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

Abstract

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 margins. Here, we present a multidisciplinary study examining the exhumation tectonics of the Permo−Triassic eclogite-bearing HP-LT terranes of the Central Qiangtang metamorphic belt (CQMB) in the central Tibetan Plateau. Field geological relations and microscopic observations show that the HP-LT rocks in the Lanling area are separated from the epimetamorphic late Paleozoic ophiolite mélange of the hanging wall by low-angle detachment faults and primarily exhibit five stages of deformation. The HP-LT terranes 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 retrograde eclogites indicate that these rocks are characterized by synexhumation mineral growth pulses with hairpin-type decompressional pressure-temperature trajectories. A compilation of previous geochronological data and our 40Ar/39Ar dating results of synexhumation minerals and shear bands in HP-LT rocks indicate continuous exhumation at ca. 244−215 Ma prior to the continental collision between the North Qiangtang Block (NQB) and South Qiangtang Block. Moreover, the CQMB is likely an autochthonous 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 Triassic lithospheric extension in the NW-SE direction.