Thermochronology of the highest Central Asian massifs (Khan Tengri -Pobedi, SE Kyrgyztan): evidence for Late Miocene (ca. 8 Ma) reactivation of Permian faults and insights into building the Tian Shan Yann Rolland, Anthony Jourdon, Carole Petit, Nicolas Bellahsen, C. Loury, Edward Sobel, Johannes Glodny To cite this version: Yann Rolland, Anthony Jourdon, Carole Petit, Nicolas Bellahsen, C. Loury, et al.. Thermochronology of the highest Central Asian massifs (Khan Tengri -Pobedi, SE Kyrgyztan): evidence for Late Miocene (ca. 8 Ma) reactivation of Permian faults and insights into building the Tian Shan. Journal of Asian Earth Sciences, Elsevier, 2020, 200, pp.104466. 10.1016/j.jseaes.2020.104466. hal-02902631 HAL Id: hal-02902631 https://hal.archives-ouvertes.fr/hal-02902631 Submitted on 20 Jul 2020 HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. The documents may come from émanant des établissements d’enseignement et de teaching and research institutions in France or recherche français ou étrangers, des laboratoires abroad, or from public or private research centers. publics ou privés. 1 Thermochronology of the highest Central Asian massifs 2 (Khan Tengri - Pobedi, SE Kyrgyztan): evidence for Late 3 Miocene (ca. 8 Ma) reactivation of Permian faults and 4 insights into building the Tian Shan 5 a* b c d c e f 6 Rolland, Y. , Jourdon, A. , Petit, C. , Bellahsen, N. , Loury, C. , Sobel, E.R. , Glodny, J. , 7 8 aEDYTEM, Université de Savoie Mont-Blanc – CNRS, Le Bourget du Lac, France. 9 bCNRS, GET, Université Paul Sabatier, Toulouse, France. 10 cUniversité Côte d’Azur, CNRS, Géoazur, 250 rue Albert Einstein, Sophia Antipolis, France. 11 dSorbonne Université, CNRS-INSU, Institut des Sciences de la Terre Paris, ISTeP UMR, 12 Paris, France. 13 eInstitut für Geowissenschaften, Universität Potsdam, Karl-Liebknecht-Str. 24-25, 14476 14 Potsdam, Germany. 15 fGFZ German Research Centre for Geosciences, Telegrafenberg, Building B, 14473 Potsdam, 16 Germany. 17 *corresponding author: [email protected] 18 19 Keywords: 40Ar/39Ar dating, Apatite Helium dating, exhumation, tectonics, reactivation. 20 1 21 Abstract 22 Geological study of the Khan Tengri and Pobedi massifs highlights two phases of 23 compressional fault displacements. A Late Permian/Triassic displacement phase is 24 highlighted (1) by biotite 40Ar/39Ar ages of 265–256 Ma, suggestive of cooling of Pobedi mid- 25 crustal granulites during >8 km of top-to-the-north motion of the Pobedi Thrust; (2) by 26 40Ar/39Ar dating of syn-kinematic phengite at 249–248 Ma, which suggests a crystallization 27 age during top-to-the-south motion of the Khan Tengri Thrust shear zone; (3) by Apatite 28 Helium (AHe) ages of 280-240 Ma on the crystalline basement below the Mesozoic 29 peneplain, which gives insight into the final exhumation stage. Cenozoic reactivation of the 30 Pobedi Thrust is indicated by AHe thermochronology with a mean age of 8.3 ± 2.5 Ma, in 31 agreement with ~3 km exhumation and ~4.2 km of top-to-the-north motion in the Late 32 Miocene. Along a north-south transect of the west Tian Shan range from Issyk-Kul Lake to 33 Tarim Basin, compiled thermochronological data outline out-of-sequence deformation 34 beginning with the activation of north-directed crustal-scale faults at 22-15 Ma along the 35 northern margin of the north Tian Shan, followed by 20-10 Ma motion at the boundary 36 between the Middle and the South Tian Shan, and ending with the <10 Ma reactivation of the 37 Pobedi Thrust in the South Tian Shan. This latter coincided with south-directed motion on the 38 South Tian Shan Front (the Maidan Fault) and fold and thrust belt propagation towards the 39 Tarim Basin. 40 1. Introduction 41 Since the Early Cenozoic, deformation has propagated towards the core of the Eurasian 42 continent in response to the India-Asia collision (e.g., Hu et al., 2016). Collisional 43 deformation is now distributed over several thousand kilometres, from the Himalayan front to 44 the interior of the Siberian craton (e.g. Avouac et al., 1993; Tapponnier et al., 1986; Sobel and 45 Dumitru, 1997). 2 b b (a) 50°E 70°E 90°E N Siberia Mongolia 75°E 80°E Chinese STS HP-UHP belt 40°N TFF b Tibet Iran 42°N Himalaya 42°N Arabia India 20°N Khan-Tengri (7010 m) 70°E 90°E Pobeda peak (7439 m) Atbashi range 40°N 40°N 0 100 km (b) 75°E 80°E Bichkek 75°E 80°E KT Issyk-kul Fig. 2 TFF Enylchek Pobeda Thrust KNT 42°N 42°N NL IMT Naryn Aksu STS ? Och Maidan Fault 40°N 40°N TARIM 0 100 km (c) PAMIRS 75°E Kashgar 80°E Blocks with Precambrian Late Carboniferous Basins Faults crustal substratum Folded belts metamorphic complexes Transtensional CZ thrusts South Tianshan (late Greenschist facies STS basin linked KNT Kokchetav-North Tianshan carboniferous) schists Late PZ strike-slip fault with TFF (J1-2) (reactivated in CZ) KT Karatau-Talas Volcano-sedimentary Eclogite facies schists, deposits PZ thrusts (reactivated IMT Ishim - Middle Tianshan gneiss and metabasites Fig. 1 Pamirs (late carboni- in CZ) Tarim ferous-jurassic) Main rivers PZ thrusts (without CZ 46 International borders reactivation) 47 Fig. 1. (a) Location of Tian Shan within the Alpine-Himalayan collisional system. (b) Satellite image 48 of the Tian Shan belt showing study areas. Insert: geographical situation of Tian Shan in Central Asia 49 (Map data: Google, Image Landsat). (c) Tectonic map of Tian Shan (Cenozoic cover removed). 50 Location of detailed geological map of Fig. 2 is shown, modified after Loury et al. (2017). 51 Abbreviations used: CZ, Cenozoic; TFF, Talas-Fergana Fault; NL, Nikolaev Line; STS, South Tian 52 Shan; NTS, North Tian Shan. 3 53 This deformation has given rise to the highest intra-continental orogen, the Tian Shan 54 mountain belt. The Tian Shan forms the south-western part of the Central Asian Orogenic 55 Belt (CAOB; Fig. 1), extending for 2500 km from Uzbekistan to western China and was built 56 up by successive continental accretionary events during the Palaeozoic (e.g. Sengör et al. 57 1993; Windley et al. 2007; Kröner et al. 2014; Loury et al., 2018b; Han et al., 2011). Active 58 deformation occurs along crustal scale faults inherited from the (post) collisional evolution of 59 the CAOB in Late Carboniferous to Permian times, which have been reactivated during the 60 Cenozoic (e.g., Sobel et al. 2006b; Glorie et al. 2011; Macaulay et al. 2013, 2014; Loury et 61 al., 2017, 2018a; Jourdon et al., 2018a-b; Rizza et al., 2019). Palaeozoic structures including 62 collisional sutures and transcurrent shear zones of the CAOB have strongly localized 63 Cenozoic deformation, while the domains between these faults behaved as rigid blocks (e.g., 64 Jolivet et al., 2010; Glorie et al. 2011; Macaulay et al. 2013, 2014; Jourdon et al., 2018a-b). 65 Prominent reactivation since the Neogene has resulted in the highest topography of Central 66 Asia, with summits above 7000 m and large flat peneplains partly preserved from the Late 67 Palaeozoic to Early Cenozoic erosional stages (e.g., Morin et al., 2019 and references 68 therein). Cenozoic erosion is spatially variable, focussed on the highest ranges, while the 69 formerly flat-lying Mesozoic peneplains are uplifted above 4000 m and form intra-orogenic 70 plateaus (Jolivet, 2017; Morin et al., 2019). Reconstructing Cenozoic Tian Shan mountain 71 building and comprehending active tectonics requires precisely locating the major faults, 72 determining their crustal scale geometry and pin-pointing their activation throughout the 73 Cenozoic with respect to the previous tectonic stages (e.g., Jourdon et al., 2018a-b). It is thus 74 necessary to disentangle the relative contributions of the tectonic phases which affected the 75 area since the Palaeozoic. 76 Previous works based on thermochronology and basin sedimentology along the Tian Shan 77 belt have enabled quantifying several crustal compartments and along-belt uplift phases 4 78 (Jolivet et al., 2010, 2015, 2018; Glorie et al. 2010, 2011, 2019; De Grave et al., 2007, 2011, 79 2012, 2013, 2014; Macaulay et al., 2013, 2014; De Pelsmaeker et al., 2017; Yin et al., 2018; 80 Zhao et al., 2019; Zhang et al., 2019). However, no such study was conducted in the highest 81 part of the South Tian Shan (STS): the Pobedi and Khan Tengri massifs, which culminate 82 above 7000 m. This most prominent Tian Shan topography is also a key zone to link the 83 history of mountain building in the Kyrgyz western Tian Shan to the frontal fold and thrust 84 belt and related foreland basin, the Tarim Basin, in NW China. For this, we have undertaken a 85 geological, 40Ar/39Ar geochronological and apatite (U-Th-Sm)/He (AHe) thermochronological 86 study. 40Ar/39Ar geochronology is applied to low-temperature mylonitic phengite in order to 87 constrain the time of ductile (Palaeozoic?) deformation (e.g., Sanchez et al., 2011). AHe 88 thermochronology is undertaken from underneath some major peneplains and on both sides of 89 major thrusts to constrain the time of the final (Palaeozoic to Cenozoic) stages of exhumation. 90 These data are integrated into a north-south crustal cross-section, and are used to reconstruct 91 the deformation sequence which gave rise to the western Tian Shan belt. 92 93 2. Geological setting 94 The main tectonic units of the Tian Shan belt correspond to at least three continental 95 blocks: the North Tian Shan (NTS), the Middle Tian Shan (MTS), the South Tian Shan (STS) 96 - Tarim basin, separated by two Palaeozoic sutures: the Nikolaev Line (NL) and the STS 97 suture (Figure 1; see a review of data in Loury et al., 2017).