Thrust tectonics orcontractional tectonics is concerned with the structures formed by, and thetectonic processes associated with, the shortening and thickening of thecrust orlithosphere. It is one of the three main types of tectonic regime, the others beingextensional tectonics andstrike-slip tectonics. These match the three types ofplate boundary,convergent (thrust),divergent (extensional) andtransform (strike-slip). There are two main types of thrust tectonics, thin-skinned and thick-skinned, depending on whether or not basement rocks are involved in the deformation. The principle geological environments where thrust tectonics is observed are zones ofcontinental collision, restraining bends on strike-slip faults and as part of detached fault systems on somepassive margins.[1]
In areas ofthrust tectonics, two main processes are recognized:thin-skinned deformation andthick-skinned deformation. The distinction is important as attempts to structurally restore the deformation will give very different results depending on the assumed geometry.[2]
Thin-skinned deformation refers to shortening that only involves the sedimentary cover. This style is typical of manyfold and thrust belts developed in the foreland of a collisional zone. This is particularly the case where a good basaldecollement exists such assalt or a zone of high pore fluid pressure.[3]
Thick-skinned deformation refers to shortening that involvesbasement rocks rather than just the overlying cover. This type of geometry is typically found in thehinterland of a collisional zone. This style may also occur in the foreland where no effective decollement surface is present or where pre-existing extensional rift structures may beinverted.[4]
The most significant areas of thrust tectonics are associated withdestructive plate boundaries leading to the formation oforogenic belts. The two main types are: the collision of twocontinentaltectonic plates (for example theArabian plate andEurasian plate, which formed theZagros fold and thrust belt) and collisions between a continent and anisland arc such as that which formedTaiwan.[5]
When astrike-slip fault is offset along strike such that the resulting bend in the fault hinders easy movement, e.g. a right stepping bend on a sinistral (left-lateral) fault, this will cause local shortening ortranspression. Examples include the 'Big Bend' region of theSan Andreas Fault,[6] and parts of theDead Sea Transform.[7]
Passive margins are characterised by large prisms of sedimentary material deposited since the original break-up of a continent associated with formation of a newspreading centre. This wedge of material will tend to spread under gravity and, where an effectivedetachment layer is present such assalt, the extensional faulting that forms at the landward side will be balanced at the front of the wedge by a series oftoe-thrusts. Examples include the outboard part of theNiger delta (with an overpressuredmudstone detachment)[8] and theAngola margin (with asalt detachment).[9]
