Thrust fault

The destructive 1994 earthquake in Northridge, Los Angeles, California, was caused by a previously undiscovered blind thrust fault.

With continued displacement on the thrust, higher stresses are developed in the footwall of the ramp due to the bend on the fault.

This process may repeat many times, forming a series of fault-bounded thrust slices known as imbricates or horses, each with the geometry of a fault-bend fold of small displacement.

Duplexing is a very efficient mechanism of accommodating the shortening of the crust by thickening the section rather than by folding and deformation.

These conditions exist in the orogenic belts that result from either two continental tectonic collisions or from subduction zone accretion.

The Himalayas, the Alps, and the Appalachians are prominent examples of compressional orogenies with numerous overthrust faults.

Here, ramp flat geometries are not usually observed because the compressional force is at a steep angle to the sedimentary layering.

Thrust faults were unrecognised until the work of Arnold Escher von der Linth, Albert Heim and Marcel Alexandre Bertrand in the Alps working on the Glarus Thrust; Charles Lapworth, Ben Peach and John Horne working on parts of the Moine Thrust in the Scottish Highlands; Alfred Elis Törnebohm in the Scandinavian Caledonides and R. G. McConnell in the Canadian Rockies.

He wrote: By a system of reversed faults, a group of strata is made to cover a great breadth of ground and actually to overlie higher members of the same series.

They are strictly reversed faults, but with so low a hade that the rocks on their upthrown side have been, as it were, pushed horizontally forward.

Thrust fault in the Qilian Shan , China. The older (left, blue, and red) thrust over the younger (right, brown).
The Glencoul Thrust at Aird da Loch, Assynt in Scotland. The irregular grey mass of rock is formed of Archaean or Paleoproterozoic Lewisian gneisses thrust over well-bedded Cambrian quartzite , along the top of the younger unit.
Small thrust fault in the cliffs at Lilstock Bay , Somerset, England; displacement of about two metres (6.6 ft)
Diagram of the evolution of a fault-bend fold or 'ramp anticline' above a thrust ramp, the ramp links decollements at the top of the green and yellow layers
Diagram of the evolution of a fault propagation fold
Development of thrust duplex by progressive failure of ramp footwall
Antiformal stack of thrust imbricates proved by drilling, Brooks Range Foothills, Alaska
An example of thin-skinned deformation (thrusting) in Montana . Note that the white Madison Limestone is repeated, with one example in the foreground and another at a higher level to the upper right corner and top of the picture.
Thrust Fault Outcrop