Origin of the West Taiwan Basin by Orogenic Loading and Flexure of a Rifted Continental Margin

Seismic and well data suggest that the West Taiwan basin developed by orogenic loading and flexure of a rift-type continental margin. The most likely source of the loading is Taiwan where oblique convergence between the Eurasian and Philippine Sea plates has produced an orogenic belt up to 300 km in length, 100 km in width and, 4 km in height. Flexure modeling shows that surface loading is unable to explain the depth of the West Taiwan basin. Other, subsurface or buried loads are required. Combined surface and buried loading explains the depth and width of the basin. It also accounts for a Bouguer gravity anomaly "high" and flanking "low" over the orogenic belt, a lateral offset of 20-30 km between the peak topography and the maximum depth to the seismic Moho and, evidence for tectonic uplift in the Penghu Islands. The "best fit" to the depth of the base of the foreland sequence in the northern part of the West Taiwan basin is for an elastic plate model with an elastic thickness, \ensuremath {T_e}, in the range 11-13 km. While these values are low when compared to most other foreland basins, they are similar to values derived from rifted continental margins. The northern part of the West Taiwan basin unconformably overlies a passive margin sequence and therefore appears to have inherited the long-term (> 1 Myr) flexural properties of the margin. In the southern part of the basin, however, the depth to the base of the foreland sequence dips too steeply to be explained by elastic plate models. This part of the basin therefore appears to be yielding rather than flexing. Differences in the flexural behaviour along-strike of the West Taiwan foreland basin lithosphere are reflected in seismicity patterns. The northern part of the basin is associated with a low level of seismic activity while the south correlates with an abundance of earthquakes, especially at shallow (< 25 km) depths. There is a cluster of earthquakes along two extensional faults that were active during rifting of the underlying margin. Therefore, lithospheric flexure and fault re-activation may be important contributors to the seismicity of the Taiwan region.