Late Neoproterozoic–early Paleozoic seismic structure–stratigraphy of the SW Tarim Block (China), its passive margin evolution and the Tarim–Rodinia breakup

Abstract

A complete structural and stratigraphic record of extensional tectonics and passive margin evolution is displayed by the late Neoproterozoic–early Paleozoic syn- and post-rift sequences exposed in the SW Tarim Block. The syn-rift strata, represented by the upper Neoproterozoic Nanhua and Sinian Systems, occur in NE–SW– and WNW–ESE–trending graben and half–graben structures and comprise clastic terrestrial and bimodal volcanic rocks that are interspersed with diamictites. Nearly 100- to 150-km-long, sub-elliptical basins with ~400-m-thick upper Neoproterozoic Nanhua System deposits are nested within these grabens. Stratigraphically higher, upper Sinian units include littoral to shallow marine clastic rocks, grading upwards into massive dolomites and marking an important transition from terrestrial to a shallow marine depositional environment in the passive margin evolution of SW Tarim. Unconformably overlying these Neoproterozoic syn–rift rock units are the lower Cambrian sandstone, mudstone and dolomite, and the middle Cambrian dolomitic limestone and stromatolites collectively making up the post–rift sequence. These rocks represent a shallow marine transgressive sequence, deposited on a submerged carbonate platform of the rifted margin of SW Tarim. The regional unconformity between the syn– and post–rift sequences marks a sharp breakup unconformity, pointing to a major tectonic transition from continental rifting to drifting in the history of the Tarim–Rodinia breakup. This extensional tectonics and passive margin evolution during the late Neoproterozoic–early Cambrian indicate that the collisional Pan–African orogenic events, which affected several other East Gondwana continents, did not involve the Tarim Block.