Tectonic transition from the Songpan-Garzê Fold Belt to the Sichuan Basin, south-western China
The formation of the Songpan-Garzê 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-Garz2 Fold Belt is composed of Triassic (T1-T2,3) turbiditic deposits and Palaeozoic greywacke-shale, whereas the Sichuan Basin consists of Sinian to middle Upper Triassic (T1) platform carbonates and Upper Triassic (T2,3) 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, 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 D2. D3 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-D3 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 clastic sequence during Late Triassic (T3x) to Quaternary times. The foreland basin history comprises an early stage during the Late Triassic (T3x 1-2), an over-fill stage during the latest Triassic to Early Cretaceous (T3x3-K1J), and a shrinking stage from the Late Cretaceous to the Quaternary (K2j-Q. These can be correlated with tectonic events in the Thrust-Nappe Belt.