JOURNAL OF GEOPHYSICAL RESEARCH VOLUME 67, No. 12 NOVEMBER 1962 Circum-PacificFaulting in the Philippines-TaiwanRegion CLARENCE R. ALLEN Division o• Geological Sciences California Institute o• Technology, Pasadena Abstract. Conflicting views of circum-Pacific tectonics have focused on the Philippines- Taiwan region, where there has been neither convincing documentation nor general agreement on the importance of transcurrent (strike-slip) faulting or the possiblesense of regional hori- zontal displacement.Structural and physiographicfeatures of the 1200-km-long Philippine fault zone are fully as spectacularas those of the better-known San Andreas and Alpine faults, and current activity is indicated by many localities in which scarpscut Recent gravels. Pre- dominance of horizontal over vertical displacementsis indicated by linearity of the fault trace, failure of one side to be consistentlyhigher than the other, disregardfor grossphysiography, and scissoring of individual scarps within the zone. Consistent stream offsets on Luzon, Masbate, and Leyte demonstrateunequivocally that the senseof Recent displacementhas been uniformly sinistral (left-handed). The Philippine fault has no obviousgeologic relation- shipto active volcanoes,but the parallelismand proximity of the fault to the Mindanao trench suggesta close causalrelationship. The remarkable Longitudinal Valley of eastern Taiwan representsanother great transcurrent fault parallelto the westernPacific rim, and grounddis- placementsduring historicearthquakes indicate a sinistralsense of displacementhere as well as in the adjacentPhilippines. This study doesnot supportthe hypothesisof counterclockwise rotation of the Pacificbasin, but more important is the further documentationof the predom- inance of transcurrentfaulting in active circum-Pacificorogenic areas. These results reinforce earlier field studiesin Alaska,California, Chile, and New Zealand,as well as emphasizingthe geologicalreasonability of the resultsof seismicfault-plane solutions indicating the world-wide predominance of transcurrent movements. TI-IE [PROBLEM placementsin thissame region. In both hypothe- ses,the assumedsense of displacementplayed The purposeof this study was to investigate a part in the interpretation of the general some of the major faults of the Philippines- mechanismof circum-Pacificorogeny. Van Bem- Taiwan region with the hope of unequivocally melen [1949] and othershave tendedto discount demonstratingtheir extent, their degreeof cur- the importanceof horizontalfault displacements rent activity, and--particularly--their sense in this region altogether. of displacement.Unusual geophysicalsignifi- Willis [1937] was apparentlythe first investi- cance is attached to this region becausewidely gator to recognizethe existenceof a through- divergentviews of circum-Pacificorogeny focus goingfault zonein the Philippinescomparable to here, especiallyconcerning the questionof the the San Andreas fault in California, although relative importance of horizontal fault dis- numerousworkers dating back to von Drasche placements.Even granting the importanceof [1878] had pointed out the existenceof vari- horizontal movements, major disagreement ous tectonic lineaments in the archipelago, has existed concerning the sense of possi- mainly on the basisof the allegedalignment of ble displacement: theories of Benio# [1959] islands, volcanoes,and earthquake epicenters. and St. Amand [1959] implied the probable Repetti [1935], in particular, had proposedthe existenceof dextral or right-handedfaults paral- 'Master fault' along the general trend of the leling the westernPacific margin in this region, Philippine fault zone, but his evidence other whereas theories of Vening Meinesz [1954] and than that from northern Luzon was based al- Biq [1960] supposedsinistral or left-handeddis- most wholly on inconclusiveseismic data. The term 'Philippine fault zone' used by Willis • Contribution 1099, Division of Geological Sci- [1937] seemsto have becomewell establishedin ences, California Institute of Technology. the literature, notwithstandingthe later use by 4795 4796 CLARENCE R. ALLEN Willis [1944] of 'Visayan rift' for the same the main zoneand is apparently at an angle of feature. at least 30ø to it. Willis further arguedthat the Von Drasche [1878, p. 3] and othershad par- regionalplan of the islandssuggested a sinistral ticularly pointed out the bifurcation or virga- senseof displacement.Other authorshave agreed tion of tectonic trends in the southern Philip- [Alvir, 1941; King and McKee, 1949].But opin- pines,the southwesterlytrend of the islandsin ion has not been unanimous; Irving [1951, p. the southwesternPhilippines contrasting sharply 70] thoughtthat many of the offsetsimplied by with the southeasterlytrend of the islands and sucharguments were 'excessive,'and Alvir [1926, mountain chainsalong the southeasternrim of p. 453] had used the same kind of argument the archipelago.The island of Masbate,x with (basedon grossphysiographic features) to indi- its two diverging 'arms,' has often been cited cate dextral displacementalong the fault in as an exampleof the acute junction of the two northern Luzon that is herein considered to be opposingtrends. Willis wasso impressed by the an integral part of the Philippine fault zone. linearity of the southeasterlytrending features Another line of evidencebearing on the sense between Ragay Gulf and northern Mindanao of fault displacementis that given by fracture that he proposedthe existenceof the Philippine patterns in Philippine mining areas, although fault zone along this line; his evidencewas none of the areas described in detail has been mittedly based almost wholly on grossphysio- within the fault zoneproper. Leith [1938], Liv- graphicforms. Other investigatorswho followed ingston [1939], and Wisser [1939] pointed out Willis have generallyagreed upon the presence that the pervasiveshear and tensionjoints of of a fault zone, althoughthere have been dis- the Baguio mining district demand east-west agreementsconcerning its possibleextension into compression,at least in Miocenetime, as does northern Luzon and its exact location in the cen- the generaltrend of the foldedmountain ranges tral part of the islands.Evidence for its existence north of Manila. Wisser [1952] cited supporting has continuedto be basedmainly on grossphysi- evidencefrom local fracture patterns in south- ography, inasmuchas there is no clear docu- eastern Luzon and northeastern Mindanao. Such mentation of displacementsalong its trace dur- east-westcompression is, of course,compatible ing historicearthquakes. Only on the island of with sinistraI transcurrent displacementon the Mindanao do publishedgeologic maps clearly Philippinefault zonebut by no meansproves it. demonstrate and delineate the various breaks Nor has there been completeagreement on the that make up the Philippine fault zone [Ranne/t regionalstress pattern; the numerousnorth- et al., 1960]. Probably the most accurate maps south lineamentsof the central Philippineswere of the fault's locationnorth of Mindanao, and visualizedby Alcaraz [1947] as tensionalin the most cogentarguments for its existence,are origin, which implies a regionalstress system those by Irving [1951, 1953], based in par• on that would lead to dextral displacementalong an analysisof the submarinemorphology of the the Philippinefault zone.Ranne/t et al. [1960] region. indicated sinistral displacementson individual Willis apparently consideredthe Philippine breaks of the zone in northeastern Mindanao but fault to be dominantlytranscurrent (i.e., 'strike- did not state why. slip,' 'lateral,' or 'wrench') becauseof its simi- It mightbe hopedthat resultsof seismicfault- larity to the San Andreasfault, particularly in its planesolutions on the numerousearthquakes of length, linearity, and generalphysiographic ex- the Philippinesregion would clear up the am- pression.But the senseof horizontal displace- biguity resulting from the limited geologic ment has been the subject of more nebulous studies.In a summary of 35 fault-plane solu- argument.Willis arguedthat the fault was sinis- tions from this region, Ritsema and Veldkamp traI, and the most often-quotedevidence is based [1960] statethat the predominantdisplacements on a single set of horizontal asymmetric slick- have indeedbeen transcurrent and sinistral,and ensidesthat were observedby Willis [1937, p. the implied directionof horizontalcompression 39] in the Aroroy mining district on Masbate, is about N 60øE. None of the larger earthquakes although this particular fault is subsidiaryto of this group, however, occurred within the • All Philippine place names used in the text Philippine fault zone, and the largest shock are indicated on Figures 1 and 2. (M -- 8.2, Jan. 24, 1948) was clearly dextral. CIRCUM-PACIFIC FAULTING 4797 The epicenterof this greatearthquake was near between the towns of Visares and Leyte. The the west coast of Panay, where there is good zone here consistsof a number of parallel active evidence of a major northeast-trending fault breaks over a width of about 500 meters, and that might reasonablybe consideredconjugate the extreme linearity of the topographywithin to the Philippine fault and thus dextral. The the fault zone stands in marked contrast to the fault-planesolution, however, which is supported topographyon both sides(Figures 3-5). Neither by the trend of the isoseismaIs[Irving and side is consistentlyhigher than the other, and Teves, 1948], indicated failure on a fault strik- the centralpart of the