Exhumation of deep crustal rocks during an orogenic cycle occurs mainly during continental collision or during post-collision extension[2] and is thus, is broadly grouped into the three mechanisms which are used to describe the burial and exhumation of the cycle namely, syn-convergent orogenic wedges,[5][6] channel flow (also known as ductile extrusion)[7] and post-convergence gravitational collapse.
During the continued convergence, the wedge maintains its shape by maintaining its critical angle of taper[6][5] by the interaction of thickening through basal accretion or foreland propagation (frontal accretion) and thinning through normal faulting and erosion at the upper part of the wedge.
[2] Tectonics of this kind result in fold and thrust belts or if they are built up over long periods, can form thick-stacked long-hot-orogens,[7] such as the Himalayas.
[7][10][11] Subsequently these rocks can decoupled from their base and begin to flow to higher crustal levels along lithostatic pressure gradients that can be caused by melt-induced buoyancy or differences in topography and lateral density contrasts.
[14][15] Alternatively, or in conjunction with the extension of the center of the orogen, propagation of the rock-mass towards the margin may lead to exhumation along a series of brittle or ductile thrusts and normal faults[11] and ultimately the formation of fold and thrust type belts along the margins of the collapsed orogen.
Characteristics of gravitational collapse include outward verging, normal sense shear zones along the margins of the core complexes and exhumation-only type P-T-t paths.