Elsevier

Gondwana Research

Volume 101, January 2022, Pages 257-277
Gondwana Research

The Mesozoic Amdo micro-block and East Asian superconvergent tectonic system

https://doi.org/10.1016/j.gr.2021.09.001Get rights and content

Highlights

  • Five-stage deformation identified in the Amdo micro-block since Mesozoic.

  • Flat subduction occurred beneath the Amdo micro-block.

  • Two collisions between the Amdo micro-block and Qiangtang Block.

Abstract

The assembly of East Asia involved multi-directional convergence of several micro-blocks during the Mesozoic. However, the geodynamic and tectonic elements associated with the southwestern convergent tectonic system of East Asia involving the Bangong-Nujiang subduction-collision system remain unclear. The Bangong-Nujiang Suture Zone in central Tibet shows a prominent east–west-trending differentiation divided by the Amdo micro-block. Here we evaluate the structural architecture of the Amdo micro-block, together with stratigraphic and paleomagnetic data, geochronological and geochemical data on the multi-stage magmatic and metamorphic rock suites with a view to trace the subduction and closure history of the Bangong-Nujiang Ocean as well as the collisions among micro-blocks. We also present new data from outcrop structural analysis which suggest that the micro-block has undergone five episodes of deformation (abbreviated as D1 to D5), which are represented by respectively NW-striking isoclinal tight folds, NE-striking tight folds, E-trending asymmetric folds, V-type conjugate strike-slip shear zones, and top-to-the-southwest thrust faults. In the Early Jurassic, initial active collision occurred between the Amdo micro-block and the Qiangtang Block followed by rifting and back-arc extension of the Bangong-Nujiang Ocean south of Amdo and passive collision between the Amdo micro-block and the Qiangtang Block in the late Early Jurassic. In the Middle-Late Jurassic, the Bangong-Nujiang Oceanic lithosphere underwent flat subduction beneath the Amdo micro-block resulting in slab tearing along the western border of the micro-block, large-scale intracontinental orogeny and thrusting. The slab tearing induced eastward mantle and crustal flows, and along with the N-S-directed compression, V-shaped conjugate shear zones formed in the interior of the Amdo micro-block. The intracontinental destruction and deep mantle lithospheric delamination in East Asia triggered the coeval convergent drops of deep materials in late Mesozoic, which drove further collision between the Lhasa Block and the East Asian continent in the Early Cretaceous.

Introduction

The assembly of the East Asian continent (EAC) involved multi-directional convergence of several micro-blocks from late Paleozoic to Mesozoic (Enkin et al., 1992, Gilder and Courtillot, 1997, Dong et al., 2008, Li et al., 2019). The late Paleozoic collisional orogenic stage formed the embryonic East Asian continent (EEA) (Carter et al., 2001, Ernst et al., 2007, Cocks and Torsvik, 2013, Guo et al., 2017a, Guo et al., 2017b, Zhao et al., 2018), known as the Indosinian Orogeny (Fromaget, 1934). Mesozoic amalgamation kinematics in East Asia is characterized by intracontinental orogeny (Li and Li, 2007, Li and Zhao, 2007, Zhang et al., 2008) and/or convergence of different micro-blocks or sub-blocks toward the EEA (Sengor, 1985, Zhang, 2001, Zhang, 2004, Oh, 2006, Kusky et al., 2007, Dong et al., 2008, Dong et al., 2018), forming three major suture zones termed as the Bangong-Nujiang subduction-collision zone to the southwest (Kapp et al., 2005, Liu et al., 2017a, Fan et al., 2018, Guo et al., 2019); the Paleo-Pacific subduction-accretion zone to the east (Suo et al., 2019, Suo et al., 2020,); and the Mongolia-Okhotsk subduction-collision zone to the north (Zorin, 1999, Fan et al., 2003, Sheldrick et al., 2020), together with some collision-derived, accretion-derived and rifting-derived micro-blocks (Li et al., 2018a) (Fig. 1a). This major deformational event is known as the Yanshanian Movement in East Asia (Wong, 1929, Dong et al., 2008). A better understanding of the Yanshanian epicontinental and intracontinental deformation and orogenesis requires detailed studies on the micro-blocks within the EAC.

The Bangong-Nujiang Suture Zone (BNSZ) formed through the subduction and closure of the Bangong-Nujiang Meso-Tethys Ocean (BNO) within the central Tibetan Plateau, and separates the Qiangtang Block (QTB) to the north and the Lhasa Block (LHB) to the south (Girardeau et al., 1984, Guynn et al., 2006a) (Fig. 1). This suture preserves the history of convergent regime of the micro-blocks that accreted northeastward toward the EAC during the late Mesozoic (Dewey et al., 1988, Kapp et al., 2007). The Amdo micro-block (ADB), located in the eastern segment of the BNSZ (Fig. 1b), is distinctly different in Mesozoic strata from the central-western sections of the BNSZ due to its unique Neoproterozoic and early Paleozoic basement (Xu et al., 1985, Guynn et al., 2006a, Guynn et al., 2006b, Guynn et al., 2012, Guynn et al., 2013, Wang et al., 2012a, Xie et al., 2013). It not only preserves the complete ophiolite suite derived from the BNO, but also provides a key region to study the transition of tectonic regimes within the intra-BNO (Coward et al., 1988, Lai and Liu, 2003).

The older basement of the ADB is composed of Neoproterozoic orthogneiss and Cambrian crystallization ages (Xie et al., 2010, Guynn et al., 2012), which were later intruded by voluminous Mesozoic granites (Harris et al., 1988, Yan et al., 2016a). It is generally agreed that the Jurassic granites are related to the northward subduction of the BNO (Sun et al., 2011, Li et al., 2017a). Sedimentary strata are mainly distributed around the ADB, but are absent in the interior of the ADB (Zhang et al., 2014). There are ophiolite outcrops on both the northern and southern sides of the micro-block (Lai and Liu, 2003, Sun et al., 2011, Bai et al., 2013). The crystalline basement has been subjected to metamorphism under high pressure (HP) granulite facies with a peak-metamorphic age of 191 Ma (Zhang et al., 2010, Zhang et al., 2014) and lower amphibolite facies with a peak-metamorphic age of 178 Ma (Guynn et al., 2013), respectively.

Previous studies mainly focused on the stratigraphy, petrology, isotopic geochronology and geochemistry (Guynn et al., 2006a, Guynn et al., 2006b, Guynn et al., 2012, Guynn et al., 2013, Zhang et al., 2010, Zhang et al., 2014, Chen et al., 2015), but several disputes exist, resulting in a lack of understanding of the evolution of ADB. Whether the missing magmatic arc on the northern side of the ADB was buried by some younger strata, or subducted northward with the LHB beneath the QTB, remains speculative (Guynn et al., 2006a; Liu et al., 2010, Liu et al., 2011a) proposed that the Amdo Jurassic plutons underwent magma mixing at 185 Ma and 175 Ma, and correlated these to mixing of crust-derived felsic magma with mantle-derived mafic magma. Furthermore, the tectonic offsetting of the Amdo ophiolites is also equivocal. Some scholars correlated these rocks to a typical island arc ophiolite (SSZ-type), which was formed in back-arc basin environment (Lai and Liu, 2003). However, others proposed that these rocks are part of oceanic crust formed in mid-ocean ridge or quasi mid-oceanic ridge environment (MOR-type) (Bai et al., 2013). In addition, the Mesozoic amalgamation history is also controversial. The peak-metamorphic age of 191 Ma for granulites indicates that the ADB deeply subducted beneath the QTB (Zhang et al., 2010, 2014); The P-T-t path shows that the Amdo gneiss experienced a tectono-thermal event related to the crustal thickening in a shortening setting at 178 Ma before the QTB-LHB collision (Guynn et al., 2013); The zircon and sphene U-Pb ages of 171 ± 6 Ma obtained by Xu et al. (1985) from the gneisses of the southern ADB may be related to the collision between the LHB and the QTB during the Early-Middle Jurassic; Many other scholars have suggested that the QTB-ADB and the LHB collided during the Early Cretaceous (Guynn et al., 2006a, Peng et al., 2011, Liu et al., 2017b).

Previous research in the ADB mostly focused on the magmatism and metamorphism with no detailed investigation on the structural aspects (Girardeau et al., 1984, Guynn et al., 2006b, Shi et al., 2012). In order to improve our understanding on the deformation history of the Amdo gneiss and the relation between Mesozoic magmatic rocks as well as the subduction of the BNO, in this paper, we present the results from a systematic field investigation and structural analysis in the ADB, and identify the deformation stages. We also integrate many petrological, isotopic geochronological and geochemical data of the Amdo magmatic and metamorphic rocks to reconstruct their tectonic settings and processes. Based on constraints from structural analysis, we discuss the response of various deformation events to magmatism and metamorphism in the ADB, as well as provide further insights into the tectonic evolution of the ADB in the context of multi-directional East Asian convergent tectonic system.

Section snippets

Strata and ophiolites around Amdo

The ADB is a roughly E-W-striking lenticular allochthonous micro-block, which is located on the eastern BNSZ between the QTB and the LHB (Xu et al., 1985, Zhu et al., 2011, Guynn et al., 2012). Its unique ancient basement distinguishes this block from the other terranes or micro-blocks around the BNSZ. The stratigraphic distribution of the ADB is nonuniform (Fig. 2a). The widely exposed Neoproterozoic Nyainrong Formation mainly comprises biotite plagioclase gneiss, amphibolite, diopside marble

Structural analysis

The major basement outcrops in the ADB are composed of highly deformed gneisses. From the late Mesozoic micro-block convergence to the Cenozoic India-Asia collision (Tapponnier and Molnar, 1976, Dong et al., 2008), the gneissic basement has experienced several tectonic events resulting in multi-stage deformation. However, the earlier gneisses, because of their high sensitivity to deformation, were replaced during the intensive late Mesozoic orogeny. Therefore, it is difficult to deduce the

Structural evidence for northward subduction polarity of the Amdo Ocean

The subduction polarity of the BNO has long remained controversial. For instance, Guynn et al. (2006a) proposed that the synchronous granitic magmatism and metamorphism of the ADB were related to the northward subduction of the oceanic lithosphere of the BNO. Zhu et al. (2013, 2016) suggested that it was related to the divergent double subduction of the BNO. Liu et al. (2011a) suggested that the Amdo island arc and QF back-arc deposition constituted within complete arc-basin system which

Conclusions

  • (1)

    Since the Mesozoic, the ADB has undergone five-stage deformation, as reflected in the NW-SE-striking tight folds, NE-SW-striking tight isoclinal folds, E-W-trending asymmetric folds, V-type conjugate strike-slip shear zones, and top-to-the-southwest thrust faults.

  • (2)

    The initial active NE-SW-striking collision occurred between the ADB and the QTB at 191 Ma, triggering Deformation D1. The slab rollback of the BNO south of the ADB caused the back-arc spreading in the QTB interior, forming a new

CRediT authorship contribution statement

Run-Hua Guo: Investigation, Conceptualization, Resources, Methodology, Writing – original draft. San-Zhong Li: Supervision, Funding acquisition, Writing – review & editing. Jie Zhou: Supervision, Writing – review & editing. Yi-Ming Liu: Investigation, Formal analysis. Sheng-Yao Yu: Resources, Funding acquisition. Yu-Hua Wang: Investigation, Formal analysis. Lin Liu: Conceptualization, Resources. M. Santosh: Writing – review & editing.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

We are thankful to Associate Editor Andrea Festa and two anonymous referees for their helpful comments which helped in improving our manuscript. This research was financially funded by National Key Research and Development Program of China (No. 2017YFC0601401), National Natural Science Foundation of China (Nos. 91958214, 41802232, 41702050), Fundamental Research Funds for the Central Universities (Nos. 202012015, 202072017), Qingdao National Laboratory for Marine Science and Technology (Nos.

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