Gravitational collapse of orogenic crust: A preliminary three dimensional finite element study

Although gravitational collapse has been widely accepted as a viable explan ation for both synorogenic and postorogenic extension, its mechanisms have not been well understood. Previous models of continental deformation that a ssumed a viscous or viscoplastic rheology are not well suited for simulatin g stress states and brittle deformation within the upper crust, where most extensional collapse occurs. Here we present preliminary results of a three -dimensional (3-D) finite element modeling of stress and faulting patterns within the orogenic crust using a viscoelastic rheology. Major parameters c ontrolling orogenic extension, including topographic loading, tectonic comp ression, basal shear, thermal structure, and 3-D tectonic boundary conditio ns are systematically explored in the model. For typical tectonic compressi on (60-100 MPa) and >2-3 km elevation the model predicts synorogenic extens ion in high plateaus and concomitant thrusting in the lowlands near the foo thills. Viscous relaxation within the ductile crust may amplify deviatoric stresses within the brittle crust by a few times, and stress amplification is greater when the crust is hotter, and therefore the brittle crust is thi nner. Most synorogenic extension occurs in the direction orthogonal to regi onal compression, while postorogenic extension is more likely to occur norm al to the trend of mountain belts. We apply the model results to the Tibeta n plateau and suggest that the age of the north trending grabens is not a r eliable proxy for the time when the plateau had reached it present mean hig h elevation. Conversely, development of the east trending South Tibetan Det achment system in the Miocene by gravitational collapse was more difficult and may require a higher elevation of the Himalayas than present or much of the Tibetan plateau to have remained low till late Miocene.