Forearc slope deformation above the Japan Trench megathrust: Implications for subduction erosion

Abstract

Subduction erosion is a commonly invoked model that is used to explain the tectonic subsidence of the Japan Trench forearc slope, although other models have explained the morphology and history of the margin. New multichannel seismic reflection and bathymetric data collected after the 2011 Tohoku earthquake provide the opportunity to investigate the detailed structure of the overriding plate near the earthquake epicenter and obtain new constraints on tectonic models. We use regional-residual separation of the local bathymetry to constrain fault scarp extents and local landward-dipping forearc basins. Seismic images of these basins clearly show landward-dipping horizons in the shallow section. The strata in these basins imply a different mechanism for formation than the surrounding forearc slope, and we propose that these basins formed from local uplift. A regional basal unconformity mapped ~150 km along-trench has highly variable relief, indicating that forearc slope subsidence occurs at multiple wavelengths in response to multiple different sources. We characterize the upper to middle slope transition and propose that this region may be the landward limit of major subduction erosion and also the main region for large mass wasting. Normal faults found in this setting have maximum lengths of ~20 km, limiting their role in margin processes. Our results place constraints on the extent of major subduction erosion at the Japan Trench margin, and indicate that subduction erosion should be revisited as the sole model of formation to include additional tectonic processes.

Event Stratigraphy in a Hadal Oceanic Trench: The Japan Trench as Sedimentary Archive Recording Recurrent Giant Subduction Zone Earthquakes and Their Role in Organic Carbon Export to the Deep Sea

“…A relatively moderate to rapid convergence rate of 7-9 cm/yr (e.g., Loveless and Meade, 2010) and relatively thin sediment cover at the trench (0.4-1 km: Lallemand et al, 1994;Nakamura et al, 2013) control development of the Japan Trench, which may favor the occurrence of subduction erosion producing tectonic subsidence (von Huene and Lallemand, 1990;Clift and Vannucchi, 2004). Although there is no typical forearc basin, isolated basins occur on the upper slope terrace (Arai et al, 2014;Boston et al, 2017). The lower slope is steeper with an average gradient of ∼5 • (Kodaira et al, 2012;Koge et al, 2014).…”

Depth-varying structural characters in the rupture zone of the 2011 Tohoku-oki earthquake

“…North of 38°N, the slope sediments from the deep-sea terrace to the middle slope have been well examined (e.g., von Huene et al, 1994;Tsuru et al, 2000Tsuru et al, , 2002Kimura et al, 2012;Boston et al, 2017). Therefore, we briefly summarize structural characteristics of the slope sediments overlying the unconformity between the Cretaceous prism and Miocene slope deposit in light of those previous studies.…”

“…2). Boston et al (2017) examined seismic sections along 15 profiles across the forearc of the central to northern Japan Trench, including the five profiles reprocessed in this study, and inferred that the sediments of the upper to middle slope transition show both extensional and contraction features. These results imply that accretion and contraction structures are common across the lower slope, whereas extensional structures are common across the middle and upper slope.…”

Slip Inversion Along Inner Fore‐Arc Faults, Eastern Tohoku, Japan

“…We identify and date growth strata that bracket the initiation of thrust displacement along these structures to 5.8-4.9 Ma along the Oritsume Fault and <2.2 Ma along the Noheji Fault (Figure 12a). Contraction along these faults is synchronous with the 5.6-3.6 Ma initiation of thrust slip on the Futaba Fault (Regalla et al, 2010) and with deformation along a network of Plio-Quaternary thrust faults and fault-related folds present along the eastern margin of the Sea of Japan (Okada & Ikeda, 2012;Okamura, 2003), across the volcanic arc (Nakajima, 2006) and offshore in the outer fore arc (Boston et al, 2017) (Figure 12). Our data also reveal an earlier phase of extensional deformation along the Oritsume and Futaba Faults between~24-20 Ma and~16-15 Ma (Figure 12a), based on the presence of synextensional strata in the hanging walls of these faults.…”