Mesozoic evolution of the eastern Pamir

Abstract

We present field and analytical results from the Tashkurgan and Waqia valleys in the southeastern Pamir that shed new light on the tectonic evolution and terrane architecture of the region. Field mapping of metasedimentary and igneous units along the Tashkurgan and Waqia valleys in the Southeast Pamir, integrated with metamorphic petrology, garnet-biotite thermometry, and zircon U/Pb isotopic analysis, help identify major structures and terrane boundaries in the region, as well as compare structural units across the Miocene Muztaghata gneiss dome. South of the Muztaghata dome, the gently northwest-plunging synformal Torbashi thrust klippe juxtaposes amphibolite facies Triassic Karakul-Mazar terrane schist and gneiss structurally above (1) greenschist facies Triassic Karakul-Mazar terrane metasedimentary rock in the north, and (2) lower-amphibolite facies schist in the south that are interpreted to be Gondwanan-derived crust (Central or South Pamir terrane). Farther south, the Rouluke thrust fault imbricates the Gondwanan crust, placing early Paleozoic schists over Permian marble and slate. Exposure of the Torbashi thrust sheet terminates in the southeast, and with it the surface exposure of the Triassic Karakul-Mazar terrane, leaving the Paleozoic Kunlun terrane juxtaposed directly against Gondwanan terrane crust. Based on lithologic and isotopic similarities of units north and south of the Muztaghata gneiss dome, we document the existence of a regionally extensive thrust nappe that stretched across the northern and eastern Pamir, prior to being cut by Miocene exhumation of the Muztaghata dome. The thrust nappe links the Torbashi thrust in the southeast Pamir with the Tanymas thrust in the northern Pamir, and documents regionally extensive exposure of lithologically continuous units across the northeast Pamir. While timing of emplacement of the Torbashi thrust klippe and displacement on the Rouluke fault to the south is not well constrained, we interpret shortening to be Cretaceous in age based on previously published cooling ages. However, a component of Cenozoic shortening cannot be ruled out. A key observation from our mapping results is that the surface exposures of the Karakul–Mazar–Songpan Ganzi terrane are not continuous between western Tibet and the Pamir, which indicates tectonic and/or erosional removal, likely sometime in the Mesozoic. Furthermore, our documentation of the Jinsha suture in the southeast Pamir on the eastern side of the Karakoram fault shows deflections of terranes across the Himalayan-Tibetan orogen were not primarily accommodated along discrete, large displacement faults (>400 km) faults. Instead, oroclinal bending of the northern Pamir, and dextral shear along the Pamir margins, may be largely responsible for the northward deflection of terranes.

Muztaghata Dome Miocene Eclogite Facies Metamorphism: A Record of Lower Crustal Evolution of the NE Pamir

“…Intensive Cenozoic lower crustal thickening appears to be partially decoupled from the upper crust of the Pamir, as upper crustal shortening is limited to localized deformation along the Central Pamir and Rushan-Pshart suture zone (Figure 14b; Robinson, 2015;Rutte, Ratschbacher, Khan, et al, 2017;Chapman, Robinson, et al, 2018;Imrecke et al, 2019). This indicates that bulk crustal shortening is not a possible mechanism to thicken the Pamir crust in the Cenozoic (Chapman, Robinson, et al, 2018).…”

“…The upper crustal deformation during this stage is characterized by intensive folding in the Permian-Triassic formations throughout the Central-Southern Pamir (Angiolini et al, 2013;Chapman, Scoggin, et al, 2018;Villarreal et al, 2020). The Cimmerian crustal shortening is broadly coeval with high-grade metamorphism in the Northern Pamir related to crustal thickening in the overriding plate during Paleotethys subduction and accretion (Imrecke et al, 2019;Robinson et al, 2004;Yang et al, 2010). The second major crustal shortening stage in the Pamir occurred during a broad interval in the Early Cretaceous and is defined by thrust faulting and upper amphibolite facies metamorphism in the Northern Pamir terrane and a broad fold-thrust belt development in the Central-Southern Pamir terranes (Chapman, Scoggin, et al, 2018;Imrecke et al, 2019;Robinson et al, 2004Robinson et al, , 2007Robinson et al, , 2012.…”

“…The Cimmerian crustal shortening is broadly coeval with high-grade metamorphism in the Northern Pamir related to crustal thickening in the overriding plate during Paleotethys subduction and accretion (Imrecke et al, 2019;Robinson et al, 2004;Yang et al, 2010). The second major crustal shortening stage in the Pamir occurred during a broad interval in the Early Cretaceous and is defined by thrust faulting and upper amphibolite facies metamorphism in the Northern Pamir terrane and a broad fold-thrust belt development in the Central-Southern Pamir terranes (Chapman, Scoggin, et al, 2018;Imrecke et al, 2019;Robinson et al, 2004Robinson et al, , 2007Robinson et al, , 2012. Deformation during the Early Cretaceous is interpreted to have occurred in a retroarc setting during north-directed subduction of Neotethys Ocean (e.g., Chapman, Robinson, et al, 2018;Chapman, Scoggin, et al, 2018;Soret et al, 2019).…”

Cretaceous Evolution of the Central Asian Proto‐Paratethys Sea: Tectonic, Eustatic, and Climatic Controls

“…To further infer the tectonic setting, we can compare the age of the increase in tectonic subsidence in the Tajik and Tarim basins to reported regional tectonic events in the Pamir. Several studies in the Pamir (e.g., He et al, 2019; Imrecke et al, 2019; Robinson, 2015; Robinson et al, 2004, 2012) provided evidence for Early Cretaceous tectonic deformation. Crustal thickening and shortening in the North Pamir have been evidenced by metamorphic and rapid exhumation events during the mid‐Cretaceous (130–100 Ma) by Robinson et al (2004).…”

“…In addition to the commonly accepted Cenozoic thickening that formed the 90 km thick continental crust (Rutte, Ratschbacher, Khan, et al, 2017; Rutte, Ratschbacher, Schneider, et al, 2017; Stübner, Ratschbacher, Rutte, et al, 2013; Stübner, Ratschbacher, Weise, et al, 2013), pre‐Cenozoic deformation involving significant Mesozoic upper‐crustal shortening and related crustal thickening in the Central and South Pamir has also been proposed (e.g., Aminov et al, 2017; Robinson, 2015). Late Jurassic and mid‐Cretaceous crustal shortening and thickening in the North Pamir and Karakoram regions to the north and south of the Central and South Pamir have also been evidenced, suggesting that the Central and South Pamir were similarly affected by the same deformation events (e.g., He et al, 2019; Imrecke et al, 2019; Robinson, 2015; Robinson et al, 2004, 2012).…”