Blueschist-bearing metamorphic core complexes in the Qiangtang block reveal deep crustal structure of northern Tibet

A 500-km-long belt of metamorphic exposures in the Qiangtang block provides an opportunity to study the internal structure of northern Tibetan crust. Metamorphic rocks exposed at two widely separated areas along this belt con sist of blueschist-bearing melange and are bounded by Late Triassic-Early J urassic, domal, low-angle normal faults. We propose that this melange was u nderplated to the Qiangtang block and was subsequently exhumed by detachmen t faulting; both the underplating and the exhumation occurred during early Mesozoic southward subduction of oceanic lithosphere along the Jinsha sutur e. This model predicts that the deeper crust of much of northern Tibet cons ists of accretionary melange, in contrast to the continental crystalline cr ust of southern Tibet, and may account for north-south variations of Cenozo ic tectonism in Tibet.