3D SUBSURFACE MAPPING OF THE TAIWAN ARC-CONTINENT COLLISION REVEALS THE KINEMATICS OF ITS PAST 20 MA

In northeastern Taiwan, two subduction zones of opposite polarity meet in a quasi-orthogonal, kinematically-stable configuration between the Eurasia (EU) and Philippine Sea (PSP) plates. We mapped in 3D the main structures involved in the collision using a combination of tomographic models and relocated earthquakes.

Our mapping reveals that the EU slab bends sharply downward beneath eastern Taiwan, steepening northward from 60° to 90+° in the upper 100 km and extending to 450-500 km depth. Below the island, the EU/PSP plate interface becomes the basal detachment of the mountain belt, located at ~ 10 km depth. Continued plate convergence results from the combination of EU lithosphere subduction, folding and thrusting in both the EU and PSP upper crust, and secondary subduction of the PSP forearc.

We used a 2D cross-sectional kinematic block model to analyze the present-day ~90 mm/yr plate convergence. Western Taiwan experiences ~30-35 mm/yr of upper crustal deformation, accompanied by ~28-30 mm/yr subduction of EU lithosphere, which has been the long-term rate since ~15 Ma. The recent increase in convergence to ~90 mm/yr in the last ~2 Ma is being absorbed by thin-skinned deformation of the forearc, arc, and retro arc, combined with ~100-125 km of secondary subduction of forearc lithosphere. The kinematic block model suggests the downward extrusion of the forearc lithosphere and accretionary complex between the stable PSP and the adjacent EU slab. Towards northeastern Taiwan, active convergence in the upper crust progressively diminishes, with the western Taiwan thrust belt (which produced the 1999 Mw 7.6 ChiChi earthquake in the center of the island) becoming inactive, while the Reisui-Suao-Hualien backthrust system in eastern Taiwan (which produced its most recent large earthquake, Mw 7.4, in April 2024) becomes active and connects seamlessly to the Ryukyu trench active structures.