WERE CENOZOIC BASINS, SHORTENING, AND MAGMATISM IN CENTRAL TIBET CAUSED BY LITHOSPHERIC DRIPPING?

Preliminary observations raise the hypothesis that Eocene – Miocene sedimentary basins in central Tibet developed initially as ~100-km-wide oval-shaped depocenters and were internally shortened above lithospheric drips, while coeval volcanic rocks were emplaced and remained undeformed atop basin-bounding topographic bulges. The most striking observation is that Cenozoic volcanic fields sit outside of, at higher modern elevations, and are much less shortened than adjacent sedimentary basins of coeval and younger age and locally >3 km thick. This suggests that Cenozoic shortening and subsidence were spatially localized, proximal to the modern elliptical outcrop limits of the basins. All documented basins (Paleocene – Quaternary) include evaporative lacustrine facies. Cenozoic volcanism was transient in a given ~100 km by 100 km region, lasting over a time interval of ~10 Myr, and spatially diachronous, initiating in the northeastern Qiangtang terrane at ~45 Ma and decreasing in age to ~26 Ma toward the west and south across central Tibet. Age control is scarce, but permissive of a southward younging in the timing of maximum Cenozoic basin subsidence: between 51 and 29 Ma in the northern Qiangtang, at ~35 Ma in the southern Qiangtang, and beginning at ~26 Ma along the Bangong suture. North to south dripping/delamination of Asian lithosphere is predicted in numerical models of India-Asia collision that include a weak, metasomatized lithosphere in the upper plate between India and stronger Asian lithosphere to the north. Within the rheologic context, it is reasonable that lithospheric removal initiated beneath the northern Qiangtang, as this region marks the northernmost extent of major Jurassic and younger magmatism and deformation in Tibet, which in turn may have weakened the underlying mantle lithosphere. Lithospheric dripping and “bobber” basin development should serve to generate internal drainage and decrease regional relief, and thus may help explain how hinterlands of high-relief and externally-drained thrust belts transition into orogenic plateaus that encompass ~100-200-km-scale elliptical and hydrologically closed basins.