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

Tectonic landforms and displacements of basement rocks reveal the tectonic history of the Rokko active fault zone (RAFZ), a major active strike-slip f ault zone in southwest Japan. Typical tectonic landforms, such as systemati cally deflected stream channels and ridges, fault saddles, shutter ridges, and fault scarps, are recognized along the intramontane segments of the RAF Z, including the Gosukebashi, Otsuki, Nunobiki, and Kikusuiyama faults. The Quaternary drainage system shows progressive displacement along the main f ault traces; the longer the upstream from the deflected point is, the large r the amount of offset is. The pre-Cenozoic rocks, however, show a constant amount of displacement along each fault. Maximum offset amounts of deflect ed stream channels are 500 m and 450 m for the Gosukebashi and the Otsuki f aults respectively, whereas offsets of the pre-Cenozoic rocks are 1.9-2.2 k m for the former and 0.2-0.4 kin for the latter. The geological and geomorp hological evidence shows that the Gosukebashi fault formed before Quaternar y time, whereas the Otsuki fault just formed in mid-late Quaternary time; b oth faults are presently active, having dextral slip rates of 1.0-1.3 mm/ye ar 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 c ertain coefficient a for intramontane active strike-slip faults. The statis tical data from some typical active strike-slip faults having a slip rate o f 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 correla tion between (I values and strike-slip rates s. This suggests that the coef ficient a may be used as a criterion to evaluate the activity of the intram ontane active strike-slip faults. (C) 2000 Elsevier Science B.V. All rights reserved.