Three-dimensional mapping and kinematic characterization of mass transport deposits along the outer Kumano Basin and Nankai accretionary wedge, southwest Japan

Abstract

Three-dimensional (3D) seismic data from the southern Kumano Basin of southwest Japan image a nested series of moderately sized mass transport deposits (MTDs) that slid from a slope along the seaward side of the forearc basin. The deposits are dated to be approximately 0.3 to 0.9 Ma. These MTDs are likely linked to the movement along a prominent out-of-sequence thrust (OOST) fault, regionally steeper slopes that would have existed during deposition, and shifts in sedimentation over the past 0.9 Ma. The spatial resolution provided by the 3D seismic data permits the identification of kinematic characteristics and the internal geometries of the MTDs which total over 2.8 km 3 in volume and cover more than 59 km 2 of the seafloor at various stratigraphic levels. Each MTD is well imaged and exhibits various kinematic indicators while most of the basal glide planes and original headwall scarps above the deposits have been partially or fully eroded by subsequent MTDs. There are at least seven individual deposits that range in volume from 0.005 to 1.16 km 3 , in area from 0.2 to 21.8 km 2 ; have runouts between 0.55 and 7.9 km, and generally translate downslope from the SE to NW. Basal, internal, and top surface kinematic indicators, such as grooves, thrust and fold systems, and pressure ridges, show that these MTDs originate from a prominent slide scar recognized in the high-resolution regional bathymetry. This, combined with a regionally shifting depocenter and faulting related to the earthquake cycle, points to regional tectonic activity as being the most likely failure trigger for these nested landslides.

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

“…Water depths vary from ~2,950 m at the seaward edge of the study area to ~2,150 m at the landward edge. Notable characteristics include the shallow extent of the MSF, various landslide scars in both the 3‐D survey and bathymetry, and large anticlines (e.g., Lackey, Moore, & Strasser, 2018; Lackey, Moore, Strasser, Kopf, et al, 2018; Moore et al, 2013; Strasser et al, 2009, 2011).…”

“…In the Nankai prism (Figure 1), a major OOST that branches (splays) from the main décollement (Park et al, 2002) and cuts through the prism to the surface has been termed the “Megasplay Fault” (MSF; Tobin & Kinoshita, 2006). Faults within the MSF region may host seismogenic slip (Hirono et al, 2009; Sakaguchi et al, 2011; Yamaguchi et al, 2011) and may trigger large slope failures (Lackey, Moore, & Strasser, 2018; Lackey, Moore, Strasser, Kopf, et al, 2018; Strasser et al, 2011). Constraining the slip history of the MSF system is therefore relevant for understanding the history of accretionary prism and basin evolution, as well as the tectonic conditions that can generate mass transport deposits (MTDs).…”

“…Hemipelagic and terrigenous sediment that is deposited on the prism during its growth accumulates in trench slope basins, creating sedimentary sequences >500 m thick (e.g., Underwood & Moore, 2012). The structural changes within the underlying prism, along with other preconditioning factors, such as oversteepened slopes, gas hydrate dissociation, or earthquake cycle related ground shaking (e.g., Bangs et al, 2010; Ikari et al, 2011; Kremer et al, 2017; Lackey, Moore, & Strasser, 2018; Sacks et al, 2013), provide an environment in which subaqueous landslides may occur, shaping the surface of the prism.…”

“…Numerous triggering mechanisms have been proposed for MTDs and surficial landslide scars that spatially correlate with the MSF (Figure 2; Lackey, Moore, & Strasser, 2018; Lackey, Moore, Strasser, Kopf, et al, 2018; Strasser et al, 2011). Free methane gas venting or hydrate dissociation from within the underlying strata can initiate landsliding locally or on kilometer scale sections of the seafloor (Bangs et al, 2010; Kremer et al, 2017; Westbrook et al, 2009).…”

“…Free methane gas venting or hydrate dissociation from within the underlying strata can initiate landsliding locally or on kilometer scale sections of the seafloor (Bangs et al, 2010; Kremer et al, 2017; Westbrook et al, 2009). Internal prism deformation can generate oversteepened slopes prone to slope failure (Lackey, Moore, & Strasser, 2018; Lackey, Moore, Strasser, Kopf, et al, 2018; Strasser et al, 2011). Coseismic rupture of a fault, such as the MSF (Figures 1 and 2), can amplify ground shaking within the hanging wall (Abrahamson & Somerville, 1996) and lead to slope failure.…”