Laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) was used to determine U and Pb isotopic compositions of detrital zircons from sandstones of the Takaharagawa, Akataki, and Makio complexes, which are parts of the Cretaceous accretionary complex of the Shimanto Belt, central Kii Peninsula, Southwest Japan. The Takaharagawa Complex is divided into three thrust-stacked units: the Upper Sandstone, Mixed-rock, and Lower Sandstone units. There are few index fossils in the rocks, so it is controversial whether the Takaharagawa Complex is part of the late Early to Late Cretaceous accretionary complex of the Sihimanto Belt, or the Jurassic to earliest Cretaceous accretionary complex of the Chichibu Belt. The youngest detrital zircon U-Pb ages of both sandstone units of the Takaharagawa Complex are ca 115 Ma (Aptian). These ages, in conjunction with lithological characteristics, confirm that the Takaharagawa Complex is part of the Shimanto Belt. This Complex has been correlated with the Yukawa Complex in the western Kii Peninsula, based on similarities in the sandstone-dominant lithologies, but the youngest U-Pb ages of the two complexes are different. The lithology and depositional age of the Akataki Complex are similar to those of parts of the Hanazono and Miyama complexes in the Shimanto Belt, but the detrital-zircon age spectra are different. The new ages suggest that the depositional age of the Makio Complex is Campanian, which is younger than previously reported.
We investigated the geological structure around the boundary between the Jurassic Chichibu accretionary complex (AC) and the Cretaceous Shimanto AC in the Yoshino area (central Kii Peninsula, SW Japan). The Chichibu AC in the study area is divided into the Daifugendake and Wasabidani complexes. Bedding plane in the Chichibu AC strikes E-W and dips gently to the north or south. The Shimanto AC, which consists of the Mugitani Complex, is characterized by the WNW-ESE striking and moderately northward dipping foliation. The boundary between the Chichibu AC and the Shimanto AC is a low-angle fault (Omine-Odai Thrust), which strikes WNW-ESE and dips gently to the south. To constrain the depositional ages of the Chichibu AC, we obtained detrital zircon U-Pb ages from four sandstone samples in the Daifugendake Complex (two samples) and the Wasabidani Complex (two samples). The youngest single grain and youngest cluster ages in the Daifugendake Complex are ca. 175-164 Ma, and those in the Wasabidani Complex are ca. 168-153 Ma. The ages are significantly older than previously reported youngest U-Pb ages from the Mugitani Complex (ca. 110-100 Ma), indicating that an age gap of ca. 55 Myr exists across the Omine-Odai Thrust. This age gap is considerably wider than the previously suggested age gap of ca. 20-30 Myr across the boundary between the Chichibu AC and the Shimanto AC in other areas (e.g., Shikoku and Kanto Mountains).
The stratigraphy and characteristics of ground motion in the Pleistocene Setagaya and Tokyo formations were investigated. These strata, which occur beneath the Musashino Upland, southwestern Tokyo, central Japan, were studied using drilling surveys and observations of microtremors. Facies analysis and tephro- and palyno-stratigraphic examination of sediment cores reveal that the Setagaya Formation is composed mainly of muddy sediments deposited as incised-valley fills in a bay environment during the early to middle Marine Isotope Stage (MIS) 5e. The valley is thought to have formed during MIS 6. The valley-filling muddy sediments are soft with standard penetration resistance (SPT N-values) between 1 and 5, and S-wave velocities of ~150 m/s. The overlying Tokyo Formation comprises gravelly and muddy sand that accumulated in the inner parts of a larger bay, most likely during the middle to late MIS 5e. Microtremor observations show that ground motion in the Setagaya area reflects the distribution of these Pleistocene strata. There appears a peak at 1 Hz in the microtremor horizontal to vertical (H/V) spectral ratio in locations where the Setagaya Formation is relatively thick (~20 m) and lacks overlying gravelly terrace deposits. When considering either geotechnical problems or geological disasters that include amplification of earthquake motions in the metropolitan area, we need to focus on the soft muddy valley-fills beneath uplands, such as those of the Setagaya Formation.