Sedimentary basins analysis through MCS profiles suggests that basins present in the landward slope of the Eastern Nankai trough originate from the compressional stress produced by the subduction of the Philippine Sea plate beneath the Southwest Japan. 26 migrated MCS profiles-oriented northwest – southeast made during Geological Survey of Japan research cruises GC98 and GA99, as well as a bathymetric map generated by GMT5 software were used to identify the basins. Once a basin was confirmed both by the MCS profile and on the bathymetric map, from the Nankai trough to the landward slope, a number (from 1 to n) was assigned to it. Then, basins with the same number were grouped together, and the basin with the greatest thickness was chosen to analyse the area deposition, thickness deposits, and structural direction. A total of 14 basins were identified and interpreted as piggyback basins (basins created by anticlines or syndepositional growth structure (Ori and Friend (1984))), and among these basins Kanasunose Trough (basin 2), Yukie Trough (basins 5-7), Shima-Oki Trough (basins 13-14) were recognized. Five preferential directions were identified: North-South (basins 8, 11, 14), North northeast – South southwest (basins 7,12), Northeast-Southwest (basins 5, 6, 10, 13), East northeast – west southwest (basins 1, 2, 3, 9), and West northwest – East southeast (basin 4). Basin 4 has a peculiarity by the fact of its west northwest – east southeast direction corresponding to the direction of the compressional deformation meaning that this basin is probably syn-tectonic. These piggyback basins with a landward tilting are composed of two seismic sequences S2 and S1. S2 (sub-parallel to migrating wave internal reflectors, continuous to discontinuous with varying amplitude events) unconformable onlaps onto the basement, and S1 (parallel internal reflectors, continuous with low to moderate amplitude) unconformable onlaps onto S2. S2 and S1 respectively can probably correspond to the Unit II and Unit I defined by Hiroki et al. (2004) in the well BH-1 giving an age comprised between 0.5-1.2 Ma for Unit 2 and 0-0.5 Ma for Unit 1, using Quaternary standard nannofossil events (events 1-10) of Sato and Takayama (1992), and corresponding to the nannofossil zone CN15-CN13b of Okada and Bukry (1980).

References

Hiroki Y., Watanabe K., and Matsumoto R., 2004. Lithology, Biostratigraphy, and Magnetostratigraphy of Gas Hydrate-Bearing Sediments in the Eastern Nankai Trough. Resource Geology. Vol. 54, no 1, 25-34.

Okada H., and Bukry D., 1980. Supplementary modification and introduction of code numbers to the low-latitude coccolith biostratigraphic zonation (Bukry, 1973; 1975). Marine Micropaleont., 5, 321-325.

Ori G.G. and Friend P.F., 1984. Sedimentary basins formed and carried piggyback on active thrust sheets. Geology,12, 475±478.

Sato T., and Takayama T., 1992. A stratigraphically significant new species of the calcareous nannofossil Reticulofenestra asanoi. In Ishizaki K., and Saito T., (eds.) Centenary of Japanese Micropaleontology, Terra Scientific Publ., 457-460.

The Oki Dogo, an island located in the southwestern part of the Sea of Japan comprises Proterozoic to Cenozoic Oki metamorphic rocks, and unconformably overlayed by volcanic and sedimentary rock strata from the late Oligocene to Miocene. Based on their age, the Oki metamorphic rocks are considered to be fragments of the continental crust (Kawabata et al., 2022), and are expected to record the geologic history prior to the opening of the Japan Sea and also tectonic process after that.

Recent studies focused on metamorphism and geochronology of the Oki Dogo, but studies intend to describe the fault activities are limited. Based on our detailed field surveys cataclastic fault zones are identified in the vicinity of the Oki metamorphic rocks.

In this study, we describe the location, structural evolution and tectonic significance of these cataclastic fault rocks. Fault outcrops are mainly identified in the metamorphic rocks along Araki river, Oku river and Nakamura river outcrops. Overall, ENE–WSW and WNW–ESE strike is prominent along the identified faults. The E–W, NE–SW, and NW–SE strike is also present in some locations. Most of the fault outcrops dip south, dip angle ranges between 40° to 70°. The width of the damage zone varies from 1–8 m. Reverse sense of movement predominant in the cataclasites, but in few locations normal and right lateral sense is also observed. In the Araki and Oku rivers, where fault outcrops observed, rhyolite intrusions are also identified in the near vicinity of the fault outcrops. Notably intrusive rocks are not affected by the fault activity, and we propose that the cataclasite faulting is possibly associated with these rhyolitic intrusions.

Our study found fault outcrops only in the metamorphic rocks, not in any other lithologies. This limitation makes it difficult to determine the relative age of the faulting. To address this challenge and better understanding of the stress fields associated with fault activities, we plan to conduct detailed fieldwork in the future. Identifying faults in Cenozoic strata and comparing them with those observed in this study will be crucial for clarifying the timing and nature of fault formation.

Reference: Kawabata et al., 2022, J. Metamor. Geol., 40, 257-286.

Earthquake along fault planes is attributed to shear rupture instabilities that usually activate damage in the fault core and surrounding damage zone. Such seismogenic faults usually exhibits several earthquake events during their lifetime. Each earthquake events augments to the heterogeneity of structure and strength of the fault zones, leading to changes in the mechanical properties and energy conditions of subsequent events. However, the evolution of the mechanical properties and energy conditions associated with multiple earthquake events is not well understood. In present study, a series of high-velocity slip-pulse experiments were conducted on simulated gabbro faults to investigate the evolution of mechanical properties of faults undergoing multiple slip events.The experiments are conducted in dry conditions with a pair of hollow cylinders of Belfast gabbro specimens that are repeatedly sheared using the Pressurized High Velocity (PHV) apparatus at slip velocity of 1 m/s and normal stresses of ~ 4 MPa. Each slip pulse representing a single earthquake event results in a total displacement of ~3.8 m. In the present study, we have attempted up to 5 slip pulses for each experiment. During the experiments, the frictional heating inevitably occurs leading to melting and temperatures ~1100–1300 ℃ at the simulated fault contact. Two stages of slip weakening are observed with one following the initial slip, and the other weakening occurs just after the second peak friction. The first weakening is associated with the flash heating at the asperity contacts, while the second weakening is attributed to formation and growth of the melt layer along the simulated fault. The two-stage slip weakening can be confirmed for all the slip pulses, however the displacement required for slip weakening decreases with subsequent slip pulses. This suggests that energy requirement decreases to initiate slip weakening associated with frictional melting in subsequent slip events. We will discuss the microstructural characteristics of the recovered samples observed by X-Radia micro-CT and SEM of the thinsections to clarify the mechanochemical processes in the fault zones. Our results elucidate the evolution of mechanical properties and breakdown energy in repeated earthquake sequences.

The Dharwar Craton, a Meso-Neoarchean terrane (3500-2500 Ma) in the southern India, is divided into three regions: the Western Dharwar Craton (WDC), Central Dharwar Craton (CDC), and Eastern Dharwar Craton (EDC). The Gadag-Mandya Shear Zone (GMSZ), which trends N-S to NW-SE, serves as the boundary between WDC and CDC.

Detailed fieldwork conducted along a 174 km stretch of the GMSZ has revealed the presence of granitic mylonite and occasionally amphibolite mylonite with a strike-slip sinistral sense of movement. Limited studies have detailed the structural evolution of this key shear zone in the Archean Eon. We will present the regional-scale variation in the microstructural properties of recrystallized quartz grains in the granitic mylonite and deformed granitic rocks of the GMSZ to unravel its microstructural evolution.

Microstructural analysis show that the dominant dynamic recrystallization structures in the studied samples are bulging recrystallization and sub-grain rotation recrystallization, which indicate low to medium grade deformation conditions. In some samples high temperature deformation characteristics are preserved possibly associated with syn-magmatic deformation.

We conducted a fractal dimension (D) analysis of recrystallized quartz grains in mylonitic samples to understand their shape characteristics. Our preliminary results show that, in the northern part of GMSZ, D value is 1.13, which plot granulite facies and synkinematic granite fields (based on Kruhl and Nega, 1996). In the central part, D is 1.13–1.18, which plot upper greenschist to lower amphibolite facies fields. In the southern part, D is 1.05, which plot close to granulite facies and synkinematic granite fields. The average size of recrystallized quartz grains in the northern part of the GMSZ is approximately 150 µm. In the central and southern parts, the averages are 60 µm and 45 µm, respectively.

Based on our initial microstructural studies, we have identified a low-to-medium grade deformation condition in the GMSZ, with a potential increase in deformation temperature towards the south within the shear zone. In future research, we aim to validate these findings through EBSD analysis of recrystallized quartz grains to ensure the consistency and robustness of our results.