THRUST-RELATED GROWTH STRATA IN THE RETROWEDGE OF ARC-CONTINENT COLLISION ZONE, SOUTHERN COASTAL RANGE OF EASTERN TAIWAN

Arc-continent collision is a common driver of mountain building and growth of continental lithosphere, but the interactions between orogen exhumation and deformation of forearc crust during collision are still poorly understood. The Taiwan orogeny is a well-studied modern arc-continent collision caused by accretion of the Luzon volcanic arc in the east (Philippine Sea plates) to the Eurasian continental margin since ~4-5 Ma. The Coastal Range of eastern Taiwan preserves a 7 km thick Plio-Pleistocene section of orogen-derived marine deposits that unconformably overlie Miocene forearc basement and formed in a syn-collisional retrowedge foredeep basin. By utilizing existing bio-magnetostratigraphic data, measured stratigraphic thicknesses, and newly recognized event-generated marker beds (tuff and pebbly mudstone layers) in the southern Coastal Range, we investigate the time-space pattern of forearc crustal deformation on the oceanward side of the collisional orogen. Our analysis reveals a coherent section of growth strata that formed by syn-depositional tilting and folding in a thrust-related syncline-anticline pair between ~3.5 and 1.0 Ma. Eastward younging of basal mudstone above an erosional unconformity, combined with a reconstructed history of accelerating subsidence, suggest these folds grew during flexural subsidence of Luzon-arc basement in response to crustal loading in the advancing orogenic wedge. Thickness and age data reveal an increase in the rate of subsidence and sediment accumulation at ca. 2 Ma during influx of huge submarine landslide deposits (pebbly mudstone) associated with accelerating exhumation in the collisional orogen. Preliminary analysis of other convergent structures and sub-basins in the Coastal Range suggests westward migration of west-vergent imbricate thrusts that culminated in rapid uplift and inversion of the foredeep basin in the hanging wall of the east-dipping Longitudinal Valley thrust fault (active suture between Eurasian and Philippine Sea plates), post-0.8 Ma. Future work will test hypotheses for (1) southward-propagating versus “simultaneous” collision, and (2) progradation of marine olistostromal facies (Lichi mélange) into the foredeep basin in front of a hypothesized eastward-migrating Plio-Pleistocene retrowedge thrust front.