Abstract

Located in the Bangong suture zone between the Lhasa and Qiangtang terranes of central Tibet, the Nima basin records Cretaceous–late Miocene sediment accumulation during a period of drastically changing paleogeography and paleoelevation. The Bangong suture zone originally formed during Late Jurassic–Early Cretaceous time as the Lhasa terrane collided with the Qiang-tang terrane. During Early to middle Cretaceous time, the region lay at northern near-equatorial paleolatitudes, near sea level. By Aptian time the Nima basin was above sea level and was strongly influenced by nearby volcanic activity and crustal shortening in the reactivated Bangong suture zone. In the southern Nima basin, an ∼50 m.y. (Late Cretaceous–Eocene) depositional hiatus correlates with major crustal shortening and ensuing voluminous ignimbrite eruptions in the Lhasa terrane. In the northern Nima basin, deposition continued during latest Cretaceous time, recording arid paleoclimate in evaporitic lacustrine and eolian dunefield deposits. By Oligocene time the Nima basin comprised two independent depocenters that accumulated coarse-grained alluvial, fluvial, lacustrine (evaporitic), and fan-delta deposits in close association with reactivated thrusts in the Bangong suture zone.

Carbon and oxygen isotope data from Oligocene paleosol carbonate, reported elsewhere, indicate that regional paleoelevation during the late Oligocene was >4.6 km, as it is today. Overall, the depositional record of the Nima basin, combined with ongoing structural and geochronologic studies, demonstrates that the Bangong suture zone was reactivated during middle Cretaceous and middle Tertiary time, that the intervening ∼50 m.y. interval was a time of regional upper crustal shortening in the Lhasa terrane followed by regional ignimbrite eruptions, and that arid paleoclimate and high paleoelevation were established by the Late Cretaceous and late Oligocene, respectively.

Within the context of other data sets from the Lhasa terrane, the record of deposition in the Nima basin is consistent with low-angle subduction of Neotethyan oceanic lithosphere and reactivation of the Bangong suture zone during the Early Cretaceous, followed by shortening within the Gangdese retroarc and northern Lhasa terrane thrust belts during the Late Cretaceous, lithospheric delamination-dripping and regional magmatic flare-up during latest Cretaceous through early Tertiary time, and underthrusting of Indian lower crust and lithosphere as far north as the Bangong suture zone during late Oligocene time.

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