Major variations in vitrinite reflectance and consolidation characteristics within a post-middle Miocene forearc basin, central Japan: A geodynamical implication for basin evolution

Forearc basin sediments near the ocean-ward margin preserve tectonic information related to plate subduction. The post-middle Miocene Boso forearc basin, central Japan, records major differences in structure, paleo-maximum temperature, and consolidation state between below (Miura Group) and above (Kazusa Group) the Kurotaki Unconformity, which formed at ca. 3 Ma. Many fault systems below the unconformity are characterized by a disaggregation-band-like inner fabric that apparently formed soon after sedimentation, whereas there are few of this type of fault system above the unconformity. Vitrinite reflectance values (Ro) are 0.38%-0.44% and 0.22%-0.25% below and above the unconformity, respectively. The consolidation yield stress (pc) in the Miura Group (23.7 MPa in the Anno Formation; 31.0 MPa in the Amatsu Formation) is much greater than that in the Kazusa Group (7.5, 7.6 and 9.6 MPa in the Umegase, Ohtadai and Kiwada formations, respectively). These clear differences in vitrinite reflectance and consolidation characteristics above and below the unconformity are attributed to a forearc basin evolution, which resulted in the Miura Group being geothermally matured, tectonically compacted, uplifted, and eroded (500 m in maximum) before sedimentation of the Kazusa Group. The forearc basin, especially near the trench-slope break, records structural and physical properties reflecting the plate-tectonic environment and the development of the trench-slope.