TECTONIC TRANSITION FROM THE SONGPAN-GARZE FOLD BELT TO THE SICHUAN BASIN, SOUTH-WESTERN CHINA

The formation of the Songpan-Garze Fold Belt and the initiation of the terrestrial Sichuan Basin are related to closing of the Palaeo-Tethys during the Late Triassic Indosinian orogeny. The Songpan-Garze Fold Belt is composed of Triassic (T-1-T-3(2)) turbiditic deposits and Palaeozoic greywacke-shale, whereas the Sichuan Basin consists of Sinian to middle Upper Triassic (T-3(2)) platform carbonates and Upper Triassic (T-3x) to Quaternary terrestrial clastics. Three principal deformation episodes during the Late Triassic (Norian to Rhaetian) were progressively localized towards the south-eastern margin of the fold belt. D-1 was a SW-directed shortening event, related to continuous subduction of the Palaeo-Tethys, and produced NW-trending structures. Differential strain between the fold belt and the Sichuan Basin was accommodated by sinistral shearing along a NE-trending transitional zone during D-2. D-3 SE-directed compression was the result of collision between the Cimmerian and Eurasian Continents and initiated the Longmen Mountains Thrust-Nappe Belt and terrestrial Sichuan Basin. Post-D-3 deformation, related to SE-directed thrusting in the Longmen Mountains, then propagated from hinterland to foreland. The Indosinian orogeny closed the Palaeo-Tethys and terminated the marine conditions that dominated the early evolution of the intracratonic Sichuan Basin. Tectonic loading from the exhumed fold belt and Thrust-Nappe Belt induced substantial subsidence in the Sichuan Basin, especially in the Western Sichuan Foreland Basin, resulting in the deposition of a terrestrial elastic sequence during Late Triassic (T-3x) to Quaternary times. The foreland basin history comprises an early stage during the Late Triassic (T-3x(1-2)), an over-fill stage during the latest Triassic to Early Cretaceous (T-3x(3)-K-lj), and a shrinking stage from the Late Cretaceous to the Quaternary (K-2j-Q). These can be correlated with tectonic events in the Thrust-Nappe Belt.