Definitions and images to illustrate geological terms, links to images and website articles


terranetexturetranspression regimes, transtension regimes (shear zones) ▪


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;

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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.

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