The structure and origin of the Vigan High off West Luzon Island

The Vigan High is located off west Luzon Island and is in the forearc basin of the Manila Trench, where the Eurasian Plate subducts eastward beneath the Philippine mobile belt. The Manila forearc basin exhibits complex structures involved in onshore faulting, seamount subduction, and slab tearing. To investigate the forearc basin of the Manila subduction system between ~16°N and 18°N, we utilized multi-channel seismic reflection data, Sparker seismic reflection data, and bathymetric data collected onboard R/V Legend in 2022 and 2023. Multi-channel seismic reflection data was also acquired on board R/V Ocean Researcher 5 in 2014. The study area can be divided into a northern region dominated by convergent environments and a southern region characterized by extensional environments. In the north region, the Vigan High consists in thrust and strike-slip faults. These faults are distributed along the central part of the Vigan High, with fault planes dipping towards the center. It indicates that the Vigan High is associated with a positive flower structure due to transpression. The strike-slip component of the structure is likely an offshore extension of the onshore Philippine Fault Zone. South of 17°N, seismic data reveal predominantly normal faults. Multi-channel seismic reflection data reveal an unconformity. Folded structures exist beneath the unconformity, and normal faults exist above it. This phenomenon suggests that the region changed from a convergent to an extensional regime. This transition may be related to a slab tearing of the subducted South China Sea slab. The structural difference between the northern and southern regions highlights the significant stress variation in the forearc basin of the Manila subduction system.