The origin of strata within the inner accretionary prism of Nankai Trough: Evidence from clay mineral assemblages along the NanTroSEIZE transect

Abstract

One of the more prominent architectural elements of the Nankai subduction margin, offshore southwest Japan, is an out‐of‐sequence thrust fault (megasplay) that separates the inner accretionary prism from the outer prism. The inner prism (hanging wall of the megasplay) is dominated by mudstone, which is enigmatic when the sedimentary facies is compared to coeval deposits in the Shikoku Basin (i.e. inputs from the subducting Philippine Sea plate) and to coarser‐grained turbidite sequences from the Quaternary trench wedge. Clay mineral assemblages amplify the mismatches of sedimentary facies. Mudstones from the inner prism are uniformly depleted in smectite, with average bulk values of 23–24 wt%, whereas the Shikoku Basin deposits show progressive decreases in proportions of smectite over time, from averages of 46–48 wt% at 10 Ma to 17–21 wt% at 1 Ma. Plate‐boundary reconstructions for the Philippine Sea region provide one solution to the conundrum. Between 15 Ma and 10 Ma, the Pacific plate subducted near the NanTroSEIZE transect, and a trench‐trench‐trench triple junction migrated to the northeast. Accretion during that period involved sediments that had been deposited on the Pacific plate. Motion of the Philippine Sea plate changed from 10 Ma to 6 Ma, resulting in sinistral slip along the proto‐Nankai Trough. Sediments accreted during that period probably had been deposited near the triple junction, with a hybrid detrital provenance. Renewed subduction of the Philippine Sea plate at 6 Ma led to reorganization of watersheds near the Izu–Honshu collision zone and gradual incision of large submarine canyons on both sides of the colliding Izu arc. Accreted Pliocene mudstones share more of an affinity to the triple junction paleoenvironment than they do to Shikoku Basin. These differences between subducting Shikoku Basin strata and accreted Pacific plate sediments have important implications for interpretations of frictional properties, structural architecture, and diagenetic fluid production.

Mantle-derived fluids in the Nankai Trough Kumano forearc basin

“…The Nankai Trough, located southeast of the eastern coast of southern Japan, was formed mainly during the Oligocene-Miocene period by a complex subduction-accretion process that probably included migration of a triple junction (Kimura et al 2014;Underwood 2018). The northwest directed subduction of the PSP beneath the Eurasian Plate initiated around~15 Ma, temporarily ceased at~12 Ma and restarted at~6 Ma (e.g.…”

“…Strasser et al (2014c) and Expedition 348 Scientists and Scientific Participants (2014) concluded that Unit V is either trench or Shikoku Basin sediment. However, most of the sediments deposited in the Shikoku Basin between 10.5 and 5.6 Ma contain substantially higher proportions of smectite (Underwood 2018) than mudstones with corresponding ages at site C0002 (depth interval 1400-2400 mbsf, see Table 1). Such discrepancies in bulk mineralogy between accreted mudstones at C0002 and coeval deposits in the Shikoku Basin call for different origins of sediments.…”

Spatiotemporal Characterization of Smectite‐to‐Illite Diagenesis in the Nankai Trough Accretionary Prism Revealed by Samples From 3 km Below Seafloor

“…Also, more widespread sources have been discussed such as detrital illite from Taiwan or fine‐grained sediment from mainland China. For a more detailed discussion the reader is referred to the studies of Underwood (), Underwood and Pickering (), and Underwood and Guo ().…”

“…Recent plate reconstruction studies relate the quiescence of subduction between 10 and 6 Ma to a clockwise rotation of the Philippine Sea plate during which subduction was highly oblique or the plate boundary underwent transform fault motion (Mahony et al, ; Wu et al, ), possibly aided by a young and buoyant Philippine Sea plate resisting subduction (Kimura et al, ). According to those plate motion reconstructions the triple junction between Philippine Sea plate, Pacific plate, and Eurasia migrated in the last 22 Ma from Shikoku Island to the NE to its present‐day position (Wu et al, ) such that the Nankai accretionary prism probably incorporated sedimentary rocks that originally accumulated in the Japan Trench (Underwood, ).…”

Simultaneous estimation of in situ porosity and thermal structure from core sample measurements and resistivity log data at Nankai accretionary prism

“…Hamada et al (2018) estimated the strength of rock using drilling parameters and found strength increases at around 2800 mbsf, which may also correspond to the depth where the estimated in situ porosity shows a stepwise decrease. Note that as the temperature reaches 100 °C at around 2600 mbsf, the transition from smectite to illite in the sedimentary material completes (Underwood 2018), which may affect the strength of rock and porosity level distributions.…”