Tectonic history of the Rokko active fault zone (southwest Japan) as inferred from cumulative offsets of stream channels and basement rocks

Abstract

Tectonic landforms and displacements of basement rocks reveal the tectonic history of the Rokko active fault zone (RAFZ), a major active strike-slip fault zone in southwest Japan. Typical tectonic landforms, such as systematically deflected stream channels and ridges, fault saddles, shutter ridges, and fault scarps, are recognized along the intramontane segments of the RAFZ, including the Gosukebashi, Otsuki, Nunobiki, and Kikusuiyama faults. The Quaternary drainage system shows progressive displacement along the main fault traces; the longer the upstream from the deflected point is, the larger the amount of offset is. The pre-Cenozoic rocks, however, show a constant amount of displacement along each fault. Maximum offset amounts of deflected stream channels are 500 m and 450 m for the Gosukebashi and the Otsuki faults respectively, whereas offsets of the pre-Cenozoic rocks are 1.9–2.2 km for the former and 0.2–0.4 km for the latter. The geological and geomorphological evidence shows that the Gosukebashi fault formed before Quaternary time, whereas the Otsuki fault just formed in mid–late Quaternary time; both faults are presently active, having dextral slip rates of 1.0–1.3 mm/year with a vertical component of 0.3–0.4 mm/year. The analysis of deflected stream channels reveals that there is a linear relation, D= aL, between the upstream length L from the deflected point and the offset amount D with a certain coefficient a for intramontane active strike-slip faults. The statistical data from some typical active strike-slip faults having a slip rate of 1–10 mm/year and the above four intramontane faults of the present study show that a values vary in the order of 0.1–1.0, showing there is a correlation between a values and strike-slip rates s. This suggests that the coefficient a may be used as a criterion to evaluate the activity of the intramontane active strike-slip faults.