The Median Tectonic Line (MTL) in southwest Japan, a major east-westtrending arc-parallel fault, has been defined as the boundary fault between the Cretaceous Sambagawa metamorphic rocks and Ryoke granitic and metamorphic rocks, which are unconformably covered by the Upper Cretaceous Izumi Group. The juxtaposition by faulting occurred after the deposition of the Izumi Group. Based on detailed fieldwork and previous studies, the authors reconstruct the kinematic history along the MTL during the Paleogene period, which has not been fully understood before. It is noted that although the strata of the Izumi Group along the MTL dip gently, east-west-trending north-vergent folds with the wavelength of~300 m commonly develop up to 2 km north from the MTL. Along the MTL, a disturbed zone of the Izumi Group up to 400 m thick, defined by the development of boudinage structures with the transverse boudin axis dipping nearly parallel to the MTL, occurs. Furthermore, east-west-trending north-vergent folds with the wavelength of 1-5 m develop within the distance up to 60 m from the MTL. The disturbed zone with the map-scale north-vergent folds along the MTL, strongly suggests that they formed due to normal faulting with a top-to-the-north sense along the MTL. Considering that the normal faulting is associated with the final exhumation of the Sambagawa metamorphic rocks, and its juxtaposition against the Izumi Group at depth, this perhaps occurred before the denudation of the Sambagawa metamorphic rocks indicated by the deposition of the Lower Eocene Hiwada-toge Formation. Dynamic equilibrium between crustal thickening at depth (underplating) and extension at shallow level is a plausible explanation for the normal faulting because the arc-normal extension suggests gravity as the driving force.
The Median Tectonic Line (MTL) in Southwest Japan, a major east-west trending arc-parallel fault, has been defined as the boundary fault between the Cretaceous Sambagawa metamorphic rocks and the Ryoke granitic and metamorphic rocks, which are unconformably covered by the Upper Cretaceous Izumi Group. Based on the detailed fieldwork and microstructural studies of fault rocks, we reconstruct the kinematic history along the MTL during the Paleogene, which can be divided into the Ichinokawa and pre-Tobe phases. While the Ichinokawa phase is defined by large-scale, top-to-the-north normal faulting, the pre-Tobe phase is represented by large-scale, high-angle right-stepping en échelon faults almost parallel to the MTL in the Upper Cretaceous Izumi Group. We found that left-handed en échelon folds have developed along the right-stepping faults, which contain 25-60 m wide cataclasite and fault gouge. Both map scale en échelon folds and microstructures (e.g., composite planar structures) in the fault rocks suggest that they were formed by sinistral-reverse faulting with top-to-the-SW kinematics. Furthermore, based on the new K-Ar age dating of authigenic illite from the fault gouge along the MTL and right-stepping faults, it can be concluded that the MTL was activated in two discrete stages at approximately 59 Ma (Ichinokawa phase) and 47-46 Ma (pre-Tobe phase). Based on these results, we reappraise the kinematic framework of the MTL in the Paleogene, which can be interpreted as the record of the movements of the subducting oceanic plate relative to the continental plate.
The M w 5.8 Awajishima earthquake occurred on 13 April 2013 in southwest Awaji Island, ∼25 km southwest of the epicenter of the 1996 M w 6.8 Kobe earthquake, southwest Japan. Analyses of aerial photographs and 3D perspective images, field investigations, and structural analysis of fault rocks reveal that: (1) a previously undocumented fault, here called the Yamada fault, strikes northwest-southeast and dips southwest at 86°along a topographic lineament at the geological boundary between Mesozoic granitic rocks and the late PlioceneQuaternary Osaka Group composed of interbedded sandstone and mudstone; (2) the main shear zone of the Yamada fault consists of a fault core with a < 10 cm wide zone of fault gouge (generally 1-5 cm), a fault breccia zone of < 100 cm wide, and a damage zone of 10-50 m wide, composed of cataclastic rocks and fractures; (3) foliations characterized by S-C fabrics that have developed in the shear zone indicate a dominantly thrust fault sense, consistent with that revealed by the focal mechanism; and (4) coseismic surface ruptures occurred locally along the main trace of the Yamada fault, consisting of numerous short fissures ranging in length from centimeters to several meters and concentrated in a zone < 5 m wide. Our findings show that the newly identified Yamada fault is an active fault and that it is probably the fault on which the 2013 M w 5.8 Awajishima earthquake occurred. Therefore, it is necessary to construct a fault model to better understand the deformation characteristics of the seismogenic source fault and for reassessing the seismic hazards on the densely populated Awaji Island of southwest Japan.
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