Crustal-scale structure of South Tien Shan: implications for subduction polarity and Cenozoic reactivation

Abstract

Based on new structural and petrological investigations, we present two crustal-scale cross-sections of the Kyrgyz South Tien Shan, and correlations of main faults and units between Kyrgyzstan and China. The overall structure corresponds to a doubly-vergent mountain belt. The Kyrgyz and Chinese areas exhibit identical structural and metamorphic histories. To the west, the Atbashi Range comprises high-pressure oceanic and continental units stacked by north-verging thrusts above a low metamorphic accretionary prism. High-pressure (HP) gneisses are bound to their south by a south-dipping detachment exhibiting mantle relicts. The high-pressure oceanic and continental units underwent similar pressure-temperature (P -T ) paths with peak conditions of around 500 8C-20 kbar, followed by rapid exhumation. The overall south-dipping structure and kinematics indicate a south-dipping subduction of the Central Tien Shan Ocean at 320-310 Ma, ending with the docking of the Tarim block to the Kazakh continent. To the east, the Pobeda Massif shows a narrow push-up structure. A major north-vergent thrust exhumes deep-crustal-level granulites, constituting the highest summits, which were thrust towards the north onto low-grade Devonian-Carboniferous schists. The southern part of South Tien Shan is made up of a south-verging thrust stack that formed later during ongoing convergence, reactivated throughout post-30 Ma phases. Downloaded from Fig. 2. (a) Satellite image of Tien Shan belt showing study areas. (Insert) Geographical situation of Tien Shan in Central Asia (Map data: Google, Image Landsat). (b) Tectonic map of Tien Shan (Cenozoic cover removed). The location of detailed maps and the geological crustal-scale cross-sections are shown. Abbreviations used: TFF, Talas-Fergana Fault; NL, Nikolaev Line; STS, South Tien Shan (kSTS + cSTS + cCTS); NTS, North Tien Shan (kNTS + cNTS).

New structural data on Late Paleozoic tectonics in the Kyrgyz Tien Shan (Central Asian Orogenic Belt)

“…In Kyrgyzstan, the At-Bashi-Kokshaal suture is characterized by an accretionary complex composed of several metamorphosed units associated with a non-metamorphic ophiolite cropping out South of the At-Bashi city (Alekseev et al, 2007;Biske, 1996;Biske et al, 1985;Loury et al, 2015b). Recent geological mapping of the At-Bashi range evidenced a South-dipping top-to-the-North tectonic stack, with from bottom to top: (1) a low-grade unmetamorphosed ophiolite thrusted by (2) a greenschist to blueschist accretionary prism and (3) eclogite-facies continental and oceanic units (Loury et al, 2015b).…”

“…Recent geological mapping of the At-Bashi range evidenced a South-dipping top-to-the-North tectonic stack, with from bottom to top: (1) a low-grade unmetamorphosed ophiolite thrusted by (2) a greenschist to blueschist accretionary prism and (3) eclogite-facies continental and oceanic units (Loury et al, 2015b). A Gzhelian to Asselian conglomerate seals the deformation phase related to the exhumation of these high-pressure rocks (Baslakunov et al, 2007).…”

“…It is composed of granitoids and gneisses presenting a greenschist facies imprint (Loury et al, 2015b) and interpreted as a part of the MTS basement (Alekseev et al, 2007). -A dismembered non-metamorphic ophiolite unit has been observed in several places of the At-Bashi range (Alekseev et al, 2009), with serpentinites, gabbros and radiolarites.…”

Geological evolution of Central Asian Basins and the western Tien Shan Range

“…Another paper takes a broad regional approach, examining the entire area while also extending further to the south to include the colliding Arabian and Indian plates. Loury et al (2015) present two crustal-scale cross-sections of the Kyrgyz portion of the Central Tien Shan and correlate the major faults and units eastwards from Kyrgyzstan to China. Based on field and seismic data, they suggest that the broad structure conforms to that of a doubly vergent mountain belt where the Chinese and Kyrgyz areas show identical structural and metamorphic histories.…”

“…The reactivation of older structures under changing geodynamic conditions is examined by Loury et al (2015) for the Kyrgyz portion of the South Tien Shan. They note that reactivation of Palaeozoic-age structures in Permian-Mesozoic times occurred mainly in a strike-slip regime, featuring left-lateral motion localized in the centre of the South Tien Shan.…”