Spatiotemporal migration of slip localization zones along the Hambaeksan Fault, South Korea

초록

Fault zones commonly contain multiple principal slip zones (PSZs) that record distinct episodes of deformation, yet the detailed processes of the migration of activity among PSZs are poorly constrained. In fault zones that juxtapose lithologies with different mechanical properties, the interplay between mechanical heterogeneity, lithological contrasts, and thermal weakening during seismic slip is expected to have a primary influence on where and how slip localizes, but direct evidence of this process in natural fault zones is scarce. Here we present a microstructural, mineralogical, and thermal characterization of the Hambaeksan Fault, a strike-slip fault juxtaposing Ordovician carbonate rocks (Joseon Supergroup) against Carboniferous-Permian siliciclastic rocks (Pyeongan Supergroup) in the Taebaeksan Basin, South Korea. Integrated field, microscopic, and spectroscopic analyses reveal a similar to 45-cm-wide fault core comprising four compositionally distinct breccia units and two PSZs that record contrasting deformation histories. The PSZ at the lithological boundary preserves evidence for hightemperature seismogenic slip, including the incipient graphitization of carbonaceous material (similar to 300 degrees C) and kaolinite dehydroxylation (similar to 400 degrees C) accompanied by injection structures resulting from thermal pressurization. In contrast, the PSZ within the shale breccia lacks these thermal indicators. We propose a model of fault evolution in which thermal modification of the primary PSZ via frictional heating-induced fluidization and destruction of the slip zone foliation resulted in an increase in frictional resistance, thereby promoting the migration of slip localization into the mechanically weaker, phyllosilicate-rich shale breccia. The results show that frictional heating and mechanical heterogeneity control the migration of slip localization between lithologically contrasting sequences in fault zones.

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