A Tectono-Magmatic Cycle that Links Shortening in Thrust Belts with Arc Magmatism, Lithosphere Delamination, and Recycling of Continental Crust
P.G. DeCelles, G. Zandt, M. Ducea, and P. Kapp
Department of Geosciences, University of Arizona, Tucson, AZ
Shortening of upper continental crust in Earth's major thrust belts often totals more than 300 km. Because the rocks involved are almost exclusively upper crustal, this requires underthrusting of an equal length of continental lower crust and lithosphere beneath the hinterlands of both cordilleran style and collisional orogenic belts. Melt-fertile continental crust fuels flare-ups in continental magmatic arcs; isotopic data suggest that up to 80% by volume of North American Cordilleran batholiths was produced in this manner. Underthrusting of millions of cubic kilometers of continental crust and lithosphere creates a room problem beneath the magmatic arc, ultimately causing a decrease in the rate of underthrusting and a consequent reduction in upper crustal shortening. Eclogites form by magmatic differentiation and phase transformations beneath the arc, promoting delamination or dripping of dense lithospheric and lower crustal roots into the upper mantle. Delamination relieves the room problem and sets the stage for a new tectono-magmatic cycle. Kinematic histories and balanced cross sections in fold-thrust belts provide a natural flux meter to reconstruct ongoing and ancient tectono-magmatic cycles. In the central Andes and Himalaya, roughly 30 million cubic km of continental crust have been recycled into the upper mantle in this manner. Cenozoic magmatic activity in Tibet has produced widespread, highly evolved crustal melts continuously since the onset of the Indo-Asian collision, suggesting that the same general type of tectono-magmatic cycle may be working in Tibet. In turn, this implies that Tibet is a previously unrecognized type of magmatic arc.