Evidence against diachronous onset of synorogenic deposition in the Qaidam Basin, northern Tibet

Determining when intense tectonic deformation encroached extensively into northern Tibet is crucial to understanding the growth model of the Tibetan Plateau and its potential geodynamic mechanism. The 0.5−1-km-thick Lulehe Formation, which is conglomeratic with red sandstone beds located in the northern Qaidam Basin, northern Tibet, represents the early fill of the basin and is routinely attributed to an immediate sedimentary response to the rapid growth of northern Tibet. Recent studies, however, suggest an up to 6 m.y. diachronism for these coarse clastic deposits. Insufficient paleomagnetic data and entirely different magnetic polarity sequences among all studies indicate that the acquired magnetostratigraphic results of the Lulehe Formation are indeterminate. We conducted integrated analyses of whole-rock Nd isotopes, geochronology, and grain morphology of detrital zircons for the Lower Cretaceous Quanyagou and Cenozoic Lulehe Formations in the northern Qaidam Basin, as well as sedimentary observations, which support the argument that the Lulehe Formation was shed primarily from an initial rush of lithic clasts driven by rapid uplift of deformed Cretaceous sedimentary cover in the fold-and-thrust belts of the northern Qaidam Basin. The relatively high erodibility of the elevated Cretaceous sedimentary rocks in the source areas, coupled with the high-energy depositional environment deduced from sedimentary facies analysis of the Lulehe Formation, favors a higher deposition rate and thus a generally coeval initiation age for syntectonic Lulehe Formation deposits. Accordingly, we suggest that the Lulehe Formation in the northern Qaidam Basin is a southward-thinning gravel wedge representing age-equivalent deposition.