Lithospheric structure of the xuefengshan belt, South China: Evidence from a seismic reflection profile

Highlights

• A deep fault cutting through the Moho surface is identified in the Xuefengshan belt.

• Collisional belt of the Yangtze and Cathaysia Blocks is revealed based on identification of geophysical relicts.

• Boundary between the Yangtze and Cathaysia Blocks is deduced based on the crustal structure.

• Two groups of layered narrow slip belts in the middle crust of the Yangtz and Cathaysia Blocks are identified.

• A narrow velocity gradient belt at a depth of ∼90 km in the upper mantle lithosphere is distinguished.

Abstract

In this study, we analyse and interpret a new seismic reflection profile across the Xuefengshan Belt (XFSB), South China, to determine the deep structure of this belt and provide insights into collisional orogenic architecture. We identify a deep regional fault located beneath the XFSB, which cuts through the reflection Moho below the belt and is interpreted as representing the collisional boundary between the Yangtze and Cathaysia blocks along the southeastern margin of the belt. We identify structural evidence for collision between the two blocks, including two groups of layered narrow slip belts (∼30 km) in the middle crust, one distributed in the Yangtze Block and the other in the Cathaysia Block. A narrow velocity-gradient belt in the upper mantle at a depth of ∼90 km is revealed by numerous strong seismic reflectors, some of which are layered. The study provides important new information on the lithospheric structure of the South China Block at the interface between the Yangtze and Cathaysia blocks.

Introduction

The South China Block (SCB), a major continental block in eastern Asia, lies south of the Qinling–Dabie orogen and to the east of the Qinghai–Tibetan Plateau (Chu et al., 2012a, b, 2012c; Shu, 2006, 2012; Shu et al., 2008; Zhao, 2015; Faure et al., 2016; Li et al., 2016; Zhu et al., 2019) (Fig. 1). The SCB was formed during the early Neoproterozoic by collision and amalgamation of the Yangtze Block in the northwest with the Cathaysia Block in the southeast (Charvet et al., 1996, 2010,Charvet, 2013; Chu et al., 2012a; Li et al., 2014; Faure et al., 2016). The Jianshan–Shaoxing Fault represents the eastern part of the Neoproterozoic ophiolitic suture zone between these two blocks (Li and Li, 2007; Faure et al., 2009, 2017; Chu et al., 2012a). The Xuefengshan Belt (XFSB) is the largest exposed Proterozoic tectonic–magmatic belt in the SCB (Shu, 2012) and is an ideal structure in which to investigate the tectonic evolution of the Yangtze and Cathaysia blocks (Fig. 1).

Several models have been proposed regarding the formation of the XFSB: (1) the XFSB was formed as an Alpine-type orogenic belt during the Indosinian (Hsü et al., 1988, 1990; Li et al., 1989), although this has not been supported by recent geophysical evidence (Wang et al., 2003, 2005, 2011a; Yan et al., 2003, 2009; Yu et al., 2005; Shu et al., 2008, 2009; Zhang et al., 2010, 2011; Yao et al., 2011, 2012, 2013, 2014a, 2014b; Chu et al., 2012a, b, 2012c; Zhang et al., 2012; Faure et al., 2016); (2) the Yangtze and Cathaysia blocks collided during the Neoproterozoic, and the final assembly of residual basins took place during the early Palaeozoic (Shui, 1987; Guo et al., 1996; Yin et al., 1999); and (3) the collision and amalgamation of the Yangtze and Cathaysia blocks occurred during the Neoproterozoic and formed a unified continental basement and plate in South China (Chen et al., 2008; Wang et al., 2011b). After collisional orogenesis, the XFSB entered a stage of intracontinental tectonic evolution within the SCB, and interactions between different continental blocks became important during this evolution (Huo et al., 2007; Charvet et al., 1996, 2010; Zhang et al., 2011). Several different views have been formed regarding the intracontinental tectonic evolution of the XFSB, including that the belt evolved as (1) a large-scale thrust-nappe structure and thrust-fold belt (Qiu et al., 1998; Yan et al., 2009); (2) a transition-type, basement-detached structure (Wang et al., 2005, 2007); or (3) a bidirectional wedge structure dominated by southeast-directed thrusting (Zhu, 1983). A lack of detailed structural knowledge about the XFSB and varying interpretations of available data have led to these different opinions regarding the tectonic evolution and structural deformation of the belt.

Studies of the lithospheric structure of South China with respect to the imprints of collision and amalgamation of the Yangtze and Cathaysia blocks (Wang et al., 2005; Chu et al., 2012a, b, 2012c; Faure et al., 2016) have revealed a need to better characterize the subsurface lithospheric structures of the XFSB if the tectonic evolution of this belt is to be better constrained and understood. Geophysical data can provide important information about the subsurface crustal architecture and tectonic history of a collision orogen (e.g., Brown, 1991; Dong et al., 2015; Zhu et al., 2019). Deep reflection profiling is a powerful tool for detecting lithospheric structure and currently has the best spatial resolution for revealing geological features of the crust and upper-mantle lithosphere compared with other geophysical techniques (Brown, 1991).

This study aimed to ascertain the detailed lithospheric structure of the XFSB by analysing and interpreting a seismic reflection profile across the belt. In this paper, we introduce the geological setting of the SCB and describe the seismic data and routine processing procedure for the studied profile. We then interpret the seismic profile and infer the structural characteristics of the crust, the reflection Moho, and the upper-mantle lithosphere beneath the XFSB. Finally, we discuss the implications of the findings for the tectonic development of the SCB.