Structural styles of an accretionary wedge south of the island of Sumba, Indonesia, revealed by SeaMARC II side scan sonar

Abstract

The accretionary wedge south of Sumba Island, Indonesia, is in the early stages of continent-island arc collision. Australian continental shelf sediments are being actively accreted to the Sunda arc at the Timor trough. We have correlated SeaMARC II side-scan images and bathymetry with 3.5-kHz and seismic-reflection profiles to construct a structural interpretation of this zone. Deformation is concentrated on the lower slope of the accretionary wedge, within 15 to 25 km of the thrust front, above which the strain rate appears to decrease abruptly. Three distinct structural styles are developed in the survey area. The characteristics of the easternmost region are (1) deformation in a fold-and-thrust geometry; (2) presence of mud volcanoes and mud ridges parallel to the thrust front, indicating elevated fluid pressure; and (3) the development of a long decollement extending 10 to 15 km from the thrust front. In contrast, the central region shows a more homogeneous thickening, resulting mainly from closely spaced faulting. Seaward-verging thrust faults and conjugate strike-slip faults are the main structures in this segment of the accretionary wedge. Mud volcanoes and a very long decollement are absent in the central region. We present a hypothesis in which structural style is controlled by the length of the decollement developed in front of the thrust front. Short, thick thrust sheets may favor uniform shortening, whereas long thin sheets may be unstable to buckling and deform by folding. High fluid pressure may facilitate propagation of the decollement. In contrast to the accretion occurring in the other two regions, the western part of the accretionary wedge is in the process of being indented and redeformed by a basement ridge.