Strain Partitioning at the Huatung Ridge, Offshore Southeast Taiwan: Evidence from Seismotectonics

The Huatung Ridge is a north-trending bathymetric high situated between the Luzon Arc and the Southern Longitudinal Trough off the coast of SE Taiwan. Strain inversions for shallow (<30 km) earthquake focal mechanisms that form a lineation on the east margin of this ridge reveal the onset of partitioned strain. We find that the dominant strain geometry in all areas is consistent with crustal thickening (thrusting), but that the direction of principal shortening varies from north to south. In particular we document a transition from principal shortening subparallel to the plate convergence vector (310°) northeast of Taitung, to principal shortening that is rotated counterclockwise (in map view) further to the south, near the islands of Lutao to Lanyu. For these latter events, those north of the Taitung Canyon yield a shortening direction oriented WNW-ESE whereas those south of this bathymetric feature yield a shortening direction oriented WSW-ENE. Furthermore, a subset of events north of the Taitung Canyon is characterized by subvertical NNW- or ENE-striking nodal planes suggesting strike slip faulting. Subvertical nodal planes are also present to the south of the Taitung Canyon but less common. Our observations suggest that previously documented forearc basin inversion, evident in the northward emergence of the Huatung Ridge, is progressively overprinted by collision-related deformation north of the Taitung Canyon. Specifically, offshore events northeast of Taitung suggest that thrusting is driven by Philippine Sea plate convergence with shortening parallel to the convergence direction. Southward toward the island of Lutao, the shortening direction associated with thrusting is rotated so that it is essentially normal to the first-order basement architecture, defined by the Huatung Ridge and adjacent Southern Longitudinal Trough. In this area, subsidiary strike-slip faults suggest a component of ~N-S lateral extrusion. Further to the south, principal shortening is likewise rotated counterclockwise and, although less common, steep nodal planes do occur. In total, our preliminary results suggest that basin inversion initiates in the south by thrusting driven by shortening normal to the basin margins, and lesser strike slip. This likely reflects partitioning of plate motion related to the high obliquity angle between the Philippine Plate convergence vector (relative to Eurasia) and the basement architecture on the west flank of the Luzon arc. To the north the component of strike-slip deformation increases as the rocks move closer to incipient collision. Finally, in the collision immediately south of the accreted arc rocks of the Coastal Range, deformation is no longer partitioned as the basement architecture is rotated clockwise (in map view) to a higher angle relative to the plate motion vector.