DEVELOPMENT OF ARC-CONTINENT COLLISION MéLANGES: LINKING ONSHORE GEOLOGICAL AND OFFSHORE GEOPHYSICAL OBSERVATIONS OF THE PLIOCENE LICHI MéLANGE, SOUTHERN TAIWAN AND NORTHERN LUZON ARC, WESTERN PACIFIC

The enigmatic Lichi Mélange has generated debate regarding its origin and evolution within studies of geophysics and tectonics of Taiwan. Two earlier models, the sedimentary slumping olistostrome model and the tectonic collision model, helped develop our understanding of the origin of Lichi Mélange. Using a time-space equivalence approach, the tectonics of the offshore continuation of the Lichi Mélange should represent its evolution. From multichannel seismic data and SeaMARC II seismic sonar image, we observed clear evidences of submarine slumping along the rear of the accretionary prism, out-of-sequence faulting in the accretionary prism, and deformed arc basement. Thus, we propose an “evolving taper angle model” which integrates and modifies the earlier models for the origin of Lichi Mélange. Our evidence suggests that the backthrusts in the accretionary prism propagated arcward on top of the deforming forearc basement and incorporated the materials of the forearc basins and basement into the rear of the accretionary prism. We interpret the deformation in light of taper wedge angle near the toe of the backthrust: the deforming arc created the evolving dip angle of the backthrust, which generated complex faulting and slumping processes in the backthrusted forearc ridge and basin. The entire complex was then exposed by progressive thrusting of the volcanic arc trench-wards together with uplift to form the Coastal Range. Similar combinations of such processes may apply to mélanges in other collision zones.