SEDIMENTATION AND LITHOFACIES PALEOGEOGRAPHY IN SOUTHWESTERN CHINA BEFORE AND AFTER THE EMEISHAN FLOOD VOLCANISM: NEW INSIGHTS INTO SURFACE RESPONSE TO MANTLE PLUME ACTIVITY

Abstract

Investigations into Permian sedimentation and reconstruction of paleogeography in SW China are aimed at characterizing sedimentary responses to the Emeishan mantle plume. In addition to erosional features on the sediments underlying the uplifted Emeishan basalts, unusual depositions of Permian age are also present in the Emeishan large igneous province (LIP). Specifically, carbonate gravity flows and submarine incised canyon fillings were developed in the western margin of the postulated uplifted area, and rifting trenches were developed along the eastern margin, alluvial fan deposits occur at the boundary between the inner and intermediate zones. These depositions all rest on the Maokou Formation and are in turn covered by the Emeishan basalts, implying synchronism between crustal uplift and depositional events. These deposits and the associated extension and normal faulting along the margin of and within the LIP represent sedimentary features resulting from dynamic behavior of mantle plume. Comparison of lithofacies paleogeography before and after the Emeishan flood volcanism highlights the determinant role of mantle plume activity in the geological evolution in SW China. The rapid, differential erosion of the Maokou Formation was likely related to plume-induced dynamic uplift. This uplift was apparently followed by subsidence, given deposition of the marine elastic rocks sandwiched between basalts and the Maokou Formation in the east and submarine basalts along the margins of the province. A second-phase uplift, attributed to underplating of plume-derived melts at the crust-mantle boundary, was characterized by prolonged (~45 m.yr.), plateau-type uplift and was responsible for the appearance of the "Chuandian old land." Integration of these erosional and depositional characteristics allows us to depict how the surface geology responds to mantle plume, which explains some complex sedimentological problems in SW China. © 2006 by The University of Chicago. All rights reserved.