Abstract
We present a new stratigraphy of the Jianchuan basin, one of the largest Cenozoic sedimentary basins in southeastern Tibet. This basin was regarded as recording sedimentation from the Eocene up to the Pliocene, and as such has been the focus of several studies aiming at constraining the environmental, tectonic and topographic evolution of the area. Within the Shuanghe and Jianchuan formations thirteen new zircon U/Pb ages and one biotite 40Ar/39Ar age of interbedded and cross-cutting ultrapotassic magmatic rocks show that a brief magmatic event occurred from ~35.7 to ~34.5Ma (35.2±0.4Ma on average). The uppermost formation (Jianchuan Fm), supposedly Pliocene in age, is related to this magmatic event and is 35.4±0.8Ma old. All sedimentary formations are thus Eocene in age, with neither Oligocene nor Miocene sediments. The coal-bearing Shuanghe Formation yields a fossil of a large amynodontid typical of the Upper Eocene Ergilian interval (37.2 to 33.9Ma). Sedimentation of the Shuanghe Formation took place in a short time interval at ~35.9±0.9Ma, after a large-scale drainage reorganization that induced the abandonment of a large braided-river system. This reorganization was possibly linked with the initiation of the left-lateral Ailao-Shan Red River fault and/or to widespread magmatism in the Jianchuan basin. Previous high paleoaltitude estimates for the Jianchuan basin are thoroughly re-evaluated and yield a value of 1200±1200m.a.s.l. for the Upper Eocene.
Highlights
Tibet is the widest and highest plateau on Earth
A new stratigraphy is proposed for the Jianchuan basin
Siltstone and mudstone beds of the lowest Menyanjing formation (E1m) correspond to a floodplain environment. They are overlain by the massive sandstones of the Baoxiangsi Fm (E2b), which corresponds to a wide braided-fluvial system
Summary
Tibet is the widest and highest plateau on Earth. It covers 200 000 km at a mean elevation of m. The Jianchuan Cenozoic basin has been the focus of several studies as it is a wide basin, which, according to geological maps, records continuous sedimentation from the Eocene to the Pliocene As such it may contain sedimentary archives allowing following the paleoenvironmental evolutions of the area since the onset of the collision, as well as altitudinal and latitudinal variations. Dating continental sediments is difficult and, whilst these contains volcanodetritic levels and are crosscut by plutonic rocks, no systematic study was previously conducted to provide the absolute ages of the sedimentary formations These ages could be used to constrain the tectonic and altitudinal variations through time.
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