True Strike-slip Displacement Accompanied by Non-brittle Vertical-axis Deformation Around an Active Fault Revealed by Paleomagnetic Analysis: a Case Study of the Enako Fault in Central Japan

In order to evaluate vartical-axis surface deformation around the fault, we carried out measurements and demagnetizations of samples from the Kamitakara pyroclastic flow deposit at thirty sites around the Enako fault in central Japan. The Kamitakara pyroclastic flow deposit is Quaternary welded tuff whose K-Ar age is 0.63+-0.04 Ma (Watanabe et al., 1999), and the deposit consists mostly of biotite ryolite welded tuff with eutaxitic structure. The Enako fault is an active right-lateral strike-slip fault. We studied tectonic geomorphology of the Enako fault. We have interpreted aerial photographs at a scale of 1:10000 taken by the Geographical Survey Institute of Japan and those at a scale of 1:40000 taken by the U.S. Armed Forces. Systematic right-laterally offset channels and ridges found along the whole traces of the Enako fault indicate that the fault is a right-lateral strike-slip fault. Dextral offset of stream channels dissecting the Kamitakara pyroclastic flow deposit is as large as 150 m along the fault. We found that all sites preserved stable primary magnetization through progressive demagnetization tests using thermal and alternating field methods. We applied tilt-corrections for site-mean directions calculated from the primary magnetizations. We obtained tilt-corrected site-mean paleomagnetic directions at fifteen sites in the Kamitakara pyroclastic flow deposit. Within several hundred meters from the Enako fault, there is tectonic rotation large enough to be detected as rotation of paleomagnetic direction for the past 0.6 million years. This suggests that paleomagnetic direction near an active fault cannot reveal regional or global tectonics without considering the effect of the fault. Differential clockwise rotations are found around the Enako fault. We found a negative correlation between relative rotations and distances from the Enako fault; the amount of vertical-axis rotation is larger for areas closer to the fault. Continuous deformation model is suitable to explain this deformation. We calculated the minimum value of strike-slip displacement by vertical-axis rotation along the Enako fault. The minimum value is about 210 m with an assumption that width of the shear zone is 500 m. The sum of this value and displacement based on offset of geomorphic features is about 360 m. The net strike-slip displacement is about 2.4 times larger than the amount revealed by geomorphic methods. To obtain more accurate information on these phenomena, denser paleomagnetic sampling is needed.