The nature of the plate boundary between the Eurasian and Philippine Sea Plates changes from subduction to collision along the northern extension of the Manila Trench, north of 21°N, offshore southwest Taiwan. To determine whether the collision-formed deformation front passes through the Tainan Basin, a region of petroleum interest, seismic profiles that cross a proposed "deformation front" are interpreted. However, only a major normal fault, that is a growth fault that forms the northwestern side of a tilted horst structure can be positively identified. The nature of this normal faulting is also supported by the modeling of gravity and magnetic data.
The observation of only tensional faults in the Taiwan Basin suggests that the deformation front is located to the Southeast, near Kaohsiung or even further to the south.
The economic basement in the Tainan Basin region, defined as preTertiary rocks that u11derlie the thick Tertia1.. y sequences alc>ng the seismic. lines \ive examined, maintains a depth of 3 to 5 km in a tilted horst-like zone, kno\vn as the Central Uplit"t, and deepens in either direction away f"rom it. The thick Tertiary deposits provide excellent potential fo1 .. source rc>ck and seal. Petroleum exploration in this area should be directed toward generating prospects related to normal faulting if struct11ral cl(>Sures can be identified.