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

• Taper angles of the Taiwan retrowedge change dramatically during collision.
• Varying taper angle is adjusted through faulting and mass-wasting, which mix rocks.
• Such mixing creates Lichi melange near the accreted arc.

Although the Lichi mélange has been studied intensively over the last decade, its enigmatic nature has generated debates regarding its origin and evolution. Two prominent models have been proposed to explain the Lichi melange: the olistostrome model with slumping sediments and the tectonic collision model with intensive shearing. Neither model can explain what causes the interpreted slumping and complex faulting processes for the sediments. Here, we study the Lichi melange using a time–space equivalence approach, in which the tectonics of the offshore continuation of the Lichi mélange should represent an earlier stage in its evolution. Our study of marine multichannel seismic data suggested that the backthrusts in the accretionary prism propagate arcward above and within the deforming forearc and arc basement to incorporate the sediment and basement materials into the rear of the accretionary prism. Therefore, we proposed the “retrowedge evolution model” whose novel key feature is that the irregular topography of the arc basement affects the taper angle of the retrowedge. The retrowedge has a greater taper than the minimum taper of the prowedge, so the slope can be steeper, and thus favors gravitational failure. Such processes would generate complex faulting and slumping processes in the backthrusted forearc ridge and basin. In sum, this retrowedge model reconciles the mixture of slumping and faulting processes found in the Lichi mélange.