The embryonic Himalayan foreland basin revealed in the eastern Yarlung Zangbo suture zone, southeastern Tibet

Abstract

Understanding of early foreland basin development is key to reconstruct the timing and nature of early collisional processes. Though the nature of the early Himalayan foreland basin system is well established in the central Himalaya range, it lacks analysis in the eastern Yarlung Zangbo suture zone. In this article, we investigate the region's lithological and structural evolution and analyze the geochemistry and geochronology of detrital zircons and bulk rocks in the Jiacha-Jindong region, southeastern Tibet. We show that: 1) The previously named Cretaceous Langxian Mélange comprises two separate stratigraphic units—the newly revised early Eocene (~56 Ma) Langxian unit and the Triassic Langjiexue Group; 2) The Langxian unit overthrusts the Oligocene to Miocene Dazhuka Formation and is overlain and/or underlain by the highly-deformed Langjiexue Group; 3) Shale-normalized REE + Y patterns of marbles with light REE depletion, La enrichment, and positive Y anomaly plus Sr isotope ratios 0.707481–0.707897, together demonstrate the Langxian unit was originally deposited in a marine environment; 4) ~1270 concordant detrital zircons from 20 rock samples indicate a bilateral provenance—from both the Gangdese arc and/or recycled forearc strata in the north and the Langjiexue Group in the south. The evidence illustrates that the Langxian unit records the early India-Asia collision and its protoliths could have been deposited in a distal foredeep basin to the embryonic eastern Himalaya orogeny. The simplest explanation is that it reflects an underfilled marine basin with sources from the early mountain range to the north and a region of forebulge erosion to the south.