This paper attempts to examine the evolutionary relationship between the North Fossa Magna region and the Toyama Trough, which includes the Toyama Deep-sea Channel system, to constrain late Cenozoic tectono-sedimentary processes at the junction zone between the NE and SW Japan arcs. Using outcrop data, 2D/3D seismic surveys, exploration-well data, and published literature, we compare tectonic regimes, paleogeography, depositional systems, and sediment supply systems between the two areas, with a focus on the connectivity of submarine-fan systems. The results reveal that the North Fossa Magna region experienced a similar tectonic history to that of the NE Japan arc, including Middle Miocene rifting and Late Miocene basin inversion, whereas the tectonic history of the Toyama Trough is more similar to that of the SW Japan arc, which experienced continuous compression after the Middle Miocene rifting stage. The obtained paleogeography and depositional-system distributions indicate that submarine fans developed in the northern part of the North Fossa Magna region were connected to the Toyama Deep-sea Channel system, and the North Fossa Magna region is regarded as the major clastic provenance of the Toyama Deep-sea Channel system until at least 3 Ma. The sediment supply system to the Toyama Trough can be characterized by multiple stepwise supply systems consisting of NE-SW-trending, parallel, terrace-like basins and interconnecting submarine fan systems, which are possibly related to the basin rifting and compression structures. The amount of clastic supply appears to have varied in response to the tectonic conditions through the late Miocene and Pliocene, causing changes in submarine-fan depositional style. When the Northern Alps of Japan were uplifted drastically, at around 3 Ma, the major sediment supply route shifted to its present position, passing through the Toyama Plain and Bay.
We present a revised Cenozoic chronostratigraphy of the Yatsuo Area, Toyama Prefecture, based on U-Pb and fission-track (FT) dating of zircon grains from tuff beds, mineralogical analysis of tuff beds, and diatom biostratigraphy. The results reveal that syn-rift volcanism (represented by the Iwaine Formation) began at ~17.5 Ma and that the Ikahama unconformity between the syn-rift Higashibessho Formation and the post-rift Tenguyama Formation represents a relatively short-duration (~0.5 m.y.) event ~15 Ma. The Otogawa Formation unconformably overlies the Tenguyama Formation and is divided into middle Miocene (Serravallian) and late Miocene, with a possible unconformity in between. The 2-4 Ma Mita Formation unconformably overlies the Otogawa Formation. These results require previous local correlations of a key tuff bed (MT2) in the Mita Formation to be reassessed.
Facies and sequence stratigraphic analysis of the lowermost Nirehara Formation confirm that this formation comprises multiple depositional cycles generated by water-level fluctuations in a coastal or lacustrine fan delta. The results also suggest that the Nirehara Formation, which is unconformably overlain by the syn-rift Yatsuo Group, was formed in an early rift basin. Detrital zircon U-Pb and FT dating of sandstone within the Nirehara Formation suggests that the sandstone may be derived from the Nohi Rhyolite.
Regional correlations of onshore and offshore Cenozoic chronostratigraphy in the Hokuriku district show multistage rifting during the Oligocene-middle Miocene opening of the Sea of Japan. Regional correlations also suggest that the post-rifting compressive regime was associated with fluctuations in the strength and direction of compressive stress fields.
Magnitude (M) >6 earthquakes triggered by reverse faults, such as the 1993 Noto-Hanto-oki earthquake (M 6.6) and the 2007 Niigata-ken chuetsu-oki earthquake (M 6.8), occur in the high-strain-rate zone of the Toyama Trough, on the eastern margin of the Japan Sea. In 2007 and 2009, we conducted marine seismic surveys using a multichannel seismic reflection system and ocean bottom seismometers in and around the trough. The results reveal differences in crustal structure between the southern and northern parts of the trough. The crust is thinner in the northern part of the trough, whereas the upper crust in the southern part is thick. In addition, the Moho and several other reflections in the crust are identifiable only in the north. Numerous anticlines and reverse faults are formed along the entire margin as well as the in the southeastern part of the trough.