The Latest Spreading Periods of the South China Sea: New Constraints From Macrostructure Analysis of IODP Expedition 349 Cores and Geophysical Data

Abstract

Macrostructures preserved in deformed rocks are essential for the understanding of their evolution, especially when the deformation is weak and hard to discriminate in regional scale or purely through geophysical data. In order to resolve the inconsistency between NS trending fracture zones and NE oriented spreading fabrics of the South China Sea during the latest spreading stage, we analyzed macrostructures identifiable from the basalt and consolidated sediment samples of the Integrated Ocean Drilling Program (IODP) Sites U1431 and U1433. These two sites are close to the East and Southwest relict spreading ridges and provide critical information on the latest spreading stages. The structures in the basalt of both sites suggest two dominant orientations of NS and NE. At U1431, sediments show mainly WNW trending slickensides, different from that of basalt. At U1433, no structures were found in postspreading sediment. Thus, NE and NS trending structures in basalt are most possibly formed by seafloor spreading. Crosscutting relationship suggests that NE trending structures formed first, followed by NS and finally WNW trending structures. These observations are consistent with geophysical features. Magnetic anomalies and ocean bottom seismometer velocity suggest that the latest relict ridge of the East Subbasin coincides with the EW trending seamount chain. Located between the relict ridges of East and Southwest Subbasins, NS trending Zhongnan-Liyue Fracture Zone had acted as the latest transform fault. Based on the above evidences, we proposed that the South China Sea may have experienced a short period of NS oriented spreading after earlier SE spreading. These results resolve the previous inconsistencies. Key Points: • Basalt and sediment show different macrostructure orientations, indicating different stress fields between the synspreading and postspreading stages • Magnetic anomalies and low upper crust P wave velocities indicate that the youngest relict ridge in the East Subbasin may coincide with a seamount chain • Cross-cutting relationship of fractures suggests a short period of roughly NS trending spreading after a SE trending spreading Supporting Information: • Supporting Information S1

Late Miocene to Present Paleoclimatic and Paleoenvironmental Evolution of the South China Sea Recorded in the Magneto‐Cyclostratigraphy of IODP Site U1505

“…The SCS receives a large number of terrigenous materials from surrounding rivers under the influence of the East Asian monsoon (Boulay et al., 2007; Clift et al., 2014; Liu et al., 2016; Wan et al., 2006, 2010b), and preserves high‐resolution sediment archives of past regional and global climate changes due to its high sedimentation rate. Over the past two decades, several international cruises have carried out scientific drilling in the SCS, obtaining high‐resolution records (Li, Lin, et al., 2014; Prell et al., 1999; Wang et al., 2014), and contributing to the understanding of paleohydrology (e.g., Miao et al., 2017, 2018), tectonic and sedimentary environment evolution (e.g., Song et al., 2017; Sun et al., 2019; Yang et al., 2019), sediment provenance (e.g., Boulay et al., 2005; Kissel et al., 2018; Li et al., 2003), carbon cycling (e.g., Bao et al., 2019; Gerotto et al., 2020), and paleoclimate (e.g., Boulay et al., 2007; Gai et al., 2020; Tian et al., 2011; Wan, Tian, et al., 2010) in the SCS.…”

Tectonic dynamics of the Zhongjiannan Basin in the western South China Sea since the late Miocene

“…A series of strike-pull basins of the SCS distributed along the fault zones, such as the Yinggehai Basin, Zhongjiannan Basin and Wan 'an Basin, were formed (Sun et al, 2003;Cai, 2014;Yin et al, 2015;Lei et al, 2021). To enter the Miocene, the tectonic dynamics of western basins in the SCS underwent a major change due to the seafloor spreading of the SCS ending and the sinistral strike-slip movements of RRF and WEFSCS weakening (Zhu and Lei, 2013;Cai, 2014;Sun et al, 2019a;Zhang et al, 2021). The basins transformed into a depression subsidence stage, and the tectonic activity was weak.…”