Late Mesozoic transition from Andean‐type to Western Pacific‐type of the East China continental margin—Is the East China Sea basement an allochthonous terrain?

Geophysical features and crustal structure of the East China Sea basement were revealed based on geophysical inversion analysis with recent gravity and magnetic data. Our results show great differences between the East China Sea continental shelf basins and the Zhemin Volcanic Belt in the coastal South China Block with respect to geophysical features and crustal structures. These areas are separated by distinct geophysical and crustal changes, approximately along a line 100 km from the coastline of China. We hypothesize that the East China Sea basement is a buoyant allochthon in the Palaeo‐Pacific Plate, and that the East China margin was an Andean‐type active continental margin until the collision of the East China Sea basement with the South China Block in the Late Cretaceous, which jammed the Late Cretaceous Trench, terminating the subduction and the related granitoid magmatism. The East China continental margin was dominated by dextral transtension with limited magmatism due to the NNW‐trend motion of the Palaeo‐Pacific Plate with a reduced convergence rate until the Middle Eocene. The renewed westward subduction of the present‐day Pacific Plate replaced the East China Andean‐type continental margin with a Western Pacific‐type one. The detailed reconstructed tectonic model for the East China Sea since the Mesozoic is presented in this paper.

active continental margin (i.e., Li & Li, 2007;Li, Santosh, Zhao, Zhang, & Jin, 2012;Niu et al., 2015;Zhou & Li, 2000); (b) magmatic activities ceased since 90 Ma and the magmatic gap continued to 60-50 Ma Niu et al., 2015;Niu & Tang, 2016); and (c) the eastern continental margin of the South China Block experienced bimodal magmatism and back-arc rifting related to the subduction of the Pacific and the Philippine Sea plates after 60-50 Ma, illustrating a Western Pacific-type continental margin (Chen et al., 2010;Chung, Sun, Tu, Chen, & Lee, 1994;Chung, Yang, Lee, & Chen, 1995;Lin, Watts, & Hesselbo, 2003;Niu et al., 2015;Niu & Tang, 2016;Suo et al., 2015;Zhou et al., 2009;Zhu et al., 2004). However, how and when the Andean-type continental margin switched into a Western Pacific-type remains unclear. Li and Li (2007) and  proposed that a new continental arc was initiated after 280 Ma along the coast after a magmatic gap owing to flat-slab subduction, which persisted until 90 Ma. After 90 Ma, a slab rollback of the Mesozoic Palaeo-Pacific subduction zone caused a retreat of the arc system and a back-arc rifting. However, those researchers did not provide further discussion about the magmatism gap between 90 and 50 Ma, or a retreat process of the subduction zone.
Studies on the Mesozoic Palaeo-Pacific subduction zone before a slab rollback began in the late 20th century, but the location of this zone is still in doubt. Guo, Shi, and Ma (1983) argued that the suture, as a relic of this Mesozoic subduction zone, is generally along the 40-m-depth contour offshore of East China. With geophysical studies on the Taiwan Straits, Wang, Chen, Cao, Pan, and Wang (1993) confirmed this opinion and pointed out that the Coastal Fault Zone is the southward extension of the suture. Studies related to metamorphic rocks within the Tananao Basement Complex in the eastern Taiwan Central Range, however, have suggested that this complex belt is the suture (Cao & Zhu, 1990;Lo & Yui, 1996). Despite In this study, we try to reveal the crustal structure and nature of the ECS basement, which includes the basement of the continental shelf basin, the Diaoyu Island Uplift-Fold Belt, the Okinawa Trough, and the Ryukyu Arc (Figure 1), based on geophysical inversion analysis with recent gravity and magnetic data and to study tectono-sedimentary features with seismic interpretation. Attempts will be made to reconstruct the tectonic evolution of the ECS basement since the Mesozoic and to constrain the transition from Andean-type to Western Pacific type continental margins to the east of the South China Block.

| GEOLOGICAL SETTING
The ECS is a 1300-km-long, nearly 740-km-wide sea distributed between 21°54′N and 33°17′N and 117°05′ and 131  The ECS is subdivided into five tectonic units, including the Zheming Volcanic Belt, the continental shelf basin, the Diaoyu Island Uplift-Fold Belt, the Okinawa Trough, and the Ryukyu Arc (Suo et al., 2014;Suo et al., 2015; (Li & Li, 2007;Niu et al., 2015;Zhou & Li, 2000). The continental shelf basin covers most of the ECS and is further divided into the West Depression, the Central Uplift, and the East Depression   China. The total length of surveying lines in the study area is about 141,000 km. The mean square roots of intersection for the gravity and magnetic surveying lines are more than ±1.47 × 10 −5 m/s 2 and ±6.69 nT, respectively. Blank areas are interpolated with a satellitederived gravity model (Sandwell et al., 2013) and Earth Magnetic Anomaly Grid 2 data (Maus et al., 2009). We normalized the data sets and generated gravity grid data and magnetic grid data with 1′ × 1′ spatial resolution. The gravity and magnetic anomaly maps are shown in Figure 2. The depths of the sediment basement were compiled from seismic imagery from Shanghai Offshore Petroleum (Lin, Sibuet, & Hsu, 2005) and a Cenozoic sediment isopach map (Liu, 1992). The grid was produced in a Mercator projection referenced to WGS84 with central longitude at 125°E and central latitude at 29°N.

| Upward continuation and reduction-to-the-pole (RTP) of magnetic anomaly
The computation of upward continuation of a magnetic anomaly reduces local magnetic disturbances and reveals deeper anomalies in the lithosphere. In this study, variable inclination RTP is employed (Li & Olderburgz, 2001;Li J.B., 2008a;Li X., 2008b), in which we divide the study area into 10 zones (3°per zone) with corresponding inclination and deflection. The inclination ranges from 60.6°to 16.8°, and the deflection ranges from −6.9°to −1.2°from north to south during the process.
To standardize resolution of magnetic grid data in the continental and oceanic regions, we further upward continued the data set and reduced some high-frequency parts in the oceanic region. The upward continuation equation is: Where u and v are the wave number in two directions, and z is the latitude. The result of 5 km upward continuation is shown in Figure 3.

| Calculation of crustal thickness
Sediment thickness is an important factor in the calculation of Moho depth and crustal thickness. Deposition on the continental shelf of the ECS is generally more than 2 km thick and can reach more than 10 km in some areas (Zhu, Mi, & Zhang, 2010). Correction for the effect of sediments is essential when calculating the Moho depth and crustal thickness of the ECS. By using a variable density correction formula for fan blocks in a spherical coordinate system, we reduced the gravity effect on sediments and obtained the residual gravity anomaly, which reflects the surface of the density contrast between the crust and mantle. The inversion is conducted based on the 3D Parker inversion method (Oldenburg, 2012), with constraints from multichannel seismic data (Zhou et al., 2013). The density contrast is calculated as 0.44 × 10 −3 kg/m 3 . The recovered crustal thickness is shown in Figure 4. The Okinawa Trough is a back-arc basin with large-scale, lowamplitude, wide, negative magnetic anomalies. The crustal thickness is less than 20 km and can be as little as 15 km. It is thinner than those of typical continental crust and is thicker than those of normal oceanic crust. Initial seafloor spreading might have occurred in the south with minimum crustal thickness (Sibuet & Hsu, 1997).

| The East China Sea basement as an allochthonous terrain
The distance between a trench and a volcanic arc cannot be unlimited and is controlled by the subduction angle, the water, and volatile contents within the subducted materials (Stern, 2002(Stern, , 2004). Recent Trough that have mature oceanic basins, show that the distance between the subduction zone and the arc/back-arc basin is generally between 150 and 350 km (Li, Ding, & Li, 2011

| Dynamic mechanism of the magma gap-trench jam?
Limited volcanic activities related to subduction were observed in East China between~90 and~50 Ma (Li & Li, 2007;Niu et al., 2015). The

| MESO-CENOZOIC RECONSTRUCTION OF THE EAST CHINA SEA EVOLUTION
Most previous attempts at an evolutionary model treated the ECS as part of the continental lithosphere of East China, which was first an Andean-type active margin caused by the subduction of the Palaeo-Pacific Plate; eastward migrating rifting then occurred, caused by slab rollback of the subduction zone (e.g., Chen et al., 2010;Chung, Sun, et al., 1994;Chung, Yang, et al., 1995;Lin et al., 2003;Suo et al., 2014;Suo et al., 2015;Yang et al., 2016;Zhu et al., 2004). However, by accepting the notion that the ECS is an allochthonous terrain and that there was a passive margin stage before the development of the Western Pacific-type continental margin, the whole evolution model should be reconstructed as follows.

| CONCLUSIONS
Geophysical inversion analysis based on recent gravity and magnetic data reveals significant differences between the continental shelf basin of the ECS and the coastal regions of the South China Block in geophysical features, as well as crustal structures. The former is characterized by low-frequency, low-amplitude magnetic anomalies, and thinner crustal thickness (generally between 15 and 20 km), and the latter is characterized by high-frequence and high-amplitude anomalies, and thicker crustal thickness (generally higher than 25 km). These facts make us doubt that the ECS basement is an offshore extension of