Thrust faults are low-angle reverse faults that form in compressional tectonic regimes, where older rock units are pushed over younger ones, leading to significant crustal shortening and thickening. Their subsurface geometries are often complex.

Planar Thrust: This is the simplest geometry, where the fault plane is relatively straight and maintains a consistent low dip, cutting uniformly through rock layers.

Ramp-Flat Geometry: A very common and diagnostic geometry, especially in layered sedimentary sequences. The thrust fault typically exploits weak, incompetent layers (e.g., shales) as low-angle flats (bedding-parallel segments). It then cuts upwards at a steeper angle through stronger, competent layers (e.g., sandstones, limestones) as ramps, before potentially flattening out again. This creates a characteristic stepped profile.

Duplex Structure: This complex geometry forms when a series of imbricate (overlapping) thrust slices, known as 'horses', are bounded by a lower floor thrust and an upper roof thrust. Each 'horse' ramps up from the floor thrust and then typically soles into the roof thrust. Duplexes are highly efficient at accommodating large amounts of crustal shortening and thickening.

Blind Thrust: A thrust fault is termed 'blind' when its fault plane does not propagate all the way to the Earth's surface. Instead, the displacement caused by the fault is accommodated by folding in the overlying, undeformed or less-deformed rock layers. This often results in the formation of an anticline or a fold-and-thrust belt structure directly above the buried fault tip.

These distinct geometries are crucial for deciphering the tectonic history and structural architecture of mountain belts.