Basement structures underneath the northern South Yellow Sea basin (East China): Implications for the collision between the North China and South China blocks

Journal Pre-proofs Basement structures underneath the northern South Yellow Sea basin (East China): implications for the collision between the North China and South China blocks Ming Xu, Jianwen Chen, Jie Liang, Yinguo Zhang, Baohua Lei, Jian Shi, Jianqiang Wang, Jun Liu, Hong Liu PII: DOI: Reference:

S1367-9120(19)30392-X https://doi.org/10.1016/j.jseaes.2019.104040 JAES 104040

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Journal of Asian Earth Sciences

Received Date: Revised Date: Accepted Date:

1 April 2019 12 August 2019 22 September 2019

Please cite this article as: Xu, M., Chen, J., Liang, J., Zhang, Y., Lei, B., Shi, J., Wang, J., Liu, J., Liu, H., Basement structures underneath the northern South Yellow Sea basin (East China): implications for the collision between the North China and South China blocks, Journal of Asian Earth Sciences (2019), doi: https://doi.org/10.1016/j.jseaes. 2019.104040

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Basement structures underneath the northern South Yellow Sea basin (East China): implications for the collision between the North China and South China blocks

Ming Xua, b, Jianwen Chena, b, *, Jie Lianga, b, Yinguo Zhanga, b, Baohua Leia, b, Jian Shia, b, Jianqiang Wanga, b, Jun Liua, b, Hong Liua, b a. Qingdao Institute of Marine Geology, Ministry of Natural Resources Qingdao, 266071, China b. Laboratory for Marine Mineral Resources, Qingdao National Laboratory for Marine Science and Technology, Qingdao,266071, China * Corresponding author at: Qingdao Institute of Marine Geology, Fuzhou South Road, Shinan District, Qingdao 266071, China

Abstract：The South Yellow Sea basin (SYSB), as the eastern part of the Yangtze Block, has undergone multiple tectonic events since the formation of its basement. The North China-South China collision affected intensely the structures of the basin. The relationship between the deep structures in the basement and collisional orogeny has long been a matter of debate. In this paper, 2D seismic, magmatic, gravity and drilling data were integrated to unravel the basement structures. In the interpreted seismic sections, the basement can be subdivided into three seismic facies. The Seismic Facies I and III correspond to two basement layers; the Seismic Facies II is

interpreted as shear zones that were formed during the Mesozoic collision and exhumation. The influence of the North China-South China collision propagated from north to south in the SYSB. According to observations from drill holes and seismic sections, the northern part of the basin, the Yantai Depression, uplifted during the earlier stage of the collision, resulting in the erosion of the upper Paleozoic and Triassic strata. In the middle and southern parts, the Laoshan Uplift and Qingdao Depression, the compression was obviously after that in the Yantai Depression.

The

exhumation of the Qianliyan basement metamorphic complex of the Yangtze Block occurred gradually.

The shear zones were initially developed during the collision

during Early Mesozoic. The exhumation process of metamorphic rocks lasted till Late Mesozoic and the Jurassic-Cretaceous strata were deposited in the hanging wall of the northern boundary fault. Keywords: South Yellow Sea basin; basement structures; North China-South China collision; shear zones; Sulu orogeny

1 Introduction Basement complexes are useful for investigating the evolution of overlying strata (Braitenberg et al., 2006; Martínez-Loriente et al., 2014; Kim et al., 2011). Structural analyses of such complexes can help to understand the structure and distribution of overlying strata and to explore petroleum systems (Fan et al. 2018; Bruvoll et al., 2012). Basin basement is referred as the oldest rocks overlain by cover sequences. Around the world, most large-scale basins had developed stable continental nucleus,

such as the Sichuan basin, Tarim basin, Pannonian basin and Canada basin (Gu et al., 2015; Wang et al., 2013; Yang et al., 2014; Zhao et al., 2018; Ross et al., 1991; Juhász et al., 2002). The basement of the Yangtze Block was mostly formed during Archean-Early Proterozoic, which had suffered rift and converge until the final formation of the Rodinia Supercontinent (Gao et al., 2005; Zhao et al., 2011). After that, basins in the Yangtze Block were largely controlled by the basement structures . The South Yellow Sea basin (SYSB) is located at the east edge of the South China Block, which served as the extension and main part of the Lower Yangtze block (Fig. 1) (Qu et al., 2005；Chen et al., 2016；Wu et al., 2016; Pang et al., 2017). The SYSB shows similar tectonic evolution as a part of South China Block. The South China Block, especially for the Yangtze Block, had experienced a series of tectonic events. Through the exposures in the western part of South China Block, the Archean-Paleoproterozoic crystalline basement can be established, which is correlated with the formation of Columbia supercontinent (Chen et al., 2013; Wang et al., 2015; Yan et al., 2018a, b). The formation of Neoproterozoic metamorphic basement was depended on the Rodinia tectonic event, since the ophiolite complex, magmatic arcs, backarc and forearc basins were identified in the western Yangtze Block (Li, 1999; Zhou et al., 2007; Yan et al., 2018a, b). During Ediacaran to Paleozoic, the most parts of South China Block had developed a passive continental margin with carbonate and clastic deposition (Qiu W.H. et al. 2018). Due to the collision between the North China and South China blocks, the South China Block had undergone extensively deformation, metamorphism and magmatism (Chen, 1988；Hao et al., 2002; Zhu et al.,

1999). Afterwards, subsequent extension formed half-graben basins in Late Mesozoic (Ding et al., 2016; Zhu et al., 2010). In Cenozoic, the South China Block was affected by the subduction of the Pacific Plate to the east and the collision between the India and Eurasia Plates to the west (Yan et al., 2018a; Rey et al., 2001; Pang et al., 2017). In general, the SYSB in the South China Block was involved in the Columbia supercontinental cycle, Rodinia supercontinental cycle, East Paleo-Tethys cycle and Pacific cycle (Yan et al., 2018a, b), which superposed different patterns of structures in the basin. The basement structures and properties have been matters of debate, largely due to potential of oil and gas exploration, development and prospective prediction (Zhang et al., 2014; Hou et al., 2008; Ding and Li, 2009). Since 1960s, efforts had been exerted for the petroleum exploration in the South Yellow Sea basin (Chen et al., 2018; Feng et al., 2002; Yao et al., 2008). Unfortunately no progresses in the exploration of industrial oil and gas were made in the past decades. Studies were mostly focused on the deep sedimentary process and structures of basins (Chen et al., 2016; Zhang et al., 2014; Qi et al., 2013), whereas studies of the basement was rather limited, especially about basement structures in multiple geophysical methods and the interplay between the basement and cover sequence above (Zhang et al., 2014). Since the basement of SYSB is deeply buried, the comprehensive geophysical results are necessary to be performed. In this study, we had applied the gravity, magnetic and 2D seismic data (Fig.1 and Fig.3), combined with the research in the land area of Lower Yangtze (Fig.2), to illustrate the basement structures and tectonic evolution of the SYSB. Besides that, the relationship between

basement and the orogenic belt of the North China-South China collision have been discussed. 2 Geological setting The Yangtze Block can be subdivided into three parts, namely the Upper Yangtze, Middle Yangtze and Lower Yangtze blocks (Fig.1) (Liu et al., 2010; Mei et al., 2008). The SYSB is located at the eastern part of the Yangtze Block, which is sandwiched by the North China Block to the north and Cathaysia Block to the south (Yan et al., 2018a; Qiu L. et al., 2016, 2018; Li et al., 2017; Zhao et al., 2018; Ye et al., 2019; Li et al., 2013). The Yangtze-Cathaysia amalgamation, North China-South China collision and subduction of the Pacific Plate all have significantly affected the Yangtze Block. The Yangtze Block is bounded by Sulu Orogenic Belt to the north, Jiangnan Orogenic Belt to the south and Tancheng-Lujiang Fault Zone to the west (Fig.1) (Pang et al., 2016; Zhu et al., 1999). According to the geological correlation with the Yangtze Block, the SYSB had developed on the same basement, with complex structural systems of EW, NE, NNE and NW directions (Xing, 2006; Gao, 2010). In the western part, the basin was mainly covered by NE-direction structures affected by the Tancheng-Lujiang Fault Zone and Sulu Orogen, and the structures turn into the NEE-EW direction in the eastern part of the basin. According to the previous results, the Mesozoic-Cenozoic terrestrial SYSB can be subdivided into the Yantai Depression, Laoshan Uplift and Qingdao Depression from north to south (Fig.1b) (Xu, 1982; Chen et al., 2018; Li, 2014). The Qianliyan Uplift and Wunansha Uplift

are situated to the north and to the south respectively (Fig.1b). Since the Ediacaran, the SYSB had evolved as stable platform until Early Triassic, with thick marine carbonate developed (Qiu W.H. et al., 2018). During the Indosinian, SYSB was uplifted to terminate the marine sediments, with foreland basin developed (Xu et al., 2019; Feng et al., 2008; Zhang et al., 2015). Since the Cretaceous, the SYSB is in the extensional or transtensional setting, with terrestrial grabens, strike-slip faults and magmatism influenced by the subduction of Paleo-Pacific slab (Shinn et al., 2010; Li et al., 2019; Qiu L. et al., 2018). During Cenozoic, the East Asia developed subsidence extensively associated with the subduction of the Pacific Plate (Xu et al., 2018; Liang and Wang, 2019). The SYSB has developed a cover sequence of Ediacaran to Quaternary depositions (Fig.2). The Ediacaran (Sinian) Doushantuo and Dengying Formations are extensively distributed in the Yangtze Block and are composed of clastic rocks and carbonate rocks, respectively. Other than the Early Cambrian Mufushan Formation with shale and mudstone, the marine carbonate had occupied the Middle Cambrian to Upper Ordovician. The Silurian to Triassic strata were confirmed by the continuous coring well of the Chinese Continental Shelf Drilling Program (CSDP-2) (Pang et al., 2019; Cai et al., 2019), the detailed information can be found in Figure 2. The Mesozoic terrestrial strata were established by a series of drilling wells (Fig. 2). The information of the cover sequence can be used for analyzing the distribution and structures of the basement. Given that the SYSB is largely covered by Cenozoic deposits, and the basement

rocks are seldom exposed, the disagreements still exist regarding the basement structures. Therefore, in studying the evolution and deep structures of the South Yellow Sea basin and orogenic belt during the Yangtze and North China collision, synthesis of available geophysical data can be useful to illustrate the relationship between basement structures and the cover sequence. 3. Basement characteristics of the Yangtze Block on land The basement was first established by the aeromagnetic data in land area of Lower Yangtze (Guo, 2005). Similarly, Ding and Li (2009) had used aeromagnetic anomaly to confirm the wide existence of Early Proterozoic or even older crystalline basement and suggested that the Yangtze and South China Blocks developed similar basement. For the depth of basement, the basement topography of in the Lower Yangtze area is quite complex. In the Lower Yangtze area, the lithologies and structures of the basement show obvious variation and the depth of basement show a stair-step deepening from SE to NW (Guo et al., 2005; Gao, 2010). The large-scale deep faults always develop as the abrupt zones for the basement depth (Guo et al., 2005; Zhang et al., 2007). Besides that, the depth of basement in land area of the Lower Yangtze block had controlled the thickness distribution of Phanerozoic strata (Ding et al., 1991, 2009). In the previous studies, it was concluded that the Yangtze Block had experienced multiple heterogeneous tectonic movement, which had induced multi-stages, multi-levels detachments (Ding et al., 1991, 2009). In the Lower Yangtze area, three major detachments could be defined, in which the detachment in the basement played

a leading role in the modification of Paleozoic marine basin (Ding et al., 1991, 2009). Through a long-time petroleum exploration in the Lower Yangtze Block, the marine Mesozoic-Paleozoic exhibit two patterns of structures, which are basement-involved and basement-uninvolved detachments. The basement-involved structures mostly demonstrate as the high-angle reverse faults and anti-listric faults, which combine to form the imbricate fault pattern and the opposite faults cause the pop up structures (Zhu et al., 1998, 1999). The basement of the Yangtze Block was made up of Proterozoic rocks covered by the Ediacaran or younger strata. In the Lower Yangtze area, a double layer basement is commonly accepted with the Proterozoic crystalline basement (Gao, 2010). The crystalline basement consists of the greenschist-granulite faces metamorphic rocks and underlies the unconformity produced by the Jinning orogeny at ~820 Ma （Zhao et al., 2011). In the SYSB, thick sedimentary sequence covers the basement. Only in the aeromagnetic map, the anomalies patterns can be correlated with the Sichuan basin, from which it can be concluded that the basement exist in the SYSB. 4 Gravity and magnetic features of the basement in the South Yellow Sea basin In this study, we combined the onshore gravity and magnetic data and offshore satellite altimetry gravity and aeromagnetic anomaly data to form the gravity and magnetic anomalies map for the SYSB and the area around (Zhang and Zhang, 2005; Liu, 1992; 909 Aero-geophysical Brigade, 1975; Fairhead et al.,2001; Zhang et al., 2007). All the available data are put together with gridding of 4 km4 km. The

Bouguer gravity and magnetic anomaly maps are shown in Figure 3. The gravity and magnetic data are beneficial for identifying the distribution of basement, geological structures and magmatism. The gravity and magnetic anomalies are mainly induced by the varied scales of geological bodies, the Bouguer gravity anomalies and magnetic maps are useful to illustrate the distribution of basement structures (Fig.3). In the magnetic anomaly map (Fig.3a), due to the absent or weak magnetism of the Phanerozoic strata, the local magnetic anomalies are mainly caused by the high-magnetic magmatic rocks. The background of the magnetic anomalies is caused by the topography of the basement. In the South Yellow Sea Basin, the magnetic anomalies show a continuous feature from the Lower Yangtze area. The zonation of magnetic anomalies in this area can be traced to the sea, which can also be concluded that the basement of the SYSB is a continuation of the Yangtze Block. Besides that, the magnetic anomalies can correspond well to the distribution of uplift and depression in the basin (Fig.3a). Along the northern boundary of the basin, a series of high magnetic anomalies develop on a background of low anomalies. The high positive anomalies are generally distributed along the northern boundary, which can be speculated that the anomalies are caused by the high-magnetic metamorphic rocks and igneous rocks related to Sulu orogenesis. In the north of the basin, the NEE direction large-scale negative anomalies correspond to the Yantai Depression, with some positive anomalies representing the volcanic rocks. According to the magnetic map, the Yantai Depression is conjoint with that in the Lower Yangtze area. Besides, in the northeastern part of the Yantai

Depression, a different magnetic anomaly pattern can be identified, in which a series of positive anomalies develop to separate the strong negative anomaly to the northeast and the main part of the Yantai Depression. The different anomalies pattern may be induced by the magmatism in the eastern part of the basin. In the middle and southern parts of the SYSB, the Laoshan Uplift and Qingdao Depression correspond to a series of positive magnetic anomalies, which cannot be correlated with that in the Lower Yangtze area. The basement depth is quite shallow in the middle and southern parts of the SYSB, compared with the Yantai Depression (Fig.1). The anomalies of NEE direction mainly develop in the sea area with magnetic anomalies increasing from west to east. Based on regional Bouguer gravity map (Fig.3b), the anomaly features can also be well correlated with the secondary tectonic units. In the Qianliyan Uplift, the gravity anomalies distribute as beads in the NE direction. According to the density analysis, the high-grade metamorphic rocks constituting the Qianliyan Uplift show the density of 2.68-2.70 g/cm3. While the Mesozoic granites in the Qianliyan Uplift had a lower density of 2.54～2.60 g/cm3, which can be related to the low positive anomalies in the background. Some local negative anomalies in the Qianliyan Uplift may be characterized by the Cenozoic deposition. To the south of the SYSB, the Wunansha Uplift is represented by low positive anomalies in general, with a few negative anomalies corresponding to the depression (Fig.2b). The gravity anomalies are mainly caused by inhomogeneous density in the upper crust.

In the northern part of the SYSB, the Yantai Depression is characterized by the NE-NEE zones with positive and negative gravity anomalies (Fig.3b), in which the negative anomalies are dominant. In most parts of the Yantai Depression, the value of gravity anomaly is below 0 mGal, with the minimum about -24 mGal. The origin of gravity anomalies is mainly caused by the thick sediments above the basement. The Laoshan Uplift is mainly characterized by low positive gravity anomalies. While in the east of the Laoshan Uplift, a large-scale area of high-positive anomalies can be recognized, which is quite different from the Lower Yangtze area. It is obvious that the eastern part of the South Yellow Sea has a basement structures differed from that of the Lower Yangtze area. The Qingdao Depression covers a relatively small area of the SYSB, with mainly the negative anomalies developed. The density difference is the main factor controlling the gravity anomalies. 5 Basement in the 2-D seismic profile For better understanding of the basement in the SYSB, we have applied an extensive date set of 2-D seismic reflection profiles to study the distribution and the geometry of the intrabasement structures (Fig.1a for locations of seismic sections). In the shallow part of seismic sections, according to the drilling data in the Yellow sea and Lower Yangtze area (Fig. 2), a series of reflections can be calibrated as critical layers. The bottom reflections of terrestrial deposition and Neogene strata are clear in the seismic sections, which indicate the termination of different evolution stages (Figs. 6 and 7). In the northeastern part of the Yantai Depression, the Jurassic strata, associated with the orogenic belt, can be calibrated according to the drilling data

(Figs. 3, 5 and 6). In general, the top reflection of Jurassic, the bottom reflections of Neogene, Palaeogene, terrestrial deposition and Upper Paleozoic in some seismic sections can be clearly interpreted (Fig.4-10). The top of the acoustic basement is mainly represented by a single, high amplitude and high frequency reflection which separates subhorizontal or gently dipping subparallel reflections in the sedimentary basin from underlying reflections with variable amplitudes, orientations and geometries (Figs. 6 and 7). It is interesting that the basement is more reflective in the places where the structures develop intensively, while the stable part in the basin show the acoustic basement of less reflectivity (Figs. 9 and 10). Below the top acoustic basement, the reflective expression of the basement can be subdivided into three different seismic facies (Figs. 3 and 4). 5.1 Seismic Facies I The Seismic Facies I is made of medium- to high-amplitude, semicontinuous, low-frequency reflections (Figs. 6 and 7). The reflections within Seismic Facies I at some locations are subparallel to the overlying reflections of cover sequence layers. The Seismic Facies I form a zone of variable thickness which can reach to up to 2 s TWT. The Seismic Facies I can be recognized in-between two or more sets of dipping medium-high amplitude reflections of Seismic Facies II and/or basement faults (Fig.8). Beneath the subparrel reflections, the Seismic Facies III can be identified. Since the basement was still not drilled in the SYSB, the petrology of the basement can only be correlated with the outcrops in the Lower Yangtze area, which share the

same basement according to the magnetic and gravity anomalies map. Accordingly, the Seismic Facies I would correspond to the metamorphic basement at the top of double basement, which was verified in the Lower Yangtze area. 5.2 Seismic Facies II Seismic Facies II is characterized mostly by medium-high-amplitude and subparallel dipping reflections which extend from 5 s TWT to about 2 s TWT (Figs. 4, 5 and 7). Seismic Facies II form a 1000-1500 ms thick packages of reflections dipping of 30-40° . Such reflection packages can be interpreted as zones of highly strained and sheared material in the deep crust. Particularly, the Seismic Facies II can only be identified in the northern part of the SYSB near the Qianliyan Uplift. This leads us to correlate the shear zones of Seismic Facies II with the orogenesis between the North China and Yangtze Blocks. Due to the comparison between the reflection pattern in the Seismic Facies II and Qingdao Uplift, the reflection packages may result from the exhumation of orogenic metamorphic belts. A particular pattern of reflections had been recognized at the northern boundary of the SYSB (Fig.4, 5), in which a series of dipping reflections separate the transparent Qingdao Uplift and the reflective SYSB. The south-dipping discontinuous reflections of boundary fault can be traced from 1 s to ~7 s, with a thickness of ~ 1500 ms. At the northern edge of the SYSB, the Seismic Facies II can be integrated into Seismic Facies III. It can be concluded that the materiel of basement shows obvious relationship with the exhumed metamorphic rocks (Fig.3). 5.3 Seismic Facies III

Seismic Facies III is the widely distributed in the SYSB and is characterized by weak-amplitude, discontinuous and high-frequency reflections (Fig.6). Seismic Facies III is mostly observed below Seismic Facies I and reaches to the depth over 8 s TWT. Seismic Facies III comprises chaotic dipping reflections. Most faults and shear zones in the SYSB (Seismic Facies II) have been terminated in Seismic Facies III, the lower part of the basement presents as the bottom of brittle deformation. Considered that Seismic Facies I would correspond to the metamorphic basement rocks and Seismic Facies II is represented as the shear zone, the Seismic Facies III is interpreted as the crystalline basement such as that in the Lower Yangtze area. Within Seismic Facies III, some chaotic dipping reflections can be observed with various directions, while most area are occupied by transparent zones of the homogeneous crystalline basement rocks. The dipping reflections may reveal some brittle fractures within the basement. According to different tectonic units, we have picked a few sections from the 2D seismic data for understanding the intrabasement structures, which are located at different tectonic units (Fig.1b). Each section can exhibit the structures of basement in different parts of the basin. The seismic facies classification described above can be used to illustrate the intrabasement structures in the three parts: the Yantai Depression, Laoshan Uplift and Qingdao Depression. 5.4 Intrabasement structures in the Yantai Depression The Yantai Depression presents the northern part of the SYSB. The acoustic basement is relatively deep (4-6s TWT), which can be well correlated with the magnetic anomaly map (Figs. 3 and 5). In the seismic profile of 1 (Fig.4), which is

located at the northeastern part of the Yantai Depression, the basement is at the depth of 4-5.5 s with several discrete high-amplitude, high-frequency reflections. In most parts of the Yantai Depression, the acoustic basement is quite reflective, with all the seismic facies in the basement recognized. The reflections in the basement are offset by a series of thrust faults. The double basement structures also develop, which can be confirmed by the two layers of different seismic facies. In some places of the Yantai Depression, the basement is characterized by middle-high-amplitude, high-frequency reflections that form ~500 ms thick packages of reflections dipping to the north (Figs. 4 and 5), which mark as the Seismic Facies II. The reflection packages are traced from the depth of 7s to the bottom of terrestrial depositions, which are identified as the shear zones with multiple episodes deformation. Most shear zones of Seismic Facies II show the north dipping direction near the Sulu orogenesis, which cannot be found at the Laoshan Uplift and Qingdao Depression. From north to south, the reflectivity of basement show obvious attenuation, coupled with structural deformation that is weaker in the south. The undulation of the top reflection of acoustic basement is in the similar shape with the strata distribution since Late Proterozoic, which show obvious coupling in the upper crust. In the Yantai Depression, the depth of basement decreases gradually to 3000 ms near the southern boundary (Fig. 6). A series of imbricate faults, which cut into the basement, cause the uplift of basement in the southern part of the Yantai Depression. 5.5 Basement reflections in the Laoshan Uplift and Qingdao Depression

Compared with the Yantai Depression, the acoustic basement of the Laoshan Uplift and Qingdao Depression shows less reflective features (Fig. 9). In the middle part of the SYSB (Laoshan Uplift), similar to the magmatic anomaly, the top reflection of acoustic basement illustrates obvious rise from north to south. The Seismic Facies I and Seismic Facies III can also be observed, with relatively thin Seismic Facies I. The Seismic Facies I develops a thickness of only 600-700 ms, with less high-amplitude reflections compared with the Yantai Depression. The faults in the Laoshan Uplift have exerted a critical control on the undulation of the basement. At the places of faults, the top reflections of the basement show obvious uplift and offset. In the southernmost SYSB, the Qingdao Depression occupies the area between the Laoshan Uplift and Wunansha Uplift. In the depression, the acoustic basement shows more obscure reflective features (Fig. 8). The thickness of Seismic Facies I demonstrates a further decrease from the north, with a thickness of ~500 ms. Beneath the Seismic Facies I, similar with the Laoshan Uplift, the less chaotic reflections of Seismic Facies III develop in the lower part of the basement, which can be traced into the middle-lower crust. Overall, the topography and structures of the basement give an important control on the distribution of cover sequence. Through the calibration of drilling well, most Mesozoic-Cenozoic terrestrial strata and upper Paleozoic marine strata had been drilled by different institutions, and the lower Paleozoic can be correlated with the outcrop and drilling results in the Lower Yangtze area (Fig. 2). In the places of

shallow basement such as the Laoshan Uplift and northern part of the Qingdao Depression, the marine Paleozoic-Mesozoic strata develop quite completely, whereas the terrestrial Mesozoic to Paleogene had been mostly eroded. In the Yantai Depression, the depth of basement is deep with Upper Paleozoic eroded and terrestrial Mesozoic-Cenozoic development. The uplift and subsidence of basement and the cover sequence above show obvious coupling, and the different distribution of the strata show that the various tectonic movement had different influences on the three tectonic units. 6 Discussion In this paper, we have described the seismic expressions of the basement in the SYSB. By integrating 2D seismic, drilling well, gravity and magnetic anomaly data, we have attempted to interpret the basement seismic facies in terms of known geological units in the land area (Figs.1 and 3) (Gao et al., 2011). We have identified three seismic facies (Facies I, Facies II and Facies III) in the Yantai Depression, two seismic facies (Facies I and Facies III) can be recognized in the Laoshan Uplift and Qingdao Depression. Here we have discussed further the nature and the significance of basement seismic facies, the influence and structures of collision between the North China and South China Blocks. 6.1 Geometry, Distribution and Seismic Characteristics of Seismic Facies II Seismic Facies II reflections are strong and semi-continuous, with dipping of 30-40°(Figs. 4 and 5), which are interpreted as the shear zones. The shear zone is characterized by E-W trending and north dipping. At most places, these shear zones

have been incorporated into the upper layer of Seismic Facies I. Particularly, the shear zones are only distributed in the north part of basin, while the Seismic Facies II cannot be found in the Laoshan Uplift and Yantai Depression. The Seismic Facies II can only be traced beneath the terrestrial strata, which is truncated by Indosinian tectonic surface. It can be concluded that the shear zones mainly formed during the collision between the North China and South China Blocks and lasted into the later exhumation. Besides that, the reflective features are similar with the northern boundary faults, which may mark the exhumation process of the metamorphic rocks in the orogenic belts. In our seismic sections, the exhumation is interpreted to derive from the basement of the Yangtze Block (even the middle-lower crust) (Fig.11). In general, the shear zones can be interpreted as the back-thrust of the exhumation progress. With our seismic data, the orogenic structures can be detected, in which the Qianliyan Uplift exhibits markable correlation with the basement of Yangtze Block (Fig.10). A series of dipping reflections at the northern boundary of the SYSB can represent the exhumation process, and the materials of the Qianliyan Uplift derived from the deep part of basement in the Yantai Depression (Figs. 4 and 6). According to the exhumation structures and shear zones, the exhumation should be concluded as a gradual process. The RC-2-1 well had drilled the Jurassic foreland sediments with a thickness of more than 2 km in the northeastern part of the Yantai Depression (Gao and Zhou, 2014; Gao et al., 2015). In the seismic sections, the shear zones (Seismic Facies II) are covered by terrestrial strata, from which the shear zones were formed

before the deposition of foreland strata (corresponding to the Jurassic drilled by RC-2-1, Gao et al., 2015). Therefore, we conclude that shear zones represent as the middle stage and branches of exhumation. After the upwards cease of shear zones, the northern boundary of basin still developed to uplift the metamorphic rocks to the Qianliyan Uplift during the Jurassic foreland deposition (Fig.11). 6.2 Role of basement on the evolution of the South Yellow Sea basin The basement of the Yangtze Block formed in the Proterozoic and underwent several stages of uplift and depression. From the 2D seismic data, the basement and the cover sequence above had been coupled during the deformation. Particularly, the cover sequences show evidently different distributions according to basement depth. On the basis of the drilling hole, seismic and magnetic data, the distribution of the marine and terrestrial strata above can be clearly identified. In the Yantai Depression, the Lower Paleozoic, Mesozoic and Cenozoic all developed well (Fig.3), with Upper Paleozoic eroded. While in the middle and southern parts of the basin, the Laoshan Uplift and Qingdao Depression have distributed Paleozoic and Cenozoic with little syn-Indosinian-Yanshanian Mesozoic strata (Figs. 8-10). The Yantai Depression corresponds to the deepest part of basement in the SYSB, with a depth of 8-12 km. The basement raises to the south inconsistently. The faults in the Yantai Depression, especially for the northern boundary fault and shear zones (Seismic Facies II), mostly cut into the lower layer of the basement. At the same time, the faults in the Laoshan Uplift and Qingdao Depression (middle and southern part of SYSB), mostly cut into the upper layer of the basement, without obvious reflections of shear zones developed.

The evident differences between three tectonic units of the SYSB show the vital control of the collision between the North China and South China Blocks and later exhumation of high pressure and ultra-high pressure metamorphic rocks. Before the Indosinian event, the basin showed relatively uniform deformation during the long-time evolution. The basement coupled with the basin had suffered multi-episodes of tectonic events and deformation during Late Proterozoic and Paleozoic, with thrust faults mostly extending into the upper part of the basement (Yan et al., 2018a, b; Qiu et al., 2018). Afterwards, the North China-South China collision caused more intense deformation, by which the basin suffered extensively uplift and erosion with a great deal of thrust faults. Most faults had cut through the Proterozoic-Paleozoic strata into the basement, which were inverted by extensional setting during Late Mesozoic. The Yantai Depression, especially in the deep crust, had recorded the impact of the collision and later exhumation. From the reflective structures in the whole basin, the deformation in the Yantai Depression is much stronger than that in the middle and southern parts of the basin. The collision to the north caused the evolution of Yantai Depression quite different from the other parts of the basin. The compressional structures develop intensely and extensively in the northern part, with most thrust faults extending into the basement, even into the lower part of the basement. The Laoshan Uplift and Qingdao Depression had been affected weaker and later by the collision, with less compressional structures developing. It is curious that the huge difference of syn-orogenic strata exists between the northern part and middle-southern parts of the basin. The RC-2-1 well had drilled the Jurassic foreland sediments with a

thickness of more than 2 km in the northeastern part of the Yantai Depression (Gao et al., 2015), while limited foreland deposition had been drilled or interpreted by other data in the Laoshan Uplift and Qingdao Depression. Combined with the topography of the basement and strata above, we can draw a conclusion that the impact of the collision had propagated southwards progressively. It is widely accepted that the collision between the North China and South China blocks initiated in the Middle Triassic. In the SYSB, especially in the Yantai Depression, the lack of Middle-Late Triassic strata can verify the time of collision between the North China and South China blocks. During the early stages of collision (~Early Triassic to Middle Triassic), the continental collision caused the intense deformation and uplift of the northern part of basin. Due to the intense collision, the Upper Paleozoic and Lower-Middle Triassic had been eroded. At the meantime, the Laoshan Uplift and Qingdao Depression had weak responses to the early orogeny. The Upper Paleozoic and Lower Triassic develop quite completely, which mean the impact of the collision had not reached the area. After the intense continental collision, the exhumation had dominated the orogenic belt during Late Triassic to Jurassic. During this time, the compressional impact of the collision had propagated to the middle and southern parts of SYSB, causing the uplift and deficiency of the Late Triassic to Jurassic strata. According to the structures in the seismic sections, the Jurassic foreland sediments were induced by the exhumation of the metamorphic rocks in the Qianliyan Uplift. The Qianlianyan Uplift would serve as the provenance for the Jurassic detrial sediments. The topography of the basement can be explained

by the collision and subsequent exhumation. In these seismic sections, the faults can be roughly subdivided into three phases according to the cross-cutting relationship. The poorly preserved faults of the first phase developed only in the Paleozoic, which show evident characteristics of thrust faults with small offset. Though uplift and erosion had developed during the Caledonian event, it can be concluded that the Paleozoic tectonic events were not critical for structural patterns nowadays. The faults of second phase were induced by the North China-South China collision during Indosinian event. In the South Yellow Sea Basin, this phase faults show different kinematic characteristics between the Yantai Depression, Qingdao Depression and Laoshan Uplift. In the depressions, the faults of Indosinian event are characterized by later negative inversion structures, for which few intense thrust faults had been preserved after the third phase structure. The extensional stress had decreased the offset along the previous thrust faults during Late Mesozoic. The third phase faults were mainly influenced by the subduction of Paleo-Pacific slab during Late Mesozoic, which were covered unconformably by Neogene-Quaternary depositions. The extensional faults in this phase had controlled the development of grabens in the SYSB. Besides that, some strike-slip faults and the flower structures are also interpreted in Late Mesozoic depositions. In general, the faults of second and third phases had important influences on the basin evolution. The Early Mesozoic thrust faults had been reactivated by Late Mesozoic extensional events. The shear zones in the basement were covered evidently by the terrestrial strata. According to the drilling well, the terrestrial strata are calibrated as Late

Jurassic-Cretaceous. Therefore, the shear zones in the basement initially developed during Indosinian event. The exhumation process of metamorphic rocks had lasted into Late Mesozoic during the third phase faults developed. Through the discussion above, we can acquire an evolution model for basement of the SYSB during the collision between the North China and South China Blocks (Fig.11). We can draw a conclusion that the impact of collision between the North China and South China Blocks was diachronous in different tectonic units.

The

deformation and distribution of the basement and deposition illustrate the Yantai Depression was affected strongly by the collision. The middle and southern parts of the SYSB had suffered hysteretic and weak influence from the collision. In Cretaceous-Cenozoic, influenced by subduction of the Pacific plate, the SYSB experienced regional extension which had little impact on the basement. According to the drilling wells and seismic sections, the shear zones first developed from the basement during Triassic since the southern branches are covered by Cretaceous terrestrial depositions. The exhumation progress of metamorphic rocks also started during the collision between the North China and South China Blocks, represented by the discontinuous reflections of shear zones originating from the basement (Fig.11a). The collision between the North China and South China Blocks may have occurred in Middle-Late Triassic (Li et al., 1993; Rowley et al., 1997; Zhang et al., 2009). Different from the north dipping shear zones, the exhumation progress had continued into the Late Mesozoic at the northern boundary of SYSB. In figures 4 and 5, the exhumation process also made a footprint in the Yantai

Depression, in which the exhumation process is interpreted as complex shear zones from the lower part of the basement even from the lower crust. The southern branches of shear zones ceased during the exhumation, and the northern branches finally exhumed to form the seaward extension of the Sulu metamorphic belt-Qianliyan Uplift. It can also be concluded that the Sulu-Qianliyan metamorphic rocks derived from the deep crust of the Yangtze Block. Through the drilling holes in the Yantai Depression, the Late Mesozoic strata are calibrated in the seismic sections. It can be concluded that the exhumation had lasted till Cretaceous and the Jurassic-Cretaceous strata were deposited

in the hanging wall of the exhumation fault (Fig. 11b).

7 Conclusions The South Yellow Sea Basin has a heterogeneous basement. We can draw the conclusions below. (1) The depths of the basement are ~12 km in the Yantai Depression, ~5 km in the Laoshan Uplift and ~10 km the Qingdao Depression. (2) Three seismic facies can be identified in the basement of the SYSB. Seismic Facies I and III correspond to the double layers of the basement. The Seismic Facies II can be identified as the shear zones, which only exist in the Yantai Depression. (3) The shear zones in the Yantai Depression can be related with the exhumation of the Qianliyan Uplift. The exhumation progressed and lasted till Late Mesozoic. The metamorphic rocks of the Qianliyan Uplift are part of the Yangtze Block. (4) The collision between the North China-South China Blocks propagated

southwards gradually from north to south. The Yantai Depression had undergone earlier and more intense compressional deformation than the Laoshan Uplift and Qingdao Depression.

Acknowledgment: This work was financially supported by the Project of China Geological Survey (DD20160512, DD20160346, DD20190379), Natural Science Foundation of Shandong Province of China (ZR201807080260), Science and Technology Development Fund Project of Shinan District (2018-4-006-ZH), Natural Science Foundation of Shandong Province of China (ZR2016DB33).

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Figure caption Fig.1. (a) Tectonic map of eastern China, showing the location of the SYSB (Fig. 1b), the North China Block, Yangtze Block, Dabie-Sulu orogen and their extensions are shown. (b) Tectonic units subdivision of South Yellow Sea basin, which are Yantai Depression, Laoshan Uplift and Qingdao Depression from north to south. (c) Basement topography map of the SYSB. The depth of basement is obtained by a large number of seismic sections (greyish lines in the map). The seismic sections interpreted to illustrate the basement structures are shown with red lines. The existing wells are shown with red dots.

Fig.2. The strata column of the SYSB. A few drilling wells were incorporated into the strata column, and the data sources from the drilling wells were labeled in the figure.

Fig.3. (a) The magnetic anomalies map of the SYSB and adjacent areas. (b) The Bouguer gravity anomalies map of South Yellow Sea basin and adjacent area. In the two maps, the boundary between the basin and Qianliyan Uplift is shown. The anomalies in the Yantai Depression are speculated as the Seismic Facies II in the seismic sections.

Fig.4. (a) The seismic section of A-A’ (location is shown in Fig.1). (b) Interpreted section A-A’, in which the abbreviations are: B.Ne for the bottom reflection of Neogene, B,Ce for the bottom reflection of Paleogene, T.Ju for the top reflection of

Jurassic, B.Te for the bottom reflection of terrestrial strata, B.UP for the bottom reflection of Upper Paleozoic, and T.B for the top of basement reflections. The abbreviations in other sections are all shown above.

Fig.5. (a) The seismic section of B-B’ northern part (location is shown in Fig.1). (b) Interpreted section B-B’ northern part, in which the abbreviations are same with Fig.4.

Fig.6. (a) The seismic section of B’-B’’ southern part (location is shown in Fig.1). (b) Interpreted section B’-B’’ southern part, in which the abbreviations are same with Figure 4.

Fig.7. (a) The seismic section of C-C’ (location is shown in Fig.1). (b) Interpreted seismic section C-C’, in which the abbreviations are same with Fig.4.

Fig.8. (a) The seismic section of D-D’ (location is shown in Fig.1). (b) Interpreted section D-D’, in which the abbreviations are same with Figure 4.

Fig.9. (a) The seismic section of E-E’ (location is shown in Fig.1). (b) Interpreted section E-E’, in which the abbreviations are same with Figure 4

Fig.10. (a) The seismic section of F-F’ (location is shown in Fig.1). (b) Interpreted section F-F’, in which the abbreviations are same with Figure 4.

Fig.11.The interpreted model shows the construction of the North China-South China collision in the SYSB. (a) The initial collision between the North China-South China Blocks, with shear zones starting to form; (b) The exhumation process of the Qianliyan Uplift metamorphic rocks during Late Mesozoic, with some shear zones ceased to develop before terrestrial depositions. The relationship between the basement and Qianliyan metamorphic rocks can be stated that the orogenic belt material had come from the deep part of the Yangtze Block.

GA

. The South Yellow Sea Basin has a heterogeneous basement with three seismic facies. The depths of the basement are ~12 km in the Yantai Depression, ~5 km in the Laoshan Uplift and ~10 km in the Qingdao Depression . The Yantai Depression had undergone earlier and more intense compressional deformation then the Laoshan Uplift and Qingdao Depression. . The basement structure was produced by the collision between the North China and South China Blocks

Declaration of Interest Statement The authors declared that they have no conflicts of interest to this work.

Jianwen Chen and Ming Xu on behalf of all the co-authors