Paleothermal structure of the Nankai inner accretionary wedge estimated from vitrinite reflectance of cuttings

Abstract

The paleothermal structure and tectonic evolution of an accretionary prism is basic information for understanding subduction zone seismogenesis. To evaluate the entire paleotemperature profile of the Integrated Ocean Drilling Program (IODP) Site C0002 located in the off‐Kumano region of the Nankai Trough and penetrate the inner accretionary wedge down to 3058.5 m below the seafloor (mbsf), we performed a vitrinite reflectance analysis for cuttings and core samples during IODP expeditions 338 and 348: Nankai Trough seismogenic zone experiment. Although vitrinite reflectance values (Ro) tend to increase with depth, two reversals of these values suggested the existence of thrust fault zones with sufficient displacements to offset the paleothermal structure. The estimated maximum paleotemperatures are 42–70°C at 1200–1300 mbsf, 44–100°C at 1600–2400 mbsf, and 56–115°C at 2600–3000 mbsf, respectively. These temperatures roughly coincide with estimated modern temperatures; however, at a smaller scale, the reconstructed partial paleogeothermal gradient (∼60–150°C/km) recorded at the hanging‐ and footwall of the presumed thrust fault zone is higher than the modern geothermal gradient (∼30–40°C/km). This high paleogeothermal gradient was possibly obtained prior to subduction, reflecting the large heat flow of the young Philippine Sea Plate.

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

“…The higher degree of illitization documented for discrete samples in our mineralogical and geochemical analysis supports the prograde smectite‐to‐illite diagenesis from ~30% to 65% illite in I‐S below 2 km bsf shown by XRD data (Figure b). Maximum paleo‐temperatures, estimated for Site C0002 based on time‐dependent maturation modeling and vitrinite reflectance measurements, are also consistent with a high‐temperature end‐member (Fukuchi et al, ). Taken together, there is multiple and consistent evidence for a thermal structure of the accretionary prism that is more consistent with the high‐temperature end‐member tested in our simulations.…”

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

“…Harris, Spinelli, & Fisher, 2017), as well as interpretations of such paleotemperature indicators as vitrinite reflectance (e.g. Fukuchi, Yamaguchi, Yamamoto, & Ashi, 2017). In addition, the contrasts between the original tectono-sedimentary environments for hanging wall versus footwall successions need to be considered when interpreting or modeling fault-slip behavior along the megasplay and the frontal megathrust.…”

Application of common reflection angle migration for imaging deformation structures in an inner accretionary wedge, Nankai Trough, Japan

“…(c) Palaeo‐geothermal structure reconstructed via vitrinite reflectance analysis using mainly borehole cuttings (modified from Fukuchi et al . ).…”

“…Fukuchi et al . () reconstructed the palaeo‐geothermal structure via vitrinite reflectance analysis using mainly drill cuttings from the C0002 borehole, and the results indicate a generally increasing trend with depth and several temperature reversals. The reversals suggest the presence of thrust faults, and one of those thrust faults with a vertical offset of approximately 700 m is inferred approximately 2400–2600 m below the sea floor (approximately 4400–4600 m below mean sea level).…”