Lithospheric extension of the accretionary wedge: An example from the Lanling high-pressure metamorphic terrane in Central Qiangtang, Tibet

Deciphering the exhumation mechanism of high-pressure-low-temperature (HP-LT) metamorphic rocks can provide important insights into the tectonic evolution of oceanic subduction zones at active continental mar-gins. Here, we present a multidisciplinary study examining the exhumation tectonics of the Permo-Triassic eclogite-bearing HP -LT terranes of the Central Qiangtang meta-morphic belt (CQMB) in the central Tibetan Plateau. Field geological relations and mi-croscopic observations show that the HP-LT rocks in the Lanling area are separated from the epimetamorphic late Paleozoic ophiolite melange of the hanging wall by low-angle detachment faults and primarily exhibit five stages of deformation. The HP-LT ter-ranes were exhumed as a metamorphic core complex and show pervasive synexhumation top-to-the-SW and-S shear structures. The results of the petrological and mineralogical analysis and pseudosection modeling of ret-rograde eclogites indicate that these rocks are characterized by synexhumation mineral growth pulses with hairpin-type decompres-sional pressure-temperature trajectories. A compilation of previous geochronologi-cal data and our 40Ar/39Ar dating results of synexhumation minerals and shear bands in HP-LT rocks indicates continuous exhuma-tion at ca. 244-215 Ma prior to the continen-tal collision between the North Qiangtang Block (NQB) and South Qiangtang Block. Moreover, the CQMB is likely an autoch-thonous accretionary wedge resulting from northward subduction of the Paleo-Tethys Ocean beneath the NQB. Combined with ca. 243-230 Ma mantle upwelling to the north of Lanling, we infer that the CQMB, namely, the accretionary wedge of the Paleo-Tethys Ocean, experienced Middle-Late Tri-assic lithospheric extension in the NW-SE direction.