Three‐Dimensional Stress Model of the Collision‐Subduction Junction East of Taiwan: Implications for the Decoupling of the Luzon Arc During Subduction

Carried by the Philippine Sea Plate converging with the Eurasian Plate, the Luzon Arc is obliquely colliding with and accreting to the Eurasian continental margin to build the Taiwan orogen. Northward along the collision suture, the Luzon Arc extends offshore and is being subducted under the Ryukyu Trench. How the northern segment of the Luzon Arc responds to both collision and subduction remains poorly understood. At present, seismic tomography and conventional stress analyses cannot fully capture images of this arc or discern how it deforms following subduction. We employed a three‐dimensional spatial clustering approach in eastern Taiwan to reveal how stress varies along the collision suture. Our results show that the maximum horizontal compression orientation, SH, is sub‐parallel to the plate motion at all depths across the lithosphere, except at shallow depths for the northern segment, where SH exhibits a clockwise rotation from convergence‐parallel by 10°–30°. Accompanying the SH rotation in the northern crustal segment is a tilt of the maximum compressive P‐axes by approximately 20°. We interpret this systematic variation along the suture and with depth as resulting from the rotation of the stress regime in the crust of the Luzon Arc forced by the oblique subduction, and likely augmented by the bending of the Central Range crustal block. The location of rotation/tilting and its disparity with depth may signify a decoupling of the volcanic arc from the mantle lithosphere during early subduction. Plain Language Summary How a volcanic arc deforms following subduction remains poorly understood despite the frequent intrusion of such structures into oceanic trenches. A subducted arc is difficult to image using seismic tomography; however, its presence and behavior are sometimes revealed via indirect evidence. The collision‐subduction zone of Taiwan is an ideal region to monitor the deformation of the Luzon Arc during subduction because of its unique tectonic setting and high abundance of earthquakes, which allow the stress field to be mapped in three dimensions. We demonstrate that the variation in the stress orientations along the subducting Luzon Arc reveals its fate. We propose that the three‐dimensional rotation of the stress field associated with the Luzon Arc represents its response to the squeezing of the Philippine Sea Plate into the collision‐subduction corner and to the bending of the Central Range–Ryukyu Arc lithospheric sliver. We infer that t