Australia: The Land Where Time Began

A biography of the Australian continent 

Thrust and Reverse Faults

A horizontal compression, which defines the main principal stress σ1, is produced and the vertical load is provided by a minor compression, σ3 are the tectonic settings in which thrust and reverse faults form. The main geotectonic settings which thrust and reverse faults form in are plate boundaries associated with convergence and collision. In continental settings thrusts and reverse faults form in fold and thrust belts that can extend for hundreds of kilometres. They appear in accretionary wedges or subduction prisms, between the trench at the plate boundary and a magmatic arc in intra-oceanic and continental active margins, in oceanic environments. Thrust faulting leads to crustal shortening and thickening. Thrust and fold belts are limited by an area that is not affected by faulting, the foreland, in front of the thrust and fold belts, as defined by the sense of movement, a place where tectonic loading can lead to the formation of a subsiding basin, and the hinterland is the area behind the thrust belt. Highly asymmetrical structures are present in thrust belts, the asymmetry being in the direction of tectonic transport or general displacement, with most faults generally dipping towards the hinterland. Thrust faults can form locally in compressive stretches of gravitational slides that developed at the foot of rock masses that are collapsing, or other processes that are associated with folding or in processes of igneous intrusion.

Reverse faults are faults of high angle in which surfaces are inclined as much as normal faults that are more than or equal to 60o. Reverse faults are less common than thrust faults, and in many compressive settings they can be important features, though they do not confirm to Anderson's theory of faulting which suggests that faults resulting from horizontal compression should have a low angle. When considering the stress conditions in Anderson's theory, reverse faults also are not consistent with Coulomb's failure criterion. Several explanations have been proposed for the high angle for reverse faults, including tectonic inversion from regimes of compression to extension and reactivation as reverse faults of faults that were  originally normal faults that had been generated previously. Curved fault surfaces that allow thrust faults to evolve to reverse faults at depth and for thrusts to evolve as faults of high angle by front ramping to the surface can result from the stress axis directions, stress trajectories, curving at depth. If stress gradients, and differences in state of stress exists in the vertical and lateral directions diverging stress trajectories can be produced. Generally the initiation of thrusts as faults of low angle but they can subsequently be deformed by the overall shape being changed by compression.

Very complex structures with thrusts, reverse faults, and folds, all associated together can be present in tectonic compressive settings, this deformation style being known as thin-skinned tectonics as a layer of crust that is relatively thin undergoes intense shortening and deformation, though the basement remains largely unaffected. Important and kinematic problems in the reconstruction of tectonic processes are posed by this situation related to thrusting as a result of the decupling that occurs between the shortening of the cover and that of the basement. A basal shear plane (decollement) that is of low-angle or horizontal, acting as detachment areas, are common structures in thrust and fold belts, that separate an upper part (cover) that is highly deformed, folded and fractured, from a substratum (basement) that is relatively undeformed. The detachment (sole fault) produced where a mechanical contact is formed by a weak layer that is less frictional, that is typically clay, shale or salts. Thrust sheets or nappes are terms often used to describe deformed rock wedges over thrust faults. Because of its displacement nature the cover is also known as an allochthonous terrain, which forms very extensive triangular rock wedges that are relatively thin, that thin in the direction of displacement or tectonic transport. The term Allochthonous is often used to describe the basement beneath the main decollement, and the rocks there remain in situ. Observation at the surface of the Earth is allowed at so-called tectonic windows by erosion of part od the allochthonous terrain. Klippes are remains of an allochthonous terrain that is surrounded by autochthonous rocks.

Flat and ramp geometries are common in thrust faults, as with normal faults, that can be perpendicular, parallel or oblique to the direction of a block. When a fault that is of low angle or horizontal rises to a shallower level in the crust, cuts competent rocks and forms a high-angle step inclined backwards, with respect to the direction of transport, running towards another incompetent layer where another decollement or flat is formed. Particular deformations are produced in the hangingwall as a result of the presence of ramps, as occurs with normal faults. A syncline on the lower reach of the ramp surface evolving towards an anticline located over the upper end of the ramp, is a very prominent structure. A syncline is formed at the toe of the ramp and an anticline forms at the top of a ramp, as the hangingwall block climbs the footwall ramp, the limbs getting progressively larger, though the axial surface of the syncline remains in the same position. The anticlinal folds formed in the hangingwall also develop ramp and flat geometries. Though there are various models for the propagation of faults, they basically form 2 kinds of thrust fault arrangement into thrust sheets.  The faults breaking the topographic surface form the first type of thrust fault arrangement, the fault trace of which can be followed in the field. The most typical type of these faults in fold and thrust belts are imbricate fans of listric faults, which are concave towards the hinterland, joining a basal sole fault, though these faults can be arranged in different forms. These structures are called schuppen zones. Duplexes in which a set of horses are confined between 2 detachment faults, a roof fault and foot fault, is the second prominent structure. Horses that form the duplex can be inclined towards the foreland, the hinterland or can stack vertically.

Sources & Further reading

  1. Leeder, Mike, Perez-Arlucea, Marta, 2006, Physical Processes in Earth and Environmental Sciences, Blackwell Publishing Ltd.
Author: M. H. Monroe
Last updated 14/05/2013

Plate Tectonics
Journey Back Through Time
Experience Australia
Aboriginal Australia
National Parks
Photo Galleries
Site Map
                                                                                           Author: M.H.Monroe  Email:     Sources & Further reading