Trench and drilling investigation of the Median Tectonic Line in Shikoku, southwest Japan: implications for fault geometry

The Median Tectonic Line (MTL) is a thousand-kilometer-long fault that extends across southwest Japan. Near the Nyugawa region of Shikoku, the MTL comprises (i) a low-angle inactive terrane boundary fault (the MTLTB) that divides the Jurassic and Cretaceous geological terranes, and (ii) a subparallel high-angle active fault zone (the MTLAFZ; Kawakami Fault). To better understand the relationship between the MTLTB and MTLAFZ fault traces, we exposed a ten-meter-long trench of approximately 2-m depth across the Kawakami Fault. We also drilled and cored five boreholes with lengths 80‒330 m along a 100 m transect to understand the cross-cutting relationship between the MTL faults and to determine the fault plane geometries and their dipping values. The Kawakami Fault was found to be a high-angle (> 70°) active fault exposed at the surface; however, it represents a non-vertical or listric fault that converges to the low-angle MTLTB fault dipping to the north at 30°. The Kawakami Fault was originally formed as a reverse fault, and subsequent dextral strike–slip displacement occurred along the same fault plane. Although the MTLTB is poorly oriented with respect to the regional stress field, it is capable of rupturing owing to its significantly weak interface; the properties of local faulted rock material are expected to play an important role in determining slip behavior.