Lack of Compressional Overprint on the Extensional Structure in Offshore Tainan and the Tectonic Implications

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.

0N and passes through the Tainan Basin as the collision zone of the tWl) plates (Figure 1) (e.g., Huang, 1993 ). Typical trench topog1·aphy is not identifiable he. re because. the nature . ot� the plate boundary changes from subdue. ti on to arc continent collision north of 21°N (Biq, 1973;Suppe, 1988;Teng, 1990). The deformation t� ront produced by this collision can serve as an indicator to trace the tectonic boundar)' in the region offs hore south\vestern Taiwan. If the deformation fr ont, such as the one proposed and shown in Figure 2 (e .. g., Letouzey and Sage, 1988;Huang� 1993) does exist within the Tainan Basin, its geological implic. ations must be different t� rom those presented in some previous studies (e.g., Hu, 1988). Eclrlier geological \V ' Orks related to petroleum exploration generally have recognized that the Basin contains pre\1alent normal faults (Hu, 1988) and int' erred that it has been under tensional stress since it was formed in the Oligocene. If the collisional det'or mation front has indeed mig1�ated into the Basin, it \Vould imply that the thrust-fault systems could have se. riously aft� ected the way that prospects or c. losures of oil reservoirs have formed, and at the compressional stage in the history· of the ev·olution and de\1elopment of the .Basin, they should, therefore, present a different scenario for prospe . ct generation from that vv·hich is currently bel ieve. d.

'.! I •\ '
Fig. 1. Regional map of Taivvan. It has been suggested that the C()llisional deformation front (after Huang, 1993) associated \Vi th the Manila Trench passes through the stud)' at�ea in tl1e . Tainan Basin which is roughly' out lined (Hu,I 988) .
To determine and e\laluate the nature of the collisional deformatic)n frc)nt in offshore Taina11, seisn1ic profiles and potential-field data acquired in the Basin 'A'ere analized and interpreta tions were made. The tectonic implications of the faulting related to the northern extension of the Manila Trench are alS() discussed in this paper.

REGIONAL GEOLOG\T AND TECTONICS
The Tainan Basin is located in the northeastern corner of the South China Sea, in \:vater depths shall0'"1'er than 2000 meters (Figure l ). The fo rmation of the Tainan Basin was relate. ct to the second stage of' the seatloor spreading of the South China Sea Basin in the early Late Oligocene (� Ho, 1982; Ru and Pigotte. , 1986;Liang and L . in, 1990). A series of northeast trending normal fa ults pr()duced the asymmetrical pattern of grabens and horsts due to a re gional tilt, characterizes the structural style l)f' the Tainan (Ho, 1982).
The deeper water and steeper slope ot' the ocean bott()ffi t()Wa1·d the south in these p1·ofi Jes reflect the shift t · ron1 the continenta1 shelf to the s1ope . A trough-like t' eature more than I km \v ide is t� ound bet\veen sh()t p<) ints 130 and 2 I 0 along Line 21.
The excellent match ot· both gravity and magnetic data in the present model calculations in fo rward modeling are alS() found in all othe. r profiles and supports our seismic interpreta tions. Slight model adjustments were normally made at the top of either the Oligocene or Cretaceous strata, and accordingly, localized n1isfits often reflect small, irregular structures within the strata.

DISCUSSION
The se. ismic data presented in this study clearly· sho\v strong evidence. of a major normal t· ault along the northwestern edge of a structural high, but no evidence of compressional struc-.
tures. In particular, the ''det' ormation t'ront'' caused by collision along the leading edge of the  Figure 15 ).

519
The. values () f susceptibilit)', velocity and density adopted i11 each laye14 in m<)deling vary f'rom line to line and may· retlect lithologic inhomogenieties due to contrasting dep()Sitic)nal envi14onments. As for seismic velocity, it is t'ound that the () Verall 14ange C)f. interval velocities is comparable among the data based on sonic logs and those derived t'14om stacking \1el<)cities, while the averaged model velocities of each laye14 conve1·red from density on the basis ot' gravity inversion are somewhat higher (Table 2). Furthermore, velocit)' anal)1ses during p140cessing show that \1elocities te.nd to increase laterally tO\V£:trd the southeast. such a trend of increasing velocity implies that either the compaction or the age of the sedi1ne11tary rock i11creases in the same direc. tion.
Although gra\1ity  A stt'UCtural map of the economic basement (i.e., the depth to the top t)f' the pre-Te1�tiary t4()Ck) was generated based on f'i nalized models ot' pote. ntial-field data ( Figure 15). The struc tural high that coincides with the Central Uplit' t maintains a depth fr om 3 to 5 km and deepens to 111ore than 5 km in either direction away t'rom the uplift zone.
To further demonstrate the characteristics ot' normal fault bounding the Central Uplit't Z(Jne, seismic Line-5 ( Figure I 6) was con1p14essed to less than one-fifth of' its length. The ve. rtically· exagge1·ated profile sh()�'S a salt dome-like t' eature, \Vith fault th[tt CC.)u]d clearly· be interpreted as normal.
Since, on the basis of our ge()physical investigation, the compression<:1l stress 14e lated de forrnation f14ont was not found in the Tainan Basin, it shot1ld be located some\vhere f' urther to the south. One such possibilit)' (Lee et c1/., 1993), \\1hich \Vas based on seismic interpretation, suggested that it is located near Kaohsiung.
The important episode ot · arc-continent collision has caused the rapid uplift of the Central Range of' Taiwan Vv'hich has consequently beco1ne the main S(1Urce. ot' sediments transported and deposited in the Tainan Basin since the Pliocene. Altht)ugh it has not at't'ected the exten sional nature of the structures deve}()ped in the Tainan Basin 'ls shown above .. the collision has hindered any' t·u rther developn1ent of the basin. Fi,<J;. 16. \! ' ertically ex,1ggerated plot 01· Lir1e-5.