In this paper, we review the general characteristics of submarine slides and their short- and long-term trigger mechanisms. Submarine slides have been reported from various sedimentary environments, including: 1) fjords; 2) active river deltas on continental margins; 3) submarine canyon-fan systems; 4) open continental slopes; 5) oceanic volcanic islands and ridges; 6) glacially influenced continental margins; and 7) continental slopes with active faults. From their geometry, submarine slides can generally be subdivided into three morphological domains: a headwall domain, a translational domain and a toe domain. Even in the initial stages of deformation, these domains can be clearly distinguished, as the headwall domain includes fissures, the translational domain includes asymmetric deformation structures due to shear deformation, and the toe domain is dominated by pressure ridges. Most of the slip surfaces correspond to clay-rich layers, although slip surfaces may also be sand layers in poorly drained environments, as reported in the Nankai Trough. The geological record over the past 20,000 years suggests there have been no large submarine slides in the last 5000 years, although data are scarce. In the Nankai Trough, there have been six large submarine slides in the last ~1 million years. The occurrence of these six slides does not correspond to sea-floor displacement resulting from large earthquakes. The trigger mechanism more likely relates to smaller earthquakes, resulting in an abrupt increase in ground acceleration and increased pore fluid pressure. The precondition for submarine slides includes many factors, such as a gradual increase in pore pressure by the decomposition of methane hydrate due to climate change, an increase in pore pressure as a result of high sedimentation rates, ground deformation due to the subduction/collision of seamounts, and/or slope steepening due to volcanic activity.

This paper presents new evidence regarding the depositional age of mudstone in the Shiraki Group within the Northern Belt of the Chichibu Composite Belt, Shima Peninsula. We address three issues: 1) the stratigraphic relationship between the Shiraki Group and the Shirataki Group (within the Kurosegawa Belt); 2) the position of the boundary between the Northern and Kurosegawa belts; and 3) the tectonostratigraphic correlation with the standard scheme for the entire Northern Belt in the Japanese Islands.

The Shiraki Group is the southeasternmost stratigraphic unit of the accretionary complexes within the Northern Belt to the north of the Gokasho–Arashima Tectonic Line. Radiolarians extracted from mudstone are characterized by Eoxitus elongatus, Eucyrtidiellum unumaense, Hsuum crassum, Hsm. maxwelli, Minutusolla aff. latusicostata, Mnt. nishimurae, Mnt. cf. michelei, Praewilliriedellum cf. convexum and Striatojaponocapsa synconexa. This fauna indicates that the mudstone of the Shiraki Group is middle Middle Jurassic (Bajocian–early Bathonian) in age.

The Shirataki Group was previously assigned to the Kurosegawa Belt to the southwest of the Shiraki Group. It shares common lithofacies, geological structures and radiolarian ages with the Shiraki Group, suggesting both groups can be regarded as a single stratigraphic unit. Integrating these units leads to a southward shift of the boundary between the Northern and Kurosegawa belts.

The scheme of standard stratigraphic units applicable throughout the entire area of the Northern Belt in the Japanese Islands encompasses five units. The lithofacies and depositional ages of the Shiraki Group show striking similarity to those of the Kamiyoshida Unit. Thus, the Shiraki Group is probably correlated with the Kamiyoshida Unit.

The Ryukyu Group in the Ryukyu islands consists of reef limestones of Pleistocene age, and records cycles of marine regression and transgression. Study of the rocks has the potential to constrain precise sea level changes around the middle Pleistocene climate transition from 40 kyr cycles to 100 kyr cycles. To assign a geochronological marker in the age of the transition, we undertook magnetostratigraphic studies of the Ryukyu Group rocks exposed on Miyakojima Island. Paleomagnetic samples were collected at 20 sites from all 5 sequence-stratigraphic units on Miyakojima Island (MY-Units 1 to 5 in ascending stratigraphic order). The conventional thermal demagnetization procedure provided reliable polarity determinations for only seven sites. For the remaining 13 sites, we developed a novel technique of reductive chemical demagnetization (RCD) combined with alternating field demagnetization. This hybrid technique successfully erased the overprinting magnetic components, revealing the primary component. Paleomagnetic directions of 20 sites show that the lower part of the Ryukyu Group (MY-Units 1-3, and the lowest part of MY-Unit 4) records reversed geomagnetic polarity, whereas the upper part (all but the lowest part of MY-Unit 4 and MY-Unit 5) records normal polarity. Combining the magnetostratigraphic data with existing calcareous nannofossil data, we conclude that the reversed-normal geomagnetic polarity transition corresponds to the Matuyama-Brunhes boundary (MBB). These magnetostratigraphic data including the MBB improve the geochronology of the Ryukyu Group, which is useful for the temporal correlation between the Ryukyu Group and the other climate records during the middle Pleistocene climate transition.

Saruodaki Falls, which is about 60 m high, is one of the highlights of the San'in Kaigan Geopark, northern Hyogo Prefecture, SW Japan. The quartz diorite exposed at the falls has previously been thought to represent a wide dike. However, we found that it is a laccolith with a horizontal diameter of <4 km and a thickness of >100 m. The base of the laccolith is not exposed. The host of the intrusive body consists of a lower Middle Miocene shaley formation, which is subhorizontal in this region. However, the formation makes a culmination centered by the body. In addition, the interface between the shaley formation and the diorite is concordant with the domal structure of the surrounding shale. Fracture patterns observed at the falls suggest that the laccolith is a composite sill made up of at least four sheets. Fission-track and U-Pb dating of zircon from the lower part of the laccolith yields ages of 15.7±1.2 Ma and 16.1±1.4 Ma, respectively. These ages are concordant with fossil data from the host rocks.

Geochemical data of metamorphosed mafic rocks can be a powerful tool for inferring the tectonic setting of their protoliths using discrimination diagrams. The metamorphic rocks from the Mt Sangun area consist mainly of mafic and pelitic rocks. In this study, we performed geochemical analyses of the metamorphosed mafic rocks from the Mt. Sangun area and plotted the data on discrimination diagrams. The rocks can be divided into two types: fine- and coarse-grained amphibolite. Most of the fine-grained amphibolites plot on the differentiation trend of basaltic magma; however, some of the coarse-grained amphibolites record accumulation according to the SiO₂/Al₂O₃ - Mg# (100×MgO/(MgO+FeO*)) diagram. The fine-grained amphibolites are geochemically similar to mid-ocean ridge basalt (MORB) or back-arc basin basalt (BABB). In contrast, the coarse-grained amphibolites, excluding samples with cumulate characteristics, resemble volcanic arc basalts (VAB).

The conventional point-counting method for modal analysis of rocks is time-consuming and depends on the user to correctly identify minerals. We introduce an alternative method using a scanning X-ray analytical microscope in conjunction with image processing and analyzing software. This method is simple and less reliant on the skill and subjectivity of the user. By using this method for thin sections or polished slabs of granitic rocks, we provide clear images showing the distribution of minerals to enable objective modal analyses to be performed quickly and efficiently.