Provenance of Jurassic-Cretaceous Tethyan Himalayan sequences in the Thakkhola Section- Nepal, inferring pre-collisional tectonics of the central Himalaya

摘要

The Jurassic-Cretaceous (Upper Tethyan) flysch well exposed in the Thakkhola Section, Nepal, was deposited prior to the continent-continent collision between the Indian and Eurasian plates. This Tethyan Himalayan Sequence (THS) has preserved the extreme northern passive margin of the Indian plate with its direct deposition onto the eroded Precambrian rocks in the lowest part of this relatively continuous section. The clastic rocks of the THS, as they are useful to constrain the paleogeography and paleotectonics of the Himalayan Orogen. In this study, we report the petrography, detrital zircon U-Pb isotopic ages, and zircon trace element data for sandstone samples from the Upper part of a Triassic stratigraphic section. The mineral modal composition data indicates that these quartz-rich units were derived from recycled orogen. This is supported that the youngest zircons analyzed can be dated to similar to 400 Ma that predates the depositional age by hundreds of millions of years. A total of 85% of the zircon ages are < 1,500 Ma and the oldest dated zircon is similar to 3300 Ma. What these sections share in terms of detrital zircon signals are peaks at pre-Rodinian, a Grenvillian, a Gondwanan, and nothing either younger, except for the weak presence or total absence, of a Cretaceous zircon age signal. More importantly, these results drive us to surmise that either sedimentary or crystalline, in the Indian plate that has provided the unusual mix of 475-675 Ma and 825-950 Ma age peaks, with the latter associated with an Eastern African super deltaic fan.