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 Permo–Triassic tectonic evolution of central Tibet. We define two metamorphic events, one that only affected pre-Ordovician basement rocks and one subduction-related Triassic high-pressure 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 non-metamorphic 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 high-pressure 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. High-pressure 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.
Highlights
Highpressure rocks, sedimentary mélange and margin sediments were thrust on top of the ophiolitic mélange that was scraped off the subducting plate
The Tibetan plateau is an amalgamation of terranes that were accreted to the southern margin of Eurasia during Phanerozoic times (Yin and Harrison, 2000)
The two terranes were separated by the Shuanghu suture that closed in Late Triassic times by northward subduction of ocean underneath the north Qiangtang terrane (NQT)
Summary
The Tibetan plateau is an amalgamation of terranes that were accreted to the southern margin of Eurasia during Phanerozoic times (Yin and Harrison, 2000). Triassic subduction mélange and high-pressure rocks that are exposed in the Qiangtang metamorphic belt in central Tibet are interpreted in radically different ways (Kapp et al, 2003b; Li et al, 2006; Zhai et al, 2011a; Liang et al, 2012; Zhao et al, 2014). Li et al (1987) interpreted the blueschists and subduction mélange as the remnants of a Late Triassic suture zone They proposed that the Qiangtang terrane was separated into a northern and a southern terrane by the Paleo-Tethys Ocean. We present the results of detailed mapping and structural reconstruction of the Rongma area in the centre of the Qiangtang metamorphic belt
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