The Role of Upper Plate Slivering in Burial of the Luzon Forearc in Taiwan

Seismogenic strain across the subduction-to-collision transition in Taiwan reveals a forearc sliver that explains an array of existing geologic and geophysical data. South of the collision the Luzon forearc experiences partitioning of oblique Eurasia-Philippine Sea plate motion into a western domain of arc-normal backthrusting, and an eastern domain of arc-parallel sinistral shearing. The forearc is therefore partially detached from the Philippine Sea plate by strike-slip motion at the same time that it is inverting in response to E-W shortening. North of ~22° N backthrusting and strike-slip motion become spatially comingled, and north of ~22° 45' N, collision-related deformation prevails, marked by oblique slip on reactivated steeply NW- and SW-dipping faults. Strike-slip motion appears to be localized on the western flank of the Luzon arc and the linkage between this motion and the collision zone is accomplished via a km-scale releasing left step. The linkage is marked by an offshore pull-apart basin south of the collided volcanic arc exposed in the Coastal Range. South of the pull-apart basin the N-S trending Huatung ridge represents a forearc basement uplift. Northward translation of the forearc into the collision zone is therefore accompanied by localized extension and deposition, and these processes conspire to bury the forearc. A protracted history of such processes is suggested by seismic tomography that reveals a gently N-plunging sliver of material with a high Poisson's ratio and inferred to be mafic in composition. This body might thus comprise forearc basement that was initially buried by pull-apart basin formation and further buried beneath the suture zone as collision progressed. Within the Coastal Range a well-documented unconformity separating volcanic arc basement and its cover from collision-derived clastics serves as a possible record of similar tectonic basins from Miocene-Pliocene time. Small collisional pull-apart basins of this type also provide a viable mechanism for the formation of mélange packages mapped along the western and southern margins of the Coastal Range. These include well-preserved turbidite sections and conglomerates that some have argued reflect olistostromal origins. Lastly, contemporary vertical motions in southeastern Taiwan are consistent with an active pull-apart basin linking forearc sliver motion to collision. Leveling and GPS data reveal rapid subsidence localized onshore from the releasing step noted above, and rapid uplift localized in the south westernmost Coastal Range where full collision is underway. This crustal architecture implies that rocks of the Coastal Range: (1) should include faulting histories that reflect margin-normal and margin-parallel slip overprinted by docking-related, vertical-axis rotation and oblique slip on reactivated faults; and (2) might include forearc basement normal faults that are oblique to the margin and that coincide with the margins of mapped mélange units and/or olistostromes.