Plio-Pleistocene Convergence and Tectonic Recycling at the Oceanward Retrowedge Margin of an Active Arc-Continent Collision, Eastern Taiwan

Arc-continent collision (ACC) plays a major role in the growth of continents by accretion of oceanic arc terranes over geologic time, but the tectonic processes and pathways by which arc crust is accreted in these settings remain poorly understood. The island of Taiwan is an active ACC where the Luzon arc is colliding with the Eurasian continental margin in a zone of rapid convergence along the Manila trench, offering unique insights into processes of arc accretion. The Coastal Range east of the Longitudinal Valley (LV) in eastern Taiwan contains a 4-6 km thick section of orogen-derived marine mudstone, turbidites, conglomerate, and olistostromes that overlie Miocene volcanic rocks and record deep subsidence in a retrowedge foredeep basin from ~5 to 1 Ma. Volcaniclastic detritus is mixed with quartz, slate and metasandstone lithics in subaqueous mass-transport deposits with SE paleoslopes, indicating that volcanic rocks were exposed in the orogen during basin subsidence and filling. Basinal deposits and underlying arc crust in the Coastal Range have been inverted, uplifted and incorporated into the collisional orogen in the past 0.6-0.8 Myr by slip on the steeply east-dipping LV fault. Immediately west of the LV in the Yuli Belt, meta-conglomerate and -breccia with rounded to angular volcanic clasts yielded ca. 17 Ma magmatic zircon (n=2). Trace-element geochemistry of 16-17 Ma (n=17) zircon from Yuli Belt metagabbro reveals elevated U/Yb ratios that indicate a subduction-related island-arc origin, consistent with Miocene zircon ages in Yuli Belt rocks of an inferred volcanic-arc protolith (Chen et al., 2017). These results suggest a history of protracted convergence between arc-forearc crust of the Philippine Sea Plate and the orogen. We hypothesize that shortening on the east-vergent Central Range fault and related structures drove eastward migration of the retrowedge foredeep basin and consumed a wide belt of arc crust beneath the orogen. Some arc-crustal fragments were accreted into the thrust belt and eroded back into the basin; others were tectonically buried up to depths of 40-50 km and are now being rapidly exhumed. Large-scale convergence and transfer of arc crust into the metamorphic retrowedge of the orogen thus appears to be a major process of accretion that needs to be further tested in this and other ACC's.