Mesozoic tectonic history and lithospheric structure of the Qiangtang terrane: Insights from the Qiangtang metamorphic belt, central Tibet

The Triassic Qiangtang metamorphic belt in central Tibet consists of eclogite- and blueschist-bearing tectonic mélange exposed in an intracontinental setting. Study of the belt has yielded insight about the tectonic history and crustal architecture of the Qiangtang terrane. Weakly deformed mafic blocks exhibiting high-pressure, low-temperature metamorphic mineral assemblages are exposed within a greenschist facies sedimentary-matrix mélange over an east-west distance of ∼600 km and north-south distance of ∼150 km. Everywhere it has been mapped, the Qiangtang mélange is exposed in the footwalls of Late Triassic - Early Jurassic domal, low-angle normal faults. The hanging walls of these normal faults are composed of late Paleozoic to early Mesozoic metasedimentary and crystalline rocks; similar lithologies are exposed in footwall mélange rocks. High-pressure metamorphism occurred during Middle Triassic time. Regional geological mapping and provenance of detrital zircon crystals from late Paleozoic to Mesozoic supracrustal rocks of the Qiangtang terrane indicate that Qiangtang crust exposed on the north and south sides of the metamorphic belt are of Gondwanan affinity. This is at odds with the widely held belief that the Qiangtang metamorphic belt marks a suture between a Gondwanan affinity southern and/or western Qiangtang terrane and a Cathayasian affinity northern and/or eastern Qiangtang terrane. Furthermore, metasedimentary rocks exposed within the meta-morphic belt yield detrital zircon age probability distributions consistent with derivation from Qiangtang terrane supracrustal strata and Paleo-Tethys affinity rocks exposed north of the Qiangtang terrane. These data, along with the significant north-south surface exposure of the metamorphic belt, suggest that a large part of the lower to middle crust is composed of silica-rich metasedimentary-dominated mélange rock that formed during Middle Triassic southward subduction along the Jinsha suture on the northern edge of the Qiangtang terrane. The implied crustal structure, which is also defined by geophysical experiments, provides an explanation for the apparent deviation from Airy isostasy from the Lhasa terrane to the Qiangtang terrane. We suggest that the replacement or partial incorporation of mafic crystalline middle to lower crust with less dense metasedimentary mélange rock led to a significant contribution of Pratt isostasy across central Tibet.