Two Cycles of Devonian–Triassic Extension and Compression in the Altai Accretionary Wedge: Insights From Tsogt Core Complex

Abstract The Altai Accretionary Wedge in the Mongolian Altai records a polyphase geodynamic evolution involving two extensional–compressional tectonic cycles, constrained by petrostructural analysis and zircon, monazite U–Pb, and Ar–Ar geochronology. The studied section includes a southwestern high‐grade and a northeastern low‐grade domain, previously considered suspect terranes. However, U–Pb zircon data indicate that both represent the same Cambrian–Ordovician tectonostratigraphic unit, variably overprinted by Devonian and Permian magmatism and metamorphism. The first cycle involved Devonian extension that produced subhorizontal fabrics in both domains, accompanied by widespread magmatism dated at 410−362 Ma. These fabrics were later folded into upright, N–S‐trending Carboniferous folds, preserved mainly in shallow crustal levels. In the high‐grade domain, this event was associated with medium‐pressure to medium‐temperature metamorphism 500−560°C, 5.5−7.5 kbar dated at 358−335 Ma. The second cycle began with the development of a crustal‐scale detachment that juxtaposed the two domains. This extensional event triggered melting of Devonian lower crust and emplacement of granite plutons in the upper crust. It was further characterized by medium‐pressure to high‐temperature metamorphism 715−785°C, 5−8 kbar and formation of subhorizontal extensional mylonites, constrained to 320−278 Ma. Continued extension produced a core complex in the southwestern domain, exhumation of partially molten lower crust, and synchronous deposition of clastic sediments onto the detachment. Subsequent compression caused strong N–S shortening and steepening of earlier fabrics, constrained by Ar–Ar muscovite and biotite ages of 268−245 Ma. Consequently, the Mongolian Altai–Trans‐Altai boundary is redefined as a verticalized extensional‐detachment zone.