A comparison of large orogenic belts formed at convergent plate boundaries indicates that orogenic evolution comprises a period of crustal thickening followed by the formation of a continental plateau and commonly syn- to post-convergence extension and thinning of the previously thickened crust. The instantaneous and finite strain patterns recorded in orogenic belts do not appear to be linked in a simple way to plate kinematics. These patterns suggest that deformation of continental crust is not solely controlled by horizontal compression due to plate convergence and that gravity-driven flow is likely to play a significant role on the deformation of thickened crust. Geophysical data in southern Tibet, an actively convergent orogen, and geologic data of the exhumed mid-crustal core of eroded orogens reveals that orogenic crust is affected by widespread partial melting. Partial melting of thermally mature zones of thickened crust results in the generation of a layer of low-viscosity rocks that may affect the behaviour of the orogenic belt. In particular, the presence of this layer favours mechanical decoupling between the subducting plate and the overlying thickened orogenic crust, resulting in flow of the crust driven by contrasts in gravitational potential energy.
Bibliographical noteFunding Information:
This work was supported by National Science Foundation Grants NSF-EAR-9526915 and NSF-EAR-9814669 and was done while O.V. was holding a postdoctoral fellowship funded by Lithoprobe in the group of Chris Beaumont and Becky Jamieson at Dalhousie University. This paper benefited from the reviews of Doug Nelson and Laurent Jolivet. Fruitful discussions with a number of colleagues including Philippe Fullsack, Ritske Huisman and Sergei Medvedev of the Dalhousie Geodynamics Group; Patrice Rey, and Jean-Pierre Burg provided food for thought, although they made clear that they did not share all of the ideas developed in this paper. The analogy between camembert tectonics and orogenic evolution is the result of a long (too long?) maturation during an extended stay away from the good wine and cheese country for both authors.
- Continental tectonics
- Gravitational collapse
- Metamorphic core complexes