EFFECTS OF IRREGULAR BASEMENT TOPOGRAPHY ON TAIWAN DOUBLY-VERGENT WEDGE: IMPLICATIONS FOR EARTHQUAKE RUPTURE BEHAVIORS IN PROWEDGE AND LICHI MELANGE IN RETROWEDGE

Following Prof. Moores’s pioneer work on ophiolites around the world, geologists have documented ophiolitic materials onland Taiwan in an arc-continent collision zone. However, how ophiolitic materials were emplaced into the Lichi Mélange between the Taiwan Wedge and the accreted arc is unclear. Here we proposed a retrowedge model to explain how the ophiolitic materials are sheared, mixed, and incorporated into the mélange. We study the Lichi Mélange 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 suggests 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 retrowedge. The novel key feature in this model is that the irregular topography of the arc basement affects the dynamic taper angle of the retrowedge throughout the collision evolution, causing thrust faulting and mass wasting as evidenced in our marine geophysical data. Basement topography has also affected deformation styles in the prowedge area. From our results of seismic waveform inversions and other geological investigation, we have documented evidence of two basement highs on the incoming plate controlling the coseismic rupture extent of an Mw7.4 earthquake in 1999 in the over-riding plate. This particular case is consistent with the interpretation that indentors can act as barriers during earthquake rupture. Due to its active tectonic processes, Taiwan is among the most studied regions. Some of these basement-related deformation processes might occur in other convergent zones.