EXTREME UP-AND-DOWN VERTICAL MOTIONS OF ACCRETING ARC CRUST IN AN ACTIVE COLLISIONAL SUTURE, COASTAL RANGE OF EASTERN TAIWAN

Vertical crustal motions in arc-continent collision zones are known to involve tectonically driven cycles of rock burial, exhumation, erosion and denudation, but the rates, timescales, and structural controls on this process are poorly understood. We have documented a well-constrained history of rapidly oscillating vertical motions in actively accreting volcanic-arc crust by inverting the stratigraphic record of subsidence and uplift over the past ~ 6 Myr in eastern Taiwan. The Coastal Range contains a thick section of deformed ~ 4.0–0.8 Ma (Plio-Pleistocene) deep-marine flysch that unconformably overlies Miocene volcanic arc basement. The basal unconformity (~ 6–4 Ma) is a broad erosive surface, discontinuously capped by thin shallow-marine limestone and epiclastic deposits. These relations record a period of slow uplift and erosion of basement near sea level followed by rapid subsidence during initiation of a retrowedge foredeep basin. We applied modern back-stripping procedures to 2 composite sections using updated magneto-biostratigraphic age constraints and foraminifera-based paleobathymetry. The results show that >4.8–6.5 km of orogen-derived sediment accumulated between ~ 3.0­ and 0.8 Ma in a rapidly subsiding basin at rates of ca. 2.6 mm/yr (in the north) and ca. 3.3 mm/yr (south). Complied paleo geotherm and porosity-effective stress estimates further indicate that ~ 0.5–1 km of sediment has been removed by erosion at the top of preserved sections. This requires an extremely rapid uplift (7.3–11.2 mm/yr in the north; 9.4–12.9 mm/yr in the south) to exhume arc basement to present elevations (1.3-1.6 km) since 0.8–0.5 Ma. These Myr-scale uplift rates are consistent with millennial rates measured in nearby marine terraces, but much faster than decade-scale geodetic rates in the north, suggesting that the northern Coastal Range may have recently entered a new subsidence stage. Abruptly oscillating vertical motions in the accreting arc crust can be explained by: (1) ~ 6–4 Ma slow uplift in an eastward migrating flexural bulge of the retrowedge foredeep basin; (2) ~ 4.0–0.8 Ma rapid subsidence and sedimentation in the same basin; (3) post-0.5 Ma extremely rapid uplift via tectonic reorganization and a shift to modern transpression; and (4) downward flexure related to subduction in the Ryukyu trench.