Exhumation by tectonic processes refers to any geological mechanism that brings rocks from deeper crustal levels to shallower crustal levels. While erosion or denudation is fundamental in eventually exposing these deeper rocks at the Earth's surface, the geological phenomenon that drive the rocks to shallower crust are still considered exhumation processes. Geological exhumation occurs on a range of scales, from smaller-scale
thrusts typically occurring within the shallow crust (less than ca. 10 km deep) which results in exhumation in the order of centimeters to meters scales, to larger-scale features originating at deeper crustal levels along which, exhumation is in the order of hundreds of meters to kilometers. The geological mechanisms that drive deep crustal exhumation can occur in a variety of
tectonic settings but are ultimately driven by the
convergence of tectonic plates through
subduction. Depending on the type of
convergent boundary, exhumation occurs by thrusting in the
accretionary wedge, by
obduction and/or as a process during the
orogenic cycle (i.e. mountain building and collapse cycle).
Obduction 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. While the exact mechanism behind the formation of ophiolites is still up for debate, channel flow (also known as ductile extrusion) and post-convergence gravitational collapse.
Syn-convergent orogenic wedge 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 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. 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. both of which are affected by erosion. 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. 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 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. == References ==