We modeled the bending deformation of the subducting plate at the southern Mariana using a 3-D elastic plate flexural model. Intraplate stresses were investigated, with the along-strike variable boundary loadings. In order to match the observed plate geometry, boundary vertical loading near the Challenger Deep had to be set twice of that in the region further east within our study area. By comparing results between 2-D and 3-D models, we found that the difference on estimating plate stress can exceed 20% when there is an along-strike variation in plate bending. Finally, we found that the sharp lateral variation in the σ and the σ corresponded to an outer-rise earthquake cluster at the southern Mariana subduction zone, indicating that along-strike variation in σ and σ may be a significant mechanism to cause non-uniform distribution of outer-rise earthquakes.

The marine pelagic brown claystone was widely recovered in the deep South China Sea (SCS) by International Ocean Discovery Program (IODP) Expedition 349, 367, 368 and 368X. The continuous deposition of brown claystone in Miocene in the SCS may represent a special sedimentary environment. IODP Site U1503A provides the ideal brown and green claystone of the early-mid Miocene to study the origin of the red coloring for sediments and their implications to the paleoclimate and paleoceanography. In this paper, the reflectance a*, grain size, clay minerals, major and trace elements, and Sr-Nd isotope analyses in brown and green claystone from U1503A have been conducted to understand the sedimentary condition and controlling factors of SCS marine brown claystone. The results show that the sediments color transition was caused by the variations of Fe (III) content. The provenance was relatively stable and sediment was mainly supplied from South China and Luzon. The source areas underwent strong chemical and physical weathering since 17 Ma, which is related to Miocene Climatic Optimum events. The data of redox proxies suggest that both brown and green claystone were deposited in the oxic condition, while the sedimentary environment of brown claystone was more oxidizing. We suggest this difference in the redox condition was mainly controlled by the variation of the oxygen-rich bottom current in the northern SCS. We attribute this variation to the larger-scale deep-water circulation change in the Antarctic and western Pacific due to glacial-deglacial cycle.

Lithospheric flexure at subduction zones is a major contributor to induce outer rise earthquakes. Here, we modeled the bending deformation of the subducting plate at the southern Mariana using a 3-D plate flexural model. Intraplate stresses were investigated, with the along-strike variable boundary loadings. In order to match the observations of plate geometry, boundary vertical loading near the Challenger Deep has to be set twice of that in other areas. We also compared results between 2-D and 3-D models and found that the difference on estimating plate stress can exceed 20% when there is an along-strike variation in plate bending. Finally, we found that the sharp lateral variation in the σ and the σ corresponded to an outer rise earthquake cluster at the southern Mariana subduction zone, indicating that along-strike variation in σ and σ may be a significant mechanism to cause non-uniform distribution of outer rise earthquakes.

The transition from active to passive continental margin of the South China Sea (SCS) is usually inferred to occur in the Late Mesozoic to Early Cenozoic. However, it is less known about the tectonic characteristics of active continental margins before the Late Mesozoic, which hampers the recognition of integral evolution of the SCS. The International Ocean Discovery Program (IODP) site U1504 has sampled greenschist facies mylonite from the basement in the Outer Margin High of the northern SCS continental margin, which potentially record the Mesozoic and Cenozoic tectonic evolution of the SCS region. The microstructure has identified two episodes of deformation in the mylonite, namely early ductile and late brittle deformation, but without age constraints. Here, we further identify three episodes of carbonate veins (pre-mylonite, syn-mylonite and post-mylonite) in the greenschist facies mylonite according to the intersecting relationship between the veins and the mylonite foliation. Then we select 10 carbonate samples for in situ U-Pb dating, and obtain three accurate ages. The pre-mylonite carbonate veins are dated to 210 ± 20 Ma and 195 ± 32 Ma, respectively, which might denote the age of the protolith clast. The age of the syn-mylonite carbonate vein is 135 ± 12 Ma. But for the post-mylonite carbonate veins, no effective age was obtained using U-Pb dating method. Post-mylonite carbonate veins and late brittle fractures were formed at the same time, and the formation environment is similar to the overlying Late Eocene bioclastic limestone. Therefore, combining the microstructure, geochemistry and seismic profile, we speculate that the post-mylonite carbonate veins and brittle fractures may be formed during the Early Cenozoic rifting. These dating ages of the three episodes of carbonate veins suggest that the mylonite records at least two main periods of continental extension in the SCS region since the Early Cretaceous. In reference to the Mesozoic tectonic settings, we infer that, due to the slab rollback of the subducting paleo-Pacific, the SCS continental margin started significant extension during the Early Cretaceous as shown by the ductile deformation of the mylonite. In the Early Cenozoic, the mylonite was exhumated to the seafloor along with further continental extension, and weak brittle deformation occurred in the mylonite. Therefore, the Early Cretaceous extension of the SCS active continental margin may have a certain promotion effect on the rupture of the passive continental margin in the Cenozoic. Keywords: Greenschist facies mylonite; Carbonate U-Pb dating; Continental margin of the SCS; Early Cretaceous; IODP 368