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(West Tibet) Mafic Rocks and Granite Resulting from Cretaceous Jinsha Subduction

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(West Tibet) Mafic Rocks and Granite Resulting from Cretaceous Jinsha Subduction *Highlights • Lungmu Co (west Tibet) mafic rocks and granite resulting from Cretaceous Jinsha subduction. • Cooling in the early Upper Cretaceous and final exhumation in Paleocene. • Paleocene NW Tibet uplift as a far field effect of India/Eurasia collision. *Graphical Abstract Section A Field observations S 7000 K1C11 Active LMC fault K1C21 6500 LMC fault zone Mangtsa leucogranite K1C20 80°E8 81°E 116.9±1 Ma 6000 0°E 50 km ? Altitude (m) Tianshuihai 5500 N 5000 Aksai Gozha L. 35°N Chin KUNLUN BLOCK 2 km ult a fa ozh GGozha fault Mafic rocks geochemistry 35°N 1000.0 B 100.0 Lo e qzung Rang 10.0 KunKun LLunun BBlocklock 1.0 TianshuihaiTianshuihai terraneterrane Lungmu Co samples/MOR 0.1 Ba Rb K Th Ta Nb La Ce P Nd Sm Zr Hf Eu Ti Tb Dy Y Y b Cretaceous supra subduction setting lt fau QiangQQiangQiaianngg TangT TangTaanngg BlockB BlockBlolockck u Co FIg.2a gm K1L38 -50 LungmuLunLungmu Co LungmuCo fault fault Co Range 900 Upper Cretaceous magmatism andcoolingK1L26 K1L25 K1L24 800 Cooling history34°N QIANGTANG BLOCKU/Pb zircon - volcanic rock U/Pb zircon - plutonic rock 700 Ar/Ar muscovite biotite K-feldsparPaleocene cooling K1L17 K1C21 6 00 U/He zircon apatite linked with Tibet uplift Quaternary K89G181 500Fault Ice cap K1C11 slow cooling since ~55 Ma East LungmuCenozoic Co range 400 Lake K1C20 Temperature (°C) structural K1C20 Cretaceous 300 mu Northwest Tibet trend Jurassic 200 K1L24 River1 K1C21 Triasic 100granite K1L23 2 Road India-Asia Deformationin Paleozoic Carboniferous-Permian 0 collision northernTibet 0 5 0152025303540 45 50 55 60 65 70 75 80 85 90 95 100 105110120 KunLun bock QiangTang block Time (Ma) *Manuscript Click here to view linked References p.1 1 1 Successive deformation episodes along the Lungmu Co zone, west-central 2 3 2 Tibet. 4 5 3 Leloup P.H.1, Arnaud N.O.2, Mahéo G.1, Paquette J.L.3, Guillot, S.4, Valli F. 5, Li H.5 &6, Xu 6 6 5 5 7 4 Z. , Lacassin R. , and Tapponnier P. 8 5 9 6 1 : Laboratoire de Géologie de Lyon: Terre, Planètes et Environnement, UMR CNRS 5276, 10 7 Université Claude Bernard – Ecole normale supérieure de lyon, 2 rue Raphaël Dubois, 69622 11 8 Villeurbanne, France. 12 13 9 [email protected] 14 10 2: ISTEEM-UM2, Géosciences Montpellier, UMR CNRS 5243, Université des sciences et 15 11 techniques du Languedoc, 4095 Montpellier cedex 5, France. 16 12 3: Laboratoire Magmas et Volcans, UMR CNRS 6524, Observatoire de Physique du globe de 17 18 13 Clermont-Ferrand, 63038 Clermont-Ferrand, France. 19 14 4 : ISTerre, CNRS, University of Grenoble I, BP 53, 1381 rue de la Piscine, 38041 Grenoble 20 15 cedex 9, France. 21 16 5 : Equipe de Tectonique, Institut de Physique du Globe de Paris, Sorbonne Paris Cité, Univ 22 17 Paris Diderot, UMR 7154 CNRS, F-75005 Paris, France. 23 24 18 6: Institute of Geology, CAGS, 26 Baiwanzhuang Road, Beijing 100037, PRC 25 26 19 Abstract 27 28 20 Field study, thermochronology and geochemistry of the east Lungmu Co (LMC) range 29 30 21 highlight some of the geological events that shaped western Tibet. The LMC fault zone has 31 32 22 long been interpreted as the boundary between the Tianshuihai terrane of Laurasian affinity 33 34 23 and the Qiangtang block of Gondwanian affinity. In the LMC range, the Paleozoic series is 35 36 24 intruded by the Mangtsa leucogranite whose zircon have a U/Pb age of 116.9±1 Ma and by 37 25 mafic rocks with U/Pb zircon ages ranging from 116.9±1 to 95.1±1.7 Ma. Geochemistry of 38 39 26 the mafic rocks indicates that they have been emplaced in a supra-subduction zone setting, 40 41 27 probably the north dipping Nujiang suture zone. 40Ar/39Ar micas ages of the granite indicate 42 40 39 43 28 that cooling below ~350°C occurred between 105 and 85 Ma. Ar/ Ar K-feldspar data 44 45 29 suggest a fast cooling event at 60-55 Ma, which we relate to the reactivation of the LMC 46 47 30 suture zone as a thrust at the onset of the India – Eurasia collision. The last, and still active, 48 31 deformation event corresponds to left-lateral strike-slip faulting along the ENE-WSW LMC 49 50 32 fault. 51 52 33 53 54 55 34 1 Introduction 56 57 35 Tibet, the highest and largest topographic plateau on earth, was essentially built during the 58 59 36 Cenozoic (e.g., Harrison et al., 1992; Tapponnier et al., 2001). However, the precise timing 60 61 62 Successive deformation episodes along the Lungmu Co zone, west-central Tibet. Leloup et al., 2011 63 64 65 p.2 37 and mechanisms of the plateau building remain highly debated. This is in part because the 1 2 38 long geological history of Tibet is still poorly known especially in remote area such as central 3 39 and western Tibet. In western Tibet, the highest part of the plateau at more than 5000m asl, 4 5 40 essential information such as detailed stratigraphy, continuity of known sutures, offset of 6 7 41 those structures by major faults and geochronological constraints are still lacking. In this 8 9 42 paper we aim to present new structural, geochronological and geochemical data from the 10 11 43 Lungmu Co range in west-central Tibet (Fig. 1). 12 13 44 The Lungmu Co (LMC) range is a noticeable topographic ridge culminating at 6192m, 14 45 located south of LMC lake that stands at an altitude of ~5100m (Fig. 2a). The northern flank 15 16 46 of the range corresponds to the eastern extremity of the active left-lateral LMC fault that can 17 18 47 be traced for more than 150 km towards the right-lateral Karakorum fault (Molnar & 19 20 48 Tapponnier, 1977) (Fig. 1a). The Karakorum fault is interpreted as the western boundary of 21 22 49 the Tibetan plateau but its precise initiation age, total offset and present day rate are still 23 24 50 debated (e.g., Leloup et al., 2011; Robinson; 2010; Valli et al., 2008; Chevalier et al., 2005). 25 51 The LMC fault appears to abut against the Karakorum fault, whilst it has been interpreted to 26 27 52 offset that fault by ~27 km (Raterman et al, 2007). Towards the Northeast, strike-slip motion 28 29 53 of the LMC appears to be transferred to the Gozha fault (Fig. 1b) that ultimately merges with 30 31 54 the Altyn Tagh fault which bounds the Tibetan plateau to the north (Fig. 1a) (Molnar & 32 33 55 Tapponnier, 1977; Peltzer & Saucier, 1996). 34 35 56 It has been proposed by Matte et al. (1996) that the LMC range also corresponds to the 36 57 boundary between the Tianshuihai terrane to the north and the Qiangtang block to the south, 37 38 58 marking the prolongation of the Triassic Jinsha suture (Fig. 1a). 39 40 59 The data presented herein document the geology of the LMC range shedding light on 41 42 60 more than 300 Ma of its geological history and its role in plateau evolution. 43 44 45 61 2 Regional geology of the Lungmu Co area. 46 47 62 2.1 The Tianshuihai terrane 48 49 63 North of the LMC range, the Tianshuihai terrane is characterized by Carboniferous 50 51 64 greenschists and greywackes overlain by Permo-Triasic flyshoïd dark slates (Matte et al., 52 53 65 1996). These series are unconformably capped by marine Jurassic black shales, and 54 55 66 Cretaceous conglomerates, red sandstones and limestones (Fig. 1b). 56 57 67 South of the LMC range the Permo-Carboniferous series consists in black shales, Tethyan 58 68 fusulinids bearing limestone and quartzite horizons. Presence of diamictites suggests a 59 60 69 Gondwanian affinity (Matte et al., 1996). Further south, near Domar, the Permo- 61 62 Successive deformation episodes along the Lungmu Co zone, west-central Tibet. Leloup et al., 2011 63 64 65 p.3 70 Carboniferous series is overlain by Triassic conglomerates and Jurassic limestone, this latter 1 2 71 being locally unconformably overlain by Cretaceaous-Paleocene sandstones and red 3 72 conglomerates (Matte et al., 1996). 4 5 73 These stratigraphic differences have led several authors to propose that the LMC fault 6 7 74 could correspond to the boundary between the Tianshuihai block to the north and the 8 9 75 Qiantang block to the South (Matte et al., 1996; Norin, 1946; Sengör and Okurogullari, 1991) 10 11 76 (Fig. 1a). In such interpretation, the Tianshuihai terrane would constitute, together with the 12 13 77 Bayan Har and Songpan terranes, a large block bounded to the North by the South Kunlun 14 78 suture, the trace of a north dipping Permo-Triassic subduction. South of this block, the LMC 15 16 79 would be the western prolongation of the South dipping Triassic Jinsha suture described in 17 18 80 central and eastern Tibet (e.g., Roger et al., 2003). However, no ultrabasites have been found 19 20 81 in the LMC area and the detailed structure and thermal history of the range are unknown. 21 22 82 Furthermore, the zone is sliced by recent strike-slip faults that may have disrupted the initial 23 24 83 relationships between the units. 25 26 84 2.2 The Lungmu Co and Ghoza faults 27 85 The Ghoza - LMC strike-slip fault zone corresponds to two distinct faults that connect 28 29 86 through an extension zone north of the LMC range at midway of its total length (Fig.
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