Gravitational collapse is permitted by a modification of lithosphere dynamics in the convergence zone and might eventually lead to opening of a new oceanic basin if collapse is followed by thinning of the lithospheric mantle. The style of extension is controlled by the rheology of the crust at the onset of gravitational collapse and its evolution as the crust thins and cools. Destruction of the orogenic crust is achieved in part by erosion but mostly by gravitational collapse. Partial melting and rheologic weakening of the thermally mature thickened crust triggers gravity-driven lateral flow of the lower crust and controls the transition from wedge to orogenic plateau. In this model, the transition from low to high geothermal gradient is associated with increased heat production in the thickened crust owing to radioactive decay and deformation. To account for these geological characteristics, a generic model, that integrates results from physical modeling, is proposed for the thermal–mechanical evolution of crustal orogenic belts and for its implication in controlling the transition between the different phases of the orogenic cycle. This evolution is controlled by complex interactions among (i) the dynamic balance among forces that arise from plate-tectonic, gravitational potential energy, and buoyancy, (ii) the thermal balance between deformation-induced and radioactive heat production and heat advection related to subduction, orogenic deformation, and magma transfer, and (iii) the mass transfer balance between uplift and erosion. These features portray the crustal orogenic cycle and are first-order indicators of the thermal and mechanical evolution of the crust within the plate boundary region. Metamorphic rocks forming orogenic crust attest to burial and exhumation under contrasted geothermal gradients. Foreland and extensional sedimentary basins in the plate boundary region are filled by the erosional products of the orogenic crust. In contrast, when the surrounding lithosphere is free to move (free-boundary divergent collapse), the thickened crust is homogeneously thinned without transfer of gravitational potential energy towards the forelands.Ĭonvergent plate boundaries are characterized by the development of crustal orogenic wedges and orogenic plateaus but also by gravitational collapse of previously thickened crust leading to the opening of intermontane and eventually oceanic back-arc basins. In orogenic domains, fixed-boundary divergent collapse implies the lateral growth of the orogenic domain at the expense of the surrounding lithosphere. This transfer is accommodated by a combination of gravitational sliding of the brittle crust and horizontal spread of the lower crust. When the surrounding lithosphere is fixed (fixed-boundary collapse), collapse occurs through a transfer of gravitational potential energy from the elevated regions towards the low lands. For each regime, two end-member modes of collapse with contrasted characteristics are defined depending on the behaviour of the lithosphere surrounding the deformed domain. This regime can be expected to occur following thinning of the continental crust. In contrast, during convergent gravitational collapse, a deficit in gravitational potential energy drives crustal material towards the deformed lithosphere. Divergent collapse is the regime that may affect the thickened crust. During divergent gravitational collapse, an excess in gravitational potential energy drives crustal material away from the deformed lithosphere. Depending on the sign of the anomaly, two fundamental regimes of gravitational collapse can be defined. When the forces that support this anomaly (i.e., tectonic forces and the strength of both the deformed and surrounding lithosphere) decrease, the gravitational potential anomaly may relax. Gravitational collapse corresponds to the decay of lateral contrast in gravitational potential energy that builds up during lithospheric deformation. The development of this concept slowed down during the late 1960s and the 1970s before reemerging in the 1980s. This concept has its roots in pioneers’ works, such as those of Jeffreys, van Bemmelen, Bucher and Ramberg, who were among the first to recognise the importance of gravity in the evolution of mountain belts. The concept of gravitational collapse has fundamentally improved our understanding of orogenic processes.
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