Seismic survey using "Bubble Pulser" was carried out in Miho Bay off eastern Shinji lowland, Southwest Japan. Based on the seismic profiles, geologic history since the Late Pleistocene was reconstructed. Three distinctive reflectors, formed before the Jomon transgression were recognized in seismic profiles. The lowest reflector was formed before the last interglacial stage. The distinctive reflector in Sakaiminato area was recognized along old scour hole. The uppermost reflector was formed during the last glacial maximum. The geologic history since the late Pleistocene in the eastern Shinji lowland can be divided into following five stages. Stage 1 (MIS 6) was the regression period before the last interglacial. At stage 2 (MIS 5), the sea level was the same as of now, and also the old Yumigahama Peninsula was formed at the position where it is at the present. The erosional scour hole at the northern end of Yumigahama Peninsula was formed at this stage by the tidal current erosion. Topographical features similar to the present basin was formed. At stage 3 (MIS 4-3), scour hole was buried by clastic materials at the northern end of Yumigahama Peninsula. At stage 4 (MIS 2), valley system was formed during the late Pleistocene at eastern Shinji Lowland. At stage 5 (Holocene), present geographical features are formed after the Jomon transgression.
The forearc basin fill turbidite succession of the Pliocene Anno Formation is well exposed in Boso Peninsula, central Japan. As the turbidite succession of the Anno Formation is distributed along a couplet of anti- and syncline axes trending WNW to ESE, three-dimensional analysis of depositional system of the formation can be carried out by using many tuff beds as key markers. Lateral distribution and successive change of turbidite facies and palaeocurrent directions of turbidites were investigated, based on the detailed lithologic map of the study area. The six sedimentary facies, 1) channel fills of pebbly sandstones and conglomerates originated from sedimentary gravity flows, 2) natural levee deposits of rhythmic alternation of thin turbidites and hemipelagic mudstones, 3) depo-center deposits in depositional lobes of thick-bedded turbidites without distinctive erosions, 4) marginal deposits 1 of alternation of thin turbidites and hemipelagic mudstones, 5) marginal deposits 2 of alternation of tuff beds and hemipelagic mudstones, and 6) extensive slump deposits composed of various depositional forms, are defined and laterally correlated by using key tuff markers. Most palaeocurrent directions of turbidites, measured from current ripple marks of Bouma Tc division, sole marks and fabric of mud clasts, show southward spreading from the central part of the peninsula, though some of them show northward in the southern part and variable directions in the western part. The southward palaeocurrent directions indicate a main feeder channel or a canyon distributed in the northern part, while the northward palaeocurrent directions may indicate the reflection from the southern local topographic highs. The variable dispersive palaeocurrent directions may be caused by winnowing processes. Occurrence of two successive depositional systems, channel-levee system in the lower part and channel-lobe system in the upper part, forming a submarine fan, is suggested by the facies distributions and palaeocurrent directions. The upward retrogradation can be detected through the formation of the forearc basin fillinig.
It is controversial how and when the Japan Sea backarc basin opened. Onshore geology is an important clue for this issue. It is believed that arc-perpendicular extension was accompanied by the opening, and that SW Japan was separated and drifted from the Eurasia as a coherent block. However, there are Early Miocene grabens trending not only parallel but also perpendicular to the arc, suggesting that the crust under present SW Japan was broken into blocks. We studied the Lower Miocene geology of the Wajima area, northern Noto Peninsula, where syn- and postrift sedimentary rocks were uplifted by Late Miocene folding. We found that fan and fan delta environments were dominant in the late Early Miocene in the area. Faults with map- and meso-scales in this horizon indicate NE-SW or E-W extensional stress. Overlying Middle Miocene offshore sediments were not affected by the extensional tectonics, suggesting that the extensional tectonics had terminated by the end of the Early Miocene. The extensional orientation was parallel to the arc. This is concordant with the orientation suggested by Early Miocene grabens to the west of the peninsula, but is perpendicular to that in the northeastern part of the peninsula. These observations suggest that SW Japan was broken into polygonal blocks to drift from Eurasia.
We determined the internal and permeability structures of the nappe boundary between the Tamba and the Ultra-Tamba belt in the Inner Zone of Southwest Japan. Protolith of hanging wall is Permian sandstone and that of footwall is Jurassic pelitic breccia. These are stuck tightly with cataclasite zone of 3 m in width. Permeability data and structural observations of the fault zone suggest the following; (1) cataclasite is distributed in the fault zone, but most deformation is localized in 0.5 to 3 cm width foliated cataclasite zones in the hanging wall, (2) experiments at effective pressures, Pe, of 40 to 100 MPa show that permeability of the hanging wall is low (10-20 m2 at Pe > 80 MPa), while it is higher by 1-3 orders of magnitude in the footwall and the fault zone. The permeability structure implies that high pore fluid pressure could be sustained within the fault zone, which could have promoted the thrust movement.
The Plio-Pleistocene Okui Formation in the Komoro Group in central Japan is composed of lake and fluvial deposits and contains many pyroclastic deposits. A thick pyroclastic flow deposit within the Okui Formation, namely the U-1 pyroclastic flow deposit, includes biotite mineral and is composed mainly of volcanic glasses and coarse pumice fragments (average maximum grain size: 10 cm). The U-1 pyroclastic flow deposit can be correlated to the Znp-Ohta Tephra Bed, which is the early Pliocene (ca. 4 Ma) widespread tephra in central Japan, by petrographic properties and stratigraphic position. This correlation enables precise correlation of the early Pliocene formations between the sea sides (the Pacific Ocean and Japan Sea) and the inland regions. The U-1 pyroclastic flow deposit is thickest among the pyroclastic flow deposits previously correlated to the Znp-Ohta Tephra Bed. This suggests that the source area of the Znp-Ohta Tephra Bed is located near the distribution area of the Okui Formation.