Forearc hyperextension by detachment faulting and ophiolite dismemberment: examples from the Yarlung Tsangpo Suture Zone (Southern Tibet)
Friday, 19 December 2014
The broad deformation zone of the Himalayan belt and Tibetan plateau is largely the product of continent-continent collision between India and Eurasia plates since the Early Eocene. Ophiolites exposed along the Yarlung Tsangpo Suture Zone (southern Tibet) demonstrate that a long-lasting intra-oceanic subduction zone must have played a significant role in accommodating closure of the >7500 km wide Neotethyan Ocean before continental collision. Their study can provide key constraints on the initial history of the Neotethyan subduction systems, and the following formation and emplacement of the ophiolite. Paleomagnetic analyses of the sheeted dykes complex of ophiolites have been successfully applied in the past to reconstruct the initial geometry of the spreading system associated to the ophiolite formation. Furthermore, oceanic detachment faults, structures widely occurring in modern magma-poor (slow-spreading) mid-ocean ridges, have been recently recognized also in ophiolites (i.e., Mirdita ophiolite of Albania), and (if present) their study may provide unique insights into the geodynamics and geometry of the associated spreading system. The YZSZ ophiolites form a 2500 km long belt mainly composed of dismembered ultramafic massifs locally covered by a crustal sequence and oceanic sediments, underlying a regionally continuous clastic Xigaze sedimentary basin interpreted as the Tibetan forearc. Our study focused along a ~250 km transect within the eastern sector of the YZSZ between the Sangsang and Xigatze ophiolite. Paleomagnetic, structural geological, and geochemical analyses evidenced the presence of fossil oceanic detachment faults that locally (Sangsang) exposed the lowermost units (mantle) directly at the seafloor. Based on this evidence, and the reconstructed rotation pattern of the region we propose a tectonic evolutionary model characterized by the interplay between magmatic crustal accretion and trench-parallel and trench-perpendicular tectonic extension, which resulted in thinning and dismemberment of the ophiolite.