Retreating Subduction-Related Intracrationic Rifting and its Effects on Differential Organic Matter Enrichment: Insights from the Late Permian Sichuan Basin (Sw China)

Abstract

The Late Permian era witnessed significant tectonic and environmental modification to Earth. Of interest is the possible role of Late Permian tectonism on organic matter enrichment (OME). This paper presents a case study of retreating subduction-related rifting and its effect on OME by an investigation of end-Permian shale of the Dalong Formation (DFS) of the Kaijiang-Liangping trough (KLT), northern Sichuan Basin, southwestern China. Our findings reveal clusters of detrital zircon weighted mean 206Pb/238U ages at ca. 256 Ma and 252 Ma suggesting that continental arc magmatism at the end of the Permian was a principal sedimentary provenance of contemporaneous clastic deposits. Furthermore, newly acquired geochemical data point to a southward retreating subduction event during Late Permian time triggered the formation of rift troughs from the northern margin to the intraplatform of the basin. Weak subduction-related extension confined organic-rich shale to the KLT in the deeper region of the northern intraplatform basin. OME of the DFS is characterized by spatio-temporal variation, including high-TOC (>2%) shale of the middle part of the Dalong Formation that varies position within the stratigraphic succession as well paleogeographically trough center to slope as well as relatively enriched TOC in the eastern KLT. OME of the DFS appears to have been intrinsically tied to paleoproductivity sustained by upwelling in the western KLT, but enhanced preservation related to the establishment of anoxic conditions in the more restricted eastern KLT. Spatio-temporal differentiation of OME is closely correlated with weak and unstable retreating subduction, which influences the nature of associated volcanic activity, upwelling, paleoredox conditions, deposition rates and detrital input, all of which are closely linked with the OME. The described case study adds to our understanding of the role of retreating subduction-related intracontinental passive rifting and its variable effects on OME of contemporaneous fine-grained sediment.

Keywords: retreating subduction, organic-rich differentiation, Paleoproductivity, Preservation, Upwelling, redox condition