STRAIN PARTITIONING OFFSHORE SOUTHEAST TAIWAN: EVIDENCE FROM FOCAL MECHANISM STRAIN INVERSIONS NEAR THE HUATUNG RIDGE

The Huatung Ridge is a bathymetric high located off the southeast coast of Taiwan in a part of the Luzon fore arc basin thought to be closing in response to the collision between the Eurasian passive margin and the Luzon arc. We use strain inversions from 144 focal mechanism solutions (FMSs) that define a seismic lineation parallel to the Huatung Ridge and aligned with the Taitung Trough to understand contemporary deformation across the zone of incipient collision. Using strain inversions on the FMSs northeast of the Huatung Ridge, we find a dominant component of crustal thickening with principal shortening oriented NW – SE, sub-parallel to the Philippine Sea Plate convergence vector (_EURV_PSP = 82 mm/yr toward 310°). Overall inversion results for the FMSs immediately east of the ridge show a dominant component of crustal thickening with principal shortening oriented E – W and a subsidiary component of shearing with principal extension oriented NNE – SSW. In detail, we find evidence of north to south counterclockwise rotation of the principal shortening direction. Inversion results for events located north of the Taitung Canyon, near the island of Lu Tao, show primarily thrust faulting with principal shortening oriented WNW – ESE. South of the Taitung Canyon, near the island of Lanyu, inversion results show primarily crustal thickening with principal shortening oriented E – W. These results suggest a north to south gradient in principal-strain orientation through the counterclockwise rotation of the D₃ strain component until it is essentially normal to the N – S trending Huatung Ridge. We suggest that the strain pattern records the north to south transition from collision to convergence that is partitioned into components of margin-normal and margin-parallel transport by thrust and strike-slip faults, respectively. In this view, the dominant N – S structural grain defined by the Huatung Ridge and Taitung Trough is accommodating both basin inversion in response to incipient collision in the northern part of the area and the onset of strike-slip due to the obliquity between the convergence vector and margin-normal direction in the south.