Tectonic evolution and high-pressure rock exhumation in the Qiangtang terrane, central Tibet

Abstract

Abstract. Conflicting interpretations of the > 500 km long, east-west-trending Qiangtang metamorphic belt have led to very different and contradicting models for the PermoTriassic tectonic evolution of central Tibet. We define two metamorphic events, one that only affected pre-Ordovician basement rocks and one subduction-related Triassic highpressure metamorphism event. Detailed mapping and structural analysis allowed us to define three main units that were juxtaposed due to collision of the north and south Qiangtang terranes after closure of the Ordovician-Triassic ocean that separated them. The base is formed by the Precambrian-Carboniferous basement, followed by nonmetamorphic ophiolitic mélange containing mafic rocks that range in age from the Ordovician to Middle Triassic. The top of the sequence is formed by strongly deformed sedimentary mélange that contains up to > 10 km size rafts of both unmetamorphosed Permian sediments and highpressure blueschists. We propose that the high-pressure rocks were exhumed from underneath the south Qiangtang terrane in an extensional setting caused by the pull of the northward subducting slab of the Shuanghu-Tethys. Highpressure rocks, sedimentary mélange and margin sediments were thrust on top of the ophiolitic mélange that was scraped off the subducting plate. Both units were subsequently thrust on top of the south Qiantang terrane continental basement. Onset of Late Triassic sedimentation marked the end of the amalgamation of both Qiangtang terranes and the beginning of spreading between Qiantang and north Lhasa to the south, leading to the deposition of thick flysch deposits in the Jurassic.

Early Cretaceous exhumation of the Qiangtang Terrane during collision with the Lhasa Terrane, Central Tibet

“…This study complements previous work (Zhao, Bons, Wang, Liu, & Zheng, ; Zhao, Bons, Wang, Soesoo, & Liu, ) with new structural data and zircon (U–Th)/He (ZHe) ages that indicate Early Cretaceous exhumation of the SQT, which we relate to closure of the Bangong Ocean leading to crustal shortening and thickening in the SQT. Our results have implications for the mode of the Lhasa–SQT collision and the lead‐up to the formation of the Tibetan Plateau.…”

“…The study area is located near the town of Rongma, in the Qiangtang Culmination south of the Longmu Co‐Shuanghu suture (LSS), which separates the North and South Qiangtang Terranes (Figure ) (Li et al., ; Zhai, Jahn, Zhang, Wang, & Su, ; Zhao et al., , ). Palaeozoic basement rocks and a Triassic subduction mélange complex are exposed in the Qiangtang culmination.…”

“…(a) In situ model of Zhao et al. (), in which HP rocks (blue) formed in a divergent double subduction system of the Palaeo‐Tethys, where the southern, short slab was pulled up. HP rocks and supracrustal rocks from that southern slab were scraped off and later thrust onto the South Qiangtang Terrane ( SQT ) as the Palaeo‐Tethys finally subducted underneath the North Qiangtang Terrane ( NQT ).…”

Sedimentary Evolution and Provenance of the late Permian‐middle Triassic Raggyorcaka Deposits in North Qiangtang (Tibet, Western China): Evidence for a Forearc Basin of the Longmu Co‐Shuanghu Tethys Ocean

“…Two major hypotheses have been proposed to explain the tectonic origin of the CQMB: (1) The CQMB was an allochthonous complex that originated from the ~200 km southward underthrust of the Songpan‐Ganzi flysch/Paleozoic arc terrane beneath a unified Qiangtang block and was subsequently exhumed by extensional detachment faults in the late Triassic‐early Jurassic (Yin & Harrison, ; Kapp et al, ; Pullen et al, , ; Pullen & Kapp, ), and (2) the CQMB was an accretionary complex of an in situ suture attributed to the northward subduction of the Longmu Co‐Shuanghu Tethys Ocean beneath the NQB in the late Paleozoic‐middle Triassic (Li, ; Li et al, , , ; Li, Zhai, Dong, et al, ; Li, Zhai, Chen, et al, ; Li, Chen, et al, ; Li, Huang, et al, ; Liang et al, , ; Wang et al, , ; Zhang, Cai, et al, , Zhang, Zhang, et al, ; Zhai, Zhang, et al, ; Zhai, Jahn, et al, ; Zhai, Jahn, Wang, et al, , ; Zhang, Cai, et al, ; Zhang, Zhang, et al, ). The allochthonous complex model was the first model indicating that the high‐pressure and low‐temperature (HP‐LT) metamorphic rocks in the CQMB were exhumed from mantle depths by a crustal‐scale detachment fault, and this finding was later corroborated by structural mappings (Liang et al, ; Zhao et al, ). In the in situ suture model, a long‐lived early Paleozoic‐Triassic ocean or the main branch of the Paleo‐Tethys Ocean separated the NQB, which showed a Cathaysian affinity from the SQB, which showed a Gondwana affinity (Li, ; Li et al, ; Li, Zhai, Dong, et al, ; Li, Zhai, Chen, et al, ; Li, Chen, et al, ; Li, Huang, et al, ; Li, Zhai, Chen, et al, , ; Liu et al, ).…”

“…Numerous studies have investigated the CQMB, especially the Permian (283–259 Ma) and Triassic (244–211 Ma) HP‐LT metamorphic rocks (Figure ; Deng et al, ; Kapp et al, ; Li et al, ; Li, Zhai, Dong, et al, ; Li, Zhai, Chen, et al, ; Liang et al, , ; Yin & Harrison, ; Zhao et al, , ; Zhang et al, ) and Paleozoic‐early Triassic ophiolites (Li et al, ; Li, Chen, et al, ; Li, Huang, et al, , ; Zhu et al, ; Wang, Pan, et al, ; Wu et al, ; Zhai, Jahn, Wang, et al, , ; Zhang et al, ). In contrast, the southern margin of the NQB, especially the late Paleozoic‐Triassic sedimentary evolution and its tectonic relationship with the CQMB, remains poorly studied.…”