Time-Series Analysis of Crustal Deformation on Longstanding Transcurrent Fault: Structural Diversity along Median Tectonic Line, Southwest Japan, and Tectonic Implications

The Median Tectonic Line (MTL) along the longstanding convergent margin of eastern Eurasia has been activated intermittently since ca. 100 Ma. In its incipient phase, propagating strike slips on the MTL generated an elongate pull-apart depression buried by voluminous clastics of the Late Cretaceous Izumi Group. In this study, the complicated deformation processes around this regional arc-bisecting fault are unraveled through a series of quantitative analyses. Our geological survey of the Izumi Group was exclusively conducted in an area of diverse fault morphology, such as jogs and steps. The phase stripping method was introduced to elucidate the time sequence of cumulative tectonic events. After stripping away the initial structure related to basin formation, neotectonic signatures were successfully categorized into discrete clusters originating from progressive wrenching near the active MTL fault system, which has been reactivated by the Quaternary oblique subduction of the Philippine Sea Plate. The method presented here is simple and effective for the detection and evaluation of active crustal failures in mobile belts where records of multiphase architectural buildup coexist.

Poly-cataclasites: Implications to the Seismic Cycle

Poly-cataclasites are rocks that have undergone multiple episodic deformational histories. These rocks retained the microstructures developed from older deformational events. They are a common occurrence in the Median Tectonic Line Japan and can be used to evaluate the changes in rock deformational processes throughout the earthquake cycle. Based on the description of mineralogical changes and the microstructures of the cataclasitic clasts, we are able to establish two main deformational events Microstructures of the co-seismic phase relates to the randomly oriented clast developed through fluidisation. Clast that are foliated formed during the aseismic phase through creeping accompanied by the precipitation of phyllosilicate minerals and the consumption of feldspathic minerals. We propose that the presence of crustal fluid circulation is essential in governing the poly-cataclasites deformational cycle providing insights into the underlying deformational processes during the earthquake cycle.