The nature of the plate boundar)'• between the Eurasian a11d Philippine Sea Plates changes from subduction to collision along the northern exten sion of the 1\ifanila T1•ench, north of 21°N, offshore southwest Tai\van. To det .ermine \\'hether the collision-formed deformation front passes through the Tainan Basin, a region ()f petroleum inte1 .. est, seismic profiles that cross a proposed ''deformation front'' are interpreted. However, only a ma. jor normal fault, that is a growth fault that forms the north,ivestern side of a tilted horst structure can be positivel)• identified. r-fhe nature of this nor mal faulti11g is also supported by the modeling of gravity and magnetic data.The observation of 011ly tensional faults in the Tai11an Basin suggests that the deformation t"ront is located to the S(lutheast, near Kaohsiung ()r even further to the south.The economic basement in the Tainan Basin region, defined as pre Tertiary 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 gener ating prospects related to normal faulting if struct11ral cl(>Sures can be iden tified.

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