Transition from subduction to arc-continent collision: Geologic and neotectonic evolution of Savu Island, Indonesia

Abstract

Geosphere, June 2009, v. 5, p. 152-171, doi:10.1130/GES00209.1, Animation 1 - Sand box experiment movie illustrating various phases of the arc-continent collision observed throughout the Banda Arc. You will need Windows Media Player or a multimedia player such as Quicktime Media Player to view this file. The model is designed to test the influence of inserting a flat-flexible backstop (forearc basement) into the accretionary wedge as observed in Savu and older parts of the Banda Orogen exposed in Timor. A stratigraphy of lime and sand is scaled to thicknesses observed for cover sequences of the Scott Plateau, while plasticine clay is used to represent rigid forearc basement. The rheological properties of these materials are well documented (i.e., Huiqi et al., 1992) and provide reasonable approximations of contrasts in mechanical strength found between Australian cover rocks and forearc basement. The model is scaled at a ratio of 1 cm = 1 km. A 1 cm layer of plasticine clay extends on top of a 3.7-cm thick section of alternating white lime and red lime/sand layers (Gondwana Sequence lower cover units of Australian continental margin). In front of the plasticine is a 1.2-cm thick section of alternating units (Australian Passive Margin Sequence of the upper cover of the Australian continental margin). Shear strengths are 161 and 176 (Pa) for the lime and sand/lime layers, respectively. The experiment and others like it produce an upper structural level of trench-ward verging imbricate thrust sheets of only the upper sections of the cover in front and above the backstop. The lower section of cover forms a duplex system of large isoclinal, recumbent folds, and out of syncline thrusts. Initially the upper thrust sheets and faults dip between 20°-30°, but with accretion of new thrust sheets to the front of the stack, older sheets are tilted progressively more arcward until they become near vertical and even slightly overturned. Maximum uplift occurs in the imbricate fan above the leading edge of the backstop. This part of the accretionary wedge would be the first to emerge above sea level and stripped by erosion, as observed in Savu. The backstop is also uplifted and folded into a concave down geometry. A highly attenuated mélange-like zone develops immediately beneath the plasticine forearc nappe due to very high shear strain. Insertion of the flat flexible backstop into the layered section produces two distinct structural domains: frontal accretion above and duplexing beneath the backstop. The same patterns of accretion and backstop deformation are found throughout the Banda arc-continent collision (Harris, 1991). Only the early stages of the model are like the Savu phase of collision. Later stages are more emblematic of the Timor region.