Basin-scale Distributed Denudation Patterns Revealed by Multiple Thermo-chronological Results: Example from Lhasa River, Southern Tibet

Abstract:

Sediments removed from the upper cover of tectonically active regimes carry abundant information of source-to-sink processes. With proper thermo-chronological tools we can detect the transporting routes of analyzed grains, unroofing history of intended study area, denudation pattern of target watershed, and potentially the activity of existing active structures.

In this study, we applied multiple thermo-chronometers to gain our understanding toward the surface process of middle Yarlung-Tsangpo, Tibet. Modern river samples were collected near the river mouth of two major tributaries of Yarlung-Tsangpo, Lhasa River and Nyang River. Basement rock samples were acquired across some transects of well-known rifting structures within the Lhasa River catchment. Target minerals were separated and subject to a series of thermo-chronological analyses: Apatite and zircon grains for fission track dating; K-feldspar grains for Ar-Ar (laser ablation) analyses; in addition, zircon grains were firstly counted for fission track results and then double analyzed for U/Pb ages.

Results from apatite/zircon fission track and Ar-Ar (K-feldspar) analyses all display a distinctive pattern showing high fraction of young thermal age component (younger than 20 Ma) dominates within the Lhasa River (over 80%). Zircon double dating results suggest that these young thermal ages are not directly linked to recent emplacement of crystalline bodies but reflecting an episode of exhumation in the Lhasa River catchment. Comparisons of Ar-Ar results between upstream and downstream samples from Lhasa River further imply that the key area generating young thermal ages lies upon the upstream regime of the catchment, where the Nyainqentanglha Range and the active Gulu rifting belt are situated. Thermo-chronological data from basement transects across the rifting belt and the modern sediments are compiled to better depict the focused denudation pattern.