Effective elastic thickness of the northern Australian continental lithosphere subducting beneath the Banda orogen (Indonesia): inelastic failure at the start of continental subduction

Pliocene-Recent continent-island are collision of the northern Australian c ontinental lithosphere across the Banda orogen from Roti to the Kai Plateau (similar to 121-137 degreesE longitude) has formed an underfilled foreland basin within the Timor-Tanimbar-Aru Trough. Continental collision on north ern Australian lithosphere is most advanced near central Timer Island in te rms of shortening and absorbing the forearc basin (Savu Basin) within the a ccretionary prism. Australian continental lithosphere north of area around central Timer Island is believed to be detached from the oceanic lithospher e. Effective Elastic Thickness (EET) of the northern Australian continental lithosphere from Roti to the Kai Plateau are derived using an elastic half -beam model. Modeled deflection is matched to the seafloor bathymetry and t he marine complete 3D Bouguer gravity anomalies. The EET varies from 27 to 75 km across the northern Australian continental lithosphere from Roti to K ai Plateau when the thickness of the elastic half-beam is kept constant. Th e highest EET values lies near central Timer. From the shelf to beneath the Banda orogen, the EET of the northern Australian continental lithosphere i s reduced from similar to 90 to similar to 30 km when the thickness of the elastic half-beam is allowed to vary down dip. Elastic half-beam modeling a pproximates the Banda orogen as a triangular load and hidden subsurface loa ds as end-point loads. Wider triangular loads modeling the load contributio n from Banda orogen need higher values of EET. Such an observation highligh ts the role of high EET in thin-skinned collisional tectonics by promoting the support of wider accretionary prisms by parts of foreland basins with h igher EET. Variations in EET may result from inelastic yielding in the nort hern Australian continental lithosphere. Oroclinal bending of the Australia n continental lithosphere in the east, from Tanimbar to the Kai Plateau, ma y create additional yielding and further decrease the EET. Change in EET oc curred at the start of continental subduction in the late Miocene-early Pli ocene boundary due to change in curvature of the northern Australian lithos phere near the shelf-slope, both in map and cross-sectional view, Evidence for the inelastic yielding of the northern Australian continental lithosphe re near the present-day shelf-slope at the continental subduction is found in: (I)the maximum change of EET near shelf-slope in laterally variable EET calculations, and (2) in the almost cessation of normal faulting in late M iocene-early Pliocene seen on seismic reflection data from northern Austral ian continental shelf-slope. Elastic half-beam models across central Timer do not require end-point loads and may imply that there are no slab pull fo rces from the oceanic lithosphere that are acting on the leading edge of th e subducted northern Australian continental lithosphere. (C) 2000 Elsevier Science B.V. All rights reserved.