Zhenhua Xue

Wei Lin
email: linwei@mail.iggcas.ac.cn

Yang Chu

Michel Faure

Yan Chen

Wenbin Ji

Huaning Qiu

An intracontinental orogen exhumed by basement-slice-imbrication in the Longmenshan Thrust Belt of the Eastern Tibetan Plateau

The double-vergent structure of the Longmenshan belt was established during the late Triassic intracontinental underthrusting 2. Boundary faults of the NW-normal faulting and SE-thrusting facilitated the SE-ward basement-slice

Introduction

The Longmenshan Thrust Belt (LMTB; Figure 1a), at the eastern margin of the Tibetan Plateau, has attracted attentions of geologists due to the paradox between low convergence rate [START_REF] Zhang | Continuous deformation of the Tibetan Plateau from global positioning system data[END_REF][START_REF] Shen | Slip maxima at fault junctions and rupturing of barriers during the 2008 Wenchuan earthquake[END_REF] and extremely high topographic gradient [START_REF] Clark | Topographic ooze: Building the eastern margin of Tibet by lower crustal flow[END_REF][START_REF] Royden | The geological evolution of the Tibetan Plateau: science[END_REF] as well as intense seismic activities (e.g., 2008 Wenchuan Mw 7.9 earthquake, 2013 Lushan Mw 7.0 earthquake, and 2017 Jiuzhaigou Mw 7.0 earthquake). This paradox has spawned a vigorous debate regarding the roles of upper crustal faulting [START_REF] Hubbard | Uplift of the Longmen Shan and Tibetan plateau, and the 2008 Wenchuan (M = 7.9) earthquake[END_REF] and lower crust channel flow [START_REF] Royden | The geological evolution of the Tibetan Plateau: science[END_REF] in building and maintaining such a high Tibetan Plateau margin. However, both models attribute shortening and crustal thickening of the LMTB to the late Cenozoic evolution, and underestimate the importance of widely recognized Mesozoic tectonics along the LMTB [START_REF] Xu | Orogenic process of the Songpan-Ganzé Orogenic belt of China[END_REF][START_REF] Burchfiel | Tectonics of the Longmen Shan and Adjacent Regions[END_REF][START_REF] Worley | Deformation partitioning and foliation reactivation during transpressional orogenesis, an example from the Central Longmen Shan[END_REF]. Furthermore, it has been reached a consensus that the LMTB is mainly inherited from the Mesozoic tectonic framework [START_REF] Burchfiel | Tectonics of the Longmen Shan and Adjacent Regions[END_REF][START_REF] Meng | Mesozoic sedimentary evolution of the northwest Sichuan basin: Implication for continued clockwise rotation of the South China block[END_REF]. Elucidating the Mesozoic tectonics of the LMTB and its adjacent areas is therefore crucial to understand the tectonic evolution of the LMTB.

The prominent deformation recorded in the LMTB and adjacent areas is indicated by observations including: 1) the peripheral foreland basin formed due to the southeastward thrusting and loading of the LMTB [START_REF] Chen | The evolution of the Western Sichuan Foreland Basin, southwestern China[END_REF][START_REF] Li | Evolution of the Longmen Shan foreland basin (western Sichuan, China) during the Late Triassic Indosinian orogeny[END_REF];

2) the early Jurassic rocks unconformable overly extensively folded late Triassic rocks [START_REF] Bgmrsp | Regional geology of Sichuan Province[END_REF][START_REF] Xu | Orogenic process of the Songpan-Ganzé Orogenic belt of China[END_REF][START_REF] Burchfiel | Tectonics of the Longmen Shan and Adjacent Regions[END_REF]; 3) the pervasive orogenparallel klippen and nappe belt sitting on the foot hills of the Longmen Mountains are believed related to the regional top-to-the-SE thrusting (Figure 2); 4) the consistent topto-the-SE thrusting indicated by different indicators at various scales [START_REF] Xu | Orogenic process of the Songpan-Ganzé Orogenic belt of China[END_REF][START_REF] Burchfiel | Tectonics of the Longmen Shan and Adjacent Regions[END_REF][START_REF] Chen | Tectonic transition from the Songpan-Garzê Fold Belt to the Sichuan Basin, south-western China[END_REF][START_REF] Chen | Emplacement of the Longmen Shan Thrust-Nappe Belt along the eastern margin of the Tibetan Plateau[END_REF][START_REF] Jia | Longmen Shan foldthrust belt and its relation to the western Sichuan Basin in central China: New insights from hydrocarbon exploration[END_REF][START_REF] Yan | Structural and geochronological constraints on the Mesozoic Cenozoic tectonic evolution of the Longmen Shan thrust belt, eastern Tibetan Plateau[END_REF][START_REF] Yan | Structural and Geochronological Constraints on the Early Mesozoic North Longmen Shan Thrust Belt: Foreland Fold-Thrust Propagation of the SW Qinling Orogenic Belt[END_REF]; 5) the pervasive upright folding in the adjacent Songpan-Ganzi Terrane was thought synchronous to the early Mesozoic deformation of the LMTB [START_REF] Burchfiel | Tectonics of the Longmen Shan and Adjacent Regions[END_REF][START_REF] Chen | Tectonic transition from the Songpan-Garzê Fold Belt to the Sichuan Basin, south-western China[END_REF][START_REF] Harrowfield | Indosinian deformation of the Songpan Garze Fold Belt, northeast Tibetan Plateau[END_REF]. This prominent deformation event was constrained to ca. 234-200 Ma by basin tectonics, timing of the top-to-the-SE/SSE shearing and syntectonic metamorphism, which indicated an early Mesozoic orogeny of the LMTB [START_REF] Chen | The evolution of the Western Sichuan Foreland Basin, southwestern China[END_REF][START_REF] Li | Evolution of the Longmen Shan foreland basin (western Sichuan, China) during the Late Triassic Indosinian orogeny[END_REF][START_REF] Yan | Structural and geochronological constraints on the Mesozoic Cenozoic tectonic evolution of the Longmen Shan thrust belt, eastern Tibetan Plateau[END_REF][START_REF] Zheng | Time constraints on the emplacement of klippen in the Longmen Shan thrust belt and tectonic implications[END_REF][START_REF] Xue | Emplacement age and tectonic implications of the brecciated limestone at the edge of the Longmenshan klippe[END_REF][START_REF] Zheng | New geochronology constraints on timing and depth of the ancient earthquakes along the Longmen Shan fault belt, eastern Tibet[END_REF]Airaghi et al., 2018a;Airaghi et al., 2018b;[START_REF] Yan | Structural and Geochronological Constraints on the Early Mesozoic North Longmen Shan Thrust Belt: Foreland Fold-Thrust Propagation of the SW Qinling Orogenic Belt[END_REF].

The early Mesozoic tectonic evolutionary mechanism accounting for the above observations, however, is still in dispute. [START_REF] Sengör | The Cimmeride Orogenic System and the Tectonics of Eurasia[END_REF] and [START_REF] Luo | The dynamical model of the lithospheric evolution in Longmenshan Orogenic Belt[END_REF] suggested that the LMTB is an early Mesozoic intracontinental subduction zone accommodating multi-stage crustal shortening. Then, [START_REF] Xu | Orogenic process of the Songpan-Ganzé Orogenic belt of China[END_REF] proposed a bivergent subduction model in which the Songpan-Ganzi Terrane sedimentary cover has been "pushed" to the southeast on top of the Yangtze Block due to the bivergent subduction along the Anymaqen and Jinshajiang Suture Zones. The current models suggested that the Songpan-Ganzi sedimentary cover were overthrust southwards on the Yangtze Block along a decollement in response to the closure of the Paleotethys [START_REF] Mattauer | Le chaine Triassique de Songpan-Garze (Ouest Sichuan et est Tibet): Une chaine de plissement-decollement sur marge passive[END_REF][START_REF] Calassou | Etude tectonique d'une chaîne de décollement. A-Tectonique triasique et tertiaire de la chaîne de Songpan Garzê[END_REF][START_REF] Dirks | Tectonic evolution of the NE margin of the Tibetan Plateau; evidence from the central Longmen Mountains, Sichuan Province, China[END_REF][START_REF] Chen | Emplacement of the Longmen Shan Thrust-Nappe Belt along the eastern margin of the Tibetan Plateau[END_REF][START_REF] Worley | Deformation partitioning and foliation reactivation during transpressional orogenesis, an example from the Central Longmen Shan[END_REF][START_REF] Roger | Timing of granite emplacement and cooling in the Songpan-Garzê Fold Belt (eastern Tibetan Plateau) with tectonic implications[END_REF][START_REF] Harrowfield | Indosinian deformation of the Songpan Garze Fold Belt, northeast Tibetan Plateau[END_REF][START_REF] De Sigoyer | Generation and emplacement of Triassic granitoids within the Songpan Ganze accretionaryorogenic wedge in a context of slab retreat accommodated by tear faulting, Eastern Tibetan plateau[END_REF][START_REF] Yan | Structural and Geochronological Constraints on the Early Mesozoic North Longmen Shan Thrust Belt: Foreland Fold-Thrust Propagation of the SW Qinling Orogenic Belt[END_REF].

Apart from the early Mesozoic top-to-the-SE/SSE thrusting along the LMTB, an increasing number of studies proposed a top-to-the-NW normal faulting that developed along the NW boundary of the orogen-parallel and stripped Neoproterozoic complexes (Figure 1b) [START_REF] Xu | Uplift of the Longmen Shan range and the Wenchuan earthquake: Episodes[END_REF][START_REF] Zhou | Structural and geochronological constraints on the tectono-thermal evolution of the Danba domal terrane, eastern margin of the Tibetan plateau[END_REF][START_REF] Tian | Late Cretaceous earliest Paleogene deformation in the Longmen Shan fold and thrust belt, eastern Tibetan Plateau margin: Pre Cenozoic thickened crust?: Tectonics[END_REF][START_REF] Billerot | Gneiss domes of the Danba Metamorphic Complex, Songpan Ganze[END_REF][START_REF] Xue | Mesozoic crustal thickening of the Longmenshan belt (NE Tibet, China) by imbrication of basement slices: insights from structural analysis, petrofabric and magnetic fabric studies, and gravity modeling: Tectonics[END_REF]Airaghi et al., 2018b). However, timing and significance of the topto-the-NW normal faulting are in dispute that have resulted in several different models, such as: 1) mid-Jurassic to early Paleogene (173-160 post orogenic extension (comparing to the early Mesozoic orogeny) [START_REF] Zhou | Structural and geochronological constraints on the tectono-thermal evolution of the Danba domal terrane, eastern margin of the Tibetan plateau[END_REF], 2) early Cretaceous (~120 Ma) crustal-scale wedge extrusion [START_REF] Xu | Uplift of the Longmen Shan range and the Wenchuan earthquake: Episodes[END_REF], 3) episodic eastward basement-slice-imbrication during the late Jurassic-early Cretaceous and late Cretaceous-Earliest Paleogene [START_REF] Billerot | Gneiss domes of the Danba Metamorphic Complex, Songpan Ganze[END_REF]Xue et al., 2017), and, 4) formation of a late Cretaceous-earliest Paleogene crustal scale duplex [START_REF] Tian | Late Cretaceous earliest Paleogene deformation in the Longmen Shan fold and thrust belt, eastern Tibetan Plateau margin: Pre Cenozoic thickened crust?: Tectonics[END_REF]. Based on the above compilation, most previous works have proposed that the top-to-the-NW normal faulting occurred in the late Mesozoic.

Major questions therefore remain unsolved: 1) Considering previous early Mesozoic tectonic models of the LMTB, whether did the significant shortening recorded in the LMTB correspond to the imbricated thrust system during the Paleotethys' closure, or to the intracontinental underthrusting along the Longmenshan belt? 2) What is the tectonic significance of the top-to-the-NW normal faulting (mostly late Mesozoic) and emplacement of the Neoproterozoic complexes? Therefore, an overall accurate reconstruction of the Mesozoic tectonics of the LMTB and its adjacent areas, especially its detailed structural analysis and timing, are of great importance.

This study presents detailed field investigations focusing on structural geometry and kinematic reconstruction, and geochronological constraints including zircon U-Pb dating of synkinematic plutonic rocks and 40 Ar/ 39 Ar dating on the synmetamorphic micas of the LMTB. In conjunction with previous published geochronological and metamorphic data, a new geodynamic model was proposed to interpret the Mesozoic tectonic evolutionary history of the LMTB.

Geological Setting

At the eastern margin of the Tibetan Plateau, the LMTB separates the intensely folded and metamorphosed (Greenschist facies grade) Songpan-Ganzi Terrane to the NW, and the weakly/undeformed Sichuan Basin to the SE (Figure 1a). The triangleshaped Songpan-Ganzi Terrane is separated from the western Qinling-Dabie Orogenic Belt to the north by the East Kunlun-Anymaqen Suture Zone, from the Qiangtang Block to the southwest by the Jinshajiang Suture Zone (Figure 1a).

Songpan-Ganzi Terrane

The Songpan-Ganzi Terrane mainly comprises ~5-15 km-thick intensely folded late Triassic turbidites [START_REF] Calassou | Etude tectonique d'une chaîne de décollement. A-Tectonique triasique et tertiaire de la chaîne de Songpan Garzê[END_REF][START_REF] Harrowfield | Indosinian deformation of the Songpan Garze Fold Belt, northeast Tibetan Plateau[END_REF]Zhang et al., 2006c;[START_REF] Zhang | Proximal provenance of the western Songpan-Ganzi turbidite complex (Late Triassic, eastern Tibetan plateau): Implications for the tectonic amalgamation of China[END_REF][START_REF] Yin | The Songpan-ganzi terrane of Tibet: was it a part of South China since the Neopreoterozoic[END_REF]. The provenance of the turbidites includes the Kunlun Mountains, North China Block, South China Block and southwest Qinling Orogenic Belt [START_REF] Yin | An indentation model for the North and South China collision and the development of the Tan-Lu and Honam fault systems[END_REF][START_REF] Enkelmann | How was the Triassic Songpan Ganzi basin filled? A provenance study: Tectonics[END_REF][START_REF] Weislogel | Detrital zircon provenance from three turbidite depocenters of the Middle-Upper Triassic Songpan-Ganzi complex, central China: Record of collisional tectonics, erosional exhumation, and sediment production[END_REF]. Voluminous felsic plutons intruded the Songpan-Ganzi Terrane at ca. 226-205 Ma, dominantly derived from partial melting of the Neoproterozoic basement and sedimentary cover of the Songpan-Ganzi Terrane, with negligible amounts of mantle material (Figure 1b) [START_REF] Roger | Timing of granite emplacement and cooling in the Songpan-Garzê Fold Belt (eastern Tibetan Plateau) with tectonic implications[END_REF][START_REF] Hu | SHRIMP U-Pb dating of zircons from granitoid bodies in the Songpan-Ganzi terrane and its implications[END_REF]Zhang et al., 2006b;[START_REF] Zhang | A-type granite and adakitic magmatism association in Songpan-Garze fold belt, eastern Tibetan Plateau: Implication for lithospheric delamination[END_REF][START_REF] De Sigoyer | Generation and emplacement of Triassic granitoids within the Songpan Ganze accretionaryorogenic wedge in a context of slab retreat accommodated by tear faulting, Eastern Tibetan plateau[END_REF]. Both geophysical and geological data indicate a continental crust of Yangtze Block affinity underlying the eastern Songpan-Ganzi Terrane [START_REF] Roger | Timing of granite emplacement and cooling in the Songpan-Garzê Fold Belt (eastern Tibetan Plateau) with tectonic implications[END_REF][START_REF] Hu | SHRIMP U-Pb dating of zircons from granitoid bodies in the Songpan-Ganzi terrane and its implications[END_REF][START_REF] Zhang | Crustal structure across Longmenshan fault belt from passive source seismic profiling[END_REF][START_REF] Weislogel | Detrital zircon provenance from three turbidite depocenters of the Middle-Upper Triassic Songpan-Ganzi complex, central China: Record of collisional tectonics, erosional exhumation, and sediment production[END_REF][START_REF] Zhang | Seismic signature of the collision between the east Tibetan escape flow and the Sichuan Basin[END_REF][START_REF] Guo | Imaging the crustal structure beneath the eastern Tibetan Plateau and implications for the uplift of the Longmen Shan range[END_REF].

Sichuan Basin of the Yangtze Block

The Sichuan Basin, located in the Yangtze Block of northwest South China Block (Figure 1a), preserves a Paleoproterozoic to Neoproterozoic basement overlain by Neoproterozoic to Cenozoic sedimentary sequences. Silurian-Devonian abyssal deposits and radiolarian chert suggested a long period of rifting during the Silurian to Devonian ages along the western part of the Yangtze Block [START_REF] Long | Several questions of geochronic evolution in the mid-northeast segment of lLongmenshan Mountains[END_REF][START_REF] Luo | The dynamical model of the lithospheric evolution in Longmenshan Orogenic Belt[END_REF][START_REF] Chang | Geology and Tectonics of the Songpan-Ganzi Fold Belt[END_REF][START_REF] Li | Composition, structural characteristics and evolution of back-Longmenshan orogen (north section) in the northwest margin of Yangtze block[END_REF]. This rift was revealed by high-resolution seismic reflection profile as well [START_REF] Jia | Longmen Shan foldthrust belt and its relation to the western Sichuan Basin in central China: New insights from hydrocarbon exploration[END_REF]. During middle-late Triassic ages, coeval with the southeastward thrusting and loading of the LMTB, the western Sichuan Basin began subsiding flexurally and evolved into an epicontinental foreland basin (Figure 2). Thrust sheets were transported to the southeast synchronously and intensely folded at the end of the late Triassic [START_REF] Chen | The evolution of the Western Sichuan Foreland Basin, southwestern China[END_REF][START_REF] Chen | Emplacement of the Longmen Shan Thrust-Nappe Belt along the eastern margin of the Tibetan Plateau[END_REF]Li et al., 2013c).

The continuous sedimentation of the middle Jurassic-upper Cretaceous deposits implies a pronounced subsidence at the western Sichuan foreland basin during this period [START_REF] Burchfiel | Tectonics of the Longmen Shan and Adjacent Regions[END_REF][START_REF] Chen | Tectonic transition from the Songpan-Garzê Fold Belt to the Sichuan Basin, south-western China[END_REF]. Episodic SE-ward thrusting and loading of the LMTB were indicated by the wedge-shaped megasequences of the western Sichuan foreland basin, namely, in late Triassic-early Jurassic, late Jurassic-early Cretaceous and late Cretaceous-earliest Paleocene [START_REF] Xu | Uplift of the Longmen Shan range and the Wenchuan earthquake: Episodes[END_REF]Li et al., 2013c;[START_REF] Tian | Late Cretaceous earliest Paleogene deformation in the Longmen Shan fold and thrust belt, eastern Tibetan Plateau margin: Pre Cenozoic thickened crust?: Tectonics[END_REF].

The LMTB

The NE-striking LMTB merges into the western Qinling Orogenic Belt to the northeast and is truncated by the Xianshuihe Fault to the southwest (Figure 1a).

Major faults of the LMTB

The regional tectonic architecture of the LMTB is defined by four major NE-SW striking faults, namely from the southeast to the northwest, the Anxian-Guanxian Fault (T1 in Figure 1b), the Yingxiu-Beichuan Fault (T2 in Figure 1b), the Wenchuan-Maoxian Fault (T3 in Figure 1b), and the Qingchuan-Pingwu Fault (T4 in Figure 1b).

The Anxian-Guanxian Fault dips to the northwest by 30-50° and crosscuts the late Triassic-early Jurassic rocks. Approaching northwestwards, the Yingxiu-Beichuan Fault dipping 40-60° to the NW separates the Sichuan foreland basin to the southeast from the LMTB hinterland [START_REF] Bgmrsp | Regional geology of Sichuan Province[END_REF][START_REF] Xu | Orogenic process of the Songpan-Ganzé Orogenic belt of China[END_REF][START_REF] Burchfiel | Tectonics of the Longmen Shan and Adjacent Regions[END_REF].

The Wenchuan-Maoxian Fault dipping 30-50° to the NW was episodically activated since ca. 220 Ma [START_REF] Burchfiel | Tectonics of the Longmen Shan and Adjacent Regions[END_REF][START_REF] Yan | The Pengguan tectonic dome of Longmen Mountains, Sichuan Province: Mesozoic denudation of a Neoproterozoic magmatic arc-basin system[END_REF][START_REF] Zheng | New geochronology constraints on timing and depth of the ancient earthquakes along the Longmen Shan fault belt, eastern Tibet[END_REF]. Seismic reflection profile showed that the ENE-striking Qingchuan-Pingwu Fault dips to the NNW or SSE at variable angles [START_REF] Wang | East Tibet Detachment: Evidence for the uplift mechanism of the Longmen Shan fault belt2012[END_REF][START_REF] Jia | Longmen Shan foldthrust belt and its relation to the western Sichuan Basin in central China: New insights from hydrocarbon exploration[END_REF][START_REF] Lin | Structural analysis of the right-lateral strike-slip Qingchuan fault, northeastern segment of the Longmen Shan thrust belt, central China[END_REF].

The northern LMTB was dissected by the Qingchuan-Pingwu Fault and incorporated to northwestward-and southeastward-thrusting that resemble a positive flower structure [START_REF] Jia | Longmen Shan foldthrust belt and its relation to the western Sichuan Basin in central China: New insights from hydrocarbon exploration[END_REF]. The Wenchuan-Maoxian Fault and Qingchuan-Pingwu Fault are characterized by mylonite overprinted by breccia and gouge [START_REF] Chen | Emplacement of the Longmen Shan Thrust-Nappe Belt along the eastern margin of the Tibetan Plateau[END_REF]Li et al., 2013a;[START_REF] Li | Lithological and structural characterization of the Longmen Shan fault belt from the 3rd hole of the Wenchuan Earthquake Fault Scientific Drilling project[END_REF].

Basement complexes in the LMTB

The basement complexes that distributes from the NE to the SW in the LMTB are composed of Neoproterozoic meta-granites overlain by a Neoproterozoic metasedimentary cover, such as the Danba (DB), Kanding (KD), Wulong (WL), Baoxing (BX), Pengguan (PG), Moutuo (MT), Xuelongbao (XLB) and Jiaoziding (JZD) complexes (Figure 1b). These basement complexes share similar lithology with the Yangtze Block basement [START_REF] Bgmrsp | Regional geology of Sichuan Province[END_REF]. Field observations, Anisotropy of Magnetic Susceptibility (AMS) and gravity modeling have suggested that these basement complexes were strongly deformed and imbricated southeastwards as basement-slices [START_REF] Ma | Tectonic deformation of Pengguan complex[END_REF][START_REF] Xu | Uplift of the Longmen Shan range and the Wenchuan earthquake: Episodes[END_REF][START_REF] Tian | Late Cretaceous earliest Paleogene deformation in the Longmen Shan fold and thrust belt, eastern Tibetan Plateau margin: Pre Cenozoic thickened crust?: Tectonics[END_REF]Airaghi et al., 2017b;[START_REF] Billerot | Gneiss domes of the Danba Metamorphic Complex, Songpan Ganze[END_REF][START_REF] Xue | Mesozoic crustal thickening of the Longmenshan belt (NE Tibet, China) by imbrication of basement slices: insights from structural analysis, petrofabric and magnetic fabric studies, and gravity modeling: Tectonics[END_REF].

Metamorphism in the LMTB

Two amphibolite facies metamorphic zones have been recognized in the metasedimentary cover in the Wenchuan-Xuelongbao area and the Danba area [START_REF] Dirks | Tectonic evolution of the NE margin of the Tibetan Plateau; evidence from the central Longmen Mountains, Sichuan Province, China[END_REF]Huang et al., 2003a;[START_REF] Zhou | Structural and geochronological constraints on the tectono-thermal evolution of the Danba domal terrane, eastern margin of the Tibetan plateau[END_REF]Airaghi et al., 2018a;Airaghi et al., 2018b).

Petrological studies (garnet-amphibolite-biotite thermometer, pseudosection using Perplex modeling and chlorite-white mica-quartz thermal dynamic equilibrium) constrain the peak metamorphic conditions at ~11 ± 2 kbar, ~620 ºC in the Wenchuan-Xuelongbao area [START_REF] Dirks | Tectonic evolution of the NE margin of the Tibetan Plateau; evidence from the central Longmen Mountains, Sichuan Province, China[END_REF]Airaghi et al., 2018a) and ~5.3-8 kbar, 570-680 ºC in the Danba area (Huang et al., 2003a;[START_REF] Weller | Quantifying Barrovian metamorphism in the Danba Structural Culmination of eastern Tibet[END_REF]. The Wenchuan-Xuelongbao metamorphic belt reached amphibolite facies conditions at ca. 210-180 Ma (Airaghi et al., 2018a;Airaghi et al., 2018c) and recorded fast exhumation at ca. 140-120 Ma [START_REF] Arne | Differential exhumation in response to episodic thrusting along the eastern margin of the Tibetan Plateau: tectonophysics[END_REF]Airaghi et al., 2018c). The Danba amphibolite-facies metamorphic belt reached peak conditions at ca. 205-180 Ma (Huang et al., 2003a;[START_REF] Weller | Quantifying Barrovian metamorphism in the Danba Structural Culmination of eastern Tibet[END_REF] and initiated enhanced rapid cooling at ca. 168-158 Ma (Huang et al., 2003a;[START_REF] Zhou | Structural and geochronological constraints on the tectono-thermal evolution of the Danba domal terrane, eastern margin of the Tibetan plateau[END_REF].

Apart from these two amphibolite facies metamorphic zones, late Triassic blueschists occur as lenses along the Qingchuan-Pingwu Fault, with peak metamorphic pressure attained to 5.2-6.5 kbar [START_REF] Wei | Metamorphism and its geological significance of the Bikou Group in the Shanxi-Gansu-Sichuan border region[END_REF][START_REF] Xu | Phase equilibria and geochronology of Triassic blueschists in the Bikou terrane and Mesozoic tectonic evolution of the northwestern margin of the Yangtze Block (SW China[END_REF]. The blueschist then have been retrograded to greenschist in a nearly isothermal decompression way [START_REF] Wei | Metamorphism and its geological significance of the Bikou Group in the Shanxi-Gansu-Sichuan border region[END_REF].

Tectonic events of the LMTB

The LMTB recorded at least three phases of deformation, including (1) D1, the late Triassic to early Jurassic regional top-to-the-SE thrusting, (2) D2, the top-to-the-NW/N normal faulting of poorly constrained timing and tectonic significance, and (3) D3, the Cenozoic shortening reactivation [START_REF] Xu | Orogenic process of the Songpan-Ganzé Orogenic belt of China[END_REF][START_REF] Burchfiel | A geological and geophysical context for the Wenchuan earthquake of 12 May 2008[END_REF][START_REF] Richardson | Extraordinary denudation in the Sichuan Basin: Insights from low-temperature thermochronology adjacent to the eastern margin of the Tibetan Plateau[END_REF][START_REF] Xu | Uplift of the Longmen Shan range and the Wenchuan earthquake: Episodes[END_REF]Li et al., 2013a;[START_REF] Li | Lithological and structural characterization of the Longmen Shan fault belt from the 3rd hole of the Wenchuan Earthquake Fault Scientific Drilling project[END_REF][START_REF] Tian | Late Cretaceous earliest Paleogene deformation in the Longmen Shan fold and thrust belt, eastern Tibetan Plateau margin: Pre Cenozoic thickened crust?: Tectonics[END_REF][START_REF] Xue | Mesozoic crustal thickening of the Longmenshan belt (NE Tibet, China) by imbrication of basement slices: insights from structural analysis, petrofabric and magnetic fabric studies, and gravity modeling: Tectonics[END_REF]. The D1 deformation is well identified by penetrative cleavages (S1), mineral and stretching lineation (L1) in the Wenchuan area, and the top-to-the-SE thrust sheets and nappes over the intensely folded late Triassic sequences in the western Sichuan foreland basin [START_REF] Xu | Orogenic process of the Songpan-Ganzé Orogenic belt of China[END_REF][START_REF] Burchfiel | Tectonics of the Longmen Shan and Adjacent Regions[END_REF][START_REF] Worley | Deformation partitioning and foliation reactivation during transpressional orogenesis, an example from the Central Longmen Shan[END_REF][START_REF] Li | Evolution of the Longmen Shan foreland basin (western Sichuan, China) during the Late Triassic Indosinian orogeny[END_REF].

Most parts of the Songpan-Ganzi Terrane show slow cooling rate at ca. 1°C/Ma since the early Mesozoic (Huang et al., 2003a;Huang et al., 2003b;[START_REF] Roger | Mesozoic-Cenozoic tectonothermal evolution of the eastern part of the Tibetan Plateau (Songpan-Garze, Longmen Shan area): insights from thermochronological data and simple thermal modelling[END_REF].

However, an enhanced cooling event (ca. 8°C/Ma) during the late Jurassic to Eocene was reported at the peripheries of the Songpan-Ganzi Terrane, especially in the Wenchuan and Danba areas [START_REF] Yuan | The uplift and cooling histories of Laojungou granite in the western margin of the central Longmen Mountain[END_REF][START_REF] Arne | Differential exhumation in response to episodic thrusting along the eastern margin of the Tibetan Plateau: tectonophysics[END_REF]Huang et al., 2003a). This rapid cooling event probably responded to the D2 top-to-the-NW/N normal faulting. The belt characterized by the top-to-the-NW normal faulting developed along the northwest/north boundary of the basement complexes was entitled as the East Tibet Detachment (Wang et al., 2012b). The D3 deformation was thought to have initiated since ca. 30-25 Ma that builds the present Longmen mountains of the eastern Tibetan Plateau [START_REF] Clark | Topographic ooze: Building the eastern margin of Tibet by lower crustal flow[END_REF][START_REF] Kirby | Late Cenozoic evolution of the eastern margin of the Tibetan Plateau: Inferences from 40Ar/39Ar and (U Th)/He thermochronology: Tectonics[END_REF]Wang et al., 2012a).

Structural analysis of the LMTB

Structural subdivisions of the LMTB

The LMTB was interpreted as an allochthonous sheet thrusting southeastward upon the autochthonous western Sichuan Foreland Basin [START_REF] Xu | Orogenic process of the Songpan-Ganzé Orogenic belt of China[END_REF][START_REF] Burchfiel | Tectonics of the Longmen Shan and Adjacent Regions[END_REF][START_REF] Worley | Deformation partitioning and foliation reactivation during transpressional orogenesis, an example from the Central Longmen Shan[END_REF]Robert et al., 2010a). Based on structural investigations, we present revised structural subdivisions of the LMTB. Firstly, the LMTB is divided into a Southeastern Zone and a Northwestern Zone by a Main Central Boundary (see Section 3.2; Figure 3a). According to the lithology, deformation intensity, and metamorphic grade, the Southeastern Zone can be further subdivided into four subdivisions (Figure 3). They are, from the southeast to the northwest: i) the undeformed Autochthon that comprises the western part of the Sichuan Foreland Basin, ii) the Para-autochthon, transported southeastwards along the Anxian-Guanxian Fault and rests over the Autochthon, iii) the Allochthon that traveled southeastwards much further on top of the Para-autochthon, and iv) the Inner Metamorphic Core that recorded the highest metamorphic conditions (i.e., amphibolite facies) among the Southeastern Zone. The four subdivisions in the Southeastern Zone are separated by the three major listric faults of the LMTB (Figure 3; see in Section 2.3.1).

i) The Autochthon, located to the southeast of the Anxian-Guanxian Fault, is mainly composed of Jurassic-Cretaceous strata and overlying Cenozoic sediments (Figures 3 and4). The Jurassic-Cretaceous strata were deposited in circumstances of alluvial, fluvio-deltaic, and lacustrine fills, characterized by medium and coarse-grained sandstone with some conglomeratic layers at the base. Total thickness of the Jurassic-Cretaceous sequences is more than 5 km [START_REF] Bgmrsp | Regional geology of Sichuan Province[END_REF]. In the northern part of the LMTB, close to Anxian and Jiangyou county, the early Jurassic and overlying Cretaceous to Cenozoic sequences are dipping to the southeast that unconformably rest on the intensively folded late Triassic rocks (Figures 2 and4c-d) [START_REF] Bgmrsp | Regional geology of Sichuan Province[END_REF].

ii) The Para-autochthon is sandwiched by the Anxian-Guanxian Fault and the Yingxiu-Beichuan Fault, with its half area covered by klippen and nappes from the Allochthon (Figure 3). The Late Triassic Xujiahe Formation dominated in this subdivision either passes up from the earliest late Triassic marine sandstone or unconformably overlies the middle Triassic limestone [START_REF] Li | Evolution of the Longmen Shan foreland basin (western Sichuan, China) during the Late Triassic Indosinian orogeny[END_REF][START_REF] Meng | Mesozoic sedimentary evolution of the northwest Sichuan basin: Implication for continued clockwise rotation of the South China block[END_REF]. The Xujiahe Formation shows markable lateral variation in both thickness and sedimentary facies. Coarse-grained sediments, such as alluvial conglomerate bodies occur close to the Yingxiu-Beichuan Fault, but change into fine-grained lacustrine sediments toward to the basin interior.

iii) The Allochthon is mainly exposed between the Wenchuan-Maoxian Fault and the Yingxiu-Beichuan Fault, and also includes the klippen and nappes resting on top of the Para-autochthon (Figures 2 and3a). Northwest to the Yingxiu-Beichuan Fault, this subdivision mainly consists of Silurian shale and greywacke with the bedding completely transposed into foliation. Greenschist-facies metamorphism is indicated by mineral assemblages of fine-grained muscovite + chlorite + quartz (Airaghi et al., 2017b;[START_REF] Xue | Mesozoic crustal thickening of the Longmenshan belt (NE Tibet, China) by imbrication of basement slices: insights from structural analysis, petrofabric and magnetic fabric studies, and gravity modeling: Tectonics[END_REF]. Metamorphic condition jumps of ~50 ºC, 3-5 kbar southeastwards across the Yingxiu-Beichuan Fault suggest that the Allochthon are more deformed and metamorphosed in a deeper structural level than the Paraautochthon (Robert et al., 2010a;Airaghi et al., 2018b). Neoproterozoic complexes including the Pengguan and Jiaoziding complexes are distributed in this subdivision.

Southeast to the Yingxiu-Beichuan Fault, klippen and nappes are mainly composed of Devonian to early Triassic thick-bedded limestone and marine sandstone (Figures 2, 3 and4a-d).

iv) The Inner Metamorphic Core comprises phyllite and micaschist that are mostly metamorphosed Silurian sediments (Figures 2 and3). Amphibolite-facies metamorphic zone that wraps the oval-shaped Xuelongbao (XLB in Figure 2) and stripped Moutuo (MT in Figure 2) complexes records the highest-grade metamorphism within the Southeastern Zone [START_REF] Dirks | Tectonic evolution of the NE margin of the Tibetan Plateau; evidence from the central Longmen Mountains, Sichuan Province, China[END_REF]Airaghi et al., 2017a;Airaghi et al., 2018a). Peak P-T condition attained to ~11 ± 2 kbar and ~620 ºC at the southernmost margin of the Xuelongbao complex calculated from metamorphic assemblages of biotite + garnet + staurolite + muscovite + kyanite [START_REF] Dirks | Tectonic evolution of the NE margin of the Tibetan Plateau; evidence from the central Longmen Mountains, Sichuan Province, China[END_REF]Airaghi et al., 2018a). Based on the enhanced metamorphic grade and deformation intensity, the Inner Metamorphic Core are therefore more deformed and metamorphosed in a deeper structural level than the Allochthon.

The Northwestern Zone includes eastern Songpan-Ganzi Terrane with Neoproterozoic sequences outcropped in the north and Paleozoic sequences outcropped in the south (Figures 2 and3). The northern Northwestern Zone mainly consists of two Neoproterozoic Groups, namely, 1) the volcanic Bikou Group defined by mafic lavas within its lower part and ranges from mafic to intermediate-felsic within its upper portion, and 2) the Hengdan Group tectonically resting on the Bikou Group is composed of black, thin-bedded pyrite-bearing shales and interbedded thinly volcaniclastic sandstone and siltstone [START_REF] Yan | SHRIMP age and geochemistry of the Bikou volcanic terrane: implications for Neoproterozoic tectonics on the northern margin of the Yangtze Craton[END_REF][START_REF] Druschke | Stratigraphic and UPb SHRIMP Detrital Zircon Evidence for a Neoproterozoic Continental Arc, Central China: Rodinia Implications[END_REF]. The southern Northwestern Zone includes Devonian Weixian Formation and the Triassic sequences.

The Devonian sequences consist mainly of quartziferous phyllite to schist and interbeded quartzite. The overlying Triassic Xikang Group is dominated by thick and highly repetitious flysch sediments. Although different rock units comprising the northern and southern Northwestern Zone, it was metamorphosed under similar greenschist facies conditions (Figures 1b and2) [START_REF] Xu | Orogenic process of the Songpan-Ganzé Orogenic belt of China[END_REF][START_REF] Liu | Deformation and metamorphism sequence of bikou terrane in the northwest margin of yangtze plate implications for extension collapse and transition of songpan garze orogenic belt[END_REF]

The Main Central Boundary

The Main Central Boundary serves as a major tectonic boundary separating the Northwestern Zone and the Southeastern Zone. It corresponds to the southern branch of the Qingchuan Fault (T4 in figure 2) in the north [START_REF] Bgmrsp | Regional geology of Sichuan Province[END_REF], and to a vertical cleavage zone within the Paleozoic rocks in the south (Figure 2). To the SE of the Main Central Boundary, all structural planes in the Southeastern Zone, including foliations, cleavages, and fold axial planes, are exclusively dipping to the NW. Various kinematic indicators in the Southeastern Zone suggest a consistent top-to-the-SE/S thrusting. To the NW of the Main Central Boundary, the Northwestern Zone is characterized by the SSE-dipping planar structures and associated top-to-the-NW thrusting (Figure 3).

The Qingchuan-Pingwu Fault, as the northern segment of the Main Central Boundary, is a major tectonic boundary. Seismic profiles, drill data [START_REF] Jia | Longmen Shan foldthrust belt and its relation to the western Sichuan Basin in central China: New insights from hydrocarbon exploration[END_REF] and field observations indicate that the Qingchuan-Pingwu Fault is a positive flower structure developed in a transpressional system. Microfabric study [START_REF] Wang | East Tibet Detachment: Evidence for the uplift mechanism of the Longmen Shan fault belt2012[END_REF] indicated that the Qingchuan-Pingwu Fault was originated from a deep-seated ductile shear zone during late Triassic and overprinted by Cenozoic brittle deformation at very least. It is the late Triassic deep-seated ductilely sheared Qingchuan-Pingwu Fault rather than the Cenozoic brittle one that separates the LMTB into the Southeastern Zone and Northwestern Zone.

The southern segment of the Main Central Boundary roughly distributes along the geological boundary between the Silurian and Devonian-Permian sequences (Figure 2).

Macroscopically, this segment of Main Central Boundary shows predominant vertical cleavage and foliation within intensely deformed argillaceous limestone, graphitic pelite and thin-bedded limestone-quartz ribbon interlayers (Figures 3a and5). Their bedding (S0) can be distinguished by the compositional variations that is parallel to the cleavages (Figure 3a and5b). Associated rootless folds imply significant shortening during the formation of the Main Central Boundary (Figures 3a and5b-c). P-wave receive function and seismic reflection profiles revealed a Moho at ∼36-40 km depth beneath the Sichuan Basin that gradually deepens northwestwards at ∼63 km depth beneath the eastern Tibet [START_REF] Guo | Imaging the crustal structure beneath the eastern Tibetan Plateau and implications for the uplift of the Longmen Shan range[END_REF]. This transition occurs beneath the Wenchuan Shear Zone (corresponding to Inner Metamorphic Core) over a horizontal distance of fewer than 30 km (Robert et al., 2010b;[START_REF] Zhang | Seismic signature of the collision between the east Tibetan escape flow and the Sichuan Basin[END_REF][START_REF] Guo | Imaging the crustal structure beneath the eastern Tibetan Plateau and implications for the uplift of the Longmen Shan range[END_REF].

Geophysical profiles, therefore, support the statement that the southern Main Central Boundary, as the northwest boundary of the Inner Metamorphic Core, serves as a major tectonic boundary.

The main deformation (D1)

The D1 main deformation is defined by a NW-SE striking stretching lineation recognized in both the Southeastern and Northwestern zones which is separated by the Main Central Boundary (Figure 2). Kinematic indicators indicate a top-to-the-SE thrusting and a top-to-the-N/NW thrusting in the Southeastern and the Northwestern zones, respectively. Since there is no evidence of superposition between these two senses of shear, it is difficult to precisely define the relative timing of these opposite shearing events. Nevertheless, to clarify the presentation of the deformation events, in the following, we distinguish D1E, the top-to-the-SE thrusting in the Southeastern Zone, and D1W, the top-to-the-NW/N thrusting in the Northwestern Zone, respectively.

Top-to-the-SE thrusting in the Southeastern Zone (D1E)

The top-to-the-SE shearing and thrusting are widespread in the Southeastern Zone at variable scales, except that the Autochthon merely recorded long-wavelength fold and monoclines toward the interior of the Sichuan Basin (Figure 4). The D1E top-tothe-SE thrusting is most developed in the Inner Metamorphic Core and generally attenuated southeastwards.

The Inner Metamorphic Core was ductilely deformed showing predominant NWdipping foliation (S1) at varied angles between 60-90°. Specifically, the foliation gradually steepens when approaching the Main Central Boundary, and eventually presents in a vertical attitude (Figures 234). This changing trend of foliation implies that the deformation intensified toward the Main Central Boundary. Foliations mostly consist of an alternation of biotite-white mica-rich and light-colored, pelitic interlayers (Figures 3a and6a-b). Along the transect from Wenchuan county to Xuecheng town, an NW-plunging mineral and stretching lineation (L1) defined by aligned newly formed biotite, white mica, elongated quartz aggregates, or iron-oxide pods are well developed (Figures 3a and6a). Kinematic indicators document a consistent top-to-the-SE thrusting in the Inner Metamorphic Core. Such as asymmetric pyrite pressure shadow in mica-schist (Figures 3a and6b) and sigma-shaped feldspar porphyroclasts in paragneiss (Figures 3a and6c) both show top-to-the-SE thrusting. Photomicrographs (perpendicular to the foliation and parallel to the lineation) show that newly formed white mica oriented as compaction plane (S) and shear plane (C), both of which composes an S-C fabric documenting a consistent top-to-the-SE thrusting (Figures 3a and6d).

In the Allochthon, the Neoproterozoic complexes such as the Pengguan and Jiaoziding complexes, mostly show brittle deformation characterized by subhorizontal joints. Inside the Pengguan complex, several NE-SW trending ductile shear zones dissected and thrust the granitic rocks to the southeast (Airaghi et al., 2017b;[START_REF] Xue | Mesozoic crustal thickening of the Longmenshan belt (NE Tibet, China) by imbrication of basement slices: insights from structural analysis, petrofabric and magnetic fabric studies, and gravity modeling: Tectonics[END_REF]. Upon the granitic rocks, the Neoproterozoic Huangshuihe Group recorded extensive shearing and sigmoid pelitic lens incorporated to the top-to-the-SE thrusting (Figures 3a and6e). Structural planes including axial planar cleavages and spaced cleavages dip to the northwest at angles of 30-65° (Figures 234). Fine-grained white mica and chlorite are developed on these structural planes demonstrating a greenschist facies grade of deformation. Sigma-shaped sandy lens in the Silurian pelitic schist indicate a top-to-the-SE thrusting as well (Figures 3a and6f).

As part of the Allochthon, most of the klippen are synclines overturned to the SE, with SE-ward reverse fault which dissected the thick Devonian limestone at the rear of the klippe (Figures 2 and6g), and show a consistent top-to-the-SE thrusting [START_REF] Bgmrsp | Regional geology of Sichuan Province[END_REF].

In the Para-autochthon, the late Triassic Xujiahe sandstone is intensively folded as indicated by the NE-SW trending π-circle-projected axis of bedding (Figures 3b-d). In the south part, multi π circle-Axis exist roughly trending NE-SW, implying multiphase of shearing and thrusting. Folds are mostly overturned to the SE indicating a top-tothe-SE thrusting (Figure 2). Axial planar cleavages are locally developed that gradually intensified and more penetrative approaching northwestward to the Yingxiu-Beichuan Fault. This cleavage-enhancing trend implies a northwestward increase in deformation intensity. In mudstone-sandstone interlayered rocks, competence contrast facilitates the formation of layer-parallel slip and shearing. Sigmoid lens as well as "cold" shearband fabrics consistently indicate a top-to-the-SE thrusting (Figures 3a and6h).

In the Autochthon, lower Jurassic strata and overlying continuous Cretaceous-Cenozoic sequences are mostly concordantly folded in a long-wavelength way. The fact that early Jurassic rocks locally overturned to the southeast (North to the Tangbazi town; Figure 2) therefore implies a post-Jurassic reactivation of the SE-directed thrusting (Figures 2 and4d).

Top-to-the-NW/N thrusting in the Northwestern Zone (D1W)

The structures and kinematics within the Northwestern Zone are different from those within the Southeastern Zone. In the northern Northwestern Zone, S-dipping foliation and associated downdip lineation are well developed. Lineations are defined by aligned calcite, fine-grained white mica, and oxide pods (Figures 2, 3a and7a). Along the lineation, a consistent top-to-the-N/NW shearing and thrusting is determined by various kinematic indicators, such as sigma-shaped pebbles in strongly sheared metaconglomerate (Figures 3a and7b) and asymmetric pressure shadow in sandy-pelitic schist. Photomicrographs show that quartz ribbon (Figures 3a and7c) and sigma-shaped feldspar porphyroclasts (Figures 3a and7d) both indicate a top-to-the-N thrusting. In the north of the Pingwu city, greywacke and siltstone were extensively folded verging to the north with associated meter-scale reverse fault cutting fold limbs (Figures 3a and7e). West to the Pingwu city, bedding (depicted by compositional banding) completely transposed to SE-dipping foliations in response to the northward shearing and thrusting (Figures 3a and7f).

In the southern Northwestern Zone, axial planar cleavage developed in the thinbedded limestone mostly dips to the SE but strikes roughly along the margin of the Xuelongbao complex (Figure 2). On the foliation, lineation plunges to the SE at ~65-70° (Figures 2 and3a). Along the lineation, asymmetric pressure shadow of calcite tail around garnet pseudomorph indicates a top-to-the-NW thrusting (Figures 3a and7g).

Asymmetric NW-verging folds are developed at different scales. The late Triassic turbidite was involved in a hundred-meter scale chevron fold verging to the NW that is consistent with the top-to-the-NW thrusting (Figures 3a and7h). In the whole Northwestern Zone, the top-to-the-NW/N thrusting is indicated by the pervasive NW/N-overturned bedding as well (Figure 2)(BGMRSP, 1991).

Syntectonic (D1W) Mupi pluton in the Northwestern Zone

In the Northwestern Zone, north of the Pingwu city, the Mupi pluton is a coarsegrained diorite and granodiorite pluton with prominent syntectonic features (Figure 2). At its southern margin, the contact between the pluton and the country rock is rather ambiguous. Strong mylonitisation with S-dipping foliation deformed both the country rocks and the south margin of the pluton (Figures 3a and8c). On the S-dipping foliation (S1), aligned biotite and sheared feldspar define a mylonitic lineation plunging south at ~45-75°. Kinematic indicators including sigmoidal feldspar porphyroclasts and shearbands both indicate a top-to-the-N shearing related to thrusting (Figures 3a and8d). About 10-15 meters northwards from the south margin of the pluton, a set of quartz + feldspar veins parallel to the shear foliation (Figures 3a and8b), in which the quartz and feldspar did not record ductile deformation. Further north ~30 meters from the south margin of the pluton, the pluton rocks show magmatic fabric with euhedral feldspar randomly distributed (Figures 3a and8a). Cracks in feldspar grains are subparallel to the shear planes and infilled with quartz (Figures 8e andS1). These cracks in conjunction with the foliation-parallel quartz + feldspar veins indicate that a brittle deformation of the feldspar occurred in the presence of residual melt. Therefore, based on the textural criteria proposed by [START_REF] Paterson | Regional, structural and strain analyses of terranes in the Western Metamorphic Belt, Central Sierra Nevada[END_REF], we prefer to interpret the Mupi pluton as a syntectonic one relating to the D1W deformation. [START_REF] Xue | Mesozoic crustal thickening of the Longmenshan belt (NE Tibet, China) by imbrication of basement slices: insights from structural analysis, petrofabric and magnetic fabric studies, and gravity modeling: Tectonics[END_REF]. Mylonite is localized along the northwest boundary of the Xuelongbao and Pengguan complexes. Along the western boundary of the Xuelongbao complex, foliation dips to the NW at 45-80° with lineation plunging to the west (Figure 2). These structural foliations and lineations are difficult to distinguish from those of the D1E deformation structures. However, sigmoidal feldspar clasts and S/C fabric indicate different kinematics of top-to-the-NW normal faulting (Figures 3a and9a). The Silurian rock directly overlies the Neoproterozoic Pengguan complex also supports NW-ward normal faulting (Figure 2).

Along the northwest boundary of the Pengguan complex, mylonitic foliation dips to the NW at high angles of ~80° (Figures 3a and9b) with lineation steeply plunging to the NW, except for a few SE-or N-dipping foliations that probably resulted from tilting of the original foliations by the Cenozoic deformation (Figure 2). Photomicrographs including asymmetric mica fishes (Figures 3a and9c), sigmoidal feldspar porphyroclasts (Figures 3a and9d), and S/C fabric of white mica (Figures S2c ande) indicate a top-to-the-NW normal faulting. The deformed rocks experienced syntectonic greenschist-facies metamorphism with a mineral assemblage of quartz + feldspar + biotite + white mica + chlorite (Figures S2e-f).

Microscopic structures from the northwest Pengguan complex show sigma-shaped porphyroclast, quartz ribbon (Figures 3a and9f), and white mica shear-band fabric (Figures 3a and9g), indicating a top-to-the-SE shearing related to the D1E deformation. However, all these structures are cut by NW-dipping shear-bands (Figures 3a and9e).

In the western flank of the Wulong complex, about 60 km southwest to the Pengguan complex, a similar top-to-the-NW normal fault with greenschist-facies metamorphism that cuts D1E-related thrust sheet has been previously reported as well (Figure 1b) [START_REF] Burchfiel | Tectonics of the Longmen Shan and Adjacent Regions[END_REF][START_REF] Tian | Late Cretaceous earliest Paleogene deformation in the Longmen Shan fold and thrust belt, eastern Tibetan Plateau margin: Pre Cenozoic thickened crust?: Tectonics[END_REF]Airaghi et al., 2018b). Thus, we interpret the top-to-the-NW normal faulting in the northwest Pengguan complex and the Xuelongbao complex as the D2 event after the D1E top-to-the-SE thrusting in the Southeastern Zone.

Geochronological constraints

In order to constrain the timing of the D1 double-vergent thrusting and the D2 topto-the-NW normal faulting, SIMS U-Pb dating on zircons from the synkinematic Mupi pluton and 40 Ar-39 Ar dating on metamorphic white mica, biotite and fine-grained white mica were carried out. U-Pb zircon dating was conducted on a Cameca IMS-1280HR SIMS following the analytical procedures described by [START_REF] Li | Precise determination of Phanerozoic zircon Pb/Pb age by multicollector SIMS without external standardization: Geochemistry[END_REF]. The primary O 2-ion beam was focused to elliptical spots with the sizes of 20 μm×30 μm. Measured Pb/U ratios were calibrated relative to the zircon standard Plešovice [START_REF] Sláma | Plešovice zircon -A new natural reference material for U-Pb and Hf isotopic microanalysis[END_REF]. Non-radiogenic Pb was subtracted from the measured Pb isotopic composition using the measured 204 Pb and the present-day average terrestrial Pb isotopic composition in the model of [START_REF] Stacey | Approximation of terrestrial lead isotope evolution by a two-stage model[END_REF]. U-Pb data were plotted and calculated using the program Isoplot 3.00 [START_REF] Ludwig | User's manual for Isoplot 3.00: a geochronological toolkit for Microsoft Excel: Berkeley Geochrongol[END_REF], and age uncertainties are quoted at the 95% confidence level except for those noted otherwise. Measurements on a second zircon standard Qinghu were interspersed with unknowns to monitor the external uncertainties, and yielded concordia ages of 159.6 ± 1.6 Ma (MSWD = 0.28, N = 6), which are identical within errors to the recommended age (159.5 ± 0.2 Ma) (Li et al., 2013b). [START_REF] Phillips | Astronomical calibration of 40 Ar/ 39 Ar reference minerals using high-precision, multicollector (ARGUSVI) mass spectrometry[END_REF]. The canister was irradiated for 40 MWhr in the CLICIT facility of the Oregon State University TRIGA reactor, USA. The measured isotopic ratios were corrected for system blanks, mass discrimination, and irradiation-induced interference. Plateau and/or total gas ages with 2σ uncertainties were calculated and calibrated based on methods proposed by [START_REF] Lee | A redetermination of the isotopic abundances of atmospheric Ar[END_REF]. Analytical data are presented in Table S2 (Xue et al., 2020), and the age spectra are presented in Figure 11. Argon age plateaus are defined by contiguous gas fractions that together represent more than 50% of the total 39 Ar released from the sample and for which no difference in age can be detected between any two fractions at the 95% confidence level [START_REF] Fleck | Interpretation of discordant 40Ar/39Ar age-spectra of mesozoic tholeiites from antarctica[END_REF]. According to this definition, all argon age spectra in this work, except for sample CX33, do not yield plateau ages (Figure 11).

We choose contiguous steps within 95% confidence close to 50% of the total 39 Ar released (CX6A white mica), or contiguous steps close to 95% confidence more than 50% of the total 39 Ar released (CX2, CX6B, CX25, CX44 white mica), to calculate weighted mean ages (Figure 11).

Results

U-Pb dating on synkinematic Mupi pluton

Zircons from the Mupi pluton are pink to colorless, 80 to 200 μm long with length/width ratios from 2 to 3. They show well-developed crystal morphologies. CL images of zircons show that most of the zircons have well-developed oscillatory zoning of igneous origin and some grains have an inherited core (Figure 10a). Under guidance of the zircon CL images, analyzed spots were positioned on the oscillatory zircon mantle/margin when there are exist inherited zircon cores. SIMS U-Pb zircon results are listed in Table S1. The SIMS U-Pb dating of the Mupi pluton yields a concordant age of 219.1 ± 1.7 Ma (MSWD = 0.99, N = 15; Figure 10b).

40

Ar-39 Ar dating Sample CX44 collected from the northern Northwestern Zone (Figure 12) is micaschist with penetrative SSE-dipping foliation and associated S-plunging lineation related to the D1W top-to-the-N thrusting. The CX44 micaschist contains domains rich in fine-grained white mica and quartz that define the shear foliation (Figures S2a-b).

Multi-grain white mica (211-152 μm) separated from CX44 yields a weighted mean age of 224.1 ± 1.7 Ma, accounting for 81% of released 39 Ar (Figures 11a andTable 1). Sample CX12 collected from the southern Northwestern Zone (Figure 12) is micaschist involved in the D1W top-to-the-NW thrusting. Shear foliation is defined by aligned fine-grained white mica (Figure 7g). Two aliquots of fine-grained white mica (104-66 μm) of CX12A & CX12B show discordant stair-shaped spectra increasing from ~65-70 Ma to ~150 Ma and ~120 Ma, respectively (Figures 11b-c and Table 1).

Samples CX6 and CX33 (Figure 12), collected from the Inner Metamorphic Core within the Southeastern Zone, are characterized by penetrative NW-dipping foliation and NW-plunging lineation that was involved in D1E top-to-the-SE thrusting. In these two samples, the foliations and associated lineations are defined by white mica (Figures 6d andS2g). Two aliquots of white mica (152-211 μm) of CX6A and CX6B yield equivalent weighted mean ages of 124 ± 1.9 Ma and 126.3 ± 3.3 Ma, respectively (Figure 11d-e). Multi-grain white mica (152-211 μm) from CX33 yields a plateau age of 148.2 ± 1.2 Ma (>60% of the total 39 Ar; Figure 11f and Table 1).

Samples CX25 from micaschist of the Xuelongbao complex and CX18, CX19 from gneiss/mylonite of the Pengguan complex (Figures 12) show well-developed NWdipping foliations related to the D2 top-to-the-NW normal faulting (Figures S2c-f).

White mica (211-152 μm) in CX25 that defines the shear planes (Figures S2e-f) was selected for 40 Ar/ 39 Ar dating. Five steps of sample CX25 with ~80% released 39 Ar yields a weighted mean age of 79.7 ± 2.7 Ma (Figure 11h). Samples of CX18 and CX19 have stair-shaped spectra, in which ages increase from ~50-60 Ma to ~110-135 Ma from low-to high-temperature steps (Figure 11i-k and Table 1). Sample CX2 from the northwest boundary of the Pengguan complex (in a structural position equivalent to the samples CX18 and CX19; Figure 12) was involved in the D2 top-to-the-NW normal faulting (Figures 3 and9g-i). Multi-grain white mica (211-152 μm) from CX2 yields a weighted mean age of 26.23 ± 0.99 Ma at low-to mediumtemperature steps (>50% of the total 39 Ar released; Figure 11g and Table 1).

Interpretation of the geochronological results

Zircon U-Pb data interpretation

In the northern Northwestern Zone, the Mupi pluton shows a continuous transition from mylonitic fabrics along the pluton margin to magmatic fabrics in the pluton interior, arguing for its synkinematic emplacement. This deduction is supported by the fact that quartz + feldspar veins parallel to the mylonitic foliation, and feldspar records brittle deformation occurred in the presence of residual melt (see details in Section 3.3.3). Thus, the D1W top-to-the-N/NW thrusting is closely related to the emplacement of the Mupi pluton. Our zircon U-Pb ages reveal that the Mupi pluton emplaced at ca. 219 Ma, therefore place age constraint on the D1W top-to-the-N/NW thrusting at ca. 219 Ma (Figure 12). 4.3.2 40 Ar-39 Ar data interpretation All samples show high radiogenic 39 Ar and almost all steps are clustered at the horizontal 39 Ar/ 40 Ar axis that does not define a real inverse isochron line (Table S2; Figure S3). Most of the step-ages calculated both by using initial 40 Ar/ 36 Ar of 298.56 [START_REF] Lee | A redetermination of the isotopic abundances of atmospheric Ar[END_REF] and from the so-called inverse isochron show nearly no difference (Table S2). Thus, the inverse isochron ages (though identical to their weighted mean ages or plateau ages) and the fake extraneous atmosphere argon are meaningless and will not be further discussed (Figure S3; Table S2).

Concordant white mica 40 Ar-39 Ar spectra, such as those yielded by samples of CX44 (81.6% released 39 Ar), CX6A (49.5% released 39 Ar), CX6B (52.4% released 39 Ar), CX33 (60.1% released 39 Ar), CX2 (53.6% released 39 Ar) and CX25 (79.7% released 39 Ar), are typical of samples that have remained a closed system since thermal/mylonitic resetting or crystallization (Figures 11a andd-h). Similar rocks to sample CX44 recorded greenschist facies metamorphism under temperatures of 260-310 °C (chlorite + white mica multi-equilibrium thermometer; [START_REF] Liu | Deformation and metamorphism sequence of bikou terrane in the northwest margin of yangtze plate implications for extension collapse and transition of songpan garze orogenic belt[END_REF], which is lower than the white mica argon closure temperature of ca. 425 °C (100 μm grain) [START_REF] Harrison | Diffusion of 40Ar in muscovite[END_REF]. Considering that the dated mica grains are strongly deformed by the top-to-the-N/NW thrusting, we, therefore, interpret that the 224.1 ± 1.7 Ma argon age of CX44 recorded the timing of the white mica recrystallization related to the D1W thrusting event.

Samples of CX6A & CX6B, and CX33 from the Inner Metamorphic Core, CX2 and CX25 from the mylonite zone along the northwest boundary of the Pengguan and Xuelongbao complexes have been deformed and metamorphosed at temperatures of >530°C (Raman Spectroscopy on Carbonaceous Material; Robert et al., 2010b).

These temperatures are higher than the closure temperature (425 °C) of the ca. 100 μm radius muscovite grain [START_REF] Harrison | Diffusion of 40Ar in muscovite[END_REF]. There is no evidence of any post late Cretaceous to Cenozoic magmatism along the LMTB and its adjacent eastern Songpan-Ganzi Terrane. Thus, the flattened spectra of CX6A (124 ± 1.9 Ma), CX6B (126.3 ± 3.3), CX33 (148.2 ± 1.2 Ma), CX2 (26.2 ± 0.9 Ma), and CX25 (79.7 ± 2.7 Ma) are probably cooling ages when deformation rapidly exhumed those rocks through the ca.

425 °C geothermal depth. These cooling ages, therefore, can represent the timing of deformations. In this scenario, considering samples of CX6A, CX6B and CX33 were Discordant stair-shaped spectra, such as those observed from the samples of CX12, CX18, and CX19 (Figures 11b-c, i-k), are commonly observed in fine-grained mineral mixes [START_REF] Tian | Late Cretaceous earliest Paleogene deformation in the Longmen Shan fold and thrust belt, eastern Tibetan Plateau margin: Pre Cenozoic thickened crust?: Tectonics[END_REF]. Those staircase spectra indicate 40 Ar diffusive loss that is possibly related to chemical alteration, a subsequent thermal event disturbance, or argon loss during the late stage mylonitisation [START_REF] Costa | Use of the 40Ar-39Ar stepwise heating method for dating mylonite zones: An example from the St. Barth616my Massif[END_REF][START_REF] Lovera | 40Ar/39Ar geothermometry for slowly cooled samples having a distribution of diffusion domain size[END_REF][START_REF] West | Dating mylonitic deformation by the 40Ar-39Ar method: An example from the Norumbega Fault Zone[END_REF]. The oldest age of the stair-shaped spectra is a minimum one for the fine-grained white mica closure age [START_REF] Mcdougall | Geochronology and thermo-chronology by the 40Ar/39Ar method[END_REF].

Samples of CX12, CX18, and CX19 have been deformed at temperatures >530°C as well (Raman Spectroscopy on Carbonaceous Material; Robert et al., 2010).

Considering the size of white mica (104-66 μm), the argon-argon system of finegrained mica should be completely reset [START_REF] Harrison | Diffusion of 40Ar in muscovite[END_REF]. According to the stairshaped spectra, samples of CX12A, CX12B, CX18, CX19A, CX19B may have recrystallized or been reset before 146.5 ± 2.7 Ma, 119 ± 1.3 Ma, 111.1 ± 0.2 Ma, 133.7 ± 3.3 Ma, and 139.3 ± 1.1 Ma, respectively (Figures 11b-c, i-k). These oldest ages of the stair-shaped spectra possibly resulted from the 148-124 Ma mylonitisation as implied by 40 Ar-39 Ar cooling ages of CX6A, CX6B, and CX33.

Chemical alteration should have led to a more profound influence on the finegrained white mica of CX12, CX18, and CX19 that may result in 40 Ar diffusive loss and flattened spectra [START_REF] Harrison | Diffusion of 40Ar in muscovite[END_REF]. In this way, considering samples of CX2, CX18, CX19, and CX25 located at the same structural position along the northwest boundary of the Neoproterozoic complexes, we may expect that CX2 and CX25, showing stair-shaped spectrum and CX18, CX19 and presenting flattened spectra, yield younger ages, however, it is not the case (Figures 11g-k). Thus, chemical alteration and related 40 Ar diffusive loss may not be the main factor to account for the stair-shaped spectra.

An alternative interpretation may suggest that the youngest age of the stair-shaped spectra gives a maximum age of the thermal overprint or late-stage mylonitisation [START_REF] Costa | Use of the 40Ar-39Ar stepwise heating method for dating mylonite zones: An example from the St. Barth616my Massif[END_REF][START_REF] Lovera | 40Ar/39Ar geothermometry for slowly cooled samples having a distribution of diffusion domain size[END_REF][START_REF] West | Dating mylonitic deformation by the 40Ar-39Ar method: An example from the Norumbega Fault Zone[END_REF]. As stated above, no magmatism of late Mesozoic-Cenozoic ages has been reported along the LMTB.

Thus, the post-cooling thermal event may not be responsible for the minimum ages of the stair-shaped spectra. The youngest ages in the stair-shaped spectra therefore most likely indicate late-stage mylonitisation after ~60-50 Ma (Figures 11i-k), which possibly related to the ca. 27 Ma top-to-the-NW faulting along the northwest boundary of the Pengguan complex. It is worth to note that the CX12A and CX12B fine-grained white mica from the Northwestern Zone (Figure 12), far away from the Pengguan complex, show older minimum ages of 64.7 ± 1.3 Ma and 72.2 ± 1.4 Ma comparing with the 50 ± 0.1 Ma, 59 ± 0.3 Ma, and 58.4 ± 0.4 Ma of the CX18, CX19A and CX19B (Figures 11b-c, j-l), which may suggest that the CX12 has been less affected by the top-to-the-NW normal faulting comparing to the CX18 and CX19.

Discussion

Summary of our newly obtained structural results

On the basis of detailed structural analyses, a Main Central Boundary and a D1W topto-the-N/NW thrusting in the Northwestern Zone are argued for the first time in the LMTB. The Main Central Boundary corresponds to a major tectonic boundary supported both by field structures and geophysical profile [START_REF] Jia | Longmen Shan foldthrust belt and its relation to the western Sichuan Basin in central China: New insights from hydrocarbon exploration[END_REF]Robert et al., 2010b), which divides the LMTB into a Northwestern Zone (part of the Songpan-Ganzi Terrane) to the northwest and a Southeastern Zone to the southeast. The Southeastern Zone is characterized by exclusively NW-dipping foliations and associated D1E top-tothe-SE thrusting, while the Northwestern Zone is characterized by exclusively SEdipping foliations and associated D1W top-to-the-NW thrusting.

This study subdivides the Southeastern Zone of the LMTB into four units based on increasing deformation intensities and metamorphic grades from southeast to northwest, namely the Autochthon, Para-autochthon, Allochthon, and Inner Metamorphic Core. This subdivision pattern is revised from the previous one that divided the LMTB into an allochthonous sheet thrust southeastwards on top of the autochthonous Sichuan Foreland Basin [START_REF] Xu | Orogenic process of the Songpan-Ganzé Orogenic belt of China[END_REF][START_REF] Burchfiel | Tectonics of the Longmen Shan and Adjacent Regions[END_REF][START_REF] Worley | Deformation partitioning and foliation reactivation during transpressional orogenesis, an example from the Central Longmen Shan[END_REF].

In the Southeastern Zone, a D2 top-to-the-NW normal faulting that cuts the D1E topto-the-SE thrusting is recognized along the northwest boundary of the Pengguan and Xuelongbao complexes.

Polyphase deformations along the LMTB

According to the geometric and kinematic features and overprinting relationship, together with 40 Ar-39 Ar and zircon U-Pb data, which allow us to draw a polyphase deformation sequence of the LMTB.

Timing of the D1 main deformation event

Timing of the D1W top-to-the-N/NW thrusting is firstly constrained by the emplaced timing of the syntectonic Mupi pluton at ca. 219 Ma (Figure 12). Synkinematic recrystallized white mica of sample CX44 that is involved in the top-to-the-N/NW thrusting yields a weighted mean age of 224.1 ± 1.7 Ma, which places a second age constraint on the D1W top-to-the-N/NW thrusting. This newly obtained argon age is consistent with previously published white mica 40 Ar-39 Ar ages at ca. 237-202 Ma from the northern Northwestern Zone (Figure 12) [START_REF] Yan | Structural and geochronological constraints on the Mesozoic Cenozoic tectonic evolution of the Longmen Shan thrust belt, eastern Tibetan Plateau[END_REF][START_REF] Yan | Structural and Geochronological Constraints on the Early Mesozoic North Longmen Shan Thrust Belt: Foreland Fold-Thrust Propagation of the SW Qinling Orogenic Belt[END_REF]. The fact that the zircon U-Pb age of the Mupi pluton is ca. 5 Ma younger than the synkinematic white mica 40 Ar-39 Ar age may suggest the Mupi pluton emplaced during the late phase of the D1W event. Based on the zircon U-Pb age of syntectonic Mupi pluton and synkinematic white mica 40 Ar-39 Ar age, we, therefore, suggest that the D1W top-to-the-N/NW thrusting event occurred at ca. 224-219 Ma.

The D1E top-to-the-SE thrusting within the Southeastern Zone has been constrained in the late Triassic for the following arguments: 1) Field observations and seismic reflection profile revealed that late Jurassic-Cenozoic rocks unconformably overly on intensively folded rocks as young as late Triassic (Figures 2 and4a-d) [START_REF] Bgmrsp | Regional geology of Sichuan Province[END_REF][START_REF] Jia | Longmen Shan foldthrust belt and its relation to the western Sichuan Basin in central China: New insights from hydrocarbon exploration[END_REF]; 2) Detailed petrological analyses and in-situ allanite U-Pb dating and biotite 40 Ar/ 39 Ar dating revealed a ca. 204-190 Ma peak metamorphism in the Inner Metamorphic Core that postdates the D1E event (Airaghi et al., 2017a;2018a;2018c);

3) The ages of the voluminous magmatism intruding the extensively folded Songpan-Ganzi Terrane range between 223 and 200 Ma, which postdates the folding in the Songpan-Ganzi Terrane and the top-to-the-SE thrusting of the LMTB (Figure 12) [START_REF] Xu | Orogenic process of the Songpan-Ganzé Orogenic belt of China[END_REF][START_REF] Roger | Timing of granite emplacement and cooling in the Songpan-Garzê Fold Belt (eastern Tibetan Plateau) with tectonic implications[END_REF][START_REF] Harrowfield | Indosinian deformation of the Songpan Garze Fold Belt, northeast Tibetan Plateau[END_REF][START_REF] Hu | SHRIMP U-Pb dating of zircons from granitoid bodies in the Songpan-Ganzi terrane and its implications[END_REF][START_REF] Zhang | A-type granite and adakitic magmatism association in Songpan-Garze fold belt, eastern Tibetan Plateau: Implication for lithospheric delamination[END_REF][START_REF] De Sigoyer | Generation and emplacement of Triassic granitoids within the Songpan Ganze accretionaryorogenic wedge in a context of slab retreat accommodated by tear faulting, Eastern Tibetan plateau[END_REF]. Thus, the regional D1E top-to-the-SE thrusting was at least activated during the late Triassic, together with the contemporary D1W top-tothe-N/NW thrusting recorded within the Northwestern Zone, both of which compose the ca. 224-219 Ma main D1 double-vergent event.

Timing of the D2 top-to-the-NW normal faulting

According to the newly obtained 40 Ar-39 Ar ages, the timing of the D2 top-to-the-NW normal faulting occurred along the northwest boundary of the Pengguan (ca. 27 Ma) and Xuelongbao (ca. 80 Ma) complexes are quite different. The top-to-the-NW/N normal faulting is widespread in an over 300 km long zone along the northern margins of the Danba and Kangding complexes, and the northwest margins of the Baoxing, Wulong, Xuelongbao, Moutuo, Pengguan, and Jiaoziding complexes (Figure 12) [START_REF] Burchfiel | Tectonics of the Longmen Shan and Adjacent Regions[END_REF][START_REF] Xu | Uplift of the Longmen Shan range and the Wenchuan earthquake: Episodes[END_REF][START_REF] Tian | Late Cretaceous earliest Paleogene deformation in the Longmen Shan fold and thrust belt, eastern Tibetan Plateau margin: Pre Cenozoic thickened crust?: Tectonics[END_REF][START_REF] Xue | Mesozoic crustal thickening of the Longmenshan belt (NE Tibet, China) by imbrication of basement slices: insights from structural analysis, petrofabric and magnetic fabric studies, and gravity modeling: Tectonics[END_REF]. This belt of top-to-the-NW normal faulting has previously been entitled East Tibet Detachment (Figure 12) (Wang et al., 2012b). Timing of the East Tibet Detachment top-to-the-NW normal faulting is different at different localities. [START_REF] Hames | Laser 40Ar/39Ar dating of Cenozoic, greenschist-facies shear zones, Longmen Shan[END_REF] reported a top-to-the-NW normal faulting and syntectonic metamorphism at ca. 25 Ma along the northwest Pengguan complex that is in accordance with the ca. 27 Ma top-to-the-NW normal faulting as indicated by sample CX2 (Figure 12).

Along the northwest boundary of the Wulong complex and north boundary of the Danba complex, the top-to-the-NW/N normal faulting was constrained to ca. 81-47 Ma ( 40 Ar/ 39 Ar dating on metamorphic biotite and white mica; Figure 12) [START_REF] Zhou | Structural and geochronological constraints on the tectono-thermal evolution of the Danba domal terrane, eastern margin of the Tibetan plateau[END_REF][START_REF] Tian | Late Cretaceous earliest Paleogene deformation in the Longmen Shan fold and thrust belt, eastern Tibetan Plateau margin: Pre Cenozoic thickened crust?: Tectonics[END_REF]. This phase of the top-to-the-NW normal faulting is recorded along the northwest boundary of the Xuelongbao complex (ca. 80 Ma 40 Ar-39 Ar age of sample CX25) as well.

A period of the top-to-the-NW/N normal faulting at ca. 166-159 Ma was reported at the northwest boundary of the Jiaoziding and Pengguan complexes, and the north boundary of the Danba complex (Figure 12) [START_REF] Dirks | Tectonic evolution of the NE margin of the Tibetan Plateau; evidence from the central Longmen Mountains, Sichuan Province, China[END_REF][START_REF] Zhou | Structural and geochronological constraints on the tectono-thermal evolution of the Danba domal terrane, eastern margin of the Tibetan plateau[END_REF][START_REF] Li | Composition, structural characteristics and evolution of back-Longmenshan orogen (north section) in the northwest margin of Yangtze block[END_REF]. Whereas, younger ages of ca. 122-120 Ma related to the top-to-the-N normal faulting at the north boundary of the Danba complex were reported as well (Figure 12) [START_REF] Xu | Uplift of the Longmen Shan range and the Wenchuan earthquake: Episodes[END_REF].

In summary, the timing of the top-to-the-NW normal faulting of the East Tibet 12). Nonetheless, a process is required to explain these four groups of well-defined argon plateau ages that do not show an age-changing trend in map view (Figure 12). We, therefore, prefer to interpret that the top-to-the-NW/N normal faulting of the East Tibet Detachment was episodically and co-planarly activated at ca. 166-159 Ma,. The late-stage activations could partially reset the fine-grained white mica argon system that resulted in stair-shaped spectra yielded by samples of CX18 and CX19 (Figures 11 and12).

Episodic basement-slice-imbrication and amphibolite facies metamorphic rock exhumation

The Western Sichuan Foreland Basin (WSFB) was formed by loading and subsidence due to the southeastward thrusting of the LMTB along the Yingxiu-Beichuan Fault and Xiaoguanzi Fault (T2 in Figure 12) [START_REF] Chen | Emplacement of the Longmen Shan Thrust-Nappe Belt along the eastern margin of the Tibetan Plateau[END_REF][START_REF] Li | Evolution of the Longmen Shan foreland basin (western Sichuan, China) during the Late Triassic Indosinian orogeny[END_REF]. Episodic terrestrial deposition in the WSFB from the late Triassic to the present day indicates that the LMTB continued to be tectonically active episodically during the late Triassic to early Jurassic, late Jurassic to early Cretaceous, and late Cretaceous to earliest Paleocene (Figure 12) [START_REF] Xu | Uplift of the Longmen Shan range and the Wenchuan earthquake: Episodes[END_REF]Li et al., 2013c;[START_REF] Tian | Late Cretaceous earliest Paleogene deformation in the Longmen Shan fold and thrust belt, eastern Tibetan Plateau margin: Pre Cenozoic thickened crust?: Tectonics[END_REF].

The late Jurassic to early Cretaceous southeastward thrusting of the Yingxiu-Beichuan Fault and Xiaoguanzi Fault (T2 in Figure 12), as the southeast boundary Fault of the Pengguan and Baoxing complexes, overlaps the ca. 166-159 Ma and ca. 122-120 Ma top-to-the-NW/N normal faulting along the NW boundary of the Jiaoziding complex and the north boundary of the Danba complex (Figure 12) [START_REF] Xu | Uplift of the Longmen Shan range and the Wenchuan earthquake: Episodes[END_REF][START_REF] Zhou | Structural and geochronological constraints on the tectono-thermal evolution of the Danba domal terrane, eastern margin of the Tibetan plateau[END_REF][START_REF] Li | Composition, structural characteristics and evolution of back-Longmenshan orogen (north section) in the northwest margin of Yangtze block[END_REF]. The late Cretaceous-earliest Paleocene southeastward thrusting of the Yingxiu-Beichuan Fault and Xiaoguanzi Fault is also coeval with the top-to-the-NW/N normal faulting along the NW boundary of the Xuelongbao (CX25 in this study) and Wulong [START_REF] Tian | Late Cretaceous earliest Paleogene deformation in the Longmen Shan fold and thrust belt, eastern Tibetan Plateau margin: Pre Cenozoic thickened crust?: Tectonics[END_REF] complexes (Figure 12). Furthermore, both the top-to-the-SE thrusting and top-to-the-NW normal faulting along the southeast and northwest boundaries of the complexes occurred under similar greenschist-facies metamorphic conditions (Airaghi et al., 2017b;[START_REF] Xue | Mesozoic crustal thickening of the Longmenshan belt (NE Tibet, China) by imbrication of basement slices: insights from structural analysis, petrofabric and magnetic fabric studies, and gravity modeling: Tectonics[END_REF] Numerical modeling of co-seismic slip vectors vs. fault angles indicates that the latest high-angle listric Yingxiu-Beichuan Fault might have facilitated the vertical imbrication of the Pengguan complex with no need of the SE-ward thrusting and loading [START_REF] Feng | Deep crustal deformation of the Longmen Shan, eastern margin of the Tibetan Plateau, from seismic reflection survey and finite element modelling[END_REF]. This feature explains the absence of a rejuvenated foreland basin contemporary to the 27-25 Ma basement imbrication. Nevertheless, the 27-25

Ma basement slice imbrication of the Pengguan complex is compatible with its 30-25 Ma fast cooling (Figure 12) (Wang et al., 2012a).

In the map view, two amphibolite facies metamorphic zones wrap the Xuelongbao and Danba complexes (Figure 13a). Both amphibolite facies metamorphic zones reached peak conditions at ca. 210-180 Ma post to the late Triassic orogeny (Figure 12) [START_REF] Dirks | Tectonic evolution of the NE margin of the Tibetan Plateau; evidence from the central Longmen Mountains, Sichuan Province, China[END_REF]Huang et al., 2003a;[START_REF] Weller | Quantifying Barrovian metamorphism in the Danba Structural Culmination of eastern Tibet[END_REF]Airaghi et al., 2018a).

Gravity anomaly indicated that the Xuelongbao complex probably extends northeastwards beneath and parallel to the amphibolite facies metamorphic rocks [START_REF] Xue | Mesozoic crustal thickening of the Longmenshan belt (NE Tibet, China) by imbrication of basement slices: insights from structural analysis, petrofabric and magnetic fabric studies, and gravity modeling: Tectonics[END_REF]. Coincidently, the Wenchuan-Xuelongbao and Danba amphibolite facies metamorphic zones experienced rapid exhumation and cooling during the late Jurassic to early Cretaceous (CX12 and CX33 in this work) that is coeval with the aforementioned late Jurassic-early Cretaceous basement-slice-imbrication (Figure 13b) [START_REF] Arne | Differential exhumation in response to episodic thrusting along the eastern margin of the Tibetan Plateau: tectonophysics[END_REF]Huang et al., 2003a;[START_REF] Tian | Late Cretaceous earliest Paleogene deformation in the Longmen Shan fold and thrust belt, eastern Tibetan Plateau margin: Pre Cenozoic thickened crust?: Tectonics[END_REF]Airaghi et al., 2018a).

Both gravity inversion [START_REF] Xue | Mesozoic crustal thickening of the Longmenshan belt (NE Tibet, China) by imbrication of basement slices: insights from structural analysis, petrofabric and magnetic fabric studies, and gravity modeling: Tectonics[END_REF] and seismic reflection profiles [START_REF] Guo | Imaging the crustal structure beneath the eastern Tibetan Plateau and implications for the uplift of the Longmen Shan range[END_REF] revealed a belt of basement-slice hidden right beneath a Mesozoic Laojungou granite (224 ± 5 Ma;[START_REF] Zhao | Mesozoic granitoids in Eastern Songpan-Garze: Geochemistry, petrogenesis and tectonic implications[END_REF] within the east Songpan-Ganzi Terrane (Figure 13a). The Laojungou granite recorded rapid exhumation and cooling during the late Jurassic to early Cretaceous as well (Figure 13b) [START_REF] Yuan | The uplift and cooling histories of Laojungou granite in the western margin of the central Longmen Mountain[END_REF].

Considering the spatial-cooling-imbricating relationships between the Laojungou granite, amphibolite facies metamorphic zones and basement-slices, we interpret that the Laojungou granite and amphibolite facies metamorphic rocks were popped up by a basement-slice-imbrication during the late Jurassic-early Cretaceous. The basementslice-imbrication was responsible for localized fast cooling and exhumation of the amphibolite-facies metamorphic rocks. However, the Wenchuan-Xuelongbao metamorphic sedimentary cover does not show fast cooling during the late Cretaceousearliest Paleocene and at ca. 27-25 Ma comparing to the basement-slice-imbrication (Figure 13b), which needs further work.

Mesozoic tectonic evolution of the LMTB

The Mesozoic tectonic evolution of the LMTB can be summarized as follows without consideration of the top-to-the-N/NW thrusting in the Northwestern Zone.

Namely, the Songpan-Ganzi Terrane subducted to the north along the Anymaqen Suture Zone and/or to the southwest along the Jinshajiang Suture Zone, which resulted in the top-to-the-S shearing of the thick Triassic flysch cover along a basal decollement layer [START_REF] Mattauer | Le chaine Triassique de Songpan-Garze (Ouest Sichuan et est Tibet): Une chaine de plissement-decollement sur marge passive[END_REF][START_REF] Calassou | Etude tectonique d'une chaîne de décollement. A-Tectonique triasique et tertiaire de la chaîne de Songpan Garzê[END_REF] and associated sinistral strike-slip to the southeastward thrusting in the LMTB [START_REF] Xu | Orogenic process of the Songpan-Ganzé Orogenic belt of China[END_REF][START_REF] Dirks | Tectonic evolution of the NE margin of the Tibetan Plateau; evidence from the central Longmen Mountains, Sichuan Province, China[END_REF][START_REF] Burchfiel | Tectonics of the Longmen Shan and Adjacent Regions[END_REF][START_REF] Worley | Deformation partitioning and foliation reactivation during transpressional orogenesis, an example from the Central Longmen Shan[END_REF][START_REF] Roger | Timing of granite emplacement and cooling in the Songpan-Garzê Fold Belt (eastern Tibetan Plateau) with tectonic implications[END_REF][START_REF] Harrowfield | Indosinian deformation of the Songpan Garze Fold Belt, northeast Tibetan Plateau[END_REF].

The amphibolite facies metamorphism in the Wenchuan-Xuelongbao area reached the peak pressure of > 11 ± 2 kbar at ca. 200 Ma (Airaghi et al., 2018b) required a sedimentary cover more than 30 km in thickness. However, high-resolution seismic reflection and gravity modeling profiles revealed a ~20 km thickness cover of the Songpan-Ganzi Terrane [START_REF] Guo | Imaging the crustal structure beneath the eastern Tibetan Plateau and implications for the uplift of the Longmen Shan range[END_REF][START_REF] Xue | Mesozoic crustal thickening of the Longmenshan belt (NE Tibet, China) by imbrication of basement slices: insights from structural analysis, petrofabric and magnetic fabric studies, and gravity modeling: Tectonics[END_REF] that is far thinner than the required ~30 km thickness.

In contrast, [START_REF] Sengör | The Cimmeride Orogenic System and the Tectonics of Eurasia[END_REF] and [START_REF] Luo | The dynamical model of the lithospheric evolution in Longmenshan Orogenic Belt[END_REF] regarded the LMTB as an early Mesozoic intracontinental subduction zone but lack details of structural observation. Considering the pervasive top-to-the-N/NW thrusting in the Northwestern Zone and top-to-the-SE thrusting in the Southeastern Zone, we thus prefer an early Mesozoic intracontinental subduction along the LMTB.

In this work, the Main Central Boundary, devoid of ophiolitic rocks separating the Southeastern Zone to the SE from the Northwestern Zone to the NW, does not represent an ophiolitic suture zone, but a tectonic "scar" corresponding to the place where the Yangtze basement was underthrusted. Like the Chenzhou-Linwu Fault in the Xuefengshan Intracontinental Orogen (South China) where the South China Block underthrusted (Chu et al., 2012a;Chu et al., 2012b). Along the LMTB, the underthrusting was coeval with important burial (> 11 ± 2 kbar) and the double-vergent thrusting in both sides of the Main Central Boundary. The basement underthrusting is also supported by the blueschist with intraplate affinity reported along the Qingchuan-Pingwu Fault [START_REF] Wei | Metamorphism and its geological significance of the Bikou Group in the Shanxi-Gansu-Sichuan border region[END_REF][START_REF] Xu | Phase equilibria and geochronology of Triassic blueschists in the Bikou terrane and Mesozoic tectonic evolution of the northwestern margin of the Yangtze Block (SW China[END_REF]. A new tectonic scenario is proposed here (Figure 14). In the pre-middle Triassic times, the Longmenshan area along the western Yangtze margin was a failed rift evidenced by radiolarian chert, abyssal deposits, slump structures [START_REF] Long | Several questions of geochronic evolution in the mid-northeast segment of lLongmenshan Mountains[END_REF][START_REF] Luo | The dynamical model of the lithospheric evolution in Longmenshan Orogenic Belt[END_REF][START_REF] Chang | Geology and Tectonics of the Songpan-Ganzi Fold Belt[END_REF][START_REF] Li | Composition, structural characteristics and evolution of back-Longmenshan orogen (north section) in the northwest margin of Yangtze block[END_REF] and seismic profile [START_REF] Jia | Longmen Shan foldthrust belt and its relation to the western Sichuan Basin in central China: New insights from hydrocarbon exploration[END_REF]. Accompany with the pre-middle Triassic rifting, a series of NW-dipping syndepositional normal faults developed in the Longmenshan area that dissected the western Yangtze basement to slices, and the lower crust thinned significantly that resulted in a local "weak zone" in the lower crust (Figure 14a).

During the late Triassic, the Yangtze basement underthrusting along the "weak zone" accommodated by inversion of the normal faults, which can be linked to the far-field effect of the Paleotethys' closure (Figure 14b) [START_REF] Luo | The dynamical model of the lithospheric evolution in Longmenshan Orogenic Belt[END_REF][START_REF] Chen | Emplacement of the Longmen Shan Thrust-Nappe Belt along the eastern margin of the Tibetan Plateau[END_REF][START_REF] Meng | Mesozoic sedimentary evolution of the northwest Sichuan basin: Implication for continued clockwise rotation of the South China block[END_REF][START_REF] Yan | Late Triassic tectonic inversion in the upper Yangtze Block: Insights from detrital zircon U Pb geochronology from south western Sichuan Basin[END_REF]. The sedimentary cover was simultaneously shortened by a top-to-the-SE thrusting and a top-to-the-NW thrusting associated with the main D1 event (Figure 14b). The domain of the top-to-the-SE thrusting in the Southeastern Zone is larger and more extensive than the top-to-the-NW thrusting in the Northwestern Zone. Similar structures were developed in the north Himalaya where a large domain experienced a top-to-the-S thrusting, compared to a small domain with a top-to-the-N thrusting due to the northward subduction of the India plate [START_REF] Dewey | The Tectonic Evolution of the Tibetan Plateau[END_REF][START_REF] Harrison | [END_REF]. We thus favor an NW-ward basement underthrusting along the LMTB (Figure 14b). The large amount of rift rocks transported southeastwards and overlain the autochthonous foreland basin. In addition, a large volume of rocks was underthrusted to crustal depth deeper than 30 km and metamorphosed, as documented by the ca. 210-180 Ma amphibolite-facies metamorphic zone that metamorphosed at a peak pressure of > 11±2 kbar (Figure 12) [START_REF] Weller | Quantifying Barrovian metamorphism in the Danba Structural Culmination of eastern Tibet[END_REF]Airaghi et al., 2018a;Airaghi et al., 2018b;Airaghi et al., 2018c).

During periods of the late Jurassic-early Cretaceous, late Cretaceous-earliest Paleogene, the accretion of Gondwana-derived Terranes to the southern Eurasia plate had resulted in episodic compression in the Longmenshan area [START_REF] Dewey | The Tectonic Evolution of the Tibetan Plateau[END_REF]Xu et al., 1992). We cannot rule out the possible contribution of the 150-140 Ma and 120-105 Ma compression resulted from the westward subduction of the Paleo-Pacific plate subduction [START_REF] Chu | Cretaceous Episodic Extension in the South China Block, East Asia: Evidence From the Yuechengling Massif of Central South China: Tectonics[END_REF]. Under the regional episodic compression, basementslices represented by the Neoproterozoic complexes along the weak zone of the Yangtze Block, including the Xuelongbao, Moutuo, and Danba complexes, episodically imbricated southeastwards along the inherited faults. Basement-sliceimbrication then exhumed the basement complexes and the amphibolite facies metamorphic rocks as documented by the synchronously fast cooling of the metasediments and the basement complexes (Figures 12 and14d) [START_REF] Yuan | The uplift and cooling histories of Laojungou granite in the western margin of the central Longmen Mountain[END_REF][START_REF] Arne | Differential exhumation in response to episodic thrusting along the eastern margin of the Tibetan Plateau: tectonophysics[END_REF]Huang et al., 2003b;[START_REF] Weller | Quantifying Barrovian metamorphism in the Danba Structural Culmination of eastern Tibet[END_REF][START_REF] Tian | Late Cretaceous earliest Paleogene deformation in the Longmen Shan fold and thrust belt, eastern Tibetan Plateau margin: Pre Cenozoic thickened crust?: Tectonics[END_REF]. Basement-sliceimbrication also reworked the late Triassic allochthonous-autochthonous LMTB into four subdivisions (Figure 14d).

Conclusions

Based on structural analysis and previous geophysical profile, we have delineated a Main Central Boundary that separates the Longmenshan Thrust Belt into a Southeastern Zone to the southeast and a Northwestern Zone to the northwest.

On the basis of different lithologies, increasing strain intensity and metamorphic grade, the Southeastern Zone is revised into four subdivisions, namely, the Autochthon, Para-autochthon, Allochthon, and Inner Metamorphic Core. T1: Anxian-Guanxian Fault; T2: Yingxiu-Beichuan Fault; T3: Wenchuan-Maoxian Fault; T4: Qingchuan-Pingwu Fault; JZD: Jiaoziding complex; PG: Pengguan complex; XLB: Xuelongbao complex; BX: Baoxing complex; KD: Kangding complex; DB: Danba complex; MT: Moutuo complex; WL: Wulong complex; RLG: Rilonguan granite; LJG: Laojungou granite. 2). Subaerial structures of these cross-sections are based on our field mapping and the deep structures are based on seismic reflection data of [START_REF] Jia | Longmen Shan foldthrust belt and its relation to the western Sichuan Basin in central China: New insights from hydrocarbon exploration[END_REF] and [START_REF] Guo | Imaging the crustal structure beneath the eastern Tibetan Plateau and implications for the uplift of the Longmen Shan range[END_REF]. The black dashed lines represent speculated folds. (e) Flower structure revealed by the seismic reflection profile processed by [START_REF] Jia | Longmen Shan foldthrust belt and its relation to the western Sichuan Basin in central China: New insights from hydrocarbon exploration[END_REF](Location is marked in Figure 4d). MCB: Main Central Boundary; AGF: Anxian-Guanxian Fault; YBF: Yingxiu-Beichuan Fault; WMF: Wenchuan-Maoxian Fault; QCF: Qingchuan-Pingwu Fault. Other symbols and acronyms are the same as in Figure 1 -39 Ar age spectra of the dated samples. We carried out a two-aliquot analysis of micas from CX6, CX12, and CX19 to exclude the influence of occasional deviation on the stair-case age spectrum, and the two-aliquot results are labeled with the suffix of A and B (eg., CX6A, CX6B, CX12A, CX12B, CX19A, CX19B). Inverse isochron diagrams are presented in Figure S3. 40 Ar-39 calculation results are summarized in Table 1, and detailed step-heating results are presented in the supporting information Table S2. Wang et al. (2012a). 31. [START_REF] Hames | Laser 40Ar/39Ar dating of Cenozoic, greenschist-facies shear zones, Longmen Shan[END_REF].

Symbols and acronyms are the same as in Figure 2. Compiled cooling histories for the Wenchuan-Xuelongbao amphibolite facies metamorphic belt (WXAB, bold dashed line) (Airaghi et al., 2018a), the Danba amphibolite facies metamorphic belt (DBAB), and migmatite zone (DBM, thin dashed line) (Huang et al., 2003a), and the Mesozoic Laojungou granite (LJG, bold gray line) [START_REF] Yuan | The uplift and cooling histories of Laojungou granite in the western margin of the central Longmen Mountain[END_REF]. Gray frames show the three episodes of basement-slice-imbrication at late Jurassic-early Cretaceous, late Cretaceous-Earliest Paleocene, ca.

Weighted mean age 224. APs233-231 9 AB73-58 7, 12 AM200-183 3 AM237±0.9 3 AM214±0.9 2A S 1 6 6 ± 1 .9

M a 1

3. 4

 4 Top-to-the-NW normal faulting along the northwest boundary of the basement complexes (D2) Most of the Neoproterozoic basement complexes in the Southeastern Zone are devoid of ductile deformation but characterized by widespread subhorizontal joints

4. 1

 1 Analytical methods 4.1.1 Zircon SIMS U-Pb dating Zircon concentrates were extracted using conventional crushing and separation techniques. After set in epoxy resin mounts, mineral grains were polished to their cores. Cathodoluminescence (CL) imaging was performed to observe their internal structure and find suitable positions for spot analyses. Geochronological data were acquired at the State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences (IGGCAS).

4. 1 . 2 40

 12 Ar-39 Ar dating Micas were separated from eight micaschist, gneiss, and mylonite samples for 40 Ar/ 39 Ar dating by step-heating technique. Minerals were irradiated and measured in the Ar-Ar Laboratory, School of Earth Sciences of the University of Melbourne. Handpicked grains were cleaned in de-ionized water and acetone. Samples were weighted and loaded into aluminum packets for irradiation. The packets were placed in a silicate glass tube (Can UM#80), interleaved with packets containing the flux monitor Mount Dromedary Biotite (Age = 99.125 ± 0.076 Ma)

  involved in top-to-the-SE thrusting, we interpret the dates of 148 Ma and 124 Ma as ages to SE-ward thrusting and rapid exhumation. Samples of CX2 and CX25 collected from the northwest boundary of the Pengguan and Xuelongbao complexes were involved in top-to-the-NW normal faulting, which may imply that the top-to-the-NW normal faulting occurred at ca. 27 Ma along the northwest boundary of the Pengguan complex, and at ca. 80 Ma along the northwest boundary of the Xuelongbao complex, respectively.

  . Conjugated kinematics of boundary SE-thrusting and NW-normal faulting characterized by similar metamorphic conditions and synchronous timing let us propose that the Xuelongbao, Pengguan, Baoxing and Wulong complexes were episodically imbricated to the southeast as basement-slices during late Jurassic-early Cretaceous and late Cretaceousearliest Paleocene.

  The detailed structural analysis allows us recognizing the multiphase deformation of the LMTB. A main D1 double-vergent thrusting event, characterized by the D1E top-tothe-SE thrusting in the Southeastern Zone and the D1W top-to-the-NW/N thrusting in the Northwestern Zone, has been recognized. The D1E and D1W events were synchronously occurred corresponding to a late Triassic intracontinental underthrusting along a "weak zone" of the LMTB. A D2 top-to-the-NW normal faulting that cuts the D1E top-to-the-SE thrusting has been recognized along the NW boundary of the basement complexes in the Southeastern Zone. Geochronological constraints obtained from this study and the previously published ones indicate that the main D1 event occurred at ca. 224-219 Ma and the D2 was episodically activated at ca. 166-160 Ma, 141-120 Ma, 81-47 Ma, and 27-25 Ma. The episodically and synchronously activated top-to-the-NW normal faulting and top-tothe-SE thrusting along the northwest and SE boundaries of the basement complexes let us propose that the basement-slices episodically imbricated to the SE during the late Jurassic-early Cretaceous and late Cretaceous-earliest Paleocene. The basement-slice imbrication exhumed the D1-related amphibolite facies rocks to the subaerial and reworked allochthonous-autochthonous LMTB into a four subdivision one.

Figure captions Figure 1 .

 captions1 Figure captionsFigure 1. (a) Simplified tectonic sketch of China. QLDB: Qingling-Dabie Orogenic Belt; EKL-ANMQS: East Kunlun-Anymaqen Suture Zone; JSZ: Jinshajiang Suture Zone; SGT: Songpan-Ganzi Terrane; LMTB: Longmenshan Thrust Belt; ICB: Indochina Block; (b) Simplified geological map of the Longmenshan Thrust Belt, modified after BGMRSP (1991),[START_REF] Burchfiel | Tectonics of the Longmen Shan and Adjacent Regions[END_REF] and[START_REF] Xu | Uplift of the Longmen Shan range and the Wenchuan earthquake: Episodes[END_REF].

Figure 2 .

 2 Figure 2. Geological map of the north and middle parts of the LMTB and its adjacent area, modified after BGMRSP (1991) and[START_REF] Xu | Uplift of the Longmen Shan range and the Wenchuan earthquake: Episodes[END_REF]. MP: Mupi pluton. Symbols and acronyms are the same as in Figure1.

Figure 3 .

 3 Figure 3. (a) Structural division, field elements including foliation, lineation, and kinematics of the LMTB. (b-d) π-circle projection of the bedding (S0) in the Para-autochthon from south to north. All field elements are shown in a lower hemisphere equal-area projection. Symbols and acronyms are the same as in Figure 1.

Figure 4 .

 4 Figure 4. (a-d) Geological cross-sections perpendicular to the LMTB. Purple solid lines represent the foliations attitude (Locations are marked in Figure 2). Subaerial structures of these cross-sections are

Figure 5 .

 5 Figure 5. Rock characteristics and deformation style of the Main Central Boundary. (a) Intensely deformed argillaceous limestone characterized by a vertical foliation [31°34.833' N, 103°24.041' E]. (b) Graphite-bearing pelite was intensely foliated to shale with vertical attitude [31°35.916' N, 103°24.008' E]. (c) Foliated thin-bedded limestone and quartz ribbon interlayer and rootless intra-folial folds coexist with vertical foliation [31°48.403' N, 103°44.080' E]. Rootless folds in figures b-c implying significant shortening. Locations are presented in Figure 3a.

Figure 6 .

 6 Figure 6. Structural features related to the Main Deformation (D1E) in the Southeastern Zone. In the Inner Metamorphic Core: (a) Lineation formed by aligned biotite plunging N335° by 30° on the Sdipping foliation [31°30.359' N, 103°34.123' E]. (b) Asymmetric pyrite pressure shadow indicating a top-to-the-SE thrusting [31°42.210' N, 103°51.067' E]. (c) Sigma-shaped feldspar porphyroclast in paragneiss indicating a top-to-the-SE thrusting [31°05.193' N, 103°18.730' E]. (d) S/C fabric observed under Cross Polarized Light (CPL) indicating a top-to-the-SE thrusting. In the Allochthon: (e) Sigmoid pelitic lens and "cold" S/C structures developed in the Neoproterozoic Huangshuihe Group suggesting a to the top-to-the-SE thrusting [31°16.195' N, 103°46.747' E]. (f) Sigma-shaped sandy lens in Silurian pelitic schist within the nappe indicating a top-to-the-SE thrusting [32°04.577' N, 104°40.599' E]. (g) At the rear of the klippe, thick-bedded limestone thrust to the SE along an NW-dipping reverse fault [31°11.530' N, 103°45.929' E]. In the Para-autochthon: (h) Sigmoid lens and "cold" S/C fabrics indicating a top-to-the-SE thrusting [30°55.403' N, 103°20.495' E]. Figure locations are marked in Figure 3a. X and Z axes are marked on the figures. Mineral abbreviations are after[START_REF] Kretz | Symbols for rock-forming minerals[END_REF].

Figure 7 .

 7 Figure 7. Structural features related to the Main Deformation (D1W) in the Northwestern Zone. In the northern Northwestern Zone: (a) Aligned calcite, fine-grained white mica, and oxide-pods define lineation plunging to the south on the S-dipping foliation [32°30.892' N, 104°08.178' E]. (b) Sigmashaped pebbles in a meta-conglomerate indicating a top-to-the-N sense of shear [32°53.562' N, 105°04.820' E]. (c) Deflected quartz ribbon observed under CPL indicating a top-to-the-N thrusting. (d) Asymmetric feldspar porphyroclast wrapped by deflected white mica suggesting a top-to-the-N thrusting. (e) Northward verging chevron fold associated with meter-scale reverse fault indicating a top-to-the-N

Figure 8 .

 8 Figure 8. Features of the syn-tectonic Mupi pluton in the Northwestern Zone [32°31.445' N, 104°32.456' E]. (a) Magmatic fabric of the Mupi pluton with euhedral feldspar randomly distributed, about 30 meters north to the south margin of the Mupi pluton. (b) Quartz + feldspar veins parallel to the shear foliation, about 10-15 meters north to the pluton south margin. (c) Strong mylonitisation at the south margin of the pluton shares similar structural geometry and kinematics with the country rock. (d) Asymmetric feldspar porphyroclasts, synthetic offset of feldspar fragments along shear band in XZ plane indicating a top-tothe-N thrusting. (c) Panorama of thin section under CPL of the Mupi pluton, the feldspar brittle cracks (marked by red arrow) infilled by quartz subparallel to the shear foliation. X and Z axes are marked on the figures. Figure locations are marked in Figure 3a. Mineral abbreviations are after[START_REF] Kretz | Symbols for rock-forming minerals[END_REF].

Figure 9 .

 9 Figure 9. Structural features related to the top-to-the-NW normal faulting along the northwest boundary of the Neoproterozoic complexes. (a) At the northwest boundary of the Xuelongbao complex, sigmoid feldspar phenocrysts associated with S/C fabric indicating a top-to-the-NW normal faulting [31°19.683' N, 103°23.874' E]. At the northwest boundary of the Pengguan complex: (b) Granitic mylonite having foliation dips to the NW [31°22.931' N, 103°30.364' E]. (c-d) Mica fish in the quartz schist (PPL) and σ-type porphyroclast (CPL) wrapped by deflected micas indicating a top-to-the-NW normal faulting. (e) Shear-band (white dashed line) distributes from the upper-right corner to the lower-left corner that cuts the whole thin section indicate a top-to-the-NW normal faulting, CPL. Sigmoid porphyroclast (e), incipient shear band inclining to the SE (f), and white mica shear-band fabric (g) indicate a top-to-the-SE thrusting related to D1E. All of them are cut by the white shear band related to the D2 top-to-the-NW normal faulting, CPL. Locations are marked in Figure 3a. X and Z axes are marked on the figures.Mineral abbreviations are after[START_REF] Kretz | Symbols for rock-forming minerals[END_REF].

Figure 10 .

 10 Figure 10. (a) CL images of the representative zircons from the Mupi pluton. The scale bar represents

Figure 12 .

 12 Figure 12. Geochronological constraints compiled from published and this study. Ages are shown by the method used to determine the age (A = Ar/Ar; K = K/Ar; U = U/Pb; F = Fission track) and mineral used (Am = Amphibole; B = biotite; M = Muscovite; Z = zircon; Mn = monazite; I= illite; S = Sericite; Wm = white mica). 1. Li (2009); 2. Yan et al. (2018); 3. Yan et al. (2011); 4. Arne et al. (1997); 5. Airaghi et al. (2017b); 6. Airaghi et al. (2018a); 7. Airaghi et al. (2018b); 8. Airaghi et al. (2018c); 9. Zheng et al. (2016); 10. Yan et al. (2008); 11. Zheng et al. (2014); 12. Tian et al. (2016); 13. Huang et al. (2003a); 14. Weller et al. (2013); 15. Xu et al. (2008); 16. Zhou et al. (2008); 17. Wallis et al. (2003); 18. Zhou et al. (2002); 19. Xia (1993); 20. Xu et al. (1992); 21. Yuan et al. (1991); 22. Hu et al. (2005); 23. Zhao (2007); 24. Kirby et al. (2002); 25. Roger et al. (2004); 26. Zhang et al. (2006a); 27. Yuan et al. (2010); 28. BGMRSP (1991); 29. Li et al. (2013c); 30.Wang et al. (2012a). 31.[START_REF] Hames | Laser 40Ar/39Ar dating of Cenozoic, greenschist-facies shear zones, Longmen Shan[END_REF].

Figure 13 .

 13 Figure 13. (a) Spatial relationship between the amphibolite facies metamorphic zones and the basement complexes along the peripheries of the Songpan-Ganzi Terrane. Isograd lines are presented in color dashed lines accompany relating mineral abbreviations. Mineral abbreviations are after Kretz (1983) except Liq representing the magmatic zone. Symbols and acronyms are the same as in Figure 12. (b)

  27-25 Ma. The Wenchuan-Xuelongbao and the Danba amphibolite facies metamorphic belt and the Laojungou granite experienced a rapid exhumation and cooling during the late Jurassic to the late Cretaceous, and 27-25 Ma.

Figure 14 .

 14 Figure 14. A possible Mesozoic tectonic evolutionary scenario of the LMTB and adjacent area. (a) Premiddle Triassic rifting of the Yangtze Block resulted in crustal thinning and a potential "weak zone" in the future Longmenshan area. (b) During the late Triassic, NW-ward Yangtze basement underthrusting along the "weak zone" resulted in extensive shortening, inversion of syn-depositional normal Fault. The sedimentary rocks were buried to a crustal depth deeper than 30 km. NW-and SE-ward thrusting, and associated nappe belt formed at this time. (c) During the late Triassic to early Jurassic, the underthrusted rocks were metamorphosed. Gradient color from green to dark blue in the Inner Metamorphic Core represents show the low-to high-metamorphic grade. (d) late Jurassic-Miocene, basement-slice episodically imbricated to the SE that exhumed the amphibolite facies metamorphic rocks to the subaerial and reworked the LMTB. Symbols and acronyms are the same as in Figure 2.

Table 1 .

 1 Fig.2

  Fig. 10

Fig. 6d

  Fig.6d

  Fig.6g

Fig. 7f

  Fig. 7f

Fig. 7c

  Fig.7c

Fig. 7d

  Fig. 7d

Fig. 7a

  Fig.7a

  Fig. 7b

Fig. 9c

  Fig. 9c

  Fig.9a

  Figure3

  Figure10

  Figure13

  Detachment are scattered at ca. 166-159 Ma (north Danba, northwest Pengguan, northwest Jiaoziding), ca. 122-120 Ma (north Danba), ca.81-47 Ma (north Danba, northwest Wulong) and ca. 27-25 Ma (northwest Pengguan; Figure

CX44 White mica CX25 White mica CX6A White mica CX6B White mica CX33 White mica CX12A Fine-grained white mica CX12B Fine-grained white mica CX18 Fine-grained white mica CX19A Fine-grained white mica CX19B Fine-grained white mica CX2 White mica

  1 ± 1.7 Ma MSWD = 8.5, 81.66% of the 39 Ar Weighted mean age 124 ± 1.9 Ma MSWD = 26, 49.57% of the 39 Ar Weighted mean age 79.7 ± 2.7 Ma MSWD = 62, 79.74% of the 39 Ar Weighted mean age 126.3 ± 3.3 Ma MSWD = 52, 52.42% of the 39 Ar Plateau age 148.2 ± 1.2 Ma MSWD = 1.77, 60.1% of the 39 Ar Total gas age 82.6 ± 1.2 Ma Total gas age 221.7 ± 3.5 Ma Total gas age 148.5 ± 1.2 Ma Total gas age 126.5 ± 1.6 Ma Total gas age 127.6 ± 1.7 Ma Total gas age 29.5 ± 2.5 Ma Weighted mean age 26.23 ± 0.99 Ma MSWD = 16, 53.63% of the 39 Ar

	146.5 ± 2.7 Ma
	64.7 ± 0.3 Ma
	119.0 ± 1.3 Ma	111.1 ± 0.2 Ma
	72.2 ± 1.4 Ma
		50.0 ± 0.1 Ma
	133.7 ± 3.3 Ma
		139.3 ± 1.1 Ma
	59.0 ± 0.3 Ma	58.4 ± 0.4 Ma

Acknowledgement

This work is supported by the National Nature Science Foundation of China (91855212, 91755205 and 41802219) and the National Key R&D Program of China (Grant Numbers 2016YFC0600401, 2016YFC0600102) as well as French LABEX VOLTAIRE (ANR-10-LABX-100-01), and EQUIPEX PLANET (ANR-11-EQPX-0036). Two anonymous reviewers are sincerely thanked for their elaborative reviews and pertinent comments. The geochronological data including SIMS U-Pb dating on zircons (Table S1) and 40 Ar/ 39 Ar dating on micas (Table S2) for this paper are available in the supplementary materials.

 S2) and almost all steps are clustered at the 39 Ar/ 40 Ar axis that do not define a real inverse isochron line. The initial 40 Ar/ 36 Ar ratio originate from the inverse isochron line is meaningless as well. Most of the step-ages calculated both by using initial 40 Ar/ 36 of 298.56 [START_REF] Lee | A redetermination of the isotopic abundances of atmospheric Ar[END_REF] and from the "inverse isochron", show nearly no differences (Table S2). Thus, the inverse isochron ages, though identical to their weighted mean ages or plateau ages, are meaningless and have not been discussed in the main text.

Table S1. Detailed SIMS U-Pb zircon results of sample CX48 Table S2. 40 Ar/ 39 Ar VG3600 Furnace Step-Heating analytical results