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.

Recognition of trench basins in collisional orogens: Insights from the Yarlung Zangbo suture zone in southern Tibet

“…In convergent plate margins close to accretionary orogens, trench-basin deposits are preserved as tectonic slices within the subduction complex, as an important component of the subduction complex. Sedimentary basins are also formed above the subduction complex, as commonly seen in seismic-reflection profiles of modern systems (Underwood and Moore, 1995;Lackey et al, 2020). The basal contact of these basins, however, is typically deformed strongly during or-ogeny, which makes the distinction between trench basins and trench-slope basins exceedingly difficult.…”

Sediment provenance, routing and tectonic linkages in the Nankai forearc region, Japan

“…Upper forearc basin sedimentation (from sample 221 to sample 212) appears generally dominated by Kumano River material, with the noteworthy exceptions of samples 222 and 225 (see below). This is likely a result of efficient seaward sediment capture, with accommodation space being provided by continued activity along the Megasplay Fault (Figure 10c,d; Buchs et al., 2015; Lackey et al., 2020; Moore et al., 2015; Ramirez et al., 2015).…”

“…2.2 Ma. Between 2.2 and 1.5 Ma, the tilting of the incipient Kumano Basin floor, and motion along the Megasplay Fault, allowed large volumes of sediment to accumulate (Gulick et al., 2010; Lackey et al., 2020; Moore et al., 2015; Ramirez et al., 2015). After 1.5 Ma, a combination of sediment subduction/underplating, megasplay faulting, and continued accretion at the toe appear to have driven continued subsidence within the wedge (C0002 upper basin samples).…”

Bathymetric Signatures of Submarine Forearc Deformation: A Case Study in the Nankai Accretionary Prism

“…Second, the KBEFZ strike‐slip system truncates the megasplay on the eastern portion of the map area (Figures 7 and 8b). These cross‐cutting relationships suggest the strike‐slip system is younger and more active than the megasplay fault, such that the KBEFZ strike‐slip faults have been active after ∼1.24 Ma when slip along the megasplay fault either ceased or decreased significantly (Lackey et al., 2020). Third, both the compression directions we infer from tectono‐geomorphic mapping and S Hmax axes from borehole breakouts change direction across the KBEFZ (Figure 9b).…”