Evolution of the Longmen Shan Foreland Basin (Western Sichuan, China) during the Late Triassic Indosinian Orogeny

Abstract

The Longmen Shan Foreland Basin developed as a flexural foredeep during the Late Triassic Indosinian orogeny, spanning the time period c. 227–206 Ma. The basin fill can be divided into three tectonostratigraphic units overlying a basal megasequence boundary, and is superimposed on the Palaeozoic–Middle Triassic (Anisian) carbonate-dominated margin of the South China Block. The remains of the load system responsible for flexure of the South China foreland can be seen in the Songpan-Ganzi Fold Belt and Longmen Shan Thrust Belt. Early in its history the Longmen Shan Foreland Basin extended well beyond its present northwestern boundary along the trace of the Pengguan Fault, to at least the palinspastically restored position of the Beichuan Fault. The basal boundary of the foreland basin megasequence is a good candidate for a flexural forebulge unconformity, passing from conformity close to the present trace of the Beichuan Fault to a karstified surface towards the southeast. The overlying tectonostratigraphic unit shows establishment and drowning of a distal margin carbonate ramp and sponge build-up, deepening into offshore marine muds, followed by progradation of marginal marine siliciclastics, collectively reminiscent of the Alpine underfilled trinity of Sinclair (1997). Tectonostratigraphic unit 2 is marked by the severing of the basin's oceanic connection, a major lake flooding and the gradual establishment of major deltaic-paralic systems that prograded from the eroding Longmen Shan orogen. The third tectonostratigraphic unit is typified by coarse, proximal conglomerates, commonly truncating underlying rocks, which fine upwards into lacustrine shales. The foreland basin stratigraphy has been further investigated using a simple analytical model based on the deflection by supracrustal loads of a continuous elastic plate overlying a fluid substratum. Load configurations have been partly informed by field geology and constrained by maximum elevations and topographic profiles of present-day mountain belts. The closest match between model predictions and stratigraphic observations is for a relatively rigid plate with flexural rigidity on the order of 5 × 1023 to 5 × 1024 N m (equivalent elastic thickness of c. 43–54 km). The orogenic load system initially (c. 227–220 Ma) advanced rapidly (>15 mm yr−1) towards the South China Block in the Carnian, associated with the rapid closure of the Songpan-Ganzi ocean, before slowing to < 5 mm yr−1 during the sedimentation of the upper two tectonostratigraphic units (c. 220–206 Ma).