faulting

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]

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geological maps

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]

mapping

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.

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