The 3D seismic geomorphological analysis for the late Quaternary sediments in the Joetsu buried submarine canyons and the western flank of the Umitaka Spur in the southeastern Toyama Trough, central Japan, reveals the characteristic depositional patterns caused by outbreaks of mass movements or turbidity currents on the surrounding flanks of the Umitaka Spur. In the Joetsu buried submarine canyons, 3D seismic survey data show evidence of lateral inflow sedimentation event, possibly caused by slope failure on the northern flank of the Umitaka Spur. At the foot of the western flank, the amplitude map on a sequence boundary horizon demonstrates a parallel elongated depositional pattern, which may reflect slope gully structures formed by abrupt turbidity currents flowing down the western flank of the spur. The 3D top surface display of chaotic mound structures seen on a seismic section demonstrates several kilometer wide debris flow lobes with an undulation pattern or a smooth pattern on the surface, reflecting the composed materials. Because previous studies pointed out that pockmarks, slope failures and related slope gully structures frequently develop on the present seafloor on and around the Umitaka Spur and Joetsu Knoll, it is suggested that these characteristic sedimentation events revealed in this study were caused by gas hydrate dissociation related to relative sea level changes during the Late Quaternary.

The development of the geological structures of the Toyama Trough on the eastern margin of the Japan Sea was reanalyzed based on recent data from seismic investigations carried out for resource exploration and earthquake disaster prevention. The results support a two-stage model of back-arc spreading with large N-S-trending shear zones along the eastern border of the Japan Basin and the Yamato Basin. The new findings from this study are:

1) Regional variance in the spatiotemporal distribution of faults, such as Miocene normal faults, and Pliocene and younger reverse faults showing evidence of translation and rotation, with active inversion structures.

2) The Toyama Trough shear zone, which is composed mainly of N-S striking faults, extends to the north along the eastern terminations of both the Yamato Basin and Yamato Rise into the southeastern margin of Japan Basin, and is regarded as a transform fault perpendicular to the spreading axis of the Japan Basin.

3) Seafloor spreading of the Yamato Basin occurred in two stages, first along E-W axes and secondly along NE-SW axes. During the final phase of the later spreading, the triangular depression at the easternmost part of Yamato Basin expanded by sinistral transtension on E-W to NW-SE faults, deforming the northern tip of Hakusan block into the present Hakusanse Shoal. The major N-S shear zone was widened by dextral transtension along conjugate faults, evolving into the present form of the trough.

4) As the Toyama Trough widened, forming a structural belt including a linear series of sedimentary basins within the Fossa Magna region, the former Miocene Honshu arc was divided into east and west segments; the timing of crustal deformation and the tectonic modes are thereafter distinct.

Syn-rift and post-rift geological structures in the Toyama Trough and adjacent areas of the southeastern margin of the Sea of Japan were interpreted from 2D and 3D seismic reflection data and other published data. The trends of the fault systems differ between the western and eastern parts of the study area, which are divided by N-S trending transcurrent faults along the eastern margin of the Toyama Trough. These faults extend to the Kashiwazaki-Choshi Tectonic Line, the border between the Southwest and Northeast Japan arcs. The western part of the study area is characterized by the superposition and intersection of six syn-rift fault systems with different trends, resulting from successive changes in the stress field during the multi-stage opening of the Sea of Japan. During the early rift stage (43-21 Ma) in the northern Toyama Trough, WNW-ESE trending rift basins formed, followed by NNW-SSE trending rift basins. During 21-18 Ma, ENE-WSW trending normal faults formed parallel to the Southwest Japan Arc. During the late rift stage (18-15 Ma), NNE-SSW trending en echelon pull-apart basins developed in the southern Toyama Trough and in the Northern Fossa Magna, related to right lateral divergent wrench deformation. The Toyama Trough is interpreted to be a composite rift basin in which the crustal thickness decreases from south to north.

After 15 Ma, the study area experienced compressional tectonics. Onlap unconformities and E-W trending reverse faults formed during the Middle Miocene (15-13 Ma) and Late Miocene (10-6 Ma). Since 6 Ma, successive changes in the direction and magnitude of compressional stress have resulted in the differentiation of uplifted and subsided areas related to the inversion of reverse faults. The post-rift structures and present course of the Toyama Deep-Sea Channel are strongly controlled by the older syn-rift structures.

Two Devonian brachiopod species, Spinocyrtia sp. and Strophopleura notabilis (Kindle), are here described from the newly proposed Takaya Formation of Takaya, Bungo-ono City, Oita Prefecture, in the Kurosegawa Belt of Kyushu, southwestern Japan. This is the first record, with systematic descriptions, of Devonian brachiopods from Kyushu. The Takaya fauna indicates a Late Devonian (Famennian) age. The Takaya Formation is thereby correlated with the Famennian formations of the Kurosegawa, South Kitakami and Hida Gaien belts in Japan.