Sm-Nd isotopic compositions of deep-marine mudstones, Xigaze forearc basin, southern Tibet: Implications for drainage evolution and expansion

Abstract

Provenance analysis of fine-grained sedimentary rocks has great importance to understanding the tectonic and paleogeographic evolutions of convergent margin systems. This study conducted Sm-Nd isotopic analysis of mudstone samples from Cretaceous deep-marine strata in the Xigaze forearc basin to evaluate the provenance, paleogeography, and regional drainage patterns in the arc-forearc region. Ten mudstone samples yield εNd(t) ranging from −7.44 to + 0.58 and Nd model age from 0.862 to 1.50 Ga. The mudstones were sourced from a mixture of the Gangdese arc and the Central to Northern Lhasa terrane. The Gangdese arc was the dominant source area, and the relative contribution of the Central to Northern Lhasa terrane sources increased up-section. Our interpretation is consistent with studies on coarser components. We interpret these results as a long-term signal of drainage capture of drainage networks that originated in the southern end of the Gangdese arc and brought Gangdese sources into the forearc basin during its early stages. Fostered by the uplift of the Lhasa terrane, drainage capture eventually reached the Gangdese mountains and regions to the north, resulting in the erosion and supply of more materials with negative εNd(t) from the Central and/ or Northern Lhasa to the basin. Similar patterns of Sm-Nd provenance evolution and drainage expansion have been observed in other well-studied forearc basins (e.g., the Great Valley forearc basin of California) and may reflect an important exhumation trend in convergent margin systems.