No abstract

The late Cenozoic Kongur Shan extensional system lies along the northeastern margin of the Pamir at the western end of the Himalayan-Tibetan orogen, accommodating east-west extension in the Pamir. At the northern end of the extensional system, the Kongur Shan normal fault juxtaposes medium-to high-grade metamorphic rocks in both its hanging wall and footwall, which record several Mesozoic to Cenozoic tectonic events. Schists within the hanging wall preserve a Buchan metamorphic sequence, dated as Late Triassic to Early Jurassic (230-200 Ma) from monazite inclusions in garnet. Metamorphic ages overlap with U-Pb zircon ages from local granite bodies and are interpreted to be the result of regional arc magmatism created by subduction of the Paleo-Tethys ocean. The northern portion of the footwall of the extensional system exposes an upper-amphibolite-facies unit (~650 °C, 8 kbar), which structurally overlies a lowgrade metagraywacke unit. The high-grade unit records late Early Cretaceous crustal thickening at ca. 125-110 Ma, followed by emplacement over the low-grade metagraywacke along a north-northeast-directed thrust prior to ca. 100 Ma. Together these results indicate signifi cant middle Cretaceous crustal thickening and shortening in the northern Pamir prior to the Indo-Asian collision. A third Late Miocene (ca. 9 Ma) amphibolite-facies metamorphic event (~650-700 °C, 8 kbar) is recorded in footwall gneisses of the Kongur Shan massif. North of the Kongur Shan massif, rapid cooling in the footwall beginning at 7-8 Ma is interpreted to date the initiation of exhumation along the Kongur Shan normal fault. A minimum of 34 km of east-west extension is inferred along the Kongur Shan massif based on the magnitude of exhumation since the Late Miocene (~29 km) and the present dip of the Kongur Shan normal fault (~40°). Field observations and interpretation of satellite images along the southernmost segment of the Kongur Shan extensional system indicate that the magnitude of late Cenozoic east-west extension decreases signifi cantly toward the south. This observation is inconsistent with models in which east-west extension in the Pamir is driven by northward propagation of the right-slip Karakoram fault, suggesting instead that extension is driven by vertical extrusion due to topographic collapse, radial thrusting along the Main Pamir Thrust, or oroclinal bending of the entire Pamir region.

[1] New detrital zircon and isotopic (Nd and Sr) analyses from the eastern Pamir provide information on the depositional age and tectonic terrane affiliation of regional metamorphic terranes. Our results show the following. First, detrital zircon analyses from metasedimentary units along the Kongur Shan extensional system dominantly yield Triassic maximum depositional ages, with a similar age distribution to the Tibetan Songpan-Ganzi terrane. Further, zircon analyses from quartzofeldspathic gneisses in the core of the Muztaghata massif show the protoliths are Triassic granites. These units are interpreted to be part of the Permian-Triassic Karakul-Mazar arc-accretionary complex terrane. Second, ɛNd (0) compositions of Triassic granites overlap with other metasedimentary units not analyzed for detrital zircons and are also interpreted to be part of the Karakul-Mazar terrane. Third, schists in the Sares-Muztaghata gneiss dome structurally above Triassic orthogneisses yield an Ordovician maximum depositional age with a distinct detrital age distribution similar to the Tibetan Qiangtang terrane and are interpreted to be part of the Central Pamir terrane. Finally, Triassic and Ordovician schists along the Muztaghata massif record an Early Jurassic metamorphic event interpreted to date south-directed subduction of the Karakul-Mazar terrane beneath the Central Pamir during final closure of the Paleo-Tethys. These results, integrated with previously published results and field relations, reveal a complex Mesozoic to Cenozoic interleaving of tectonic terranes in the eastern Pamir with emplacement of the Karakul Mazar terrane both above and below the Kunlun and Central Pamir terranes to the north and south, respectively.Citation: Robinson, A. C., M. Ducea, and T. J. Lapen (2012), Detrital zircon and isotopic constraints on the crustal architecture and tectonic evolution of the northeastern Pamir, Tectonics, 31, TC2016,

No abstract

The Tarim basin between the Tibetan plateau to the south and Tian Shan to the north in the Indo-Asian collision zone is little deformed as indicated by flat-lying Cenozoic strata across much of the basin. Due to the lack of direct observations from its crystalline basement, the geologic setting for the existence of such a rigid Cenozoic block remains elusive. Hypotheses for the nature of the Tarim basement include (1) Precambrian basement, (2) late Paleozoic trapped oceanic basin, (3) a late Precambrian failed rift, and (4) a Precambrian oceanic plateau. These models make specific predictions about the age and composition of the Tarim basement. To test these hypotheses, we conduct geochemical and geochronologic analyses of samples recovered from a deep well that reached a depth of . 7000 m and drilled into the crystalline basement for , 35 m beneath the central Tarim basin. Mineralogical composition and major element analysis suggest that the crystalline from the drill core is a diorite. Under think sections the rocks samples consist of fine-grained (0.1-0.4 mm in the longest dimension) and medium-grain domains (2 -3 mm in the longest dimension). The contact between the two domains is sharp and the change in grain size across the boundary is abrupt. The rock under thin section shows undeformed igneous textures. Rare earth element patterns and isotopic compositions of Sr and Nd suggest that the central Tarim diorite was derived from an arc setting. The minimum age of the diorite is determined by 40 Ar/ 39 Ar dating of hornblende, which yields three ages from three different samples: 790.0^22.1, 754.4^22.6, and 744.0^9.3 Ma, respectively (uncertainty is reported at 1s). The older age is associated with the fine-grained sample while the younger ages are associated with the medium-grained samples. We are unable to determine whether the different ages are due to argon loss as the rock was located in partial retention zone or caused by different phases of igneous intrusion. In any case, the initiation of the pluton intrusion in the central Tarim region must predate 790.0^22.1 Ma. The possible existence of a Proterozoic magmatic arc (. 790 Ma) in the central Tarim region may be spatially correlated to a late Precambrian blueschist belt in the southern Tian Shan and a 970-920 Ma plutonic belt in the central and eastern Altyn Tagh range east and west of the drilling site where our samples were obtained. This regional correlation implies the existence of an east-trending Precambrian subduction system underneath the Tarim basin, which might be related to a north-dipping subduction zone. Our new data do not support the Tarim to be floored by a remnant oceanic basin or an oceanic plateau. Our results in conjunction with the fact that the Tarim interior has experienced major deformation as recent as the Jurassic do not support the hypotheses that the Tarim basement is compositionally different from and mechanically stronger than its surrounding areas during Cenozoic deformation. Differences in thermal structures or stress stat...