The role of transtension and transpression in incipient collision: insights from offshore SE Taiwan

Offshore SE Taiwan is regarded as the northern extension of the North Luzon Trough and reflects the early stages of arc–continent collision resulting from the oblique convergence between the Eurasian continental margin and the Luzon Arc. Although the region is bounded seaward by the offshore strike-slip-dominated Lutao–Lanyu Fault (LLF) and is likely connected landward to the onshore oblique-slip Longitudinal Valley Fault (LVF), the structural relationship between the LVF and LLF remains poorly constrained. This is largely due to the lack of comprehensive understanding of the transitional structures between the onshore (southern Coastal Range) and offshore the Taitung Plain, as well as the limited knowledge regarding the seaward extensions of the boundary faults, namely the Yungfeng Fault and the Lichi Fault. Newly acquired and reprocessed archived marine seismic data offshore the Taitung Plain demonstrate that the W–E–trending, en-echelon bathymetric ridges-and-troughs observed in the nearshore area are structurally controlled by a fault system comprising F1, F2, F3, and F4. Faults F1 and F2 are located seaward of the Yungfeng and Lichi faults and define the outer boundaries of the ridge-and-trough system, whereas faults F3 and F4 lie between F1 and F2, bounding the central trough. Focal mechanism solutions and GPS velocity data further document strike-slip deformation throughout the southern Coastal Range and adjacent nearshore areas, including SSE-directed motion at the southern tip of the Coastal Range. These observations collectively support a model of eastward-directed structural extrusion and imply the existence of a localized strike-slip fault system offshore. We propose that the trough between faults F3 and F4 represents a transtensional pull-apart basin that developed during a period of dominant left-lateral slip between the LVF and LLF (~ 3 Ma). In contrast, the ridges between F1 and F3 and between F2 and F4 are interpreted as products of fault reactivation (F1 and F2) and local transpressional extrusion that developed as convergent motion became more pronounced after ~ 2 Ma. This two-stage development of the Yungfeng–Lichi strike-slip duplex nearshore Taitung Plain provide an interpretation not only aligned with regional structural framework but also correspond well with recent geomorphological and geophysical observations.