Deformation of the Nankai Trough inner accretionary prism: The role of inherited structures

Abstract

Accretionary prisms commonly grow seaward, with the strata of the inner prism consisting of older, previously accreted outer prism rocks overlain by thick fore-arc basin strata. We focus on the Nankai Trough inner accretionary prism using three-dimensional (3-D) seismic data and logging data from the Integrated Ocean Drilling Program (IODP). We update the 3-D seismic volume using well velocity data to better constrain deeper horizons. Interpretation of these horizons reveals multiple folds with axial surfaces that strike near parallel to modern outer prism thrust faults, and we interpret that these folds formed as a result of thrust faulting. Reactivation of one inner prism thrust fault continued until at least 0.44 Ma, after the modern fore-arc basin formed, indicating that the inner prism had continued deformation until that time. Structural restorations of these folded seismic horizons demonstrate that 580 m of slip occurred on this steeply dipping reactivated thrust after fore-arc basin formation. Structural interpretation and analysis of logging-while-drilling data, including borehole images, in the deep inner prism revealed intense deformation of a generally homogenous lithology characterized by bedding that dips steeply (608-908), intersected by faults and fractures that have a range of dips and densities. Our study of the deep Kumano Basin provides new insights into the structure of the inner prism and reveals that although the inner prism has partially preserved inherited outer prism structures, these older folds and faults are steeply rotated and cut by multiple fracture populations during subsequent deformation.

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

“…The unorthodox interpretation presented above is based largely on sedimentological criteria, but it is consistent with the analysis of seismic and logging data by Boston et al (), who documented large‐scale reactivation of thrust faults, steep rotation of older folds and faults, and multiple fracture populations within the inner prism. It seems likely that the younger structural fabric caused by Philippine Sea plate subduction is superimposed on an older fabric imparted by Pacific plate subduction.…”

“…A combination of riserless and riser drilling during IODP Expeditions 315, 338, and 348 extended boreholes through the forearc basin into intermediate depths of the inner accretionary prism, reaching a maximum depth of 3058 mbsf (Ashi et al, ; Strasser et al, ; Tobin et al, ). Analysis of 3‐D seismic data in the vicinity of Site C0002 provides clear evidence of thrust fault reactivation after the forearc basin formed (Boston, Moore, Jurado, & Sone, ). In addition, deformation of the inner prism is heterogeneous and intense, and the prism's semi‐transparent acoustic character is indicative of a generally uniform lithology with steep bedding dips (Boston et al, ).…”

“…Seismic line runs roughly perpendicular to the trench axis through Kashinosaki Knoll. (c) Simplified interpretation of seismic‐reflection profile crossing the Nankai accretionary prism (modified from Boston et al, ), with locations of IODP Sites C0002 and C0001. See Figures and for expanded seismic‐reflection details near those sites…”

Impact of sedimentation on evolution of accretionary wedges: Insights from high-resolution thermomechanical modeling

“…Recently, Ramirez et al . [] pointed out for the Nankai wedge that stabilization due to the formation of the KFB could have led to deactivation of the ancient OOST underneath the KFB [ Tsuji et al ., , ; Boston et al ., ].…”

“…Reflection seismic and borehole data from accretionary margins have provided precise position and timing of internal deformation structures inside the accretionary wedges. For instance, seismic stratigraphy and borehole data from Nankai Trough, SW Japan, have revealed the existence of prominent OOST adjacent Kumano forearc basin (KFB) (referred to as the Megasplay Fault [ Park et al ., ]) and its temporal variation in fault activity [ Strasser et al ., ; Gulick et al ., ; Kimura et al ., ; Ramirez et al ., ; Boston et al ., ]. Recent attempts to reconstruct deformation history of the accretionary wedge adjacent to the KFB, using the stratigraphy of wedge‐top basins, have strongly corroborated control of sedimentation on wedge stabilization and temporal variation of the Megasplay Fault activity [ Hayman et al ., ; Moore et al ., ; Ramirez et al ., ].…”