Smith Lecture: Early Plateau Growth in Central Tibet and the Subduction History of the Greater Indian Lithosphere
During Cretaceous to early Eocene time, continental collision in central Tibet and the development of an Andean-style retroarc thrust belt in southern Tibet resulted in >50% of regional shortening. This deformation history, together with the history of exhumation revealed by thermochronologic studies, suggest that plateau-like conditions were established in central Tibet by 45 Ma. Initial subduction of 'Greater Indian' lithosphere- the lithosphere that was previously positioned between India and Asia but has since been consumed beneath Tibet- ignited a magmatic flare-up in southern Tibet. Beginning at ~46 Ma, magmatism swept >400 km north of the Indus-Yarlung suture (IYS), and may record northward underthrusting of Greater India during Eo-Oligocene time. Subsequent rollback of Greater India during Oligo-Miocene time can explain an observed, southward return sweep in magmatism and localized extension along the IYS (and South Tibetan Detachment) at this time. Renewed northward underthrusting of Greater India lithosphere initiated at ~16 Ma and led to the initiation of E-W extension and eventual cessation of volcanism in southern Tibet. Epsilon Hf values of zircon in igneous rocks attributed to underthrusting and rollback of Greater India show systematic temporal sweeps to more evolved and juvenile signatures, respectively, adding support to the tectonic interpretations. The proposed scenario shows that more than half of the >3000 km of convergence between India and Asia since initial collision could have been accommodated by northward subduction of Greater Indian lithosphere. This result is consistent with paleomagnetic studies and kinematic restorations of Cenozoic deformation in Asia, but is at odds with estimates of shortening (<900 km) in the Himalaya.