Fractures & Faults II

Advanced Tectonics
James S. Aber


Faults and fault-zone features

Normal fault (< >) exposed in the walls of Boquillas Canyon of the Rio Grande, Big Bend National Park, western Texas.
Closeup view of normal fault in Boquillas Canyon shows a zone of multiple faults and brecciated rocks.
Hot Springs of the Rio Grande, Big Bend National Park, western Texas. Springs are often associated with fault zones.
Santa Elena Canyon of the Rio Grande, Big Bend National Park, western Texas. The massive cliff is the uplifted side of a major fault.
Santa Elena Canyon of the Rio Grande, Big Bend National Park, western Texas. The river here has eroded through thick lower Cretaceous limestone that caps the uplifted side of the Santa Elena fault.
Front of the Santa Elena fault. Lower Cretaceous limestone forms the cliff on the uplifted side of the fault (background), and upper Cretaceous chalk can be seen on the downthrown side at the base of the cliff.
Closeup view of the Santa Elena fault, marked here by numerous veins of large calcite crystals. Such secondary mineral deposits are typical of major faults.
Slickensides of a fault on Lost Mine ridge, Chisos Mountains, Big Bend National Park, western Texas. Car keys near top right for scale.
Bocono Fault zone marks the boundary between the Caribbean and South American plates in the Andes Mountains of western Venezuela. Here, the Chamas River follows a deep valley along the fault zone.
The small river and lake are situated on the Bocono Fault adjacent to a key highway in the Andes Mts. of western Venezuela. This fault has produced earthquakes historically, but none have taken place in recent decades.
Crest of anticline in the upper Cretaceous Whitemud Fm. in the Cactus Hills of southern Saskatchewan, Canada. The anticline was produced by ice pushing, and it is disrupted by small faults (see below).
Normal faults on the crest of anticline. These faults were created by glacial loading after ice advance overrode and truncated the fold crest.
Lignite seam (black) is repeated in a thrust fault on the upice flank of the Cactus Hills anticline. Note how lignite is smeared along the fault for a distance of several meters.

Overthrust faulting

The major structures underlying mountain belts and ice shoved hills are long, low-angle thrust faults called over thrusts. A major overthrust may represent a décollement, which is a structural break separating complexly deformed rocks above from undeformedbasement rocks below. The displaced rock mass above the decollement is said to be allochthonous, in contrast to the unmoved autochthonous basement rocks below the décollement. The allochthonous rocks have often been transported laterally for great distances (10s to 100s km). Décollements are known to underlie complexly folded strata in many mountain belts: Swiss Juras and Alps, Appalachians, Ouachitas, Canadian Rockies, and Himalayas. Décollements are associated also with large ice-shoved ridges in many formerly glaciated regions.

How such large rock masses could be pushed or slid laterally for great distances was long a puzzle to geologists. The physical conditions necessary for overthrusting of large bedrock blocks were elegantly explained by Hubbert and Rubey (1959), who analyzed subsurface pressure conditions. The lithostatic pressure developed in deeply buried rocks is comprised of two components.

  1. Intergranular pressure - pressure transmitted directly from grain to grain within the rock.

  2. Hydrostatic pressure - pressure transmitted indirectly by fluid filling pore spaces or fractures between grains.

Below the water table, all rocks are completely saturated with fluid of some kind--water, oil, brine, or gas. Thus, subsurface pressure conditions can be represented as:

P(lithostatic) = P(intergranular) + P(hydrostatic)

In most cases, the lithostatic P is borne mainly by intergranular P, and hydrostatic P is much less than lithostatic P. However, in some situations, hydrostatic P can approach or equal lithostatic P, in which case the overburdern mass of rock essentially floats on a cushion of high-pressure fluid. Hubbert and Rubey (1959) demonstrated that, as hydrostatic P reaches lithostatic P, the shear stress required to move the overburden approaches zero. This condition is nicely illustrated by the game of "air hockey," where the puck slides effortlessly over a film of high-pressure air coming through tiny holes in the playing surface.

The necessary fluid pressure may be developed in the crust when porous, mechanically weak strata of low permeability, such as claystone, lignite, chalk, or salt, are subjected to rapid loading. The grain-to-grain structure of the rock may collapse, and weight of the overburden is then borne by a fluid cushion trapped within the rock. This raises the fluid pressure of the rock, expells some fluid from the rock, and compacts the rock.

Rapid loading may compact incompetent strata raising the hydrostatic P in various settings: thick building up of orogenic sediments (flysch), prograding delta, advancing ice sheet, or pushing of a thrust block onto previously undisturbed strata. The high fluid pressure sufficient to facilitate overthrusting may develop whenever the rate of loading for whatever reason is greater than the rate of fluid escape from compacted, weak strata.

Once a cushion of high-pressure fluid is created, lateral movement of the overlying rock may be accomplished by pushing, as when plates collide or a glacier advances, or simply by gravity sliding down a very gentle gradient. In any case, overthrusting will continue only so long as high fluid pressure is maintained within the décollement.

Simpson Pass thrust fault, Canadian Rocky Mts. front range, western Alberta. Cambrian strata (left, dark) are thrust over Ordovician rocks (right, light). This is a classic example of a montane thrust in which older strata come to rest on top of younger rocks.
Small overthrust (^) with drag folding in Jurassic sand beneath glacial gravel. Ice-push deformation, island of Bornholm, Denmark.
Overthrust of brown Eastend Fm. (left) onto white/gray Whitemud Fm. (right), Dirt Hills, Saskatchewan, Canada. The Dirt Hills are a huge ice-shoved ridge complex built of displaced Cretaceous strata. In the normal stratigraphic sequence, Eastend Fm. underlies the Whitemud Fm.
Closeup view of drag folding in the Eastend Fm. thrust over the Whitemud Fm. Dirt Hills, Saskatchewan, Canada.

Reference

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Notice: Advanced tectonics is presented for the use and benefit of students enrolled at Emporia State University. Others are welcome to view the course webpages. Any other use of text, imagery or curriculum materials is prohibited without permission of the instructor. All text and imagery © J.S. Aber (2012).