Characterization of a fault gouge region of the Median Tectonic Line in Shikoku, southwestern Japan
Abstract
Markers for discriminating between aseismic and seismic rupture behaviors are very important and expected for evaluation of faulting activity. In this research, we focused on grain characteristics to tackle the crucial issue through the fault gouge region in the fault damage zone of the Median Tectonic Line (MTL) in Shikoku.
The MTL is the longest geological terrane boundary fault in Japan, and some parts have, repeatedly, reactivated in Quaternary. We drilled a borehole to obtain a geological core sample penetrating into the inactive MTL. The total core length is 250 m-long. The fault plane of the MTL is within 37.45 m-depth. The hanging wall and foot wall zones are Izumi Group (alteration of sand and mud rocks) in Ryoke Belt and Sanbagawa high P/T metamorphic rocks, respectively. A remarkable thick fault gouge region (granular material) with c.a. 3 m-wide evolves around the fault plane of the MTL. We measured grain characteristics, grain size (GS) distribution and grain shape (grain angularity: GA), of the fault gouge region (9 samples) to describe the slip property. The fractal dimension (D) values generally increase from immature to mature cataclastic rocks as fragmentation mechanism change from fracturing to abrasion. In this measurement, D values showed variation of 1.59-1.99 (2-D measurement) in the fault gouge region, suggesting heterogeneous slip property. In addition, we can recognize two-type granular material properties based on the GA values (wide or narrow range). Most of measurement samples show narrow range of GA (N-GA type) values with 10-30, while some samples give wide range of GA (W-GA type) values with 10-45. As the GA values are independent on GS in the N-GA type sections, the rupture behavior of the N-GA type sections represents abrasion processes with grain rolling. On the other hand, the W-GA type sections are characterized by higher GA values in tiny and sorted GS. Therefore, the W-GA type sections might correspond to high slip localization domains. These measurement data suggest that creeping and seismic faulting sections exist in the gouge region (like a fault core) in the MTL. We also discuss the relationships between the measurement results of grain characteristics in the gouge region and other petrophysical data (CT, porosity, and Vp-Vs) in this presentation.- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFM.T33D0386I
- Keywords:
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- 8118 Dynamics and mechanics of faulting;
- TECTONOPHYSICS