Offset rivers, drainage spacing and the record of strike-slip faulting: The Kuh Banan Fault, Iran

Abstract

This study concerns the ways in which rivers can record part, but not necessarily all, of strike-slip fault offset. The focus is the active right-lateral Kuh Banan Fault in eastern Iran, within the Arabia–Eurasia collision. Plate convergence has caused thrust and strike-slip faulting across SW Asia. The active slip rate of the Kuh Banan Fault is ~1–2mm/yr. Total displacement is ~5–7km, as determined from offset geological markers and the length of a pull-apart basin. A component of thrusting has generated ~1km of relief, which preserves the offset of rivers displaced laterally by the strike-slip motion. Ridge half-widths (W), and river outlet spacings along the fault (S) are related by the drainage spacing parameter R, where R=W/S. Published data for older, larger mountain ranges have extremely characteristic drainage spacing (R=2.1). Drainage spacing along the Kuh Banan Fault has a mean value of 1.8 and is much more variable (R=1.1–3.1), due to local structural complexities which have influenced river courses. Most river offsets along the Kuh Banan Fault are small (<100m); the maximum observed offset (~3.5km) is smaller than the total fault displacement of at least ~5km. The most likely explanation for this discrepancy is stream capture, whereby reaches of rivers downstream of the fault are juxtaposed by fault slip against rivers upstream of the fault. In this way, offset of individual rivers is repeatedly reset to zero. Stream capture is influenced by outlet spacing such that the largest rivers can accumulate large offsets, while smaller, closely spaced rivers are captured more often, inhibiting large offsets. The mean offset of the main rivers along the Kuh Banan Fault is one-third their mean drainage spacing, indicating that the spacing of smaller rivers controls the size of the maximum offset.