The Central Asian Orogenic System (CAOS) is one of the largest Phanerozoic accretionary orogens in the world and may represent a significant site of continental growth. Its origin has been explained by two competing models: syn-subduction strike-slip duplication of a single (>1000 km) long-lived arc (ca. 630–360 Ma) or collision of multiple arcs and micro-continents. Central to the debate are the relative roles of syn-subduction strike-slip faulting versus thrusting. In both models, the Ertix fault figures prominently, either as a roof fault of a large strike-slip duplex system developed during oceanic subduction or as a suture of arc-continent or continent-continent collision. In order to differentiate between the above models, we conducted field mapping, detailed kinematic analysis, and geochrono-logical dating of the Ertix fault zone in the Chinese Altai. Our work indicates that the fault is a crustal-scale thrust that was active in the Permian. Its hanging wall records two pulses of magmatism ca. 450 Ma and ca. 280 Ma and experienced peak pressure and temperature of 6.2–7.7 kbar and 560–670 °C. Our geologic observations, together with the existing geologic information, favor a tectonic model that involves two episodes of subduction below the Altai arc: first, in the Ordovician, along a south-dipping sub-duction zone; and second, in the late Carboniferous and early Permian along north-dipping subduction of the Junggar ocean. It was during the latter event that a mélange complex was underplated below the older Ordovician arc, metamorphosed at lower crustal depths, and then exhumed to the upper crust along the south-directed Ertix thrust zone.

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