VOL. 77, NO. 23 JOURNAL OF GEOPHYSICAL RESEARCH AUGUST 10, 1972 Plate Convergence,Transcurrent Faults, and InternalDeformation Adiacentto SoutheastAsia and the WesternPacific THOMAS J. FITCI-I 2 Lamont-Do.herty GeologicalObservatory o.• Columbia University,Palisades, New York 10964 A model for oblique convergencebetween plates of lithosphere is proposed in which at least a fraction of slip parallel to the plate margin results in transcurrent movements on a nearly vertical fault located on the continental side of a zone of plate consumption.In the extreme caseof complete decoupling,only the componentof slip normal to the plate margin can be inferred from underthrusting.Recent movementsin the western Sunda region provide the most convincingevidence for decouplingof slip, which in this region is thought to be oblique to the plate margin. A speculative model for convergencealong the margins of the Philippine Sea is constructedfrom an inferred direction of oblique slip in the Philippine region. This model requires that the triple point formed by the junction of the Japanese and Izu-Bonin trenches and the Nankai trough migrate along the Sagami trough. Absence of an offset between the trenches can be explained by rifting within the Izu-Bonin arc. Transcurrent movements in southwest.Japan and the Tai.wan region suggest that similar movements within well-developed island arcs are initiated during a nascent stage in the development of such plate margins and are often imposed on a pre-existing zone of weakness. The bulgelike configurationof the western margin of the Andaman basin is explained as a result of interarc extension within the western part of the basin. In the Indian Ocean plate south and west of this basin, strike-slip movements inferred from four mechanism solutions, two o.f which are reported in this work, suggest• distribution of stressin which axes of both maximum and minimum compressi.onare nearly horizontal, the stress maximum trending west of north. The larger recent earthquakes in the eastern Sunda region have resulted in deformation within the arc rather than shallow underthrusting at the inferred plate margin. A narrow region between the Celebes basin and the Philippine trench, including the Talaud Islands and a northern extensi.on of the Molucca basin, contains several zones of shallow thrusting. Mechanism solutions show that these movements trend E-W consistent with shortening normal to the arc-related structures in the region. Recent movements near the east coast of Luzon reveal a zone of underthrusting along the inner wall of a trough, marginal to the coast. There is an apparent hiatus in underthrusting between this zone and the one that follows the inner wall of the Philippine trench. Several of the earth's most extensive zones sibly the Nicaragua trough [Maldo•ada-Koer- of transcurrentfaulting are locatedwithin and dell, 196.6].With the exceptionof the Atacama strike parallel to active island arcs and arclike [Arabasz and Allen, 1970], the other fault structures.Allen [1965] discussedevidence for zonesshow evidenceof recent strike-slipmove- movements on three such faults: the Atacama ments contemporarywith plate consumptionin in Chile, the Longitudinal in Taiwan, and the the sameregion. In the work presentedhere such Philippine. To this list can be added the a systemof contrastingmovements is explained Semangkofault in Sumatra [e.g., Katili, 1970], by a model of convergencein which slip that is pbssiblythe median tectonic line in southwest obliqueto the plate margin is at least partially Japan [Karneko,1966; Okada, 1971], and pos- decoupled between parallel zones of trans- current faulting and underthrusting.A similar x Lamont-Doherty Geological Observatory con- model is proposedby Karig and Mammerickx tribution 1830. [1972] to explain an en echelon distribution 2 Now with Department of Geophysics and Geochemistry, The Australian National Univer- of oceanictroughs east of the New Hebrides sity, Canberra, Australia. arc south of 12øS. A schematic cross section of a decoupled Copyright • 1972 by the American Geophysical Union. plate margin is shown in Figure 1. The trans- 4432 •)LATE CONVERGENCEAND TRANSCURRENT FAULTS 4433 Asthenospher• • Asthenosphere Fig. 1. Vertical sectionof a well-developedisland arc in a region of oblique convergence. Decoupling hypothesisis illustrated by transcurrent movement on a vertical fault adjacent to the center of active volcanism. current fault is near an axis of a chain of active either the vertical or the inclined fault are volcanoes,as the Philippine and Semangko obtained by equating horizontal shear stress faults are (Figure 2). One mechanismsolution to the product of the normal stress and the for a recent earthquake shows evidence for coefficientof friction •: strike-slipmovement [Mo.l•ar and Sykes,1969] alongthe Nicaragua trough, a volcanic gra.b•n Vertical fault thatis nearlyparallel to anadjacent section F(cosa- /zsin a) - /zNH (1) of the Middle America trench. Coexistence of activevolcanism and transcurrent. faulting is Inclinedfault extensionunderstandablecan alsoin bethat weaka zone in horizontalof weakness shear,in F(cos a -- /zsin a sin$)sin $ - /zNH (2) and vice versa. A zone of weakness of a dif- where N• is the contribution to normal stress ferent origin can be inferredfor right-lateral from the hydrostaticload. When (1) > (2), movementsalong the median tectonic line horizontalshear occurson the vertical fault [Kaneko, 1966; Okado, 1971]. These move- rather than on the inclinedfault. When a dip mentsparallel 'a zoneof underthrustingalong of 30ø is assumedfor the inclinedfault, the the inner wall of the Nankai trough [e.g., inequalitysimplifies to cosa • (3 • sin e•)/2, Fitch and Scholz, 1971]. In place of recent in whichcase angles of obliqueslip as largeas volcanism,of whichthere is nonein this part 45ø satisfythe inequal!ty,if it is assumed of Japan[e.g., Matsuda e't al., 1967],narrow that • is about 0.7. This argumentis only metamorphicbelts, considered relics of Meso- slightlyweakened by usingan expressionfor zoicplate margins [Miyashiro, 1967], provide obliqueshear rather than horizontalshear in a zone of weakness conducive to horizontal (2). shear. Of the four regions of oblique convergence Decouplingof obliqueslip in the manner revealedby modelsbased on the 'assumption proposedhere is favoredin manyreal situa- of rigid plates[e.g., Le Pichon,1968], one tions becausea nearly verticalsurface con- servesas a type exampleof a plate margin centrates horizontal shear more effectively suchas that illustratedin Figure1. The region than an inclinedsurface of equal or greater is the westernSunda, where horizontal shear strength.This argumentis only weaklyde- and underthrustingoccur concurrently on the pendenton the angleof obliqueslip. Consider Semangkofault and alongthe inner wall of the simplemodel in Figure3, in whichuniform the Java trench,respectively. Directions and horizontalforce per unit area F in the direc- rates of convergencein this regionare com- tionof the slipvector is appliedto a.plate mar- puted from a simplifiedmodel that doesnot gin. Simpleconditions for horizontalshear on includeslip betweenthe Chinaand Eurasian 4434 THOMAS J. FITCH nearly parallels the island arc [Gumper, 1971]. 60N Similarly, underthrustingnorth of Puerto Rico parallelsthe plate margin in this regionmarked by the Puerto Rico trench. Near New Guinea, slip between the major plates is obscuredby movements between small intervening plates [Johnson and Molnar, 1972] and by possible internal deformation in western parts. of this region. A model for decoupled slip similar to that proposedfor the western Sunda arc is used to infer a direction of oblique convergencefor the Philippine arc. From this inference, al- though it is tenuous,a model is constructedfor o convergence along the eastern and western lOS margins of the Philippine Sea. For the western margin, directions and rates of convergence comefrom recent movements (measureddirectly or inferred from seismic evidence or both) Fig. 2. Plate boundaries and major transcur- rent faults in the Far East. Solid boundaries are within selected zones of shallow earthquakes. those that are well defined by recent shallow earth- The spreading history of the mid-oceansystem quakes [e.g., Bc•razangi•d Dorm•n, 1969]. Dashed of active ridgescannot be used,because no part curves extrapolate between well-defined bound- of this system passes through the Philippine aries. Dotted curves represent traces of known Sea. transcurrent faults of great length. MTL, median tectonic line; LF, Longitudinal fault; PF, Philip- No attempt is made to include specialregions pine fault; SF, Semangko fault; ST, Sagami such as interarc basins [Karig, 1970] in these trough. Plate margins east o.f New Guinea are models. Active extension within several of the from Johnso.n c•nd Molnar [ 1972]. basins marginal to the Philippine Sea and southeastAsia probably influencesthe rate and plates (Figure 2). In the absenceof a quan- direction of convergencealong adjacent frontal titative measure of this slip, large corrections arcs. The western part of the Andaman basin to slip vectorsin the Sundaregion are possible; (Figure 4) contains one or more centers.of however,such correctionsare unlikely because spreadingthat became active for the first time computed centers of rotation for the Indian Ocean and Eurasian (including China) plates are nearly 90 ø distant from this arc. The other regionsof oblique convergenceare the western Aleutians [e.g., McKenzie and Parker, 1967], northern