Abstract

The Songpan-Ganzi complex is composed of deep-water depositional systems that accumulated in the northeasternmost Tethys during the Middle to Late Triassic, and which are coeval with the continent-continent collision between the South China and the North China blocks that produced the Qinling-Dabie orogen to the east. Middle to Upper Triassic turbidite strata within the Songpan-Ganzi complex are an estimated >10 km (>6 mi) thick, and cover an area of >200,000 km² (>77,000 mi²), equivalent to the size of Colorado. Abundant sand-size detritus derived from the uplift and erosion of this collisional orogen was transported by orogen-parallel drainages into the Songpan-Ganzi deep-water basin. These deposits were intensely deformed and subjected to low-grade metamorphism during Early Jurassic time. They outcrop as thrusted dip panels along heavily vegetated, steep river valleys of the eastern Tibet Plateau. Thus, limited outcrop exposures preclude in-depth, three-dimensional analysis of the geometries and evolution of these deep-water deposystems. However, one-dimensional analysis of facies present in individual outcrops (generally less than 20 m [66 ft] thick) provides valuable constraints on the dominant depositional mode and nature of the deep-water systems. Transport in the basin was achieved primarily through turbidity currents, with only very uncommon occurrences of debris flows and other sediment-gravity-flow deposits. The dominance of sand produced a low-gradient depositional slope, upon which developed an extensive, constructional, mid-fan depositional environment characterized by broad channel and channel-levee complexes. Given its overall depositional setting, its configuration of paleotransport systems, and its association with continent–continent collisional tectonics, the Songpan-Ganzi complex is best compared to the modern Indus and Bengal Fans.