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 Â 10 23 to 5 Â 10 24 N m (equivalent elastic thickness of c. 43±54 km). The orogenic load system initially (c. 227±220 Ma) advanced rapidly (>15 mm yr À1 ) towards the South China Block in the Carnian, associated with the rapid closure of the Songpan-Ganzi ocean, before slowing to < 5 mm yr À1 during the sedimentation of the upper two tectonostratigraphic units (c. 220±206 Ma).

The tectonic evolution of the Songpan-Garzê (North Tibet) and adjacent areas from Proterozoic to Present: A synthesis

“…1A). To the east, the Longmen Shan thrust-nappe belt separates the Songpan-Garzê fold belt from the Sichuan basin (Zhang et al, 1984;Chen et al, 1995;Chen and Wilson, 1996;Yong et al, 2003) (Fig. 1B).…”

Episodicity of Mesozoic terrane accretion along the Pacific margin of Gondwana: implications for superplume-plate interactions

“…Deposition in the flexural foredeep to this fold belt, the Longmen Shan foreland basin (LS: Fig. 1), spanned the time period from 227 Ma to 206 Ma (Yong et al 2003). High greenschist to amphibolite grade metamorphism occurred during Late Triassic-Middle Jurassic terrane collision in the Fujian Province of southeast China (FP: Fig.…”

U-Pb dating, geochemistry, and tectonic implications of the Songpan-Ganzi block and the Longmen Shan, China

“…The allochthon is composed mainly of thick succession of strongly folded Middle and Upper Triassic SongpanGanzi flysch (Burchfiel et al, 1995). Silurian thick deposition of fine-grained clastic and carbonate rocks marked the initial subsidence of the western Yangtze craton and formation of a continental margin (Burchfiel et al, 1995 Yong et al, 2003). Black circles represent locations of samples used in zircon U-Pb dating.…”

“…The present-day Longmen Shan front coincides approximately with the position of a Mesozoic collisional plate margin that developed as the Paleo-Tethys closed. As a result, the North Tibetan (Qiangtang) block sutured to the colliding North China craton-Kunlun Shan-Qaidam block and Yangtze craton (Yong et al, 2003). Apart from Middle Paleozoic collision between the North China and Yangtze cratons in the Qinling orogen (Meng and Zhang, 1999), the DabieHong'an-Tongbai-Qinling orogen formed chiefly in the Triassic (Liu and Hao, 1989;Hacker et al, 1996Hacker et al, , 1998.…”