Geochronology, Geochemistry, and Geodynamic Implications of Permo-Triassic Back-Arc Basin Successions in the North Pamir, Central Asia

Abstract

The Permo-Triassic period marks the time interval between Hercynian (Variscan) orogenic events in the Tien Shan and the North Pamir, and the Cimmerian accretion of the Gondwana-derived Central and South Pamir to the southern margin of the Paleo-Asian continent. A well-preserved Permo-Triassic volcano-sedimentary sequence from the Chinese North Pamir yields important information on the geodynamic evolution of Asia’s pre-Cimmerian southern margin. The oldest volcanic rocks from that section are dated to the late Guadalupian epoch by a rhyolite and a dacitic dike that gave zircon U-Pb ages of ~260 Ma. Permian volcanism was largely pyroclastic and mafic to intermediate. Upsection, a massive ignimbritic crystal tuff in the Chinese Qimgan valley was dated to 244.1±1.1 Ma, a similar unit in the nearby Gez valley to 245±11 Ma, and an associated rhyolite to 233.4±1.1 Ma. Deposition of the locally ~200 m thick crystal tuff unit follows an unconformity and marks the onset of intense, mainly mafic to intermediate, calc-alkaline magmatic activity. Triassic volcanic activity in the North Pamir was coeval with the major phase of Cimmerian intrusive activity in the Karakul-Mazar arc-accretionary complex to the south, caused by northward subduction of the Paleo-Tethys. It also coincided with the emplacement of basanitic and carbonatitic dikes and a thermal event in the South Tien Shan, to the north of our study area. Evidence for arc-related magmatic activity in a back-arc position provides strong arguments for back-arc extension or transtension and basin formation. This puts the Qimgan succession in line with a more than 1000 km long realm of extensional Triassic back-arc basins known from the North Pamir in the Kyrgyz Altyn Darya valley (Myntekin formation), the North Pamir of Tajikistan and Afghanistan, and the Afghan Hindukush (Doab formation) and further west from the Paropamisus and Kopet Dag (Aghdarband, NE Iran).

Late Miocene to present synchronous extension and contraction in the eastern Pamir: Insights from inversion of thermochronologic data across the southern Muztaghata dome

“…1A), formed due to the late Paleozoic-early Mesozoic accretion of the Kunlun, Karakul-Mazar, central Pamir, and southern Pamir terranes (Fig. 1B; Burtman and Molnar, 1993;Burtman, 2010;Robinson et al, 2004Robinson et al, , 2012Robinson, 2015;Angiolini et al, 2013;Villarreal et al, 2020;Chen et al, 2021;Li et al, 2022;Rembe et al, 2022) and the late Mesozoic-Cenozoic intracontinental deformation caused by northward subduction of the Neo-Tethys oceanic slab and subsequent collision of the Indian and Asian plates (Burtman and Molnar, 1993;Sobel and Dumitru, 1997;Robinson et al, 2004;Cowgill, 2010;Bershaw et al, 2012;Sobel et al, 2011Sobel et al, , 2013Cao et al, 2013a;Robinson, 2015;Rutte et al, 2017aRutte et al, , 2017bChapman et al, 2018;Worthington et al, 2020;Cai et al, 2021;Li et al, 2022;Villarreal et al, 2023). In this convergent setting, igneous and medium-to high-grade (575-830 °C/6-22 kbar) metamorphic rocks were initially exhumed in the early Miocene in the Pamir interior in a series of gneiss domes that formed by detachment faulting associated with ∼N-S extension (Fig.…”