Tectonic development of the southern Chinese Altai Range as determined by structural geology, thermobarometry, 40Ar/39Ar thermochronology, and Th/Pb ion-microprobe monazite geochronology

摘要

The 2500-km Altai Range is located in the central part of the Central Asia Orogenic System, a tectonic collage comprising oceanic and continental fragments that were assembled during the Paleozoic continental growth of Eurasia. We conducted field mapping, 40Ar/39Ar thermochronology, metamorphic petrology, and Th/Pb ion-microprobe monazite dating in the southern Chinese Altai Range. This study demonstrates the presence of a south-vergent, Permo-Triassic thrust belt active across the region. Metamorphic conditions of 610 ± 35 °C and 5.7 ± 1.8 kbar were reached by schists with Permo-Triassic monazite ages. Mica 40Ar/39Ar ages range from Late Permian to Jurassic, and cooling in these rocks is correlated with thrust faulting. This shortening was synchronous with localized left-lateral, strike-slip shear deformation. Our work suggests that the high-grade schists of the Altai orogen were buried to depths of more than 18–20 km and were exhumed in the Permian to Jurassic. The Permo-Triassic Altai thrust belt was reactivated locally by Late Jurassic contraction after ca. 160 Ma, which may result from the final closure of the Mongol-Okhotsk Ocean or the collision of the Lhasa block onto the southern Asian margin.