EXOTIC TERRANES AND THE DYNAMICS OF ACCRETIONARY OROGENS

The dynamics of accretionary orogens is influenced by many processes affecting the subducting and overriding plates as well as the underlying asthenospheric mantle. Amongst them, the velocity and direction of plates motion, the steepening or shallowing of the subducting slab, the folding of the slab the base of the asthenosphere, the frictional strengh of the upper/lower plate contact, the asthenospheric flow in the vicinity of the subducting slab, the progressive hydration and weakening in the mantle wedge above the subducting slab, and the thermal weakening of the overriding continental plate can contribute to the switch from contractional to extensional tectonics in overriding plates. On the basis of 2D numerical thermo-mechanical experiments, and using the mid-Cretaceous to Paleocene evolution of the East Gondwana margin as a prime example, we investigate the stability of accretionary orogens as a function of the trench-normal component of motion between the converging plates and as a function of the mantle wedge buoyancy. We show that, upon lowering the trench-normal velocity component, and for a moderate mantle wedge buoyancy, the volume forces overcome compressional forces and drive a range of processes dynamically linking the gravitational collapse of the accretionary orogen to the fragmentation of the plate margin via the formation of a marginal basin, the forcing of trench retreat and slab rollback, and to detachment of continental micro-blocks. During most of the Phanerozoic, Gondwana north and east margins were progressively eroded pieces by pieces sending waves of micro-blocks and continental ribbons across the Proto, Paleo and Neothethys, as well as across the Pacific Ocean. Gondwana-derived exotic terranes contributed to successive accretionary orogens resulting in the southward build-up of Asia, as well as the build-up of Western North America.