THE ROLE OF THE MANTLE LITHOSPHERE IN THE DEVELOPMENT OF CONVERGENT SYSTEMS: COMPARISON OF THE ANDEAN AND HIMALAYAN-TIBETAN OROGENS

Recent geophysical results from several international projects are integrated with geological studies to compare and contrast the Andean and Himalayan-Tibet orogens. Both are fundamentally bivergent orogens in which the high elevations of the central plateaus are maintained by thick crust built from crustal material squeezed between two strong converging lithospheres. The dominant crustal thickening mechanism is horizontal shortening of weak continental upper crust sheared off one or both of the underthrusting lithosphere, then accreted to and deformed in the overlying fold and thrust belt, while the remaining strong lower crust and upper mantle is thrust under and behind the fold and thrust belt. The nature of the underthrusting lithosphere mantle may partly explain the differences between the two orogens and, more generally, play a critical role in controlling the final width and duration of the overlying orogen. The underthrusting lithosphere can be classified as oceanic, tectonic continental, stable continental, or depleted continental (Archean) type. The differences in density and fluid content of these different types of lithospheres (with the upper crust removed in the continental lithospheres), and the interactions between them, may control the evolution and ultimate fate of the convergent system. First, the dip of the subducting lithosphere, controlled largely by its density, affects the volume of the mantle wedge that is available to accommodate the underthrusting of the opposing lithosphere. In highly asymmetric orogens, like the Andes, the underthrusting oceanic slab forms a floor beneath much of the orogen that restricts underthrusting of the opposing foreland continental lithosphere and continued shortening requires episodic breaking off of the leading edge of the foreland lithosphere or re-initiation of underthrusting further inland. Second, the fluid content of the underthrusting lithosphere can affect the style and rate of mantle deformation and the degree of magmatism in the orogen. During the life span of a major convergent orogen, the type of underthrusting lithosphere can change abruptly with important tectonic consequences.