Tectonic Influences on Trench Slope Basin Development via Structural Restoration Along the Outer Nankai Accretionary Prism, Southwest Japan

Abstract

Three-dimensional (3-D) seismic reflection data and sediment cores record ~2.87 million years of structural and depositional history of a trench slope basin along the outer Nankai accretionary prism, southwest Japan. Numerous mass transport deposits (MTDs) and fault structures are present in the basin. Here, we investigate the links between slope failures, slope basin development, and movement along a prominent out-of-sequence thrust (OOST) fault and development of a large anticline. Three two-dimensional (2-D) cross sections are restored to ~2.2 Ma using stratigraphic and structural relationships interpreted in the 3-D data. The restorations are then compared and combined to provide a 3-D perspective of basin development. We find that total shortening across all faults and folds was accommodated by different displacement styles along multiple branches, with 5.3, 5.5, and 7.3 km of shortening from northeast to southwest over the past ~2.2 Ma. We believe that the majority of this displacement occurred prior to ~1.7 Ma, followed by a dramatic decrease in slip rate within the study area as slip was transferred to the more seaward portions of the prism. In the northeast, deformation is primarily accommodated by the main branch of the OOST and anticlinal faulting, while deformation in the southwest is primarily along deeper branches of the OOST. This differential motion explains the occurrences of various mass wasting events, and lateral differences in trench slope basin geometry within the study area.