Exhumation (geology)

During the subduction of an oceanic plate underneath the continental crust, some fragments of the oceanic crust can be trapped above the continental crust through obduction. The resulting rocks obducted on the continental crust are called ophiolites.[4]  While the exact mechanism behind the formation of ophiolites is still up for debate,[4] those rocks still show an example of rocks being exhumed and exposed at the surface by the tectonic process of obduction and then exposed.

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 subduction to the collisional phases of the orogenic cycle, a tectonic wedge forms on the prowedge (side of the subducting plate) and commonly the retrowedge (continental side) of the orogen. 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. Erosion of the wedge significantly impacts the dynamics within the wedge, which promotes exhumation by thrusting mid-crustal rocks to the hanging wall of the wedge.[8][9] Characteristics of this mode of exhumation include, evidence for strong non coaxial reverse-shearing, pro-grade metamorphism, cooling ages are progressively younger towards deeper structural levels and that exhumation at higher structural levels is coeval to burial of the structural levels.[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.

Channel flow typically occurs in long-hot orogens when the orogen is sufficiently thick to promote partial melting in the middle-lower part of the orogen to a point where the rocks reach a critically low viscosity enabling them to flow.[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.[12] both of which are affected by erosion.[13] Characteristics of this mode of exhumation include simultaneous normal shearing and reverse shearing along the roof and the base of the channel respectively, high-temperature retrograde metamorphic assemblages, cooing ages should be younger to the front of the channel and P-T-t paths suggesting prolonged burial and synchronous exhumation throughout the channel.[2]

Post-convergent gravitational collapse (extension) occurs once the convergence forces can no longer support the gravitational force of the orogen that was built up during collision. During collapse, high-grade rocks from the core of the orogen are exhumed through upward flow towards now thinned crustal areas forming domal shaped metamorphic core complexes.[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.[2]