RIFTING, SEAFLOOR SPREADING AND SUBDUCTION ZONE RECORD OF THE NEOTETHYAN OCEANIC LITHOSPHERE IN SOUTHERN TIBET AND THE CRUSTAL ARCHITECTURE OF THE YARLUNG-ZANGBO SUTURE ZONE

Remnants of the Cretaceous Neotethyan oceanic lithosphere, ophiolitic mélanges, Triassic-Jurassic flysch units and continental rocks are exposed discontinuously along the >2000-km-long Yarlung-Zangbo suture zone (YZSZ) in Southern Tibet. Collectively, these tectonic units display a long record of rift-drift, seafloor spreading and subduction zone magmatism that occurred in a Neotethyan ocean north of the proto-Indian continent during the Mesozoic. The YZSZ ophiolites are diverse in their internal structure and geochemical makeup and range in affinity from plume-influenced continental margin types to N-MORB to E-MORB mid-ocean ridge and suprasubduction zone (forearc and backarc) types. Timing of the magmatic evolution of these ophiolites from west to east within the Neotethyan realm was diachronous, explaining the wide range in their ages reported from along-across the YZSZ. Blocks of high-grade metapelites-amphibolites within and south of the YZSZ represent the exposed remnants of microcontinents that once separated different seaways within Neotethys. YZSZ ophiolites were first emplaced southwards onto the northern margin of proto-India as a result of an arc-continent collision in the late Cretaceous, and were subsequently deformed and backthrusted northwards due to the India – Asia collision in the Paleogene. The ophiolites, ophiolitic mélanges and Triassic flysch units were tectonically displaced northwards onto the younger, Eocene conglomerate and the Gangdese magmatic belt along the northern boundary of the YZSZ. Continued oblique continental collision and significant dextral strike-slip deformation throughout the Cenozoic locally produced large-scale reclined folds with steeply plunging fold axes, and caused major attrition of the Xigaze forearc sequence.