A Strike-Slip Fault System Trending NE in and Around the Northwestern Corner of the West Philippine Basin

The southwestern part of the R�1ukyu arc-tre11ch S)·stem to the west of 126<>E is obviously bent north\\1ards horizontally. The mechanism of this horizontal bending has been discussed in the literat11re, but it is still am­ biguous. The \\1 est Philippine B asin (\\lPB) is generally believed to be pro­ duced by spreading along the Central B asin Ridge (CBR). However, the CBR does not extend to the northwestern C(>rner of the \\'PB. Previous geomagnetic and geomorphological studies suggested that the CBR termi­ nates or is displaced at its northwestern end. Howe,1er, the detailed picture is still unclear. In this paper, \Ve have examined bathymetry, gra\1ity and seismicity in and around the northwestern corne1· of the WPB, and discov­ ered a NE-trending right-handed strike-slip fault system '''hich ca11 be used to interpret the h<>rizontal bending of the sot1thwestern part of the Ryukyu arc .. trench system. A plausible model shO\\iing h<>W the right-handed fault system results in the bending of the Ryt1ky11 arc-trench S)rstem is de1non­ strat.ed. As the southeastern part ot· the right-handed strike-slip fault sys­ tem, the Luzon-Okina\\1a Fault Zone, \\'hich is an obvious fault zone be­ t\\'een the Luzon and Okinawa islands, is described in terms of bath"·metric . ., .

However, it is li kely· to be tl1e main c�1use f'o1· the be. ndi ng, because the scale ot' the southe1·n pc11·t ot· the Ryuky'U arc t1·ench is large, and whether the el£1sticity· {)f the lithosphere could stand being bent is dou btf' ul. The1·ef�o1·e� the mechan isrn ()f. the bending still 11eeds t() be resolv'ed.

CENTRAL TAI\\TAN NE .. S\V FAL:LT ZONE
The frequent earthquakes in the Tai\.\1an region (Figu1·e 5) a1·e generally bel ieved to be the result ot' the complicated and intense interaction betwee11 the Eurasian and Philippine Sea plates. In Figure 5� it is known that the earthqt1akes in the nc_)1·the.astern Tai\van   • I �.. ·'" ..
I O(l · : 1 . : ' . ! . ·: "t � ·�:-� ·:  zone. Therefore� we reC()gnize the NE-trending seism ic zone as a fau lt zone. We. refe1� to this seismic zone and its northeastern extension as the Central Taiwan NE-SW Fault z. one. Ac cording to the fault plane solutions in the literature (e.g.� Chang and Shin, 1994;Jiang, 1994;Shin et a. I., 1994), the faults in the Taiwan Fault zone are right-handed, like those in the Luzon Okinavv·a Fault zone.
In Figure 5, we. c.an see that along the t'ore-arc basin of" the Ryuk)'U arc-trench system in the northeastern offshore region of Taiwan, there is an obvious earthquake belt. This belt • bends northwards obviously when it intersects with the Central Tai\\i1an NE-SW Fault Zone. Figure. 7 is a sketch sh owing hc>w the bencling of this se is111ic belt is 1nade possi bly by succes sive slips in the t · ault zone. We believ·e that successiv·e slips in S()Tne fault zones are. the main mechanism of the bending of the Ryukyt1 arc-trench syste11 1.

NE-TRENDING FAULT SYSTEM IN WPB
Here w·e conside14 a NE-trending 1·ight-handed strike-slip fa ult system in the north\vestern c<.>r11er of the \\iTest Philippine Basin (\'v1PB) and the surrounding areas, such as the R)1ukyu arc, the Tai wan island and the Luzon Llrc. Because the plate. tectonics is very complicated in this regi<.)n, the fault s:y·stems are also \1ery· complicated. To simplify· the discussion, faults not striking NE are neglec.ted. The Central Tai\van NE-S\\l Fault Zone is considered to be a north western part of� the NE-trending fault system, and the Luzon-Okina\�ia Fault Zone a southeast ern part. Figure 8 show·s the shaded topography in the Taiwan region. From the. shaded topog raphy, we can see some clear NE-trending depressional structural lineaments. If we trace these NE-trending structural lineaments (the so lid line segments in Figure 9), and then dra\v SC)me straight lines fo llowing the tende.ncy· ot· these lineaments (dashed lines in Figure 9), then the straight lines show the approximate locations of some major faults. It is \iery intere.sting to see that the boundary lines ot· the NE-trending seismic zone in southern Taiwan, and their north eastern extensions mentioned in the last section, are among these straight lines. These majo1· faults extend northeast\\'ards across the Ryuky-u arc-trench system, and south\vest\\1ards across the Taiwan island and the Luz{)n arc.
The data are released by Sand�1el l and S1nith (_ 1995), and have sam pling spacing of 3 mint1tes (0.05 degree). The C()ntour spacing of the figure is 50 mgal .

501
sur14ounding areas, including the R)1ukyL1 <:1rc-trench system, the 1�aiwan island and the Luzon arc-trench system. This t'ault system includes the Central Tai \\1an NE-S\\l Fault Zone, the Luzon-Okinawa Fault Z-one, and a series c)f approximately par,1lle.1 majc)r t-�tults in this region . Tl1is t · au lt systen1 is the 111ain cause C. )f' the no1·thward be-nding of the southvvestern part of the Ryt1kyu arc -trench system. It is alsc> the main cat1se of terminati(lll or displacement C)f .. the C. entral Basin Ridge ot' the \vr est Philippine Basin at its i101-thweste-rn e11d.