The influence of lower plate tectonism on the evolution of accretionary forearc systems

The application of advanced geophysical and remote sensing techniques, to the study of modern accretionary forearcs, has yielded new insight into their tectonic evolution. Emerging, from these studies, is a clearer understanding of the important influence of lower plate deformation on strain in the overlying forearc. Two studies, using 3-D seismic reflection and coherence imaging, were conducted of the Barbados accretionary wedge. Combined coherence and seismic amplitude imaging of the décollement, zone reveal anomalous NE-trending lineaments parallel and abutting zones of high amplitude negative polarity reflections. Detailed mapping of structures within the lower plate and through the décollement, reveal high angle faults cutting the décollement, zone and penetrating the upper plate. These structures apparently channel migration of deeply sourced fluids through the décollement, zone. Evidence from seismic data, structure contour maps and isopach maps, indicates these faults are right-lateral strike slip faults. Basement structure maps show offset of prominent relic topography across this zone of up to 1.3km. Also cutting the décollement, are NNE-trending structures which show complex history of interactions with the NE-trending faults and which mirror basement topography. A number of out-of-sequence faults are recognized inboard from the deformation front and also mirror basement topography. Analysis of sea floor surface and near surface reflectors indicate that several NNE-trending out-of-sequence faults display normal offset while the majority are consistent with thrusting. Detailed mapping of these structures in the lower plate indicates that normal faulting controls the geometry of out-of-sequence normal and thrust faulting.; Nine left-lateral arc-oblique strike-slip faults cut the Cascadia accretionary wedge off the coast of Oregon and Washington and are strikingly similar in character to the oblique strike-slip fault discovered cutting the Barbados accretionary wedge. An integrated study involving structural analysis and remote sensing was conducted of the Cascadian forearc of Oregon. The combined results of this study and that of the Barbados Ridge décollement, support a mechanical model of shear failure along arc-oblique strike-slip faults in response to compression across the forearc. This model may explain the existence of arc-oblique strike-slip faults in accretionary forearc systems globally.