Subduction Reversal in a Divergent Double Subduction Zone Drives the Exhumation of Southern Qiangtang Blueschist‐Bearing Mélange, Central Tibet

Abstract

Ultra) high-pressure (HP) rocks can be exhumed rapidly by subduction reversal or divergent plate motion. Recent studies show that subduction reversal can in particular occur in a divergent double subduction zone when the slab pull of one slab exceeds that of the other, shorter one, which then experiences a net upward pull. This recent hypothesis, first proposed for Triassic HP-rocks exposed in the central Qiangtang mélange belt in central Tibet, can explain the exhumation of (ultra) HP rocks through upward slab movement. However, this model lacks the support of kinematic evidence. In this study, based on the recognition of multiple deformational phases, we analyze the kinematics of the HP-bearing mélange in central Qiangtang. Based on new 40 Ar-39 Ar geochronology data and those collected from the literature, we present a temporal framework for the new observations. We recognize a switch in sense of shear between the prograde (D1) and exhumation (D2-3) paths. The change of shear sense reflects the reversal from downward to upward movement of the oceanic slab below. Early D2 represents the early exhumation stage that caused retrograde metamorphism from eclogite to blueschist facies. No magmatism occurred during this period. Continued exhumation from blueschist facies to greenschist facies resulted in D2-D3 structures. Voluminous igneous activity occurred during this stage. We suggest that subduction reversal in a divergent double subduction zone can best explain the kinematic evolution and temporal framework above. This exhumation model may provide a new perspective on the exhumation mechanism for other HP rocks around the world.

A late Permian–Triassic trench‐slope basin in the Central Qiangtang metamorphic belt, Northern Tibet: Stratigraphy, sedimentology, syndepositional deformation and tectonic implications

“…The genesis of the CQMB has largely been debated under three radically different hypothetical models (Figure 12) (cf. Li et al., 2020; Liang et al., 2020; Pullen & Kapp, 2014): (a) the CQMB was an allochthonous accretionary complex from the Jinshajiang suture zone (JSSZ) to the north that experienced the ca. 200 km southward underthrust beneath a single Qiangtang block in the Triassic (Figure 12a) (Kapp et al., 2003; Pullen & Kapp, 2014; Pullen et al., 2008, 2011; Yin & Harrison, 2000); (b) the CQMB was an in situ accretionary complex that was generated during the northward oceanic subduction of the Longmu Co–Shuanghu Tethys Ocean beneath the NQB (Figure 12c) (Li, 1987; Li et al., 1995, 2006, 2016, 2019, 2020; Liang et al.…”

“…Li et al., 2020; Liang et al., 2020; Pullen & Kapp, 2014): (a) the CQMB was an allochthonous accretionary complex from the Jinshajiang suture zone (JSSZ) to the north that experienced the ca. 200 km southward underthrust beneath a single Qiangtang block in the Triassic (Figure 12a) (Kapp et al., 2003; Pullen & Kapp, 2014; Pullen et al., 2008, 2011; Yin & Harrison, 2000); (b) the CQMB was an in situ accretionary complex that was generated during the northward oceanic subduction of the Longmu Co–Shuanghu Tethys Ocean beneath the NQB (Figure 12c) (Li, 1987; Li et al., 1995, 2006, 2016, 2019, 2020; Liang et al. ,2012, 2017, 2020; Lu et al., 2019; Wang et al., 2009, 2017; Xu et al., 2020; Zhai et al., 2011, 2016; Zhai, Jahn, Wang, et al., 2013; Zhang et al., 2006, 2017); and (c) the CQMB was a composite accretionary complex resulting from the double subduction of the Longmu Co–Shuanghu Tethys Ocean in Central Qiangtang (Figure 12b) (Li et al., 2020; Zhao et al., 2015).…”

“…The >500-km-long and up to 100-km-wide E-W-trending Central Qiangtang metamorphic belt (CQMB) (Figure 1a) is of particular relevance because of the good preservation of the Permo-Triassic subduction zone tectonics and ophiolites of the Palaeo-Tethys Ocean (Figures 1 and 2) (Kapp et al, 2000(Kapp et al, , 2003Li et al, 1995Li et al, , 2006Li et al, , 2016Liang et al, 2012Liang et al, , 2017Liang et al, , 2020Lu et al, 2019;Pullen & Kapp, 2014;Pullen et al, 2008Pullen et al, , 2011Xu et al, 2020;Zhai et al, 2011Zhai et al, , 2016Zhang et al, 2016;Zhao et al, 2014. The previous geological investigations of the CQMB mainly focused on the Permo-Triassic high-pressure and lowtemperature (HP-LT) metamorphic rocks (Deng et al, 2000;Kapp et al, 2003;Li et al, 2006Li et al, , 2008Li et al, , 2020Liang et al, 2012Liang et al, , 2017Zhai et al, 2011) and the tectonic origin of ophiolites (Fan et al, 2017;Li et al, 1995Li et al, , 2016Wu et al, 2010;Zhai et al, 2016;Zhang et al, 2017). In contrast, the Permo-Triassic stratigraphic architecture and sedimentary evolution in the CQMB remain very poorly studied.…”

“…The Tibetan Plateau is widely deemed as a compelling orogenic archive for the study of Tethys evolution and plate tectonics (Figure 1b) (Kapp & Decelles, 2019; Searle et al., 2011; Yin & Harrison, 2000). The >500‐km‐long and up to 100‐km‐wide E–W‐trending Central Qiangtang metamorphic belt (CQMB) (Figure 1a) is of particular relevance because of the good preservation of the Permo‐Triassic subduction zone tectonics and ophiolites of the Palaeo‐Tethys Ocean (Figures 1 and 2) (Kapp et al., 2000, 2003; Li et al., 1995, 2006, 2016; Liang et al., 2012, 2017, 2020; Lu et al., 2019; Pullen & Kapp, 2014; Pullen et al., 2008, 2011; Xu et al., 2020; Yin & Harrison, 2000; Zhai, Jahn, Su, et al., 2013; Zhai et al., 2011, 2016; Zhai, Jahn, Wang, et al., 2013; Zhang et al., 2016; Zhao et al., 2014, 2015). The previous geological investigations of the CQMB mainly focused on the Permo‐Triassic high‐pressure and low‐temperature (HP‐LT) metamorphic rocks (Deng et al., 2000; Kapp et al., 2003; Li et al., 2006, 2008, 2020; Liang et al., 2012, 2017; Zhai et al., 2011) and the tectonic origin of ophiolites (Fan et al., 2017; Li et al., 1995, 2016; Wu et al., 2010; Zhai et al., 2016; Zhai, Jahn, Wang, et al., 2013; Zhang et al., 2017).…”

Impact of upper mantle convection on lithosphere hyper-extension and subsequent convergence-induced subduction

“…Another example of divergent double-slab subduction is presumably the Paleo-Asian Ocean, which has been subducted beneath both the southern Siberian Craton in the north and the northern margin of the North China Craton in the south during the Paleozoic (Yang et al, 2017). Furthermore, a divergent double-slab subduction was also suggested between the North Qiangtang and South Qiangtang terrane (Li et al, 2020;Zhao et al, 2015). Our models show that a divergent double-slab subduction is a thermo-mechanically feasible process during convergence of tectonic plates.…”