GRAVITATIONAL COLLAPSE OF THE CONTINENTAL CRUST: DEFINITION, REGIMES AND MODES

Abstract

All geodynamic processes should be explained by the fundamental principle of the strive for gravitational equilibrium. [Ann. Soc. Geol. Belge Bull. 64 (1965) 95-123]The concept of gravitational collapse has fundamentally improved our understanding of orogenic processes. 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 development of this concept slowed down during the late 1960s and the 1970s before reemerging in the 1980s. Gravitational collapse corresponds to the decay of lateral contrast in gravitational potential energy that builds up during lithospheric deformation. 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. Depending on the sign of the anomaly, two fundamental regimes of gravitational collapse can be defined. During divergent gravitational collapse, an excess in gravitational potential energy drives crustal material away from the deformed lithosphere. Divergent collapse is the regime that may affect the thickened crust. In contrast, during convergent gravitational collapse, a deficit in gravitational potential energy drives crustal material towards the deformed lithosphere. This regime can be expected to occur following thinning of the continental crust. 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. 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. This transfer is accommodated by a combination of gravitational sliding of the brittle crust and horizontal spread of the lower crust. In orogenic domains, fixed-boundary divergent collapse implies the lateral growth of the orogenic domain at the expense of the surrounding lithosphere. 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.