The Margin-Oblique Kumawa Strike-Slip Fault in the Banda Forearc, East Indonesia: Structural Deformation, Tectonic Origin and Geohazard Implication

Abstract

The northeast Banda margin is affected by the arc-continent collision between the Australian continental margin and the Banda arc. Unlike margin-parallel strike-slip faults in other subduction zones, the margin-oblique Seram-Kumawa Shear Zone (SKSZ) has greatly deformed the Banda forearc with 80 km displacement along plate boundary. It trends ∼ WNW-ESE comprising the Kawa shear zone (KSZ, >200 km) on the Seram island and the Kumawa fault (KF, >200 km) offshore SE Seram. Using multibeam bathymetry, marine seismic reflection and earthquake data, we studied the active tectonics and driving mechanism of the SKSZ with focus on its offshore part (KF). Our results show: (1) the KF is a young (∼2 Ma), linear crustal structure with three segments from NW to SE: pull-apart basin (dextral), diffuse shear zone (sinistral) and distinct fault trace (sinistral); (2) It propagates southeastward and cross-cuts the entire upper plate with prominent seafloor expression, but without seismicity along it, similar to the Banda detachment caused by forearc extension; (3) By incorporating the oblique collision and regional tectonics with our results, we proposed that the KF is formed primarily by the upper plate extension induced by subduction rollback during the last 2 Ma, with minor contribution from oblique arc-continent collision; (4) The slip rate of the KF has reduced from 40 mm/yr over 2 Ma to <14 mm/yr at present. These observations combined with the clear seafloor faulting, 80-km displacement and occurrence of a Mw 5.2 earthquake on its horsetail fault suggest it is currently locked awaiting a large earthquake.