crystals settle to the bottom of the magma body, they become segregated from the residual melt. Rocks that are formed by settling crystals are termed cumulates, and the rocks are often zoned, with the densest, first-formed crystals accumulated at the base of the magma chamber. Cumulates formed by the lighter crystals occasionally float to the top, with the lightest at the very top. This process produces layering in igneous rocks. The crystals of cumulate rocks are typically cemented by residual magmatic fluids.

▪ Bowen's Reaction Series

[links: animations: fractional crystallization/magmatic settling; webpages: Kurt Hollocher's webpage on Greenland's Eocene Skaergaard Intrusion has a gallery of excellent photographs of microrhythmic layering (and other interesting phenomena); Mining: Rock Formation]

fusibles and refractories

2007-07-04T17:19:00.000-07:00

Fusibles are rocks or minerals that melt easily, and are the opposite of melting-resistant refractories.

Sedimentary rocks, which are stable at the Earth's cool surface, tend to be fusible because they consist mostly of stable minerals that have resisted weathering. Crystalline rocks tend to be refractory and to resist melting because they consist mostly of minerals that crystallized out of melts. The most refractory rocks, such as gabbro and peridotite, are stable in the lower crust and the mantle.

▪ Bowen's Reaction Series

G

2007-06-30T23:59:00.000-07:00

▪ gabbro ▪ geological maps ▪ glassy texture ▪ Gondwana ▪ gneissic texture ▪ granite ▪ graphic representation ▪ greenstone belts

gabbro

2007-06-24T22:08:00.000-07:00

Gabbro is a coarse-grained, plutonic igneous rock that forms at spreading centers in rift zones and mid-ocean ridges (so underlies oceanic crust). Gabbros can form as massive uniform intrusions or as layered ultramafic intrusions formed by settling of pyroxenes and plagioclase (pyroxene-plagioclase cumulate).

As an essential component of the oceanic crust, gabbros are found in many ophiolite complexes in the sheeted dyke zone to massive gabbro zone (zones III and IV). Long belts of gabbroic intrusions are typical at proto-rift zones and around ancient rift zone margins, where they intrude into the rift flanks.

Gabbro is a dense rock that is greenish or dark-colored and comprises varied percentages of pyroxenes, plagioclase, amphiboles, and olivine. Where olivine is present in large quantities, the rock is termed olivine gabbro.

A finer grained rocks with the same composition as gabbro is termed diabase.

[images : layered gabbro, North Cascades : Salem gabbro-diorite cut by a a composite dike with felsic margins and a central core of basaltic rock : White Mountain Magma Series : pegmatitic gabbro : oceanic crust exposed on Cyprus : oceanic crust gabbro, 2 : thin section Oman Ophiolite gabbro : thin section of olivine gabbro - pyroxene and olivine show bright colours, striped grey rectangular crystals are plagioclase feldspar : thin section of gabbro with plagioclase and hypersthene (orthopyroxene) : hypersthene gabbro : thin section with pyroxene and (striped) plagioclases : thin section orthopyroxenes crystals surrounded by alteration (uralite) : thin section : thin section with twinned plagioclases ]

geological maps

2007-06-12T23:08:00.000-07:00

A geological map provides a graphic representation of selected geological features within a desired surface topographic or subsurface area. The size and relative position of each feature on the map corresponds to its correct geographic situation according to an established scale and projection. Mapped features included in the map key include geologic units, stratigraphic contour lines, fault lines, strike and dip lines, and other symbols. Information can be extrapolated from surface mapping in order to postulate the distribution of geological features in the subsurface. Resulting hypothetical structural models can provide the basis for exploring the landmass in search of its resources.

Commonly, geological features cannot be measured continuously over large areas, so other forms of evidence are employed to infer the delineation of large or hidden features. Surface geological features may be traceable in bedrock outcroppings (ground surveys), from air photographs (photogeological reconnaissance) and/or from satellite images. Subsurface geological features may be traceable in boreholes using cores, cuttings and/or geophysical logs. Geophysical surveys (measuring the magnetic, gravitational, or seismic properties) provide information that helps delineate geological features in the subsurface.

A Geographic Information System (GIS) is an efficient way to manage, analyse and display spatial data. Data from a variety of different sources can be rapidly computer-overlaid for viewing and analysis.

[links: images: LANDSAT image gallery, Geology - Oman; Seafloor Mapping, Bahia, Brazil; websites: Geological Map of Canada : Structural Geology (animations) : Geologic Maps from USGS National Park Service : Geological Maps, UWisc : USGS National Geologic Map Database : USGS Geology in the Parks : Animations : Plate Tectonics : Rocks & Minerals : Geologic Time : Glossary : Geomorphic Provinces : Sand Dunes : Caves : Glaciers : Coasts : NPS Park Geology Tour home]

Gondwana

2007-06-10T10:40:00.000-07:00

Gondwana, or Gondwanaland (yellow at left) was a supercontinent that assembled (animation) during the late Precambrian-early Cambrian (550-500 Ma).

Gondwana, with East Antarctica as its center, began to break up during Late Triassic to Early Jurassic time (182 Ma to 64 Ma) : A Tight fit-Early Mesozoic Gondwana: A Plate Reconstruction Perspective : pdf.

granite

2007-06-07T23:10:00.000-07:00

Granite is typically a medium to coarse grained felsic, intrusive igneous rock (plutonic) that is usually pink to dark gray, sometimes black, depending on its chemistry and mineralogy. Granites are the commonest basement rocks of the continental crust, many dating from the Precambrian.

In some granites, individual crystals are larger than the groundmass (porphyrys). Granites primarily comprises orthoclase and plagioclase feldspars, quartz, hornblende, muscovite and/or biotite micas, with minor accessory minerals such as magnetite, garnet, zircon and apatite. Rarely, a pyroxene is present. Very rarely, iron-rich olivine, fayalite, occurs.

Granites are classified according to the QAPF diagram for granitoids and phaneritic foidolites (plutonic rocks) that compares the percentages of quartz, alkali feldspar (orthoclase, sanidine, or microcline) and plagioclase feldspar.

As a plutonic rock, granite is often exposed in weathered tors, dykes and as massive batholiths.

greenstone belts

2007-06-07T19:37:00.000-07:00

Greenstone belts are zones named for the green hue imparted by chlorite (left) minerals within the rocks, which are variably metamorphosed (greenschist) mafic to ultramafic volcanic sequences associated with sedimentary rocks. Greenstone belts are found between granite and gneiss bodies in Archaean and Proterozoic cratons.

Greenstone belts are interpreted as having formed at ancient oceanic spreading centers and island arc terranes.

◙ greenstone-hosted quartz-carbonate vein deposits ◙ GQCV

Shield greenstones are located in:
● Slave craton of Canada (Archaean and Proterozoic)
● Pilbara craton of Australia (Archaean)
● Barberton greenstone belt in Kapvaal craton of southeastern Africa (Archaean)
● western Africa (Archaean and Proterozoic)
● Madagascar (Archaean and Proterozoic)
● Isua greenstone belt of southwestern Greenland (Archaean)
● shield area in Brazil (Archaean)
● Baltic craton of Scandinavia and the Kola Peninsula (Archaean and Proterozoic).

[links: images: hand-specimens: greenschists; 3.5 Ga komatiite from Lower Onverwacht Group of Barberton Greenstone Belt; formations: greenschists; Storø shear zone; geological maps: world distribution of world class greenstone-hosted quartz-carbonate vein deposits; Canadian greenstone-hosted quartz-carbonate vein districts; northeast-trending "greenstone" belts of ancient gneiss alternating with belts of metamorphosed volcanic and sedimentary rocks in Minnesota and Canada, Superior Province greenstone belts; x-c, and diagramatic model of northeast-trending belts most likely result from structural deformation of the crust following the formation of the volcanic and sedimentary rocks; diagrams: greenstone belts; model for deposition of volcanic and sedimentary rocks; setting of greenstone -hosted quartz-carbonate vein deposit]

H

2007-05-31T23:59:00.000-07:00

▪ hiatus ▪ hinge (fold) ▪

I

2007-04-30T23:59:00.000-07:00

▪ ice quake ▪ igneous structures

igneous structures

2007-04-18T10:36:00.000-07:00

Magma is molten (igneous) rock formed when Earth's radioactivity heats rocks – because of Earth's geothermal gradient, the deeper in the Earth, the greater the temperature.

◙◙ Rock Index: Igneous Rocks ◙◙

Intrusion:
Magma that is emplaced below the surface when molten rock intrudes into or across strata of country rock cools and crystallizes in a variety of intrusive structures, including large plutonic and small hypabassal structures:

intrusive emplacement / structures: ▪ aplite, aplite dike (◙ aplite) ▪ batholith ▪ boudin ▪ bysmalith ▪ concordant ▪ conformable ▪ diapir ▪ diatreme ▪ dike ▪ discordant ▪ enclaves ▪ hypabassal ▪ laccoliths ▪ lopoliths ▪ pluton ▪ sills ▪ strata ▪ volcano ▪ vein ▪

structures reflecting flow/crystallization within magma chambers: ▪ cumulates ▪ enclaves ▪ schlieren ▪ xenoliths ▪

magmatic processes: ▪ anatexis ▪ assimiliation ▪ exchange of volatiles ▪ fractional crystallization ◘ igneous rocks ◘ igneous structures ◘ magma ▪ magmatic differentiation ▪ magmatic mixing ◘ ophiolite complexes ◘ tectonics

Extrusion:
When melted rock flows extrusively or erupts explosively at the surface is called lava – Hawaii's and Iceland's basalt lavas fountain or flow freely, while other lavas are sticky and explosive, producing deadly pyroclastic flows (Mount St. Helens, Vesuvius, Pinatubo).

extrusive: ▪ aphanitic texture ◘ lava ◙ tectonism ◘ volcanoes ▪ vulcanism

Links: Maps of North American rock types : rock types - metamorphic, plutonic, sedimentary, volcanic; tapestry of time and terrain, terrain.

K

2007-02-28T23:59:00.000-08:00

▪ klippes ▪

L

2007-01-31T23:59:00.000-08:00

▪ laccoliths ▪ Laurentia ▪ Laurentian Craton ▪ layering (fractional crystallization and cumulates) ▪ limb (fold) ▪ lopoliths ▪

laccoliths

2007-01-24T21:46:00.000-08:00

Laccoliths are moderately large concordant intrusions that cause uplift and folding of the preexisting, country rocks above the intrusion. Laccoliths tend to form at relatively shallow depths beneath relative light sedimentary rocks, ensuring that the pressure of intruding magma is sufficiently high to lift overlying rock strata, forming a domed or mushroom-like form with a generally planar base. Laccoliths typically arise from relatively viscous magmas, such as those that crystallize to diorite, granodiorite, and granite.

[images laccolith, 2, 3, 4, 5; laccolith (l) and set of sills (r); monocline and syncline; Henry Mt. laccoliths, Capitol Reef, UT, anaglyphic images (needs 3D glasses); Mt Hillers, Sawtooth Ridge; Mt Ellesworth, UT; DEM Black Mesa laccolith; diagram laccolith Black Hills, SD; Bear Butte, SD, photo; Mt. Elden laccolith; Los Cuernos; topo map of Mount Katahdin laccolith]

Laurentia

2007-01-24T03:06:00.000-08:00

Laurentia, or the Laurentian Shield, or the Canadian Shield is the geological term for the North American Craton.

The North American Craton (brown) has comprised a portion of a series of supercontinents, and has remained stable for about 600 million years. This cratonic region comprises a basement of Precambrian metamorphic and igneous rock, exposed in the north as the Canadian Shield, and covered by a relatively thin cover of younger sedimentary rock on the Interior Platform.

The geologically stable rocks of the cratonic core are surrounded by deformed rocks (purple) and more recently deposited sedimentary material (green). Compare to bedrock of North America.

Maps of North American rock types : rock types - metamorphic, plutonic, sedimentary, volcanic; tapestry of time and terrain, terrain.

lopolith

2007-01-10T08:09:00.000-08:00

Lopoliths are saucer shaped concordant emplacements that lie parallel to the strata of intruded country rock.

Lopoliths are relatively small plutons that typically developed an upper surface that is concave downward. This sagging shape may be attributable to volume reduction when magmas crystallize. The weight of the overlying strata would cause collapse into the volume previously occupied by more voluminous liquid magma.

Lopoliths formed by a similar mechanism to laccoliths, but they are composed of dense, mafic magma that allows depression by the overlying strata on cooling. Many lopoliths contain layered gabbroic rocks. Some lopoliths are very large, with thicknesses of many kilometres. The Bushveldt lopolith in southern Africa is several hundred kilometres across and contains the richest platinum deposits known.

image Bushveldt volcanics

M

2006-12-31T23:59:00.000-08:00

▪ magmatic differentiation ▪ magmatic mixing ▪ mapping ▪ mélange ▪ minerals ▪ monoclines ▪

◊◊◊ Minerals Index ◊◊◊

magmatic differentiation

2006-12-24T17:43:00.000-08:00

Magmatic differentiation is a complex process whereby a single melt can produce a wide variety of different igneous rocks. Some degree of differentiation typically develops across space and time in exposed magma bodies (intrusive or extrusive).

Most melts develop in the lower crust or in the asthenosphere (upper mantle), so, such melts have a primitive mafic or basaltic composition, whereas melts developing in the upper crust have a higher initial silica content.

Regardless of depth of formation, melts ultimately produce igneous rocks that depend on the composition of the original melt, on the properties of wall rocks encountered during ascent, and on rate/s of cooling. The main processes involved in differentiation are assimiliation, exchange of volatiles, fractional crystallization, and magmatic mixing.

▪ Bowen's Reaction Series

[links: animations: fractional crystallization/magmatic settling]

magmatic mixing

2006-12-24T11:01:00.000-08:00

Magmatic mixing in areas of active magmatism is the process wherby adjacent magma bodies can develop transient subsurface communications before their eruption or final subsurface emplacement.

Anatexis-related magmatic mixing involves the secondary melting of mid- to lower crustal rocks upon contact with much hotter, rising mafic melts of mantle origin, and produces felsic (feldspar- and quartz-rich) magmas in arc and continental rift settings. Such melts may reach high crustal levels carrying both mantle heat and mantle material.

mapping

2006-12-24T08:08:00.000-08:00

A geological map provides a graphic representation of selected geological features within a desired surface topographic or subsurface area. The size and relative position of each feature on the map corresponds to its correct geographic situation according to an established scale and projection. When mapping out a region, standardized terms are employed to describe the spatial orientation (attitude) of planar and linear elements of folds, faults, and other geological structures (diagram):

├ . The strike of a plane is the compass direction of the line formed by the intersection of the horizontal plane with the inclined plane under consideration. So, strike marks the geographical direction perpendicular to dip. ├ Strike and dip are depicted on geological maps by a long line (strike) with a short perpendicular line (dip) and a number indicating the angle of dip (degrees).

In North Amercia, strike is often expressed as the angle E or W of true North (0º-90º). In the European system, compass directions are expressed as azimuths. The azimuth is measured clockwise along an horizontal plane from the true North direction to the strike line (0º-359º). N=0º, E=90º, S=180º, W=270º.

A strike tending 26º east of true north would be expressed as N26ºE in Canada and the US, and as 026º in the European system. Similarly 74º west of true north would be expressed as N74ºW in the North American system, and as (360-74=) 286º in the European system.

├ .. The dip of a plane is analogous to plunge, and is the angle in degrees measured from an horizontal plane down to the inclined plane under consideration. That is, dip is the angle between the inclined plane and the horizontal plane, and is measured along a vertical plane perpendicular to the strike line of the plane.

Strike and dip directions of a fold are always mutually perpendicular, though two planes could have the same numerical strike (direction) and dip (angle). That is, a plane inclined at 45º to the horizontal (dip) that is facing SSE (135º) could have a strike (direction) of 45º East (o45º).

Strike and dip are differentiated in North America by expressing the direction according to the geographic quadrant faced by the planes. By European convention, strike is expressed as a three digit azimuthal number, and dip as a two digit angular number. Thus, a plane striking 25º and dipping 45º toward the southwest would be noted: 025-45SW.

mélange

2006-12-20T00:54:00.000-08:00

A mélange is a mappable-sized, breccia containing varied rocks jumbled together with little continuity of contacts. The diverse blocks within a mélange are supported and separated by a matrix of fine grained material (typically shale, slate, or serpentinite) with a tectonic fabric. Mélanges originate either as components of tectonic accretionary prisms, as a result of gravitational submarine sliding (olistostromes), or through diapirism (diagram).

An olistostrome, or "gravitational mélange", is a mappable, chaotic sedimentary deposit composed of heterogeneous olistoliths (blocks) derived from submarine gravity siliding or slumping of unconsolidated sediments. Such slides may have traveled several dozen to several hundred kilometers, resulting in large, thick, heterogeneous stratiform units that accumulated somewhat chaotically from an active fault escarpment, in various tectonic settings.

Olistostromes may range from several meters up to several hundreds of meters thick, and component olistoliths (blocks)may have preserved their internal coherence to the extent that the original facies can still be established. Olistoliths are immersed in a fine-grained matrix (typically mudstone or serpentinite).

Olistostromes are mélanges formed by semi-fluid accumulation of submarine, gravitational flow. Slide masses composed of hard rocks plus semi-indurated and soft sediments fail when the softer and friable materials form a basal mobile phase. Such a slide may even have liquefied and progressively disintegrated during displacement. So, olistostromes are stratigraphic units with chaotic bedding or without true bedding, yet which are intercalated between normal sedimentary bedding sequences.

[links: images: formations: mélanges: tectonic mélange rocks; melange enclosed in dark matrix of serpentinite and containing a block of S-type, coarse granite, eastern flank of the Cordillera Occidental; a mélange is a mappable body of rock that includes fragments and blocks of all sizes, embedded in a generally sheared matrix; Franciscan mélange, another, Central Valley; Franciscan mélange - isolated blocks of resistant Franciscan rocks of various types (known as knockers) mixed into a sheared mudstone matrix; 3D of Point Bonita, Kirby Cove; mélange, Marin headlands, another, and close-up of mélange with chert clasts; close-up of disarticulated Hartland granofels block in a tectonic mélange near Cameron’s Line; mélange close-up; close-up of fragment of greenstone in mud-rich mélange, Malua Bay, south of Batemans Bay, south coast of New South Wales, fragments of sandstone in mud-rich mélange, Garden Bay, near Malua Bay, small fragments in mud-rich mélange, Sunshine Bay, south of Batemans Bay, chert mélange from the southern side of Burrewarra Point, south of Batemans Bay; exotic breccia blocks, eroded from the Hoh mélange, near Goodman Creek, 2; mélange, Kiryu-Kurohone Complex, Japan; tectonic mélange at the base of a large thrust sheet of Bravo Lake volcanic rocks; quartzite blocks in Gwna Mélange, Llyn Peninsula and Anglesey, Wales, and pillow-basalts, Porth Dinllaen ; Dunnage Mélange near Gander, Newfoundland; mélange, Nfld; Cretaceous rock in mélange in CA; thrust fault composed of imbricated structure, coherent and mélange facies, shear fabrics, Japan; amphibolite-grade mélange in Catalina Schist; olistrosomes: olistrosome; olistostromes (brownish strata in lower part of hillside) and ophiolites (dark material in upper part of section) exposed in eastern Cuba; webpages: trenches and mélanges]

minerals

2006-12-16T10:35:00.000-08:00

Mineral are naturally occuring homogeneous solids with definite chemical compositions and ordered atomic arrangements, and are either elements or chemical compounds that have been formed by geological processes.

◊◊ Minerals Site Map ◊◊

See also IMA Commission On New Minerals And Mineral Names:

monoclines

2006-12-10T10:09:00.000-08:00

A monocline is a step-like geological fold without a change in dip direction across the fold hinge because the layers dip in the same direction.

This is contrasted with anticlines, in which limbs dip away (curve downward) from the hinge, and with synclines, in which the limbs dip toward the hinge (curve upward).

(images at left - click to enlarge - top, schematic of monocline; middle, looking along the eroded strata of Waterpocket Fold in Capitol Reef National Park, mid-res, hi-res; bottom Waterpocket Fold from Strike Valley Overlook, mid-res; image below right, schematic cross section through Waterpocket Fold.)

The Waterpocket Fold in Capitol Reef National Park is a classical monocline that is almost 100-miles long (160 km). It is a huge regional fold with one very steep side in otherwise nearly horizontal layers.

During the Laramide Orogeny, between 50 and 70 Ma, the rock layers on the west side of the Waterpocket Fold were elevated more than 7000 feet (> 2000 m) higher than the layers on the east. The entire Colorado Plateau was subsequently uplifted again, and erosion has exposed this fold within the last 15 to 20 million years.

[links: formations: classic monocline in which Mesozoic strata dip 45º, near Mexican Hat, Utah, 2, 3; monoclinal drape fold, Shell Canyon, Bighorn mountains; Dinosaur National Park monocline; Waterpocket Fold, Capitol Reef, Utah, and eastern flank of Waterpocket Fold, 2; Waterpocket Fold area, as viewed from Boulder Mountain; Strike Valley overlook, 2; Capitol Reef, 2, 3, 4, from the Burr Trail, Chimney Rock trail, Sunset Point; Cross sections of Strike Valley of Waterpocket Fold showing formations and their topographic expression; View of tilted beds along the Waterpocket Fold; satellite: monocline and syncline crossed by transverse stream, Capitol Reef, UT, and nearby monocline and syncline; Henry Mt. laccoliths, Capitol Reef, UT; satellite image; webpages: Capitol Reef; Folding Satellite Images Set]

N

2006-11-30T23:59:00.000-08:00

▪ nappes (allochthonous) ▪ nonconformity (angular unconformity, blended unconformity, disconformity, hiatus, nonconformity, paraconformity, unconformities) ▪

nappes

2006-11-24T08:08:00.000-08:00

A nappe is a large, sliver of rock that has been thrusted far from its original position (allochthonous) by thrust faulting during continental plate collisions.

1. klippe 2. nappe 3. window
gray = allochthon (thrusted), brown = autochthon (in situ)

A thrust fault is a special case of low dip-angle fault that formed during regional compressional deformation. The folds associated with nappes are sheared to such an extent that they fold back over on themselves and break apart, producing large-scale recumbent folds of the hanging wall block.

The underlying autochthonous (original, in-place) rocks are visible in tectonic windows beneath nappes when erosion removes the allochthonous rocks of the nappe. An isolated island of allochthonous nappe rock surrounded by autochthonous rock is termed a klippe.

Nappes or nappe belts arose during the complex tectonic history of the European Alps. The Austroalpine nappes of the eastern Alps comprise three thrust faulted nappe stacks that lie atop three older Penninic nappes [Subalpine nappes].

◙ subduction zone magmas ◙

[links: images: formations: nappe: folds in the Musconetcong Nappe within the Reading Prong nappe megasystem in Lehigh County; unconformity between coarse basal breccia-conglomerate and the underlying Quarff Nappe metasediments; Morcles nappe; overturned folds in the Liassic limestone and marls of the Helvetic Doldenhorn Nappe, Ferdenrothorn, 3180 m (Loetschental Mountains between the Bernese Oberland and Valais); Flysch turbidite of the Ligurian nappe (constructed of "olistrostromes", undersea landslides), and another overturned turbidite of the Ligurian nappe, siliclastic in the lower (younger) layer, carbonate in the upper (older) layer; the upper part of the Matterhorn is a fragment of Africa, and the lower part is the Jurassic Tethys oceanic crust (Zermatt-Saas ophiolite (lower photo) including pillow lavas and serpentinite bodies (foreground knob on left); klippe: Chief Mountain klippe; Cam Loch klippe; klippe, Salto de gitano; klippe; Carrancas Klippe, Andrelândia Sequence, Minas Gerais; limestone klippe, near Nowy Targ, erosional remnant of a thrust sheet that rests on younger flysch deposits; petrology: eclogites (resulting from prograde metamorphic history of garnet growth during subduction of the oceanic crust) are common in both the Zermatt-Saas complex and older units such as the Sesia nappe (SE) and reflects sections of African continental margin, and HP granitic rocks, including jadeite- and phengite-bearing granites (and relict biotite) in the Sesia nappe; closure of the Donara nappe at the metamorphic core of the Himalaya and Karakoram; Viyazón-Reigada Syncline, Somiedo nappe; maps: 3D topographic/bathymetric render looking down on the Alps from the NW into the Mediterranean Sea and northern Africa (upper). Prior to opening of the Mediterranean, the Apulian plate ("Africa") was thrust onto Europe in the late Mesozoic to early Cenozoic, forming the HP-UHP terranes of the Alps, as well as Greece (upper left) - geological map; geologic map of the Gades Cove geologic window, created as the Great Smoky Thrust Fault was eroded; Map of Cam Loch klippe]

O

2006-10-31T23:59:00.000-08:00

▪ olistrosome ▪ open-space filling (veins) ▪ ophiolites ▪ overgrowth (crystals) ▪ overturned and recumbent folds ▪

ophiolites

2006-10-09T16:17:00.000-07:00

Ophiolite complexes, or ophiolites (for 'snake stones') are uplifted or emplaced sections of oceanic crust and subjacent upper mantle that have become exposed within rocks of the continental crustal. The age of ophiolite formation is often quite close to the age of their emplacement into the continental crust. (left - click to route to larger image - Ophiolites (Mohorovičić discontinuity) in Gros Morne National Park, Newfoundland.)

Most ophiolites are assigned to either the Tethyan or Cordilleran groups, which have different modes of emplacement yet are both SSZ in origin. The episodic emplacement of ophiolites throughout geological history suggests that the complexes formed and emplaced at times of super-continent break-up and dispersal when large ocean basins adjacent to super-continents subducted as rifting progressed.

Tethyan ophiolites are named for the ancient Tethys ocean, and are found in the eastern Mediterranean areas ( Troodos in Cyprus, Semail in Oman). These are relatively complete classic ophiolite assemblages that were emplaced intact onto a passive continental margin.

Cordilleran ophiolites are named for the Cordillera mountain belts of western North America. These ophiolites are not associated with a passive continental margin and sit on subduction zone accretionary complexes (subduction complexes). Cordilleran ophiolites include the Coast Range ophiolite of California, the Josephine ophiolite of the Klamath Mountains (California, Oregon), and ophiolites in the southern Andes of South America.

Ophiolite assemblages in collisional mountain belts, such as the Alps, represent incipient ocean crust at thinned continental margins (formed during rifting and continental drift) that was emplaced into the collision zone.

The stratigraphic sequences observed in some ophiolites suggest origins in lithosphere-forming, spreading centers at mid-oceanic ridges. Supra-subduction zone (SSZ) ophiolites are more closely related to island arcs than to ocean ridges, and are formed by rapid extension of fore-arc crust during subduction initiation followed by rebound of the continental crust carrying forearc lithosphere (ophiolite) atop it. The occurrence of ophiolite complexes within orogenic belts documents the former existence of ocean basins now consumed by subduction, so providing supporting evidence for plate tectonics.

◙ subduction zone magmas ◙

more information plus image links

overgrowth

2006-10-04T19:08:00.000-07:00

Crystal overgrowth involves the partial mantling of a mineral:
● by material of the same composition, or
● by material of the same mineral species but different solid-solution composition, or
● by an unrelated mineral.

Overgrowth implies crystallographic continuity between the two participating minerals so far as permitted by their differing crystal structures. Where the overgrowth forms a more or less continuous rim around enclosed mineral, it is termed a mantle.

overturned and recumbent

2006-10-04T06:39:00.000-07:00

Folding may be so pronounced that strata are overturned beyond the vertical.

In an overturned fold, the beds dip in the same direction on both sides of the axial plane because one of those limbs being rotated through an angle of at least 90º. An extreme example of an overturned fold occurs when the axial plane is horizontal – this is called a recumbent fold.

(To visualize this concept, place an opened book flat on a table with the spine (hinge) uppermost. Close the book by squeezing the edges toward one another, and then flip the book onto one side. One half of the book (one limb) has rotated through more than 90º.)

[links: animations: folding; axial surface, plunge, fold; images: hand-specimen: overturned fold in limestone; formations: thrust and overturned fold; recumbent, isoclinal folds in dolomitic marble, Mosaic Canyon, Death Valley (fingers point to the same layer, which can be traced around a fold hinge to the left); recumbent folds and recumbent folds and extension veins, St. Bude, Cornwall; overturned folds in the Alps; overturned folds with thinning and detachment of the lower limb, Herdla island, near Bergen, western Norway, overturned fold of Fur Fm. (Paleocene) bedrock, Hanklit, Denmark; close-up of recumbent fold; overturned fold, 2; overturned section, Five Springs, Bighorn Mountains]

P

2006-09-30T23:59:00.000-07:00

▪ paraconformity ▪ pegmatite dike ▪ peridotite ▪ phacoliths ▪ phaneritic texture ▪ phenocryst ▪ phyllitic texture ▪ plunge, plunge direction (fold) ▪ pluton, plutonic ▪ porphyritic texture ▪ porphyry ▪

peridotite

2006-09-20T06:14:00.000-07:00

Peridotite is an ultramafic, ultrabasic (less than 45% silica), dense, plutonic igneous rock comprising mostly olivine and pyroxene. Most of the Earth's upper mantle (asthenosphere) is composed of peridotite that originated during the accretion and differentiation of the Earth, or that has differentiated, by precipitation of olivine ± pyroxenes, from basaltic or ultramafic magmas in turn derived from partial melting of the upper mantle peridotites. Deeper in the crust, olivine is replaced by a high pressure polymorphs, so peridotites do not occur at depths greater than 400 km.

Peridotite emplaced in the continental crust is typically found in obducted ophiolite complexes, as xenoliths in basalts and kimberlite pipes, and as orogenic peridotite massifs and alpine peridotites. Olivine is unstable at shallow depths and reacts rapidly with water, so that much surface peridotite has been altered to serpentinite by a process in which the pyroxenes and olivines are converted to green serpentine.

phacoliths

2006-09-17T23:46:00.000-07:00

Phacoliths are concordant plutonic bodies that lie parallel to the bedding plane or foliation of folded country rock. They occur along the crests of anticlines or the troughs of synclines in folded sedimentary strata.

Rarely, the body of a phacolith may extend as a sill from the crest of an anticline through the trough of an adjacent syncline, such that it has an S shape in cross section. The hinge of folds in intensely folded terrains are areas of reduced pressure, so are potential sites for magma migration and emplacement.

[links: images: close-up: autolith of biotite-rich, enclave bearing, porphyritic granite in another more phenocrystic granite, Wolf Mountain Intrusion (Phacolith), Proterozoic Llano Uplift, central Texas]

phenocryst

2006-09-17T19:10:00.000-07:00

A phenocryst is a conspicuous, large crystal embedded in a finer-grained matrix of smaller crystals in a porphyritic igneous rock.

(left - click to enlarge image - a plagioclase feldspar phenocryst, Lambert Dome, Yosemite, courtesy Daniel Mayer)

Porphyrys are formed by a two-stage cooling of rising magma. First, deep crustal magma cools slowly, allowing formation of large phenocrysts (diameter 2 mm or more). Second, the magma cools rapidly at shallower depths having been injected upward or extruded by a volcano, allowing for formation of small crystals in the groundmass.

[images : phenocrysts of feldspar in matrix of quartz, feldspar and mica; large feldspathic phenocrysts in granite; large zoned plagioclase phenocryst; andesites with phenocrysts; formation: Death Valley vitrophyre, a phenocryst-bearing obsidian; thin-section feldspar and clinopyroxene phenocrysts in Mt. Fuji basalt; thin-section clinopyroxene phenocrysts, ppl; thin-section augite phenocryst, same augite phenocryst in cross-polarized light; resorbed quartz phenocryst, in PPL; thin-section anhedral amphibole phenocryst; igneous rocks in thin section]

plutonic

2006-09-13T03:04:00.000-07:00

Pluton is a general term for any large igneous intrusion of crystallized magma. Rocks that emplaced in large intrusive structures are termed "plutonic", whereas those that form in small intrusions are called "hypabyssal".

Plutonic igneous rocks cooled and crystallized from intrusive or irruptive magma within the Earth's crust. Plutonic rocks exhibit a fine-grained aphanitic texture if the magma cooled close to the surface in volcanic necks or feeder pipes, with grain size increasing through to coarse-grained, phaneritic textures for magmas that cooled very slowly at great depth in very large magma chambers. (image - click to enlarge - plutonic rock of North America)

Plutonic rock types include granite, diorite, gabbro, and rhyolite. Plutonic structures include huge solidified magma chambers called batholiths, laccoliths, stocks, bysmaliths, and lopoliths, while smaller hypabyssal structures include diapirs, dikes and dikelets, ring dykes, sills, volcanic necks and plugs, and cone sheets. Less commonly encountered plutonic structures include: phacoliths, cactoliths, ductoliths, harpoliths, sphenoliths, akmoliths, and ethmoliths. Chonoliths are intrusive igneous structure with shapes so irregular that they do not fall into the normal categories. Any intrusive structure may later become exposed at the surface by erosion.

(Veins are finite volumes within rocks, which are filled with crystals of minerals precipitated from hot (aqueous) fluids.)

intrusive emplacement / structures: ▪ batholith ▪ boudin ▪ bysmalith ▪ concordant ▪ conformable ▪ diapir ▪ diatreme ▪ dike ▪ discordant ▪ enclaves ▪ hypabassal ▪ laccoliths ▪ lopoliths ▪ pluton ▪ sills ▪ strata ▪ volcano ▪ vein ▪

structures reflecting flow/crystallization within magma chambers: ▪ cumulates ▪ enclaves ▪ schlieren ▪ xenoliths ▪

magmatic processes: ▪ assimiliation ▪ exchange of volatiles ▪ fractional crystallization ◘ igneous rocks ◘ igneous structures ◘ magma ▪ magmatic differentiation ▪ magmatic mixing ◘ ophiolite complexes ◘ tectonics extrusive: ◘ lava ◘ volcanoes ▪ vulcanism

igneous rocks: ◘ basalt ▪ cumulates ◙ dunites ▪ enclaves ◘ felsic (sial) ◘ gabbro ◘ granite ◙ granodiorite ◊ groundmass ◙ keratophyres ◘ lava ◘ mafic (sima) ◘ magma ◘ ophiolite complexes ◙ pegmatites ◘ peridotite ▪ peridotite ▪ porphyry ▪ schlieren ▪ tonalites ◊ xenocryst ▪ xenoliths

Maps of North American rock types : rock types - metamorphic, plutonic, sedimentary, volcanic; tapestry of time and terrain, terrain.

[images : dike, sill, diapir, laccolith, batholith]

porphyry

2006-09-10T06:08:00.000-07:00

Porphyry is classically a reddish-brown to purple igneous rock containing large phenocrysts of minerals such as feldpar or quartz embedded in a fine-grained matrix or groundmass of feldspar. More generally, the term porphyry encompasses any rock with a texture of phenocrysts embedded in a finer textured matrix (such as granite) or with visible crystals in an aphanitic matrix (such as basalts, aphanites or phanerites).

Porphyrys are formed by a two-stage cooling of rising magma. First, deep crustal magma cools slowly, allowing formation of large phenocrysts (diameter 2 mm or more). Second, the magma cools rapidly at shallower depths having been injected upward or extruded by a volcano, allowing for formation of small crystals in the groundmass. Differential cooling permits separation of dissolved metals into distinct zones, creating rich, localised metal ore deposits (gold, copper, molybdenum, lead, tin, zinc and tungsten).

[images - roll-over link for preview (where available); large images (well worth a visit) show only as a corner on preview : hand-specimen diabase porphyry : hand-specimen rhyolite porphyry : hand-specimen andesite porphyry, 2 : porphyry : hand specimen of "ore" from the Coed y Brenin porphyry-copper deposit : Wolf porphyry : red porphyry : red Imperial porphyry Egypt : 4th C porphyry head : xenolith-bearing hybrid porphyry : porphyrite dike cutting undeformed post-tectonic granite : closeup of hybrid porphyry texture, 2, microphenocryst of titaniferous augite rimmed with brown sodic ferrohornblende : outcrop of hybrid porphyry from pipe cluster : gabbro xenolith in hybrid porphyry : Guanella Pass porphyry : Porphyry Basin, 2, 3, 4, 5, 6, g : porphyry Trentino : porphyry quarry : porphyry copper mine : gold-copper porphyry at Quispe : thin-section plagioclase, augite, oxides porphyry, 2, 3, Skaergaard intrusion photomicrographs]

R

2006-07-31T23:59:00.000-07:00

▪ recumbent and overturned folds ▪ refractories and fusibles ▪ ribbon (banded) vein ▪

▫ Rodinia supercontinent

Rodinia

2006-07-10T20:16:00.000-07:00

Rodinia was a Neoproterozoic supercontinent that began to form roughly 1.3 Ga during the Grenville Orogeny.

The configuration of Rodinia has been hypothetically reconstructed based upon paleomagnetic data and the mountain chains (orange) formed by the Grenville orogeny, which span several modern continents.

Rodinia probably existed as a single continent from 1 billion years ago until it began to rift into eight smaller continents about 800 million years ago.

S

2006-06-30T23:59:00.000-07:00

▪ schistose texture ▪ schlieren, schlieren arch, schlieren dome ▪ shear strain stress ▪ shear zones ▪ sills ▪ slaty texture ▪ slump, slumping ▪ sills ▪ stock ▪ strata ▪ strain stess shear ▪ stress strain shear ▪ strike (fold) ▪ symmetric fold ▪ syncline ▪

schlieren

2006-06-22T16:18:00.000-07:00

Schlieren (geology) are fragile, usually elongate concentrations of mafic material. A schlieren could be, for example, a tabular zone in a granite with either more or less of some of the minerals in the surrounding granite, typically the dark (mafic) minerals.

The origins of schlieren are not always clear; they may be produced by differential magma flow, or disaggregation of xenoliths, or by other mechanisms. Schlieren are usually interpreted as having arisen by one of four mechanisms:
1. shearing of heterogeneities (enclaves or xenoliths),
2. crystal sorting during convective flow,
3. crystal sorting during magmatic flow, or
4. crystal settling.

At the time of formation or crystallization of a magma chamber, mafic minerals such as biotite, rare earth elements of the lanthanide and actanide series, allanite, and the phosphate mineral apatite can orient in a preferred manner that creates bands. Schlieren bands vary in geometry ranging from deformed, tubular, planar, and rings, to arachnid (spider-like) formations.

A schlieren arch is an intrusive igneous body with flow layers that occur along its borders, but which are poorly developed or absent in its interior. A schlieren dome is an intrusive body that is almost completely outlined by flow layers that culminate in one central area.

[images: schlieren in biotite-rich mantle with granodiorite inside and outside, and close-up of the margin of the schlieren; a prominent schlieren that defines a structure rather like the hinge region of an isoclinal fold, and close-up of the upper left side of the schlieren showing dark, biotite-rich prominent part of the schlieren (curving to the right) with thinner, less prominent biotite -rich streaks extending upwards (the K-feldspar phenocrysts are approximately parallel to the schlieren margin); spidery "arocknid", composed of two sprays of thin schlieren; thick portion of schlieren with irregular convex surface, parallel alignment of K-feldspar phenocrysts, and K-feldspar phenocrysts in the host granodiorite that are nearly perpendicular to the convex margin of the schlieren (top center); K-feldspar-rich mass in normal foliated granodiorite; schlieren.

Schlieren (from the German for 'streaks') are optical inhomogeneities in transparent material that are not visible to the human eye. Schlieren, shadowgraph, and interferometric techniques are used to study the distribution of density gradients within a transparent medium.

shear zones

2006-06-17T19:54:00.000-07:00

Shear zones involve volumes of rock deformed by shearing stress under brittle-ductile or ductile conditions, typically in subduction zones at depths down to 10-20 km. Shear zones often occur at the edges of tectonic blocks, forming discontinuities that mark distinct terranes. Shear zones can host ore bodies as a result of hydrothermal flow through orogenic belts, are commonly metasomatized, and often display some retrograde metamorphism from a peak metamorphic assemblage.

Close to the Earth's surface, cool rocks respond to tectonic stresses with fracture and faulting. At greater depths than ductile shear zones, migmatites result from high temperature/high pressure prograde Barrovian regional metamorphism, and at still higher temperatures, rocks melt to form magmas.

Transpression regimes, such as the Alpine Fault zone of New Zealand, form during oblique collision of tectonic plates and during non-orthogonal subduction. Transpression typically generates oblique-slip thrust faults, strike-slip faults, or transform faults. Microstructural evidence of transpressional regimes include rodding lineations, mylonites, augen-structured gneisses, and mica fish.

Transtension regimes are oblique tensional environments that result in oblique, normal geologic faults and detachment faults in rift zones. Microstructural evidence of transtension includes rodding or stretching lineations, stretched porphyroblasts, and mylonites.

Shear zones can extend from centimeters to several kilometres in width, and display deformation, folding, and foliations in dynamically altered rocks (breccias, cataclasites, mylonites, S-L-L-S breccia or cataclasite is formed, with the rock milled and broken into a mélange of random fragments.

Pseudotachylites form at depths from 5-10 km, where confining pressures are focused into discrete fault planes and are sufficient to prevent brecciation and milling. The frictional heating at these depths can melt the rock to form pseudotachylite glass or mylonite, and adjacent to these zones, can result in growth of new mineral assemblages.

At greater depths, angular breccias transform into ductile shear textures and mylonite zones, as ductile shear zones accommodate compressive stress through dislocation creep within minerals, fracturing of minerals and regrowth of sub-grain boundaries, or by lattice glide along preferred orientation foliation planes in phyllosilicates.

Within the depth range of 10-20km, ductile deformation conditions prevail and frictional heating is dispersed throughout shear zones, resulting in distributed deformation and a weaker thermal imprint. Here, deformation forms mylonites, with dynamothermal metamorphism observed rarely as the growth of porphyroblasts in mylonite zones.

◙ subduction zone magmas ◙

[links: images: animation: fabric in simple shear; shear zone experiment; formations: mylonitic migmatitic granite-gneiss in shear zone, Epupa Complex, S of Red Drum, NW Namibia; Golden Eagle Shear Zone, Yukon; melt enhanced shear zone, along the base of an intruding batholith; shear zone in the axial zone of the Pyrenees, Parc natural del Cap de Creus, Spain; dike cutting a shear zone, Snake Range, Nevada; sheath fold in boulder, Tarfala Valley, Sweden, and sheath folds, nSweden; fold in high strain zone, NZ; close-ups: ultramylonite core (~1 cm thick) from ductile shear zone of the Diana Syenite of the NW Adirondacks; shear zone related fold in the Kohistan Arc Complex, Northern Pakistan; rock texture in shear zone; rock in ductile shear zone; right-lateral, ductile shear zone; anorthosite in ductile shear zone, Adirondacks; close-up of dextral shear zone; leucosome cuts gneissic layering; 1.7 Ga foliated quartz monzonite of Boulder Creek batholith in Idaho Springs-Ralston shear zone with strong mylonitic (sheared) fabric that parallels the shear zone, 2, 3; 1.7 Ga metapelite (metamorphic marine claystone) that includes large porphyroblasts of pink quartz and andalusite (dull dark gray blocky crystals), and wavy alignment of porphyroblasts in this rock with a mylonitic fabric indicates a complex deformation history; with en echelon antithetic veins in dextral shear zone, Baraboo Quartzite; sigmoidal antithetic fractures in a dextral shear zone, Tiddiline Conglomerate, Bou Azzer inlier, Morocco; mylonitic marble in shear zone, Escambray Massif, Central Cuba; lower greenschist facies shear zone cutting basement schists, assymmetric clast of pegmatite, assymmetric pod of leucogranite in schist, ptygmatic folds of leucogranite in schist, assymmetric pod of schist, Cap De Creus, neSpain; thin-sections: thin section of Lower Ordovician Pinnak Sandstone showing multiple tectonic foliations, the most prominent of which is a crenulation cleavage that overprints an early fine foliation; euhedral staurolite (yellow pleochroic in PPL) overgrows shear zone between large light coloured plagioclase porphyroblasts (graphite inclusions outline shear zone, staurolite crystals postkinematic); garnet with spiral-shaped inclusion trails indicating synkinematic growth, and a dextral sense of shear; diagrams: cataclasite-mylonite in shear zone; block diagram - shear zone host for gold, geometric relationships between structural elements of zone and veins; region within macroscopic shear zone illustrating bimodal porosity distribution within shear zone; model of shear zone]

sills

2006-06-16T00:12:00.000-07:00

Sills are tabular slabs or concordant intrusive (plutonic) sheets of igneous rock that intruded laterally and horizontally, or nearly horizontally, as magma. Sills lie between and parallel to layers of older sedimentary rock, beds of volcanic lava or tuff, or along the direction of foliation in metamorphic rock.

slump

2006-06-13T03:11:00.000-07:00

A slump (geology) is a mass movement process of slope failure, in which a mass of rock or unconsolidated material drops along a concave slip surface. Slump units move downslope as an intact block (without internal deformation of the landslide material) and frequently rotate backwards.

Slumps appear as discrete block movements, whereas slides usually break up and travel downslope. The term 'slump' is also used to refer to the material that breaks off in a slumping slide.

(images - click to enlarge - above left, diagram of slumping; below right, slump features; bottom right, cross-section of a Toreva block.)

Slumps are sometimes caused by clear cutting on unstable soils, and the sagging and rotational movement of the mass of soil and rock is due in part to water infiltration and lubrication of clay-rich soils below. Coastal cliffs are subject to slumping when wave action undercuts lower layers. A submarine slope slump movement may be result from tidal forces acting on an unstable slope, or from a large seismic event near the affected body of water.

Toreva block slumping is a distinctive form of landslide in which huge backrotated blocks of resistent strata such as sandstones and limestones collapsed when weaker underlying beds such as shales were undercut. The existence of Toreva blocks suggests that an earlier warmer or wetter earlier climate existed in the region. (The Transantarctic Mountains have Toreva blocks, implying temperate conditions in Cenozoic Antarctica)

[links: fresh slumping: slumping in unconsolidated sediments, the Naze; recent slumping: Norfolk coastal slumping; slump folds in shale, Chisholm Creek, outside Wichita; slumping, 2, Milpas; old slumping: slump structures in friable sandstones of the Bournemouth Marine Beds (Eocene), channeling and slump structures, close-up of slump structures in friable sandstone, sandstone with slumps, truncated at reactivation surface; slump in cliff, Italy; slump; slump block, Clifton Creek, 2; Toreva blocks abound in the Grand Canyon area where the Bright Angel Shale has failed: two massive backrotated Toreva blocks of the Cogswell East Landslide, close-up view of the upper slide block of the East Cogswell Landslide; multiple slump blocks; Vermillion Cliffs; Surprise Valley Toreva block; webpages: 218-Mile Toreva Block; Cogswell Butte Landslides And River Diversions]

stock

2006-06-05T09:48:00.000-07:00

Stocks are smaller structures than batholiths, having exposed surfaces of less than 100 sq.km..

Stocks are composite bodies that have probably been fed by deeper level batholiths and may have been feeders for volcanic eruptions. However, because considerable erosion is necessary to expose a stock or batholith, the associated volcanic rocks rarely remain.

strata

2006-06-05T06:56:00.000-07:00

Strata (sing. stratum) are distinctive layers or beds, of sediments or sedimentary rocks deposited consecutively, or with interruption by unconformities, atop other rocks. Each sedimentary stratum has approximately the same composition throughout, reflecting conditions at the time of deposition.

Biostratigraphy is employed to correlate the age of rocks within strata according to their fossil assemblages.

[images : Cerro de Cristo Rey, New Mexico, rock layers in the Anapra formation, 2, 3, close-up of cross-bedding; cross-bedding and flowing water; cross-lamination Pease Bay; cross-bedding Navajo Sandstone; cross-bedding Hawkesbury Sandstone; cross-bedding Kaibab; Scout Lookout; sandstone layering; cross-bedding Glenshaw Formation; slumped and deformed lacustrine sediments of Morale Claim Maar]

For a comprehensive glossary of terminology related to rock strata, see GGIPAC BeddingPattern (pdf) or html version.

stress strain shear

2006-06-05T06:35:00.000-07:00

Stress is defined as a force applied over an area, and has the dimensions F/A.

Shear is a stress that results from the opposition of forces that are not aligned (F/A).

Strain is defined as deflection divided by the original dimension, and is a measure of deformation, that is, the differential change in size, shape, or volume of a material. Strain is dimensionless.

The modulus of elasticity is stress divided by strain.

Lithostatic stress or confining pressure is uniform stress that operates equally in all directions in rocks. Confining pressure is due to the burden of overlying rock, just as seawater exerts equal pressure from all directions at depth.

Differential or divertorial stress results from compressional or extensional tectonic stresses that are not equal from all directions: tensional stress (stretching), compressional stress (squeezing), or shearing stress (side to side shearing). Compressional stresses act along the direction of maximum principal stress, whereas extensional stresses act along the direction of minimum principal stress.

For the mutually orthogonal directions along which stress is applied, the
▪ maximum principal stress direction is designated σ₁ ,
▪ minimum principal stress direction is σ₃ , and
▪ intermediate principal stress direction is designated _σ2 .
▪ normal stress, directed perpendicular to a given plane is designated, σ_η
▪ shear stress, directed parallel to a given plane is designated, σ_τ
▪ vector components of stress can be expressed according to Cartesian coordinate system as 9 components (x,y, z combinations) relative to the 3 mutually perpendicular x-y-z axes

Materials differ in their responses to stress, depending upon composition, conditions of temperature and confining pressure, and strain rate. However, regardless of intrinsic degrees of brittle or ductile qualities, all strained materials pass through 3 successive stages of deformation: elastic, ductile, and fracture (failure, or brittle deformation). Provided that the strain rate is sufficiently slow to allow minerals to accommodate structurally, minerals can adjust to applied stresses by a variety of mechanisms.

In elastic deformation, rock changes shape by a very small amount and the deformation is not permanent. Elastic deformation occurs only with small differential stresses, which are less than the rock's yield strength. Rock adjacent to failed rock in earthquakes propagates seismic waves by elastic deformation, then springs back to its original shape in elastic rebound. For this reason, structural damage to rocks and formations provides the only evidence of passage of past seismic waves.

In ductile deformation, rock deeper than 10-20 km is subjected to enormous lithostatic stress, and the high temperatures of burial render the hot rock softer and more malleable. At these depths, in the lower continental crust and mantle, rock undergoes plastic deformation and flows in response to application of a differential stress that is stronger than its yield strength. Rock undergoes ductile deformation by gradual creep along crystal grain boundaries and planes within crystal lattices. Ductile deformation in generates folding and ductile shear zone features, such the dynamic metamorphic mineralogical and textural changes seen in foliated schistose, banded, lineated, and augen-structured dynamic metamorphic rocks, mylonites, distorted porphyroclasts and mica fish, phyllonites.

In brittle deformation close to the Earth's surface, where rocks are comparatively cool, rock behaves in a brittle fashion, fracturing in response to differential stress greater than the rock's yield strength. Rock failure and fracture generates faulting, brecciation, pseudotachlites, cataclasis, and slickenside striations. Rock adjacent to the failed rock springs back to its original shape in the elastic rebound that is responsible for earthquakes.

syncline

2006-06-02T10:43:00.000-07:00

A syncline is a concave geological fold, with layers that dip downward toward the center of the structure. This arrangement is opposite to that of an arching anticline.

Provided that the syncline has not been overturned, strata within synclines have progressively younger rock layers toward the center of the syncline, with the youngest layer at the fold's center or hinge, mirrored by the same layers in reverse sequence on the opposite side of the hinge. Elongate circular or circular fold patterns create basin structures.

Folding typically develops during crustal deformation as the result of compression that accompanies orogenic mountain building.

Strata folded as the Rocky Mountains formed. Near Mount Withrow in northeastern B.C., a prominent syncline is outlined by beds of sandstone that form ridges because they are resistant to weathering. A low area at the centre of the fold is underlain by shale, which weathers very readily. The syncline stretches off into the distance.

Wyoming's Powder River Basin is another notable example of synclinal folding.

[ links: images: formations: synclinal folds outlined by ridges of resistant sandstone, Rocky Mountains near Mt. Withrow; syncline core in Miocene sandstone and shale from the western Tarim Basin, China; Syncline near Indus Suture, Tibet; syncline and unconformity; Mojave syncline that formed at a bend in a strike-slip fault in Rainbow Basin, 2, aka Barstow syncline; Mexican Hat Syncline, UT; overturned syncline in Taconic slates, VT; Scotch Hill syncline, VT; synclinal; sycline in Lockhart Basin, UT; 3D Camargo syncline in central Bolivia (3D glasses needed); Camargo syncline, 2; Camargo syncline center foreground and the San Juan de Oro surface unconformably over it in the background, view looking south with LandSat image draped over the topography; syncline exposed in Monarch Canyon in the Funeral Mountains, Death Valley; Tremont syncline in Llewellyn Formation, PA, Alleghanian Orogeny 290-250Ma; syncline and some accommodation structures near Bude, Cornwall; The Glen Lake Syncline; anticline and syncline and complicated folds near Calico Ghost Town, Yermo, California; Sideling Hill, western Washington County, Maryland Syncline; syncline in Maryland I-68 roadcut, 2, 3, 4, 5; anticline-syncline pair, Canadian Rockies; Gibson Peak syncline; tightly folded syncline that formed as a drag fold along the boundary of a thick, Miocene, andesite dike (upper left), Monterey Shale at Crescent Bay in Laguna Beach; Monument Fold, Colorado River; syncline, 2; satellite: Tindouf syncline; plunging anticlines and synclines, Dinosaur National Monument, UT; monocline and syncline; Henry Mt. laccoliths, Capitol Reef, UT; monocline and syncline crossed by transverse stream, Capitol Reef, UT; Atenango Del Rio Syncline, pseudo false color, false color composite, principle components, digital elevation model; diagram: fold types; websites: Fault Block Mountains Folded Mountains Upwarped Mountains Accreted Mountains; Geology of the Sideling Hill Road Cut; Camargo Syncline Geology]

T

2006-05-31T23:59:00.000-07:00

▪ terrane ▪ texture ▪ transpression regimes, transtension regimes (shear zones) ▪

terrane

2006-05-20T06:08:00.000-07:00

A terrane, as distinct from the more general topographic term 'terrain', is a crustal block or fragment that is typically bounded by faults and that has a geologic genesis distinct from those of surrounding areas.

In paleogeography, a terrane is the accreted block that has sutured to a craton (continental nucleus) and that contains distinct rock strata of distinct genesis. Thus, accreted terranes have become attached to continents as a result of tectonic processes. Superterranes are defined as composite terranes that comprise groups of individual terranes and other assemblages that share a distinctive tectonic history.

The Canadian Cordillera is an example of a complex, accreted terrane that is composed of five sub-parallel morphotectonic belts that result from Mesozoic and Cenozoic collision and deformation and accretion of allochthonous superterranes to the North American Craton. The Intermontane superterrane was accreted approximately 180 Ma, and the Insular superterrane was accreted approximately 100 Ma. The Coast Belt contains the suture resulting from the mid-Cretaceous collision between the exotic Insular superterrane and the previously accreted Intermontane superterrane. The accretionary suture was subsequently overprinted by the evolving subduction-related magmatic arc that persists as part of the modern Cascadia subduction zone. The Omineca belt represents the suture to the east of the Intermontane superterrane.

◙ subduction zone magmas ◙

[links: images: formations: rusty Archaean greenstones with gold mineralisation located adjacent to major Archean terrane boundary, Storø, Godthåbsfjord, Greenland; boudinaged pegmatites in Ordovician Hebron gneiss, a major rock type of the Merrimack terrane; Avalon Terrane Boundary, Deep River; maps: North America: Canadian Cordillera, 2, Great Slave Craton, SCLM (subcontinental lithospheric mantle) evolution beneath Slave Craton, cross-section of Buffalo Head Terrane; and terrane and tectonic elements, wNA, simplified; terranes associated with the Alaska Range (wp); Talkeetna Volcanic FormationPeninsula Terrane, south central Alaska, x-section; accreted-terrane map of Idaho (wp), western Idaho suture line; Virginia Piedmont and Blue Ridge; Geologic Terranes of Eastern New York and Connecticut; neUS; Ordovician Mid-Atlantic prior to Taconic collision, and Taconic collision, (wp); Geologic terrane map of Precambrian Basement Rocks (pdf) in Wisconsin, Minnesota and Iowa, and Geologic map of Precambrian Basement Rocks (pdf); Penokean Volcanic Terrane; Late Mesozoic and Early Cenozoic terrane translation along western North America: The Baja-BC hypothesis (wp); maps: non-NA: Australia; New Zealand terranes; Scotland; Mozambique; suture and other major shear zones associated with terrane amalgamation in the Arabian-Nubian shield showing estimates of the ages of convergence (wp); major geologic terranes of Taiwan and the faults that separate them (wp); Mongolian terrane map;

texture

2006-05-20T01:04:00.000-07:00

The texture of a rock is determined by the size and configuration of its constituent minerals and any presence of gas bubbles. Rock fabric refers to the general appearance of a set of crystals that have grown together to produce a distinctive shape or texture. The term "fabric" is best applied to groups of crystals, or to invidividual crystals within groups, which have grown together so that their growing surfaces have encountered each other. Crystal habit is the general appearance of a crystal that results from the nature and prominence of crystal forms, such as faces or sets of faces. The term "habit" is best applied to invidividual crystals that have grown without their growing surfaces encountering any pre-exisiting solid (encounter = fabric). The microstructure of a rock is its set of structural features, such as grain boundaries, grain size and structure, which can be observed in thin-section (under a microscope).

igneous rocks:
aphanitic – all crystals are too small to be visible to the naked eye or with a hand lens, giving the rock a dull appearance; resulting from rapid cooling in volcanic or hypabyssal (shallow subsurface) environments. Eg. basalt
phaneritic – all crystals sufficiently large to be clearly visible to the naked eye; resulting from slow cooling of magma deep underground in plutonic structures. Eg. granite, gabbro
porphyritic – larger phenocrysts embedded in a finer textured matrix; resulting from two-stage cooling of rising magma, first at depth and subsequently at the surface or shallow subsurface. Eg. porphyry
glassy – non-crystalline rocks; resulting from very rapid cooling of lava at or very near the Earth's surface. Eg. obsidian
vesicular – porous rock with vesicles (holes, pores, or cavities); resulting from gas expansion within rapidly cooling ejected lava. Eg. vesicular basalt
fragmental or pyroclastic – irregular grains welded together, sometimes with glassy shards; resulting from pyroclastic volcanic eruptions. Eg. volcanic tuff, ignimbrites

metamorphic rocks:
foliated – in which mineral constituents are oriented in a parallel or subparallel arrangement resulting from imposed pressure during regional metamorphism. From low to high metamorphic grades:
--slaty – parallel orientation of microscopic grains; Eg. slate
--phyllitic – parallel arrangement of platy minerals, usually micas, that are barely visible to the naked eye; Eg. phyllite
=-schistose – subparallel to parallel orientation of platy minerals such as chlorite or micas; Eg. schists
--gneissic – coarsely foliated texture in which minerals have segregated into discontinuous hands, each of which is dominated by one or two minerals; Eg. gneisses
...granuloblastic – typical of granulites, which have even-sized, granular mineral grains with weak preferred orientation. Microscopic structure reveals small, rounded grains forming a closely-fitted mosaic.
non-foliated – in which constituent minerals lack an ordered arrangement, retaining roughly the orientation of grains present in the country rock before thermal metamorphism; Eg. quartzite, marble, metaconglomerates, hornfels, anthracite coal.

Euhedral crystals are distinct, well-formed crystals with sharp, easily-recognized faces (almandine garnet in quartzitic gneiss at left).

Euhdral is opposite to the interlocked grains of anhedral textured rocks that have cooled in the crowded environment of magma chambers. Subhedral crystals are intermediate in character between distinct euhedral crystals and enmeshed anhedral textures.

U

2006-04-30T23:59:00.000-07:00

▪ unconformity (angular unconformity, blended unconformity, disconformity, hiatus, nonconformity, paraconformity, unconformities) ▪