The Xigaze forearc basin: evolution and facies architecture (Cretaceous, Tibet)

Abstract

The mid-Cretaceous to Eocene flysch deposits of the Xigaze forearc basin in southern Tibet were investigated in a 120 km segment along the Indus-Yarlung suture zone. The basin evolved south of the magmatic arc (Gangdise belt) of the Lhasa block on top of trapped oceanic or transitional crust. Remnants of shelf carbonates are preserved along the northern edge of the basin. The mapped segment was shortened by about 65%; metamorphism reached low-grade conditions along the northern margin of the basin. The flysch sequence reaches a thickness of at least 5 km and consists to a large degree of volcaniclastic (andesitic to dacitic) material shed from the magmatic-arc Gangdise belt. Particularly in the western part of the study area, plutonic and sedimentary rocks from deep erosion levels and/or more distal sources contributed to the basin fill. Rivers from the Lhasa block acted as point sources and fed five major deep-sea channel systems. Turbidity currents in the channels were directed towards the growing accretionary wedge of the subduction zone, thus indicating that the basin was continuously filled up to outer ridge level and gradually shallowing. The forearc flysch is subdivided into at least three megasequences, which begin with wide (up to several km) incised (10 to 50 m), coarse-grained channel fills and their associated fan deposits. The upper parts of the megasequences contain hemipelagic dark shales and marls (deposited above the calcite compensation depth). Lateral channel migration, channel-lobe switching, but also volcanic pulses generated a predominantly fining-upward, high-frequency cyclicity. After continental collision, the marine sedimentation in the forearc basin was replaced by fluvial deposits of the Eocene-Oligocene Qiuwu formation, which is time-equivalent to the Kailas and Indus molasses farther west and rich in coarse gravel derived from the Gangdise belt. Both forearc flysch and Qiuwu formation were deformed simultaneously during the Miocene. We assume that the molasse-type Qiuwu formation represents the final continental facies of the forearc basin filling.