Flow dynamics of Nankai Trough submarine landslide inferred from internal deformation using magnetic fabric

Abstract

Submarine landslide deposits in an active subduction zone were investigated by Integrated Ocean Drilling Program Expedition 333 as the ''Nankai Trough Submarine Landslides History.'' The expedition recovered a Pleistocene to Holocene sequence of stacked mass-transport deposits at Site C0018, located within a slope basin on the footwall of the megasplay fault in the Nankai Trough off the Kii Peninsula, southwest Japan. Six mass-transport deposit units intercalated with coherent intervals were recovered from the upper 190 m of the drilled succession. Anisotropy of magnetic susceptibility variations within mass-transport deposit units was used to ascertain their rheology. Shape parameters and magnetic fabric orientation reveal inhomogeneity through the sequences, indicating that different compaction and shear occurred within individual units. The upper intervals of younger units generally represent a magnetic fabric formed under vertical compression. However, the lower intervals involve magnetic fabrics indicating lateral shear with in-depth gradual change. In the older mass-transport deposit, a distribution of magnetic foliation forms tightly folded strata, indicating different sliding patterns from the younger mass-transport deposit units. Using available paleomagnetic data, the shear directions of basal intervals of units are reoriented, producing two different sliding orientations. Variation in the flow type and supply route is interpreted to reflect the slope condition (e.g., slope gradient and susceptibility to ground motion), which has been controlled by local tectonic evolution of the accretionary wedge.

Kinematics of Mass Transport Deposits revealed by magnetic fabrics

“…Kanamatsu et al . [] reconstructed the shear direction of six submarine MTDs from AMS data. In one of these MTDs, they inferred a direction based on the K 1 and K 3 axes, while for the other five, a direction that contains the K 2 and K 3 axes was inferred.…”

“…These studies suffer from having very narrow windows of observation (a few centimeters across) that may hamper the kinematic interpretation. A few previous studies [ Schwehr et al ., ; Meissl et al ., ; Kanamatsu et al ., ] have taken advantage of the continuous cores through MTDs, to portray their kinematics with AMS. Such studies have to use available paleomagnetic data in order to reconstruct the transport directions of MTDs.…”

Seafloor morphology and sediment magnetic fabric in a putative 1771 Meiwa tsunami source region in the southern Ryukyu Islands, SW Japan

“…The K min inclinations of the fabric induced by slumping are largely divergent from the vertical and ultimately become horizontal (e.g. Schwehr et al 2007, Kanamatsu et al 2014. However, no such fabric was observed in the AMS results.…”

Possible climate preconditioning on submarine landslides along a convergent margin, Nankai Trough (NE Pacific)

“…This uplift resulted in slope steepening, thereby creating the slopes necessary for the initiation of slope failure and perhaps explaining the onset of mass transport deposition at~0.86 Ma with MTD 6. For the period around 0.5 Ma, as inferred from the reconstructed flow directions of MTDs and bathymetric analyses (Kanamatsu et al 2014), seamount subduction could have affected the study area ( Fig. 1; Kimura et al 2011) and resulted in a change in slope orientation from NE-SW to NW-SE (the present slope).…”