DEVELOPMENT OF THE KUMANO FOREARC BASIN, NANKAI TROUGH SUBDUCTION ZONE

The forearc basin of the Nankai Trough accretionary prism has developed over the past 6 Ma as a product of subduction of the Philippine Sea Plate (PSP) under Japan. The basin’s tectono-sedimentary history has been controlled by feed-back between subduction, volcanism, subaerial erosion, sedimentation and accretion.

The Nankai Trough experienced slow subduction or strike slip from ~9-6 Ma. During that time sediment eroded from Japan was transported out onto the PSP. Rejuvenated subduction at ~6 Ma led to rapid accretion of those sediments and formation of an outer ridge, creating accommodation space and trapping sediments in the nascent Kumano Basin.

Accreted strata form the basin’s basement. The overlying lower trench-slope deposits are a condensed section of hemipelagics separated from the underlying prism by an angular unconformity. Strata above the unconformity increase in thickness from 50 to 750 from SE to NW. The unconformity between the prism and overlying forearc strata is time transgressive: at the SE edge of the basin it separates 5 Ma prism rocks from 3.65 Ma basin deposits; in the center of the basin, it separates 5.6 Ma prism from 3.8 Ma basin strata.

The oldest FAB strata in the SW part of the basin are ~1.67 Ma, whereas those in the central part of the basin are younger than ~0.9 Ma due to progressive landward onlap onto the slope strata. The landward-onlapping sequences tilt progressively landward due to regional uplift along the basin’s seaward edge. Regional tilting shifted the depocenter progressively landward, expanding the basin from <10 km in width to >30 km. Turbidites in the upper FAB sequence progressively onlap the lower FAB strata in a seaward direction. Total thickness of FAB strata is 750-800 m.

Pleistocene-Holocene sedimentation has been controlled by changes in sea level and source supply. Three distinct seismic facies are related to the different stages of the deglaciation phase: the low stand stage is defined by chaotic debris- and mud-flow deposits and cut-and-fill structures, with sediments accumulating mainly in the deep basin. As sea-level rose, there was an increase in sand supply, so the transgressive stage deposits are mainly sandy turbidites. Progradation of the sandy materials toward the deep basin was followed by retrogradation of the sandy deposits toward the canyon mouths.