The intra-arc basin, : Basin genesis and in situ ophiolite development in a strike-slip setting

DANIEL R. SAREWITZ Department of Geological Sciences, Cornell University, Ithaca, New York 14853 STEPHEN D. LEWIS U.S. Geological Survey, Menlo Park, California 94025

ABSTRACT INTRODUCTION geophysical data, including SeaMARC II side- scan sonar imagery and swath bathymetry, and The Marinduque basin is a marine intra- A growing body of data from modern single-channel seismic reflection profiles. Our arc basin in the north-central Philippine vol- convergent-plate boundaries show that volcanic results suggest that the Marinduque basin in- canic arc system. A suite of marine geophysi- island-arc systems evolve under the influence of itiated as a pull-apart basin in a major strike-slip cal observations show the basin as a whole to many complex and variable tectonic processes fault zone, and that strike-slip faulting has been be rhombic in shape, with its long axis trend- (Jarrard, 1986a; Hamilton, 1988). This increas- the dominant controlling process throughout the ing north-northwest. A conspicuous volcanic ingly sophisticated understanding of island-arc evolution of the basin. We also document an ridge trends east-northeast across the basin, tectonics, however, has not been accompanied episode of probable crustal extension similar to dividing it into two smaller active depocen- by new models describing the evolution of sed- sea-floor spreading within the basin that has im- ters. Symmetric linear magnetic anomalies, imentary basins in island-arc systems. The tri- portant implications for the generation and em- striking parallel to, and centered over, the partite arc-related basin nomenclature—forearc, placement of ophiolites within island arcs. central volcanic ridge indicate that it formed intra-arc, and backarc—still prevails, and actual- by extension in a north-south direction by a istic models for basin development have not REGIONAL TECTONIC SETTING process analogous to sea-floor spreading. changed significantly in the past decade (Inger- Geometric and temporal relations between soll, 1988; Karig, 1974; Dickinson and Seeley, The Philippines volcanic island-arc system the central volcanic ridge and several en 1979). lies within the obliquely convergent plate echelon sets of north-trending, left-stepping In the Philippines volcanic-arc system (Fig. boundary zone between Eurasia and the Philip- faults indicate that sea-floor spreading oc- 1), major sedimentary basins occur in marine pine Sea. The major tectonic elements currently curred in the extensional stepover zone of a and nonmarine settings, both along the margins active in the Philippines are the , , left-lateral, strike-slip fault system, and that and within the interior of the archipelago. Few and trenches on the west side of the the Marinduque basin as a whole is a compos- detailed studies of these basins have been under- arc; the Philippine Trench and East ite pull-apart basin whose floor is in part taken, but existing data suggest that basin evolu- Trough on the east side of the arc; and the Phil- composed of oceanic-type crust. The evolu- tion is directly related to the complex tectonic ippine fault zone, which transects the interior of tion of the central volcanic ridge presents an development of the arc, and most sedimentary the arc (Fig. 1; Cardwell and others, 1980). Rel- actualistic model for in situ development and basins in the Philippines appear to have evolved ative interplate motion between Eurasia and the emplacement of ophiolites in an island-arc through several distinct tectonic phases (for ex- Philippine Sea plate, which is in a generally setting. ample, Hashimoto, 1981; Bachman and others, northwest-southeast direction, is oblique to the The presently active, left-lateral Philippine 1983; Schweller and others, 1983; Lewis and overall north-south trend of the archipelago and fault zone truncated the eastern flank of the Hayes, 1984; Karig and others, 1986a). For this to most of the trenches (Seno, 1977; Ranken basin as well as the central volcanic ridge, reason, standard classification and evolutionary and others, 1984). Focal mechanisms from shal- following their initial formation. Shortening schemes for island-arc basins cannot be satisfac- low earthquakes show that this overall north- associated with a probable restraining bend torily applied to most basins in the Philippines. west-southeast oblique plate convergence is in the Philippine fault has resulted in uplift of Those that fit into conventional models, such as separated into an east-west component of con- basin flank sediments, which have been thrust the forearc basins inboard of the Manila Trench vergence accommodated by subduction, pre- back toward the basin interior. The Philip- (Lewis and Hayes, 1984), are young features dominantly along the Manila and Philippine pine fault has also displaced fragments of the that have not yet experienced changing tectonic trenches, and a north-south component of strike- ridge and basin floor toward the north. regimes. slip offset, predominantly along the Philippine The overall history of the Marinduque In this paper, we investigate the morphology fault zone (Fitch, 1972; Cardwell and others, basin further suggests that strike-slip proc- and tectonic development of the Marinduque 1980). Broadly distributed shallow seismicity esses may play an important role in the origin basin, a modern, marine intra-arc basin located throughout the archipelago indicates that de- and growth of intra-arc basins. Such basins in the , south of Luzon (Fig. 1). We formation occurs across the entire arc (Fig. 2; may ultimately become isolated in a forearc present a detailed tectonic model for the evolu- Cardwell and others, 1980). The Philippine arc or backarc setting. tion of the basin based on an array of marine thus cannot be viewed as a simple, rigid plate

Geological Society of America Bulletin, v. 103, p. 597-614, 17 figs., May 1991.

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fault-bounded geologic units of regional extent that have geologic histories different from neighboring terranes (Howell and others, 1985), juxtaposed across numerous major structures, active and inactive, and knitted together by Cenozoic volcanoes and successor basins (Karig, 1983; McCabe and others, 1985; Haw- kins and others, 1985). Terranes include a Pa- leozoic and Mesozoic continental fragment, complexly deformed and metamorphosed pre- Tertiary island-arc "basement" assemblages, and several ophiolite complexes. Paleomagnetic data (Fuller and others, 1983) and plate reconstruc- tions based on regional geological relations (Sarewitz and Karig, 1986a; Jolivet and others, 1989) show evidence for northward transport of several terranes along faults that probably had a significant component of strike-slip motion. The present combination of flanking trenches and complex, internal, strike-slip-dominated defor- mation has probably been typical of the Philip- pines at least since the Eocene (Karig and others, 1986b). The geologic evolution of the Philip- pines during the Tertiary thus reflects changing subduction geometries, internal deformation of the arc by strike-slip faulting and small plate collision, widespread igneous activity along nu- merous distinct volcanic axes, and sedimenta- tion in a variety of basinal settings (Murphy, 1973; Karig, 1983; Mitchell and others, 1986; Jolivet and others, 1989).

DATA ACQUISITION

Data were acquired during April 1988 aboard the R/V Moana Wave, operated by the Hawaii Institute of Geophysics. An array of un- derway geophysical techniques was used to de- termine the three-dimensional geometry of the Marinduque basin. SeaMARC II side-scan sonar imagery and swath bathymetry, single- channel seismic reflection (SCS) profiles (120 in.3 airgun acoustic source), 3.5-KHz shallow- penetration reflection data, and gravity and magnetic data were acquired continuously along track (Fig. 2). SeaMARC II side-scan sonar data provide a Figure 1. Tectonic map of the Philippines. The Marinduque basin is shaded dark gray. Other digital, gray-scale image of the sea floor along a marine intra-arc basins are light gray. Onshore basins are shown by diagonal lines: 1. swath from 5 km up to 10 km width (Fig. 3; see Basin, 2. Luzon Central Valley, 3. - Trough. Blackinton and others, 1983, for a description of the system). It also maps bathymetry with a depth resolution of about 50 m along a swath boundary, but rather it comprises a wide zone of west side of the archipelago north of about 14°N approximately 3.4 times the water depth (Fig. diffuse interplate motion. latitude to the eastern side of the archipelago 4). The SeaMARC II system is capable of imag- Widespread Quaternary volcanism has oc- south of 14°N latitude. This probably reflects a ing linear features with bathymetric relief of 5 to curred on Luzon, Negros, and , as long-term transition in the main locus of plate 10 m and is particularly effective for identifying well as on numerous smaller islands (Fig. 1; Phil- consumption from the Manila Trench to the fault scarps, volcanic structures, and sediment- ippine Bureau of Mines, 1962; Divis, 1981; De- Philippine Trench. distribution features. Sea-floor features that do fant and others, 1989). In the northern Philip- The crust of the Philippines is a heterogene- not have a sharp bathymetric expression, such as pines, the main volcanic chain swings from the ous assemblage of tectonostratigraphic terranes, changes in rock type or breaks in slope, are more

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Figure 2. Tectonic setting of the Marinduque basin, with ship track lines. Heavy track-line segments locate subsequent figures. Epicentral locations shown for shallow earthquakes with known focal mechanisms. Strike-slip solutions indicated by trend and slip sense of nodal planes. Normal solutions indicated by opposing arrows in trend of T-axis. Reverse solutions indicated by converging arrows in trend of P-axis. (Shallow underthrust events associated with the Philippine Trench not shown. Earthquake data from Cardwell and others, 1980.)

difficult to resolve, but such features can be iden- This subsidiary basin reaches a maximum depth trending central ridge that bisects the basin (Fig. tified by changes in tone, texture, or both, on a of about 1,200 m. 4). This ridge, which is about 35 km long and 11 scale of hundreds of meters. The three-dimen- The flanks of the Marinduque basin are km across at its widest point, reaches to within sional character of sea-floor structures can be strongly gullied, with numerous small submarine 375 m of the sea surface and stands more than assessed by combining side-scan imagery and canyons trending downslope (Figs. 3, 4). Al- 1,100 m above the surrounding basin floor. The bathymetry with seismic reflection data. though the morphology of the basin flanks is western end of the central ridge merges into the complex in detail, the bathymétrie map (Fig. 4) basin flank in a zone of small gullies and isolated BASIN MORPHOLOGY shows that, in general, the western and eastern knobs (Fig. 4). The central ridge does not extend sides of the basin are characterized by steeper across the entire basin floor but is abruptly trun- The Marinduque basin is rhombic in shape, slopes and more linear segments than are the cated about 4 km from the base of the eastern measuring about 75 km along its northwest- southern and northern flanks. The average gra- basin slope. trending long axis, and 38 km in the shorter, dient of the eastern and western sides of the The eastern end of the ridge is characterized east-northeast dimension (Figs. 3 and 4). Water basin is almost 9°, whereas the average slope of on side-scan imagery and bathymetry by several depths within the basin reach more than 1,700 the northern and southern sides of the basin is conical bathymetric highs that vary in width m in a small region in the northern part of the just 3°. The most strikingly linear segment of the from several hundred meters to more than 2 km basin; most of the northern basin floor is about basin margin lies along the eastern side of the (Figs. 3 and 4); the best developed of these oc- 1,600 m deep, whereas south of the central basin. cupy the highest portion of the central ridge. ridge, the basin floor is -1,500 m deep (Fig. 4). Each cone is spatially associated with a zone of An elevated ridge that forms the northwestern CENTRAL RIDGE uniform tone on the side-scan imagery, and in boundary of the central part of the Marinduque some cases these zones display a dense fabric of basin also serves to isolate a smaller, flat-floored, The most prominent bathymétrie feature of lineaments that radiate away from the cone. The perched basin farther to the northwest (Fig. 4). the Marinduque basin is an east-northeast- acoustic character of these conical bathymetric

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features on the side-scan imagery, in combina- tion with the prominent magnetic anomalies as- sociated with the central ridge (see below), suggest that these are volcanic cones with asso- ciated lava fields that have flowed down the ridge flanks. Single-channel reflection profiles indicate that the ridge is not covered by a substantial thick- ness of sediment (Fig. 5). Layered reflections occur near the base of the ridge where basin sediments lap onto the ridge flank. SeaMARC II imagery shows a series of rectilinear features as much as several hundred meters in width and up to 10 km in length that trend generally east- northeast, parallel to the ridge axis. Bathymetric data indicate that these features are steep (30°-35°), linear slopes that in some cases bound elongate depressions.

NORTHERN SUB-BASIN

The central ridge separates two distinct sub- basins (Fig. 4). The northern sub-basin is gener- ally rhombic in shape, with its long axis oriented about west-northwest. SCS profiles show that the sediments of the northern sub-basin are highly variable in thickness, and exceed 1.1 sec (two-way traveltime) or ~ 1 km in some areas (Fig. 6). This sediment package thins gradually toward the western basin margin where shallow- sediment units unconformably overlie a basal- sediment sequence that is concordant with the underlying acoustic basement. The geometry of the northern basin-fill sequence is highly variable, both laterally and vertically, due to irregularities of underlying acoustic basement. Several north- to north- northwest- trending folds, some apparently in- volving acoustic basement, are partially to completely buried beneath onlapping basin-fill sediments (Figs. 6 and 7). Total structural relief on these ridges exceeds 1.0 sec in some areas. The position of the folds is a first-order control on the distribution of sediment within the north- ern sub-basin. They separate several distinct, asymmetric sediment packages. These observations indicate that the basement beneath the northern sub-basin is comprised of several structural blocks that defined local depo- centers early in the history of the Marinduque basin. Individual seismic sequences are com- monly thinned and flexed above areas of posi- tive structural relief. In fact, numerous internal unconformities expressed by onlap of basin-fill reflections onto older, gently folded sequences show that deposition and structural disruption of the basin floor occurred synchronously during much of the duration of basin evolution. Con- tinued sedimentation led to burial of basement Figure 3. SeaMARC II side-scan sonar mosaic of the Marinduque basin. relief and coalescence of younger basin-fill strata

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Figure 4. SeaMARC II bathymetric map of the Marinduque basin. Contour interval is 100 m. Dashed lines indicate areas of no data. ' 7 tiO \ / S

/ / »,11\\ ' / ' 1 ! ! v x • V ! ' - -< I 1 t k * v \ ' to define the present, larger depositional area of the northern sub-basin.

SOUTHERN SUB-BASIN

The southern sub-basin is smaller and less in- ternally complex than its northern counterpart. It is approximately triangular in shape, with one side trending east-northeast, adjacent to the cen- tral volcanic ridge, and the two other sides defin- ing the gently sloping southern flank of the basin (Fig. 4). SCS data show about 0.3 sec (-300 m) of horizontal to very gently dipping, well- defined, parallel, basin-fill reflectors that lap onto older, dipping reflectors of the southeast basin flank (Fig. 5). The southern basin se- quence thickens slightly toward the central ridge and displays several internal unconformities, but it is more homogeneous than the northern sub-basin in terms of both geometry and reflec- tive character.

SOUTHEAST BASIN FLANK: THE PHILIPPINE FAULT

South of about 13°N latitude, the eastern margin of the Marinduque basin is defined by a well-developed network of parallel and anas- tomosing lineaments on the sonar imagery (Fig. 8). These northwest-trending lineaments mark the western side of the active, left-lateral, Philip- pine fault zone. The major fault trace in the survey area comprises numerous discontinuous colinear fault segments that can be followed for more than 60 km on the side-scan image and that probably continue beyond the limits of the sonar coverage to the southeast. The major fault trace runs along the eastern edge of a 10-km-wide, northwest-plunging val- ley, just above the break in slope at the valley floor (Fig. 9). A secondary fault trace runs along the western edge of the valley. Correlation of surface lineaments with SCS data shows that the Philippine fault zone marks the transition from the relatively undeformed sedimentary se- quences of the southern sub-basin and basin flank to the more intensely deformed east-basin flank. South-flank sediments are offset by a series of steeply dipping minor faults that cut up through the section to the sea floor (Fig. 9). toward the major strands of the fault, as shown plunging fault valley, where both the acoustic These faults correspond to a subtle northwest- by the progressive eastward increase in disrup- basement and overlying sedimentary sequences trending fabric seen on sonar imagery. The de- tion of reflections in the sediments. A major are downdropped at least 0.5 sec. gree of deformation increases progressively structural discontinuity marks the edge of the The eastern end of the central volcanic ridge

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Figure 5. Seismic reflection profile P-P', interpretive line drawing, and magnetic anomaly across Marinduque basin and central volcanic ridge. See Figure 2 for location.

terminates against the Philippine fault zone. flows on the side-scan image. A SCS profile NORTHEAST BASIN FLANK Near the fault zone, the volcanic ridge is bathy- across this zone (Fig. 10) shows that the metrically subdued, but its presence is docu- volcanic ridge is onlapped by basin-fill strata on The northeastern flank of the basin is defined mented by a symmetrical magnetic anomaly and its southeast flank, but to the north it is abruptly by a series of three en echelon, arcuate side-scan by the identification of a volcanic cone and lava truncated by the fault zone. lineaments north of about 13°05'. Each of these

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Figure 6. Seismic reflection profiles I-I' and K-K', with interpretive line drawings across the northern sub-basin. Line I shows pronounced, south-plunging bathymétrie ridge that controlled distribution of sediments throughout basin development. Line K shows gently folded reflec- tions perhaps overlying the down-plunge projection of the ridge in line I. Note structural inversion of west-basin flank on line K, which is minimized on the profile because of extreme vertical exaggeration. See Figure 2 for location.

arcuate features comprises a linear, 350°- face traces of faults with a significant component and in most cases appears to extend below the trending segment that curves sharply but contin- of reverse offset. zone of seismic penetration. uously toward the east. These lineaments, which A similar seismic package has been recog- run parallel to the coastline of the Bondoc pe- SOUTHERN FLANK nized to the east, between Island and ninsula (Fig. 7), define a sharp morphological the Bondoc Peninsula of Luzon (unpub. data break between relatively undeformed basin-fill Seismic data show that the south flank of the from the Philippine Office of Energy Affairs). sequences and the disrupted and seismically in- Marinduque basin is underlain by a package of Onshore proprietary well control suggests that coherent northeast-basin flank. The northwest- relatively coherent, parallel reflectors that over- the middle, nonreflective sequence is middle trending basin morphology of the Philippine lie a highly irregular acoustic basement surface Miocene to early late Miocene, and that the fault zone appears to be truncated by these lin- and dip gently toward and beneath the southern upper, reflective sequence is late Miocene to eaments, but more easterly strands of the sub-basin. The thickness of this seismic package latest Pliocene. Philippine fault at this latitude—those to the varies from about 1.0 sec to greater than 1.5 sec. Most faults in this region show a down-to- east of the survey area—continue farther north Changes in thickness correspond to structural the-basin sense of slip (Fig. 11). A contour map (Fig. 2). relief on the underlying acoustic basement. On of two-way traveltime to the base of the middle, Where SCS lines cross these arcuate linea- most profiles, the reflectors of the south-basin transparent sequence (Fig. 12) shows that the ments, they show that the basin-fill sequence flank can be divided into three distinct sequences structural fabric beneath the south flank of the extends beneath the basin flank at depth (Fig. 6, (Figs. 9 and 10): Marinduque basin generally follows the ba- SCS line K). Gently dipping basin-fill reflections (1) an upper sequence characterized by thymetry, indicating that disruption of the basin- between =2.5 and 3.2 sec can be traced beneath well-defined, continuous parallel reflectors and flank sequences was probably coincident with the basin flank for 1.5 km. Other profiles show variable thickness (up to 1.0 sec); the formation of the southern sub-basin and that basin-fill reflections extending as much as 2.5 (2) a middle sequence of relatively uniform the morphology of the basin flank directly re- km beneath and beyond the modern basin edge. thickness (H3.1 sec) that is almost transparent in flects its structural evolution. Changes in thick- This indicates that undeformed basin sediments some areas; and ness of the uppermost seismic sequence corre- are structurally overlain by basin flank units and (3) a lower sequence of strong reflectors of spond to changes in relief of the two older that the arcuate side-scan lineaments are the sur- variable continuity that is at least 0.3 sec thick, sequences, indicating that some or much of the

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structural relief developed during deposition of this youngest basin-flank sequence, tentatively dated as late Miocene to Pliocene. In the south- east corner of the survey area, all three sequences are disrupted by the northwest- trending Philippine fault zone (Fig. 9).

WESTERN AND NORTHERN BASIN FLANKS

Side-scan imagery shows the western and northern flanks of the basin to be composed of steep slopes dissected by dense, dendritic sys- tems of erosional submarine channels and can- yons (Fig. 3). These submarine canyons and channels supply sediment not only to the north- ern sub-basin, but also to a smaller basin perched at about 1,250-m depth on the northern flank (Figs. 4 and 6). Reflection profiles across the northernmost basin flank show a southward-dipping reflective sequence that thickens toward, and is partly con- tinuous with, the reflectors of this perched basin (Fig. 13). In contrast, at the western basin flank, reflectors in this perched basin are truncated against acoustic basement and cannot be traced onto the basin flank itself. The truncation sur- face dips beneath the basin, and basin-fill reflec- tions flex upward as they approach it, suggesting that it is a fault rather than an unconformity. On the sea floor, this abrupt western margin of the perched basin appears as several poorly defined north-south-trending lineaments that correspond to sharp breaks in slope. The perched basin displays a series of distinc- tive, east-west-trending lineaments that can be traced for more than 10 km across the entire basin floor. These features actually comprise numerous, short (< 1 km), east-west linear seg- ments that are separated or offset along subtly defined north-south lineaments. Shallow-pene- tration, 3.5-KHz data show minor sea-floor re- lief (25 to 30 m) across the east-west lineaments, and SCS profiles show that they are down-to- the-basin faults that penetrate and offset or flex subhorizontal reflectors in the perched basin to a depth of at least 0.85 sec sub-sea floor (Fig. 13). These observations suggest that the perched basin is structurally active, with relative subsi- dence occurring across east-west- and north- south-trending faults along the north and west basin flanks, respectively.

MAGNETIC SIGNATURE OF THE CENTRAL RIDGE

The Marinduque basin displays a distinctive pattern of symmetrical magnetic anomalies cen- tered over the axis of the central basin ridge. A negative anomaly of about 170-nanoteslas (nT) amplitude runs along the bathymétrie crest of Figure 7. Synoptic map of the Marinduque basin, showing the location of major features the ridge for more than 35 km (Fig. 14). Flank- identified from side-scan, seismic, and magnetic data. ing positive anomalies are well developed ~6

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/103/5/597/3381142/i0016-7606-103-5-597.pdf by guest on 30 September 2021 Figure 8. Detail of southeast portion of side-scan mosaic, showing Philippine fault and eastern end of the central volcanic ridge.

Figure 9. Seismic re- flection profile S-S' and interpretive line drawing across southeast-basin — flank and Philippine fault zone. Tentative ages of o seismic sequences are as

latest Pliocene; II. middle

0) OL

Main Western Branch

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Figure 10. Seismic reflection profile R-R', interpretive line drawing and magnetic anomaly showing truncation of the central volcanic ridge and southeast-basin flank by the Philippine fault zone. Seismic sequences as in Figure 9. See Figure 2 for location.

Figure 11. Seismic reflection profile Z-Z' and interpretive line drawing across south-basin flank, showing major fault that controlled the location of the basin margin. In addition to down-to-the-basin slip displayed on the profile, this fault probably also accommodated signifi- cant strike-slip motion. See Figure 2 for location.

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km to the north and south, with amplitudes of 180 nT to 240 nT, respectively, and maximum peak-to-trough amplitudes of 440 nT. All anomalies trend east-northeast; the central anom- aly shows the best lateral continuity. Profiles P-P', Q-Q', and R-R' (Fig. 14) cross the cen- tral ridge, and display well-developed "axial" anomalies, whereas profile S-S' crosses the basin to the east of the eastern end of the central ridge. The negative anomaly characteristic of the central ridge is absent on profile S-S'. Where the central ridge is widest, a small positive anomaly occurs within the larger, axial negative anomaly. This smaller anomaly is centered over the large volcanic cone located at the crest of the ridge. No other magnetic anomalies similar in character to these have been reported from the Visayan region. The pattern of magnetic anomalies, in combi- nation with the apparently igneous origin of the central ridge, suggests that the ridge developed by progressive, symmetrical accretion of igneous material about a central, east-northeast-trending axis, in a manner analogous to sea-floor spread- ing. No other plausible explanation can account for the linear distribution and symmetrical char- acter of the magnetic anomalies, their alignment parallel to the central ridge, and the spatial asso- ciation of the anomalies with the central ridge. The observed magnetic anomaly pattern across the central ridge can be modeled by the symmetrical global geomagnetic reversal se- quence of positively and negatively magnetized blocks centered on the ridge (Fig. 14), with the ridge itself positively magnetized, and using a phase shift of-160° calculated from the present- day location and orientation of the Marinduque basin. Due to the short sequence of lineated magnetic anomalies preserved within the Marin- duque basin, however, we cannot reliably correlate the observed anomaly sequence with the global geomagnetic reversal sequence. The synthetic model profile shown in Figure 14 was calculated from the magnetic reversal pattern from 5 Ma to the present with a spreading rate of 32 mm/yr, and it matches the wavelength of the central anomaly well. Figure 12. Contoured map of two-way traveltime to the base of the transparent sequence (II) on the south-basin flank (contour interval = 0.1 sec). Note that the general pattern of isotime STRUCTURAL EVOLUTION OF THE lines appears to be controlled by faulting on the basin flank, and contour geometry is similar to MARINDUQUE BASIN present bathymetric contours.

Sonar imagery in combination with seismic reflection data is a powerful technique for de- have imposed a variable kinematic history on morphologies of individual structures, but analy- termining the three-dimensional geometry of any given structure. sis of the geometric distribution of families of geologic structures. The kinematic history of Transcurrent faulting is known to be an im- structures in an area of suspected strike-slip mo- structures, however, is not easily constrained, portant process of intra-arc deformation in the tion can indicate such motion. Experimental and especially in areas that have a protracted history Philippines, based on modern seismicity (Card- theoretical studies of strike-slip fault zones pre- of deformation. The Philippine archipelago as a well and others, 1980; Acharya, 1980) and on dict a suite of structures whose orientations and whole has undergone an extremely complex detailed field studies (Sarewitz and Karig, kinematic histories are related to the orientation Tertiary structural evolution, and so individual 1986a, 1986b; Karig and others, 1986b). Evi- of the stress field (Tchalenko, 1970; Sanderson structures may have experienced multiple epi- dence of strike-slip offset in any area is often and Marchini, 1984). The kinematics of indi- sodes of activity, and changing stress fields may difficult to document based on geometries and vidual structures has often been inferred by

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amples, such as single-event fault ruptures (Tchalenko, 1970; Terres and Sylvester, 1981). Second, most experimental studies deal with a homogeneous deforming medium, whereas the island-arc crust of the Philippines is almost cer- tainly Theologically heterogeneous even on the scale of this study, due to its long history of internal deformation and igneous and metamor- phic evolution. Third, pre-existing structures may be reactivated in a new stress field even though they are not oriented in a theoretically optimal position. Finally, the stress field itself is variable both through space—due to complex decoupling of plate motions—and in time—due to changes in regional plate motions. There are two observations of kinematic indi- cators that can be applied to developing a model for the evolution of the Marinduque basin: (1) the left-lateral, strike-slip motion of the Phil- ippine fault and other north- to northwest- trending structures, which is amply demon- strated by earthquake focal mechanisms; (2) the approximately north-south (340°-350°) exten- sion direction of the basin, demonstrated by the generally east-west orientation of symmetrical magnetic anomalies associated with the central volcanic ridge. East-west-trending scarps on the central volcanic ridge are thus interpreted as normal faults, consistent with the observation of the east-west-trending, down-to-the-basin nor- mal faults that transect the perched basin on the north flank of the basin. In the following sections, we describe four important families of structures that can be dis- tinguished in the Marinduque basin area, based on orientation, physical appearance, apparent sense of offset, and relative age.

EAST-WEST-TRENDING STRUCTURES

East-west-trending structures, which are in- terpreted to be normal faults (see above), are developed on the central volcanic ridge and in the area of the northern perched basin (Fig. 7). Offsets across these structures cannot be deter- mined because there is no penetration on SCS data, but narrow, elongate valleys that occur along the west end of the ridge may be small grabens (Fig. 4). The symmetrical shape of the central volcanic ridge, and the good correlation Figure 13. Seismic reflection profiles F-F' and C-C', and interpretive line drawings across between breaks in slope and east-west linea- perched basin. Basin reflections show onlap and offlap relation with north-basin flank, where- ments suggest that normal faults are more or less as the southwest-basin margin is fault-bounded. Note subtle relief on basin floor corresponding symmetrically disposed about the ridge axis, to recently active, east-west-trending faults. See Figure 2 for location. with slip in a down-to-the-basin sense. The rela- tive age of these faults cannot be determined, but comparing the relative orientations displayed by the Philippines arc as a whole, for several rea- symmetrical onlap of the youngest basin-fill re- actual faults and folds to those predicted from sons. First, experimental studies are usually con- flectors onto both the central ridge and the experimental studies (Harding, 1974; Christie- cerned with low-strain conditions. It is worth south-basin flank (Fig. 5) suggests that evolution Blick and Biddle, 1986). This approach is clearly noting that the field studies that best conform to of the ridge occurred for the most part prior to inappropriate to the Marinduque basin, and to the experimental results are also low-strain ex- the deposition of the youngest undeformed units

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extension is best explained if left-stepping, north- trending fault zones were left-lateral transcurrent faults. The transition or stepover between en echelon faults would be divergent, and the pre- dicted minimum principal stress direction within the stepover zone would be parallel to the re- gional, north-south fault trends (Giraud and Seguret, 1985). Divergent, strike-slip stepovers lead to the development of pull-apart basins 20 40 Km (Crowell, 1974; Mann and others, 1983), and in this context, the Marinduque basin may best be Figure 14. Magnetic anomaly pro- characterized as a composite pull-apart in the MODEL files across the Marinduque basin. See terminology of Aydin and Nur (1982). The Figure 2 for locations. A synthetic rhombic shape of the basin is also typical of model magnetic profile calculated from pull-aparts. the sea-floor-spreading magnetic block Regional considerations also predict left- • in • m I model using the methods of Schouten lateral slip on north-trending structures. On-land and McCamy (1972) is also shown. geological studies have shown that sinistral slip on generally northward-striking faults was a dominant structural element in the northern Philippines through much of the Tertiary (Karig and others, 1986b). In particular, Haeck and Karig (1983) suggested that north-south trans- current fault systems in east-central Luzon rep- resented a "proto-Philippine fault" that was active through much of Miocene time. A S ' left-lateral transcurrent history for north-trend- ing fault systems in the Marinduque basin area is thus consistent with regional geological observations. of the southern sub-basin, but synchronously see Fig. 6). The overall parallelism of north- with deposition and/or subsequent tilting of the trending faults with basin depocenters suggests NORTHWEST-TRENDING upper seismic sequence (late Miocene to late that there was a component of normal slip on STRUCTURES: THE PHILIPPINE Pliocene?) on the south-basin flank. these structures. Although some north-trending FAULT SYSTEM East-west-trending normal faults in the north- structures show evidence of recent activity, es- ern perched basin are also likely to have ac- pecially in the northern sub-basin and perched As discussed, the Philippine fault zone defines commodated extension directly related to subsid- basin, tentative correlation of seismic strati- the southeastern boundary of the Marinduque ence within the basin. SCS profiles across these graphic units on the south-basin flank (see basin, where it is imaged on both side-scan and faults often show increasing offset of reflectors above) to onshore stratigraphy suggests that the seismic data. The Philippine fault is the current with increasing depth (Fig. 13), which suggests structural development of the Marinduque basin locus of major left-lateral strike slip in the Phil- that they have been active during basin infilling. occurred mostly during late Miocene to Pliocene ippines (Cardwell and others, 1980). At the Surface scarps on side-scan and 3.5-KHz data time. Deposition subsequent to that time has latitude of the survey area, the Philippine fault show that these faults are active, consistent with resulted in burial of many faults and the suppres- zone is almost 50 km wide (Fig. 2; Philippine the shallow intraplate earthquake focal mecha- sion of structural bathymetry. Bureau of Mines, 1963; Morante, 1974), and nisms in the central Philippines, most of which Although it appears that north-trending struc- seismicity patterns indicate activity across this show generally northward-trending tensional tures accommodated normal slip during basin entire width (Fig. 2), including a major shallow axes (Cardwell and others, 1980). genesis, it is also likely that these structures un- event (Mb = 5.3; International Seismological derwent significant strike-slip motion as well. Center, 1983) located along the major fault NORTH-TRENDING STRUCTURES The geometric and temporal relations between strand imaged in this region. The focal mecha- the central volcanic axis and north-trending fault nism from this event shows left-lateral motion On the scale of the entire Marinduque basin, zones strongly implies a genetic link between on the nodal plane that is parallel to the fault major north-trending (=345°-000°) features these features. SCS data show that young, rela- trend (Fig. 7). This strand of the Philippine fault show an overall left-stepping en echelon geome- tively undeformed basin-fill reflectors of the corresponds to the "Sibuyan Sea branch," re- try that is intersected by the central volcanic southern and northern sub-basins onlap, and cently identified by Bischke and others (1988). ridge (Fig. 6). Structural relief on acoustic thus postdate, both the volcanic ridge flanks and Within the survey area, the Philippine fault basement and overlying sediment packages is the deformed basin flanks (Fig. 5). Protracted zone deforms the entire sequence of basin-fill associated both with discrete, north-trending north-south extension, best documented by the strata (Fig. 9) and therefore postdates any major fault zones that truncate basin-fill reflectors (Fig. orientation of magnetic anomalies, is almost activity on north-striking faults, which do not 11), and with broad, inclined surfaces that are parallel to the trace of the north-trending substantially deform the youngest basin-fill se- onlapped by reflectors and do not show evi- structures that dominate the morphology of the quence. Whereas north-trending structures con- dence of structural discontinuity (for example, basin. The tectonic environment for north-south trol the morphology of the basin itself, including

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the geometry of basin-fill packages, the location faults. The north-south segments may therefore fault toward a more northerly orientation, or by of the Philippine fault zone does not seem to be zones of transfer or tear faulting. an eastward (right-handed) stepover. Either of influence the internal character of the basin se- In spite of the evidence for shortening, the these geometric solutions would result in local quences (Fig. 12). Activity on the Philippine arcuate faults are in some respects anomalous. In compression that would generate the shortening fault zone within our survey area therefore particular, their distinctive shape is not typical of seen at the northeast-basin flank. probably postdates the formation of the Marin- reverse faults documented from other zones of On the Bondoc Peninsula, onshore imme- duque basin. strike slip (Aydin and Page, 1984; Sylvester, diately to the east of the arcuate offshore faults, a The Philippine fault also truncates, and thus 1988). Conversely, this morphology is charac- series of north- to northwest-trending folds de- postdates, the central volcanic ridge. The bath- teristic of boundary faults in many pull-apart form uplifted Neogene marine sediments in the ymetric expression of the ridge dies out adjacent basins (Aydin and Nur, 1982; ten Brink and zone between the fault trace imaged in this sur- to the fault zone. Volcanic flows identified on Ben-Avraham, 1989). Given the extensional vey and the adjacent major trace to the east side-scan imagery, as well as the magnetic signa- origin of the Marinduque basin, we interpret the (Figs. 2 and 7; Philippine Bureau of Mines, ture associated with the ridge, do not continue to arcuate faults as initially transtensional features 1963; Kimura and others, 1968). A zone of the east of the main fault strand. In addition, the that were later reactivated. These structures shortening about 25-30 km wide occurs on- overall trend of the central volcanic ridge is de- formed as north-trending, left-lateral, strike-slip shore northeast of the Marinduque basin, appar- flected near the fault zone. Whereas the ridge faults that curved into east-west normal faults in ently in response to this restraining bend or trends =070° across most of the basin, it curves the stepover zone. The convex-southward step- stepover. The arcuate faults thus mark the west- to —045° near the Philippine fault. This may over geometry suggests that pull-aparts opened ern margin of this zone of shortening. The paral- reflect passive rotation ("drag"), perhaps due to to the north. A more recent phase of activity lelism between these faults and the coastline of offset on parallel, secondary faults that decreases then led to reactivation of the transtensional the Bondoc Peninsula suggests that emergence of incrementally away from the main fault zone. In structures as reverse faults. Basin-fill sequences the Peninsula is related to uplift on these struc- fact, several north west-trending lineaments can that thicken toward and beneath the northeast- tures and inversion of the basin. be traced on side-scan imagery from the south- basin flank (Fig. 6, line K) indicate that a depo- center has been structurally overridden and that basin flank through the eastern end of the vol- STRUCTURAL SYNTHESIS canic ridge, but they do not display measurable the rocks of the northeast-basin flank compose, at least in part, elevated and disrupted basin-fill offset. We interpret the Marinduque basin to be a units. complex, or composite, pull-apart basin that was ARCUATE STRUCTURES The younger, contractional phase of motion subsequently modified along its eastern flank by on these structures can be attributed to the re- activity on the Philippine fault. The pull-apart The arcuate lineaments that define the north- gional geometry of the Philippine fault. The interpretation encompasses the principal charac- east margin of the Marinduque basin (Figs. 7, main strand of the Philippine fault that was im- teristics of the basin within a single model. These 15) on side-scan imagery represent the surface aged by our side-scan survey cannot continue characteristics include: (1) the north- to north- traces of faults that display reverse offset (Fig. 6, along strike to the northwest for more than northwest-trending orientation of the basin line K). Reflection profiles that cross these struc- about 40 km, because the arcuate reverse faults margins and major faults within the basin; tures in several places show that reverse slip is truncate the projected fault trace. Thus, motion (2) the regional left-stepping distribution of accommodated primarily along the southern on this seismically active strand of the Philippine north-trending faults; (3) evidence for north- curvilinear segments of these north-dipping fault must be resolved either by a bend of the south extension based on the orientation of

Figure IS. Detail of northeast portion of side-scan mosaic, showing well-defined arcuate lineaments that define the slope break.

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Figure 16. Genetic model for the evolution of the Marinduque basin. A. Pull-aparts develop across north-trending, left-stepping, en echelon faults. Fault system migrates westward. B. Continued crustal stretching in broad stepover zone leads to sea-floor-type spreading on central volcanic ridge. C. Cessation of motion on north-trending structures; basins fill in and coalesce; structural topography is buried or subdued. Activity on the Philippine fault begins, leading to truncation of the east end of the volcanic ridge. Deformation at restraining bend results in shortening across northeast-basin flank.

symmetric magnetic anomalies within the basin; riety of structures. The only kinematic require- of the Marinduque basin, we do not suggest that (4) relative synchroneity of central-ridge evolu- ment for such a zone should be that most the resulting igneous product is petrologically tion and basin development; (5) absence of syn- structures will show a component of normal slip, identical to oceanic or backarc lithosphere. On sedimentary tectonic shortening during basin regardless of orientation (Harding and others, the other hand, the great structural and chemical genesis; and (6) the rhombic shape of the basin 1985). The change from transtension to trans- diversity of ophiolites emplaced in orogenic as a whole. pression (stage C), marked by the truncation of belts (Coleman, 1984) suggests that there is sig- A simplified genetic model for the evolution the east-basin margin by the Philippine fault and nificant variability in the processes that lead to of the basin (Fig. 16) shows a progressive widen- the reactivation of the arcuate structures as re- ophiolite formation, and we would expect that ing and lengthening of the stepover zone, ac- verse faults, may correspond to a major change the rocks of the central volcanic ridge would fall companied by the development of sea-floor in Philippine-Eurasia relative plate motion in within this range of variability. spreading across the central volcanic ridge Pliocene time (Seno and Maruyama, 1984; The association of sea-floor spreading and (stages A and B). The complex internal charac- Sarewitz and Karig, 1986a). major strike-slip boundaries has been docu- ter of the basin suggests that it originated as mented in the Andaman Sea (Curray and others, several localized depressions that became amal- OPHIOLITE GENERATION IN 1979) and the Cayman Trough (Holcombe and gamated with continued tectonism and sedimen- AN INTRA-ARC SETTING Sharman, 1983). Diffuse generation of oceanic tation. This composite internal morphology may crust may also be occurring in pull-apart basins be typical of many pull-apart basins (Aydin and Although our data indicate that a process sim- of the Gulf of California (Moore and Curray, Nur, 1982). A broad zone of transtension should ilar to sea-floor spreading about the central basin 1982). These areas differ from the Marinduque result in heterogeneous deformation along a va- ridge was an important aspect of the formation basin and from each other in terms of both scale

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and tectonic setting, but they illustrate that the land arc or Andean margin with a passive con- ophiolites may be created, transported, dis- generation of oceanic-type crust in transten- tinental margin (Roeder, 1979; Gealey, 1977). rupted, and emplaced in an intra-arc setting by sional environments is not unusual and that it is The history of the Marinduque basin suggests strike-slip processes. Ophiolite generation and not limited to strictly intra-oceanic settings. The a third model for generating and emplacing emplacement therefore need not always be asso- Marinduque basin, however, is the only example ophiolites within an island arc (Fig. 16). Recog- ciated with basin closure, arc collision, plate re- recognized to date of localized sea-floor spread- nition of sea-floor spreading within an intra-arc, organization, or other discontinuous or "termi- ing in a known intra-arc, strike-slip setting. strike-slip fault zone addresses the petrologic, nal" events, but rather may occur as part of the Geochemical and paleogeographic data sug- stratigraphie, and chronologic data from ophio- progressive evolution of an island-arc system. gest that many, and perhaps most, ophiolites lites that point to formation within an island arc Similarly, the occurrence of ophiolitic fragments formed in, or adjacent to, island arcs, rather than ("supra-subduction zone") setting. The ultimate in major fault zones need not necessarily mark at mid-ocean ridges far from any convergent fate of the Marinduque basin cannot be pre- the sites of closed ocean basins, but may instead margin (Pearce and others, 1984; Leitch, 1984). dicted, but it is already "emplaced" into the arc, indicate the disruption of an ophiolite by strike- At the same time, the internal structure and stra- although it is not yet exposed. Vertical slip asso- slip faulting. tigraphy of most ophiolites indicate that they ciated with continued transpression on the Phil- were generated by sea-floor spreading rather ippine fault may result in uplift and exposure of INTRA-ARC BASINS: than by long-term plutonism in the roots of an the basin floor without any significant change in EVOLUTIONARY MODELS island-arc system (Casey and Dewey, 1984). regional plate kinematics, analogous to the Eo- Most models for ophiolite genesis fall into cene ophiolite of Luzon. The Zam- The Marinduque basin is only one of several two general groups—a backarc origin and a bales ophiolite is interpreted to have moved interior Philippine marine basins that occur in a forearc origin. Backarc ophiolites may be distin- northward more than 1,000 km (Fuller and oth- generally intra-arc setting (Fig. 1). The origins of guished by the presence of conformably overly- ers, 1983) by predominantly strike-slip motion most of these basins have not yet been studied. ing sediments derived from the coeval volcanic along the West Luzon Shear (Karig, 1983). Re- Some, such as the Ragay Gulf, are located along arc (Moores, 1982) and/or the older, remnant gional tilting, uplift, and exposure of the ophio- the Philippine fault and are almost certainly arc (Wyld and Wright, 1988). Modern, exten- lite took place after most or all of this related to strike-slip processes. Others may sional backarc basins (Karig, 1971; Taylor and displacement had occurred (Bachman and oth- have formed by different mechanisms. Basins Karner, 1983) provide a reasonable analogue ers, 1983; Schweller and others, 1983). Sim- preserved on land have been explained by colli- for the generation of backarc ophiolites. Pearce ilarly, the Marinduque basin is now being sional models (Davao-Agusan trough of Min- and others (1984) suggested that many ophio- displaced relative to the terranes of east Luzon, danao; Hawkins and others, 1985) or oblique- lites were generated by "supra-subduction zone" which are moving relatively northward due to or strike-slip processes (Luzon Central Valley; sea-floor spreading prior to the development of motion on the Philippine fault zone. Bachman and others, 1983). Early stages of arc volcanism, although there are no known An important, if implicit, corollary of conver- backarc rifting, and trapping of small pieces of modern examples of this process. gent models for ophiolite emplacement is that oceanic crust by plate-boundary reorganization Forearc ophiolites, in contrast, are thought to the occurrence of disrupted ophiolitic mate- may also create intra-arc basins. In fact, the be created by the initiation of new subduction rial—including ophiolitic mélanges—within complex tectonic evolution of the Philippines zones that trap pre-existing ocean crust between major fault zones indicates the location of a "su- during the Tertiary suggests not only that several the interplate zone and the nascent volcanic arc ture," or a closed ocean basin (Dewey, 1977). mechanisms may lead to the development of (Karig, 1982; Casey and Dewey, 1984). These Several workers, however, have recently sug- intra-arc basins, but also that any single intra- conditions may commonly occur during fre- gested that strike-slip faulting may play an im- arc basin may undergo a composite evolution. quent and rapid changes in plate geometry that portant part in the generation and/or emplace- This is certainly the case for the Marinduque apparently characterize the evolution of com- ment of some ophiolites and ophiolite-bearing basin, which formed by extension in a strike-slip plex island-arc systems such as the Philippines sutures (Saleeby, 1977; Woodcock and Robert- setting and continues to receive sediment while and Indonesia (Hamilton, 1979, 1988). Such son, 1982; Taira and others, 1983; Sarewitz and undergoing compressional deformation at its processes may also act to trap oceanic crust in Karig, 1986a). Mitchell and Reading (1978) northeastern margin. backarc regions, and in many cases it may not have outlined a "strike-slip" cycle analogous to The future evolution of the Marinduque basin be possible to distinguish between forearc and the Wilson cycle for ocean basins. depends on the large-scale evolution of the Phil- backarc crustal fragments after they are em- In the northern Philippines, the association of ippine Sea-Eurasian plate boundary. For exam- placed as ophiolites (Searle and Stevens, 1984). major strike-slip faults with coherent or dis- ple, a change from oblique plate convergence to Numerous structural models have been pro- rupted ophiolites has been documented in sev- perpendicular convergence would probably re- posed for the emplacement of ophiolites into or- eral areas (Karig, 1983; Karig and others, sult in the cessation of intra-arc strike-slip mo- ogenic belts. Most models emphasize the impor- 1986b). This association, in combination with tion (Beck, 1986) and could lead to deformation tance of thrust faults or zones of lithospheric paleomagnetic and paleogeographic data, has of the arc by internal shortening. In this case, the underthrusting in the emplacement process. Two been used to support a model of transport and Marinduque basin sequence might be disrupted favored mechanisms are the collapse of a back- emplacement of ophiolites by predominantly and emplaced by overthrusting onto adjacent arc basin with subsequent thrusting of the basin strike-slip processes and has led to the recogni- terranes. This scenario is similar to interpreta- floor onto the adjacent arc or continental margin tion of strike-slip "sutures" (Karig and others, tions of the Neogene evolution of southwest (Bruhn and Dalziel, 1977; Harper and Wright, 1986b). Island, where Oligo-Miocene strike- 1984), and the collision of an intra-oceanic is- The Marinduque basin example indicates that slip basin sequences were thrust over the North

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M. to L. Eocene Strike-slip phase

Figure 17. Evolution of the Luzon Central Valley (modified from Bach- man and others, 1983), showing how a strike-slip-dominated basin can under- go a complex history, including back- arc and forearc phases of development. Changes in subduction polarity and angle control location of the basin rela- tive to the migrating arc. Eocene and Oligocene stages are generally analo- gous to the Marinduque basin; they in- clude generation, transport, and initial uplift of an ophiolite that now under- lies the basin. Miocene to Recent stages show gradual shallowing up- Quaternary £> BackarcTCIntra-arc phase ward in the basin and reduced influ- --S.br •-•-.,•••.-. .. .„fc ence of strike-slip processes. Incipient ® subduction Philippine Fault

Palawan terrane as a result of a change from to explicitly correlate strike-slip activity with the REFERENCES CITED strike-slip to convergent kinematics (Sarewitz genesis of particular island-arc basins (but see Acharya, H. K., 1980, Seismic siip on the Philippine fault: Geology, v. 8, and Karig, 1986a). Bachman and others, 1983) or to incorporate p. 40 42. Aydin, A., and Nur, A., 1982, Evolution of pull-apart basins and their scale Alternatively, the Marinduque basin could these processes into generalized models for basin independence: Tectonics, v. 1, p. 91-105. development. We suggest that localized pull- Aydin, A., and Page, B. M., 1984, Diverse Pliocene-Quaternary tectonics in a continue to receive sediment, and exposure transform environment, San Francisco Bay region, California: Geologi- could ultimately occur simply as a result of tilt- aparts in an intra-arc environment may ulti- cal Society of America Bulletin, v. 95, p. 1303-1317. Bachman, S., Lewis, S., and Schweller, W., 1983, Evolution of a forearc basin, ing of one basin flank, perhaps in response to mately evolve into regionally significant forearc Luzon Central Valley, Philippines: American Association of Petroleum or backarc basins such as the Luzon Central Geologists Bulletin, v. 67, p. 1143 1162. ongoing transpression associated with the Phil- Beck, M. E., Jr., 1986, Model for late Mesozoic-early Tertiary tectonics of ippine fault. This is exactly the scenario that has Valley, Andaman Sea, and the Great Valley of coastal California and western Mexico and speculations on the origin of the San Andreas fault: Tectonics, v. 5, p. 49-64. been interpreted for the Luzon Central Valley California. The history of the Marinduque basin Bischke, R. E., Suppe, J., and del Pilar, R., 1988, Implications of a newly may therefore represent an early evolutionary discovered branch of the Philippine fault system: International Sympo- (Fig. 17; Bachman and others, 1983; Karig, sium on Evolution of Eastern Eurasian Margin, Paris, France. 1983), where the Zambales ophiolite and con- phase that is common to several types of island- Blackinton, J. G., Hussong, D. M., and Kosalos, J. G., 1983, First results from a combination side-scan sonar and seafloor mapping system (SeaMarc II): formably overlying marine strata have been arc basins that are often considered to be geneti- Offshore Technology Conference, Houston, Texas, v. 4478, tilted up and exposed on the west basin flank, cally distinct. p. 307 311. Bruhn, R. L., and Dalziel, I.W.D., 1977, Destruction of the Early Cretaceous adjacent to a major strike-slip boundary. Thus, marginal basin in the Andes of Tierra del Fuego, in Talwani, M„ and Pitman, W. C., III, eds.. Island arcs, deep sea trenches and back-arc the Marinduque basin may represent an early ACKNOWLEDGMENTS basins: Washington, D.C., American Geophysical Union, p. 395 405. Cardwell, R., Isacks, B., and Karig, D., 1980, The spatial distribution of earth- stage in the development of a much larger, and quakes, focal mechanism solutions, and subducted lithosphere in the longer lived, basin. Philippine and northeastern Indonesian Islands, in Hayes, D. E., ed., We thank the officers and crew of the R/V The tectonic and geologic evolution of Southeast Asian seas and islands. The importance of strike-slip faults at both Moana Wave for their work during the data- Part 1: Washington, D.C., American Geophysical Union Geophysical Monograph 23, p. 1-35. modern and ancient obliquely convergent plate acquisition phase of this project. In particular, Casey, J. F., and Dewey, J. F., 1984, Initiation of subduction zones along transform and accreting plate boundaries, triple-junction evolution, and boundaries is well documented (Fitch, 1972; we thank J. Erickson and S. Stahl for their tire- forearc spreading centres—Implications for ophiolitic geology and ob- Karig, 1983; Taira and others, 1983; Beck, less efforts. K. Abarra, J. Hibbard, E. Ramos, duction, in Gass, I. G., Lippard, S. J., and Shelton, A. W., eds., Ophio- lites and oceanic lithosphere: Oxford, England, Blackwell Scientific 1986; Jarrard, 1986b; Woodcock, 1986), and it M. Sara, K. Tillman, and K. Wirth provided Publications, p. 269-290. directly implies that strike-slip basins should Christie-Blick, N., and Biddle, K. T., 1986, Deformation and basin formation able assistance and lively scientific discussions at along strike-slip faults, in Biddle, K. T., and Christie-Blick, N., eds., commonly form in a variety of island-arc set- sea. This project was supported by National Strike-slip deformation, basin formation, and sedimentation: Tulsa, Ok- lahoma, Society of Ixonomic Paleontologists and Mineralogists Special tings. There have been few attempts, however, Science Foundation Grant OCE 87-11382. Publication 37, p. 1-34.

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