thematic article a bathymetric overview of the mariana forearcrjstern/pdfs/sternsmoot98.pdfthematic...

Thematic Article

A bathymetric overview of the Mariana forearc

ROBERTOBERT J. STERNTERN11 ANDAND N. CHRISTIANHRISTIAN SMOOTMOOT2

1Department of Geosciences, University of Texas at Dallas, Box 830688, Richardson, TX 75083±0688, USA and2Sea¯oor Data Bases Division, Naval Oceanographic Of®ce, Stennis Science Center, Bay St Louis,

MS 39522±5001, USA

Abstract Bathymetric data at a 200-m contour interval for the entire Mariana forearc,from south of 13°N to 25°N, permits the ®rst comprehensive overview of this feature.The Mariana forearc represents a sediment-starved end-member. The forearc in itssouthern and central sections is divisible into a structurally complex eastern provinceand a less-deformed western province. Despite the absence of an accretionary complexthe Mariana forearc has a well-de®ned outer-arc high; this probably results from agreater concentration of low-density serpentinized mantle lithosphere beneath theouter forearc relative to the inner forearc. This serpentinization gradient is coupled withdiffering deformational styles of thinner and more brittle lithosphere beneath the outerforearc compared to thicker and more ductile lithosphere beneath the inner forearc. Thebathymetric data also support models calling for extension along-strike of the forearc,re¯ecting an increase in arc length accompanying the crescent-shaped opening of theMariana Trough back-arc basin. Both northeast and northwest ridges and grabens canbe identi®ed, with the latter restricted to the southern part of the forearc and theformer widely distributed in the central and northern forearc. Northeast-oriented ex-tensional structures are supplanted northward by long, linear northwest-trendingstructures that are interpreted as left-lateral strike±slip faults. These variations indeformation along-strike of the forearc manifest a transition from nearly orthogonalconvergence in the south to highly oblique convergence in the north.

Key words: bathymetry, forearc, Mariana arc.

INTRODUCTION

The forearc is the part of a convergent platemargin that lies between the trench and themagmatic front. Forearcs are important for anumber of reasons. They overlie subductedlithosphere and sediments where these are ®rstsqueezed, and so represent sites where ¯uids andmatter begin to be transferred from the sub-ducting to the overlying plate. Forearcs are thebest place to reconstruct arc history. The accre-tionary wedge, if present, records what wassubducted, while sediments in forearc basinsrecord the eruption and uplift history of the arc.Because it forms or is trapped when subduction

begins, studying forearc igneous basement canreveal how subduction began. A better under-standing of forearcs is important for other rea-sons, such as mountain building and hydrocarbonexploration. Forearcs make up the vanguardwhen arcs collide, so reconstructing orogenicevents depends on our ability to recognize de-formed forearc igneous and sedimentary se-quences. Most ophiolites represent forearcbasement. Forearc basins are important sites ofsediment and hydrocarbon accumulation andstorage.

In spite of their importance, forearcs are dif-®cult to study for two reasons. First, they arelargely submerged. Second, many forearcs lie inregions of high sediment supply, so most of theforearc infrastructure is covered by sediments.

The Island Arc (1998) 7, 525±540

Accepted for publication February 1998.

Accretionary prism development also re¯ectssediment supply, with large accretionary prismsfound where sediment ¯ux to the trench is high.Conversely, forearc basement is exposed wherethe sediment supply is low; Le Pichon et al.(1993) state that no active accretionary wedgeexists where the average trench ®ll is less than1 km. High-sediment-supply forearcs are char-acterized by thick sequences of relatively unde-formed sediments derived from the arc andcontinent lying inboard and above deformedsediments that are scraped off the subductingplate, as well as shed from the arc and continent.Most forearc models are based on high-sediment-supply forearcs such as the Aleutians or Suma-tra, which attract study because these lie closerto population centers and may contain oil. Fore-arcs from regions of low sediment ¯ux, such asthe Mariana forearc, are more poorly known.This is unfortunate, because `naked forearcs'such as the Marianas provide opportunities forstudying features of convergent margin pro-cesses that are dif®cult to examine in high-sedi-ment-supply forearcs. In particular, nakedforearcs provide unparalleled opportunities toexamine the nature of forearc basement and theorigin of the trench-slope break. The presentpaper focuses on the Mariana forearc as a superbexample of a naked forearc by means of abathymetric overview, based on US Navy sonararray sounding system (SASS) bathymetry.

METHODS

STUDY AREA

The location of the Mariana arc system and thebathymetric coverage presented here are shownin Fig. 1. The southernmost limit of the Marianaforearc for the purposes of this contribution liesat � 13°N, which is the northernmost extent ofmajor east±west-trending faults which truncateall forearc elements further south (Karig et al.1978; Karig & Ranken 1983). The northernmostlimit of the Mariana forearc lies at � 24°N, wherethe active Mariana arc and the remnant arc ofthe West Mariana Ridge converge. The westernlimit of coverage is de®ned by the active volcanicarc; in some cases, coverage extends east of thetrench axis because this allows us to see whereseamounts are colliding with the forearc, andcollision of seamount chains may be importantcontrols of forearc structure.

The study area is large, and this dictates thesize of features we discuss. The Mariana forearcis 200±220 km wide, and the present study fo-cuses on an expanse that stretches � 1200 kmfrom south to north. These dimensions are ap-proximately those of Honshu, the largest of theJapanese islands. Another size comparison iswith the state of California; the distance fromSan Diego to San Francisco is similar to thatfrom Guam to the point of greatest curvature ofthe arc at 18°N. The reader should keep inmind the great extent of the study area, whichmeans that many important but smaller fea-tures may be overlooked. For example, struc-tural features observed in more detailed studies

Fig. 1 Location of the Mariana arc system in the Western Paci®c.Dashed boxes labeled 2, 3, and 4 approximate the position of mapsshown in Figs 2±4.

526 R. J. Stern and N. C. Smoot

(Karig & Ranken 1983; Mrozowski & Hayes1980; Wessel et al. 1994) are mostly missed atthe scale of our study. Our intent is to providea broad overview of the Mariana forearc toserve as context for more detailed studies, anduse the results of previous studies to guide ourinterpretations.

DATA COLLECTION AND PROCESSING

Data collection and processing for this projectbegan in the late 1960s by the US NavalOceanographic Of®ce. LORAN-C and the Navy'stransit satellite was used for horizontal control.The Sonar Array Sounding System (SASS) wasused to collect depth information. The SASS, amultibeam sonar collector, employs a burst ofacoustic energy that is transmitted from trans-ponders on the ship's hull. Time delays betweenthe highest and lowest transponders are pre-programmed with roll and pitch compensators,and a `fan of sound' is sent out. An array of hy-drophones allows the returned energy to beprocessed into range and angle data, which arefurther translated into raw depth and location.With the sound-velocity pro®le having been en-tered into the program, ray-bending correctionsare made and applied. The output is then incorrected depths and lateral positions, and this isthe sonar bottom. Further details are presentedby Glenn (1970).

Bathymetric maps are components of a long-running study of the northwest Paci®c region ingeneral and the Izu±Bonin ± Mariana (hereafter,IBM) arc system in particular (Bloomer et al.1989; Fryer & Smoot 1985; Smoot 1983a,b, 1984,1988, 1990, 1991, 1997; Stern et al. 1984, 1989;Taylor & Smoot 1984). The total-coverage ba-thymetry has been shown in a variety of formats,mostly in fathoms and in bits and pieces. Thisoverview is a composite of all these previousstudies. A computer program (GRASSGRASS 4.1) wasused to show projections of the bathymetric datain a way that gives the appearance of being threedimensional (3-D).

BATHYMETRIC MAPS OF THE MARIANA FOREARC

The bathymetric maps are shown in Figs 2, 3,and 4. The forearc is depicted on three panels ofapproximately equal size and at the same scale.We can refer to the central Mariana forearc(Fig. 2), the southern Mariana forearc (Fig. 3),

and the northern Mariana forearc (Fig. 4). We®rst discuss the central Mariana forearc becauseit is the best known of the three segments.

THE CENTRAL MARIANA FOREARC

The best known part of the central Marianaforearc lies between 18° and 19°N. The largestvolcanoes in the Mariana arc (Pagan and Agri-gan) bound the forearc to the west. Two mor-phological provinces lie between the line of arcvolcanoes and the trench, 200 km to the east(Fig. 2). Smooth sea ¯oor de®nes the westerntwo-thirds of the forearc, which is inclined gentlyto the east and lies mostly at depths of 3±4 kmbelow sea-level. This `smooth' province re¯ectsdeposition of distal volcaniclastic turbidites, ash,and pelagic sediments. In contrast, the regionbetween the smooth province and the trench hasa rough, mountainous morphology. This `rough'region is characterized by local relief of over2 km. Rough and smooth morphological prov-inces are readily distinguished from perspectiveviews in Figs 5, 6, and 7.

Rough and smooth forearc sectors correspondto two structural provinces: a western and aneastern structural province (Mrozowski & Hayes1980). The western structural province (WSP) istypi®ed by a thick sequence of well-strati®ed,relatively undeformed sediments above an east-ward-shoaling basement re¯ector. Sediments ofthe WSP thicken to greater than 3 km in the¯exural moat east of the active volcanic arc(Mrozowski & Hayes 1980). From Deep SeaDrilling Program (DSDP) sites 458 and 459 weinfer that a relatively undeformed sedimentarysequence extending back at least into the lateEocene or early Oligocene is preserved in theforearc basin constructed on the WSP (Hussong& Uyeda 1981). A basaltic sill of late Miocene oryounger age was penetrated at Ocean DrillingProgram (ODP) site 781A (Marlow et al. 1992)and similar sills have been encountered in theTonga forearc (Taylor et al. 1995), so forearcvolcanic ¯ows and intrusions comprise a minorcomponent of the supracrustal succession in theWSP forearc basin. Although this broad basin isstructurally much more simple than the easternstructural province (ESP), it is cut by numeroushigh-angle normal faults that parallel the arc andtrench, with surface offsets that range from 10 to250 m (Mrozowski & Hayes 1980). The DSDPdrilling at sites 458 and 459 con®rms that thedeeper, more consolidated sediments are cut by

Mariana forearc: Bathymetric overview 527

numerous normal faults, further indicating thatthe forearc is under tension (Hussong & Uyeda1981). The WSP±ESP boundary is de®ned by theintersection of acoustic basement with the sea¯oor. The ESP is characterized by Mrozowskiand Hayes (1980) as having a very high density ofsmall and uncorrelatable normal faults.

In most of the central Mariana forearc, the sea¯oor of the WSP is deeper than the ESP. Thisindicates that the forearc basin has generally notreached sill depth, the point at which sedimen-tary ¯ux from the forearc basin equals that intothe trench. This may be one of the reasons thatthere is little evidence for submarine canyons,because it is argued that extensive submarinecanyon formation begins after the forarc basin is

®lled to the spill point of the outer-arc high(Klaus & Taylor 1991). The spill point is reachedeast of Pagan, and this may be responsible forthe development of a short (� 30-km-long) can-yon leading into the trench northeast of site 459(another apparent canyon trends northeast fromnear the ESP±WSP boundary at 18°30¢N to thetrench just north of 19°N). Because canyons ini-tiate in the outer-arc high and grow by headwarderosion (Taylor & Smoot 1984; Klaus & Taylor1991), continuing tectonic instability in the ESPmay prevent canyon formation.

Drilling this part of the Mariana forearc duringDSDP Leg 60 demonstrated two importantthings. First, the forearc is underlain by an ig-neous basement of middle±late Eocene age. This

Fig. 2 Bathymetric map of the centralMariana forearc. Contour interval is200 m, with heavy contours shown every1000 m and labeled in km below sea-level. All of the islands, from Alamaganin the south to Uracas in the north, areactive or dormant volcanoes, except forMaug, which is extinct. The MarianaTrench lies along or near the easternlimit of coverage, and can be tracedthrough deeps of 8 km in the south to7 km in the north. Note that the locationsof Conical and Pacman seamounts areshown, along with the approximate po-sitions of DSDP sites 457, 458, 459, and460.


result has been con®rmed by studies of Eocenevolcanic sequences on Saipan and Guam, dredg-ing the inner wall of the Mariana Trench, andresults from ODP Legs 125 and 126. Marianaforearc basement consists of depleted igneousrocks produced during and shortly after the be-ginning of subduction (Bloomer 1983; Bloomer &Hawkins 1983, 1987; Hickey & Frey 1982; Stern& Bloomer 1992). Scant evidence supports mod-els calling for trapped oceanic crust to underliethe forearc; second, very little material has beentransferred from the subducting Paci®c plate tothe Mariana forearc. Subsequent dredging stud-ies have recovered what appear to be relatively

minor amounts of mid-ocean ridge basalt(MORB) and ocean island basalt (OIB) lavas as-sociated with Cretaceous chert, which could beaccreted material (Johnson et al. 1991), but mayhave been subcreted and then brought to thesurface by entrainment in serpentine diapirs(Fryer et al. 1995). Dredging the inner trenchwall more commonly recovers Tertiary igneousrocks with strong arc af®nities. A controversycontinues regarding how much of the forearc hasbeen removed by tectonic erosion. Some of theforearc basement must have been downfaultedand subducted in order to expose the igneousbasement (frontal erosion of von Huene & Scholl

Fig. 3 Bathymetric map of thesouthern Mariana forearc. Contourinterval is 200 m, with heavy contoursshown every 1000 m and labeled ev-ery km below sea-level. The islandsfrom Guam to Ferdinand de Medinillamanifest structural uplift along thefrontal arc; active arc volcanism liesslightly to the west of the frontal arc.The Mariana Trench lies along or nearthe eastern limit of coverage, and canbe traced through deeps of 9 km inthe south to 8 km in the north. Notethat the locations of Chamorro (C1)and Chamorro 2 (C2) forearc sea-mounts are shown, along with theapproximate position of DSDP site 60.


1991), but earlier estimates that 50±60 km ormore must have been removed to expose arcbasement (Bloomer 1983) are rendered unnec-essary by the subduction initiation model ofStern and Bloomer (1992).

Given that the ESP and WSP are underlain bysimilar crustal materials, what is responsible forthese very different morphologies? Clearly theWSP is much less deformed than the ESP, andthe bathymetric data presented here allow us toidentify three main structural trends and mor-phologies for the ESP. First, there are numerousconical features which are now understood to bewhere serpentinite diapirs breach the sea ¯oor as

serpentinite mud volcanoes (Fryer et al. 1995).These serpentinite seamounts may be as large as30 km or more in diameter. The relief developedby the serpentine seamounts largely de®nes thetrench-slope break for this part of the Marianaarc system (Fryer & Pearce 1992). There is also astrong N30°W fault orientation that extendssouth from the northern Mariana forearc andintersects the trench near its point of maximumcurvature � 18°30¢N. Nevertheless, this struc-tural fabric is much less important in the centralMariana forearc than it is in the northern Mari-ana forearc. The N30°W fault system becomesincreasingly important as Paci®c±Mariana con-vergence increasingly parallels the strike of thetrench, as is observed northward from 18°N(Seno 1989). Finally, there is a well-de®nedN50°E structural trend that can be identi®edfrom � 18°N to 21°10¢N. This is de®ned by ridgesand intervening lows; intersection of the north-west and northeast structural fabrics may pro-duce structures such as the 2-km-deepdepression at 19°30¢N, 146°50¢E. The origin of thenortheast-trending structure is probably relatedto forearc extension resulting from progressiveincrease in the radius of curvature of the Marianaarc, caused by differential opening of the MarianaTrough back-arc basin (Carlson & Melia 1984;Fryer & Pearce 1992). The differential opening ofthe Mariana Trough could also be related to`pinning' of the arc in the north by the Michelson/Marcus-Necker Ridge and in the south by theCaroline Ridge (Hsui & Youngquist 1985; Smoot1984). Karig et al. (1978) suggest an average ex-tensional strain of 10% along the forearc associ-ated with the opening of the Mariana Troughback-arc basin. The model of Hsui & Youngquist(1985) implies considerably greater (40±50% to-tal) extension. Regardless of which model iscloser to the truth, extension and deformationrelated to it are concentrated in the ESP relativeto the WSP and in the central and southernsegments relative to the northern forearc.

Serpentine mud volcanism is facilitated byextension such as appears to be occurring in thecentral Mariana forearc, and it is noteworthythat the greatest density of serpentine sea-mounts is located where the northeast-trendingextensional structures are concentrated. Thissuggests that forearc extension is required tomobilize ascent of serpentine diapirs, an inter-pretation that is consistent with the absence ofactive serpentine mud volcanoes in forearcs thatare not being stretched, such as the northern

Fig. 4 Bathymetric map of the northern Mariana forearc. Contourinterval is 200 m, with heavy contours shown every 1000 m andlabeled as km below sea-level. The Mariana Trench lies along ornear the eastern limit of coverage, and can be traced through deepsof > 7 km in the south to 8 km in the north.


IBM or Tonga forearcs (Ishii et al. 1996). Oneinterpretation of the sharp WSP±ESP boundaryis that this is de®ned by the westward limit ofminimum extension required to permit ascent ofserpentine diapirs. Alternatively, the boundarymay re¯ect deeper processes, such as the inter-action between decreasing water ¯ux with depthand increasing lithospheric thickness to the west.Lithosphere above the subduction zone and eastof the boundary is < 50 km thick, whereas it is> 75 km thick west of the boundary. These twoeffects may combine so that mantle beneath the

ESP is largely serpentinized, whereas beneaththe WSP serpentinization may be limited to thatpart of the mantle adjacent to the subductedslab. In this case, the uppermost mantle beneaththe WSP may be relatively unfaulted so thatdiapiric ascent is precluded.

THE SOUTHERN MARIANA FOREARC

This segment of the arc is subducting the Paci®cplate along a convergence vector that is per-pendicular to the strike of the trench or nearly so

Fig. 5 Perspective view of the Cen-tral Mariana forearc, viewed from thesouth end of region covered by Fig. 2,looking towards the north. This per-spective encompasses � 400 km of theforearc. Vertical exaggeration � 5´.

Fig. 6 Perspective view of the centralMariana forearc, viewed from the Paci®cplate looking towards the west. Themiddle of the image is � 19°30¢N. Thisperspective encompasses � 400 km ofthe forearc. Vertical exaggeration � 5´.


(Seno 1989). The southern Mariana forearc isshown in Fig. 3. The most striking aspect of thissegment is the well-developed structural ridgethat lies just east of the active arc and extendsfor � 500 km north-northeast of Guam. This isthe `frontal arc' of Karig (1971) and includes theislands of Guam, Rota, Aguijin, Tinian, Saipan,and Ferdinand de Medinilla. The forearc is tiltednorthward so that its expression is subduedprogressively to the north. A submerged equiv-alent of this structural ridge can be identi®edalmost as far as 18°. The frontal arc south ofGuam is reduced rapidly to the south by impor-tant east±west faults so that the frontal arc is not

expressed south of 12°30¢N. It is not clear whythe frontal arc is so well developed in thesouthern Marianas, nor why the active arc issubmerged where the frontal arc is emergent,while the frontal arc is missing where active arcvolcanoes stand highest above sea-level. It isnoteworthy, however, that the frontal arc is bestdeveloped in that sector of the arc where con-vergence is most nearly perpendicular to thetrend of the trench. This suggests that conver-gence normal to the trend of the trench is re-quired for development of the frontal arc.

The southern Mariana forearc provides goodexamples of the improvement in visualization

Fig. 8 Perspective view of the southern Mariana forearc, viewed from the Paci®c plate looking towards the west. The southern limit of imagedregion lies at � 13°30¢N, and the northern limit extends almost up to the northern limit of the region covered by Fig. 3. This perspectiveencompasses � 400 km of the forearc. Vertical exaggeration � 5´.

Fig. 7 Perspective view of the CentralMariana forearc, viewed from the northend of the region covered by Fig. 2,looking towards the south. This per-spective encompasses � 400 km of theforearc. Vertical exaggeration � 5´.


provided by swath-mapping over earlier re¯ec-tion pro®les. Compare Figs 6, 7, and 8 of Karigand Ranken (1983) with Figs 3 and 8 of thepresent report. Figure 6 of the older study (pro-®le along 14°30¢N) clearly traverses the broadridge de®ning the trench slope break between14°10¢ and 15°10¢N. The high that extends along50 km of the lower slope traversed on the13°40¢N pro®le in Fig. 7 of the older study isshown to be a complex faceted structural ridge.Two bathymetric highs shown on the east-northeast pro®le in Fig. 8 of the older study are

revealed to be serpentine seamounts Chamorroand Chamorro 2 (Fig. 3).

The boundary between the WSP and ESP ofMrozowski and Hayes (1980) can be extrapolatedalong a bathymetric and morphologic break intothe southern Mariana forearc, although the reliefof the WSP becomes increasingly subdued southof 15°20¢N.

The WSP occupies a 70±80 km width of theforearc and lies immediately east of a relativelysteep slope that de®nes the eastern margin of thefrontal arc. The WSP in the southern Mariana

Fig. 9 Perspective views of thenorthern Mariana forearc. (a) Highlyoblique view from the southeast, look-ing north-northwest; (b) less obliqueview from the south, looking north. Thearea imaged is the same as that shownin Fig. 7. This perspective encompasses� 400 km of the forearc. Vertical ex-aggeration � 5´.


forearc is de®ned by relatively smooth sea ¯oorthat deepens eastward from � 2±4 or 5 km waterdepth. This is succeeded to the east by rougherbathymetry de®ned by, from south to north: (i)serpentine mud volcanoes; (ii) a broad, trench-parallel ridge; and (iii) steep, short ridges strik-ing at high angles to the trench. Serpentine mudvolcanoes de®ne the ESP from 13°40¢ to 14°25¢N,and probably are sea ¯oor expressions of ser-pentine diapirs like those from the centralMariana forearc. These are several tens of kilo-meters in diameter and 500±1500 m high. Thetwo southern seamounts have steep trenchwardslopes, and it is likely that debris ¯ows fromthese seamounts are directed to the lower trenchslope. The southernmost is called ChamorroSeamount (13°47¢N, 146°03¢E) by Fryer (1996Fig. 10b) and has been investigated by dredgingand Shinkai 6500 diving. This is a site of active

venting of ¯uids, with aragonite chimneys andrich biological communities (Fryer 1996). Recentserpentine mud¯ows containing clasts of ultra-ma®c rocks up to 2±3 m in size were observed.Chamorro Seamount is clearly still tectonicallyactive, issuing ¯uids and serpentine mud ¯ows.The northern serpentine seamount (called Cha-morro 2 Seamount by Fryer (1996)) is charac-terized by subdued summit relief and marks thesouth end of the previously mentioned broadridge. This ridge rises as much as 600 m abovethe adjacent ESP sea ¯oor and extends for� 40 km across the forearc before dropping pre-cipitously into the lower trench slope. We spec-ulate that this ridge marks a zone where buoyantserpentinite has not yet been organized intodiscrete diapirs.

We see no good evidence for erosional subma-rine canyons. A submarine canyon that was

Fig. 10 Tectonic sketch maps of the Mariana forearc. (a) Generalized structures. The lightly stippled area near 22°N shows the location of thedetailed study area of Wessel et al. (1994). (b) Generalized structural elements of frontal arc uplift, forearc extension zones, and regions of strike±slip faulting. These tectonic elements re¯ect the interaction of stresses associated with opening of the Mariana Trough back-arc basin andnorthwest-directed relative motion between the Paci®c and Philippine Sea plates, as discussed in text.


identi®ed by Karig (1971) between 16°N and 15°Nappears to be tectonic in origin, almost certainlyrelated to the L-shaped Lapulapu Ridge (cen-tered on 15°30¢N, 147°E). Lapulapu Ridge and itstectonic `moat' may result from the collision ofMagellan seamounts with the Mariana forearc(Smoot 1997), as a result of processes discussed byFryer and Smoot (1985) and von Huene and Scholl(1991). Alternatively, this may mark the south-ernmost expression of normal faults, orientated athigh angles to the trench, that characterize theESP farther north. The 50-km-long southern limbof the Lapulapu Ridge trends �N70°W, subpar-allel to a 90-km-long northwest-trending ridgethat dominates the ESP at 16°20¢N. The 80-km-long northern limb of the Lapulapu Ridge trendsnortheast, parallel to the dominant structuralgrain of the ESP. Several serpentine seamounts(P. Fryer pers. comm., 1997) are found on andnorthwest of the Lapulapu Ridge and on the un-named, northwest-trending ridge; an unusuallylarge serpentine seamount lies north of thenorthwest-trending ridge at 17°N.

Sedimentation rates in the WSP are very low,even in regions above the carbonate compensa-tion depth (CCD), which lies at a depth of� 4.5 km (Seibold & Berger 1996). The DSDP site60 was drilled in 3700 m of seawater, betweenChamorro seamount and Guam. This encoun-tered late middle Miocene marl at 50 m.b.s.f. andbottomed in early or middle Miocene ash at 350 m(Party 1971).

THE NORTHERN MARIANA FOREARC

This segment is conspicuously different from theother two segments in several ways. First, thereis little evidence for serpentine seamounts. Rel-atively small candidates exist near 21°30¢N,23°20¢N and 24°N, but the large seamountscommon in the forearc farther south are missing.Second, it is dif®cult to extrapolate the ESP±WSP boundary into the northern segment. In-stead, the entire width of the forearc is de-formed. We can here describe in general termsan inner and outer forearc, but the sharp boun-dary between forearc provinces found to thesouth is missing (Fig. 9). As ®rst noted by Fryerand Fryer (1987), deformation of the inner fore-arc is particularly clear in the region between21°N and 22°15¢N, where a northeast-trendinggrain is readily apparent. The detailed Sea-MARC II study of Wessel et al. (1994) of theinner forearc east of the Kasuga volcanoes

showed that this is a tectonic fabric. Theyidenti®ed the northeast-trending fabric of theinner forearc between 21°35¢N and 22°10¢N as abroad zone of normal faulting. Two generalmodels were entertained by Wessel et al. (1994):radial tension due to increasing curvature of thearc and highly oblique plate convergence. Thenortheast-trending structural grain is dif®cult toidentify north of 22°15¢N, where the inner fore-arc is dominated by broader swells and sags.Deformation of the inner forearc is less apparentfarther north, but is still much greater than thatobserved in the WSP of the central and southernsegments.

The northeast-trending fabric of the innerforearc is truncated by the great northwest-trending faults that dominate the outer forearc.The northwest-trending system could manifestnormal or strike±slip faults, or both. Althoughthis forearc segment is characterized by abroad zone of shallow (< 50 km) seismicity,scant focal mechanism data give con¯icting re-sults (Eguchi 1984), and new data will be re-quired to answer this question. We note,however, that convergence between the Mari-ana forearc and Paci®c plate is highly obliquealong this segment, and strike±slip faulting ischaracteristic of convergent margins undergo-ing highly oblique convergence (Beck 1983;Ryan & Scholl 1989). At present it is impossibleto quantify the extent to which convergencebetween the northern Mariana forearc is par-allel to the trend of the arc, for three reasons.First, relative motion between the PhilippineSea and Paci®c plates is poorly constrained.Second, sea ¯oor spreading and rifting in theMariana Trough forms a Mariana `platelet' andthe Euler pole between the Mariana plateletand the Philippine Sea plate is poorly con-strained, or may be unde®nable (Baker et al.1996). Finally, the Mariana platelet appears tobe undergoing considerable internal deforma-tion, so that even if we knew Euler poles be-tween the Philippine Sea and Paci®c plates andbetween the Philippine Sea and any part of theMariana `platelet', this would still be insuf®cientto completely describe relative motion betweenthe Paci®c plate and Mariana forearc. Never-theless, reasonable models such as that of Seno(1989) results in a relative convergence direc-tion of 285° between the Paci®c plate and thePhilippine Sea plate at 24°N. This is not verydifferent from the trench azimuth of 320°, sothe general conclusion that the northern Mari-


ana forearc lies in a region of highly obliqueplate convergence is inescapable. This is farmore oblique than the 20° to 30° of obliqueconvergence suggested by Beck (1983) to berequired to activate strike±slip faulting alongconvergent margins. We therefore concludethat the northwest-trending set of long, linearfaults in the Mariana forearc re¯ect left-lateralstrike±slip faults.

DISCUSSION

The bathymetric data reported here demon-strate that the Mariana forearc is a complexgeological entity, but with systematic lateralvariations both along- and across-strike. Thescale of our overview allows us to isolate fourimportant causes for this variability. The ®rstcontributing factor is transfer of material(mostly water) from the subducted Paci®c plate.The second factor re¯ects the changing rheologyof the westward-thickening mantle wedge. Thethird factor expresses along-strike extension ofthe forearc associated with opening of the Mari-ana Trough back-arc basin. The last factor is theoblique convergence of the Paci®c plate. The ®rsttwo factors are largely responsible for across-strike variations, whereas the fourth factor isimportant for along-strike variability. The thirdcause contributes to both along-strike andacross-strike variability. The following discus-sion focuses on these processes.

Subduction engenders a complex transfer ofmatter across the plate interface associated withthe Mariana forearc, with signi®cant amounts ofrock being removed from the hanging wall at thesame time that large volumes of water are addedto it. The inner wall of the Mariana Trenchmostly exposes igneous basement of the forearc(Bloomer 1983; Bloomer & Hawkins 1987), and isa Type 2 (non-accretionary) trench according tothe classi®cation of von Huene and Scholl (1991).This indicates that not only is material not being`bulldozed' off the down-going plate and added tothe overriding plate by frontal accretion, matterhas been removed from the forearc by frontalsubduction erosion (Von Huene & Lallemand1990). Additional material may be removed fromthe base of the Mariana forearc (Cloos & Shreve1996). In contrast, water derived from the sub-ducted plate is moving into and through theforearc (Kastner et al. 1991). The abundance ofserpentine mud volcanoes and active ¯uid vent-

ing sites in the ESP suggests that transfer ofwater from the subducting plate to the base ofthe forearc is important for evolution of theMariana forearc.

Dehydration of the subducted slab and sedi-ments occurs by porosity reduction and mineraldehydration reactions (Kastner et al. 1991). Wedo not yet have good estimates of the water ¯uxfrom the subducted plate but it is likely that thewater yield generally decreases with increasingdepth, and this will be re¯ected in changes seenin the forearc with increasing distance from thetrench. This effect is magni®ed because withgreater depth in the subduction zone (greaterdistance from the trench), less water is releasedfrom the slab and sediments and this acts on agreater thickness of overlying lithosphere. Theresult is that, whereas the lithospheric mantleadjacent to the trench is extensively (but notcompletely) serpentinized, the extent of ser-pentinization decreases rapidly away from thetrench. Serpentinization results in a reduction ofdensity to � 75±80% of the original peridotite(Coleman 1971), resulting in a volume increase of25±33%. Completely serpentinized mantle issimilar in density to basaltic or gabbroic oceaniccrust and should rise through unserpentinizedmantle. Portions of the forearc that are underlainby highly serpentinized peridotite will thereforebe elevated relative to those portions underlainby unserpentinized peridotite. In the southernand central Mariana forearc the outer-arc high(OAH) is well-de®ned in spite of the fact thatthere is no accretionary prism. The MarianaOAH is composed of serpentine mud volcanoesand autochthonous basement fault blocks. TheOAH of other arc systems may re¯ect a transi-tion from accretionary prism to forearc `buttress'(Dickinson 1995), but gradients in serpentinizat-ion across the forearc may be more important incontrolling forearc architecture than heretoforeacknowledged. Thus, much of the difference be-tween the rugged ESP and smooth WSP may beascribed the more extensively serpentinizedmantle beneath the former relative to that be-neath the latter.

A second cause of geological complexity acrossthe Mariana forearc re¯ects rheological varia-tions of the forearc wedge, and how this interactswith subducted seamounts. Along non-accretingconvergent margins like the Marianas, the sub-duction of seamounts fractures the inner trenchwall (von Huene & Scholl 1991). Subducted sea-mounts continue to focus stress for a consider-


able distance into the subduction zone (Cloos1992). The restriction of serpentine mud volca-noes to the outer half of the Mariana forearc wassuggested by Fryer et al. (1985) to mirror theway the mantle wedge responded to thesestresses. They argued that the upper 30 km ofthe forearc wedge behaves in a brittle way, butat greater depths it behaves in a ductile way.Subduction of seamounts fractures the mantlewedge only where this is thinner than 30 km, andthese fractures allow channelized ¯uid ¯ow andascent paths for serpentinized mantle. Atgreater depths the mantle wedge deforms with-out fracturing, and ascent channels are not cre-ated. Fryer et al. (1985) noted that the forearclithosphere was 30 km thick � 100 km west of thetrench, and this corresponds approximately tothe ESP±WSP boundary.

The third cause of geological complexity in theMariana forearc is arc-parallel extension causedby the `bowing-out' of the arc associated with thecrescent-shaped opening of the Mariana Trough.We commented on this earlier, but it is worthemphasizing that the very portions of the forearcthat are subjected to the greatest arc-parallelextensional stresses lie closest to the trench(except in the Northern Mariana forearc, whereoblique convergence apparently translates intodominant strike±slip faulting), where the forearcis most weakened by serpentinization and frac-turing due to subducted seamounts. This portionof the forearc also overlies mantle lithospherethat is most likely to undergo brittle deforma-tion, and thus rupture during extension.

Increasing radius of curvature of the Marianaforearc induces arc-parallel extension, manifest-ed as radial, high-angle, normal±slip faults withhorst-and-graben structures that are best de-veloped near the trench. Arc-parallel extensioncan be approximated with a set of ®nite strainellipsoids, with their major semi-principal axeseverywhere parallel to the strike of the arc. Theorientation of conjugate sets of maximum shearstrain which accompany extension varies fromnorth-northwest and east±west in the northernMariana forearc to northwest and northeast inthe south. Oblique convergence of the Paci®cplate favors a northwest-directed compressionalsemi-principal axis of the regional stress ellip-soid. The implication of this stress±strain con-®guration is: (i) predominance of northwest-trending faults in the southern Mariana forearc;(ii) predominance of northeast-trending faults inthe central part of the forearc; and (iii) develop-

ment of regionally signi®cant north-northwest-trending sinistral strike±slip faults in the north-ern Mariana forearc.

A summary of Mariana forearc tectonic ele-ments is presented in Fig. 10(a), where it can beseen that northwest-trending extensional struc-tures in the ESP are restricted to the regionbetween 15° and 17°N. Northeast-trending ex-tensional structures in the ESP become commonnorth of 17°N, and are much more importantoverall for controlling the ESP structural fabricthan the northwest-trending set. This may berelated to the increasing importance of north-west-trending left-lateral faults in the northernpart of the forearc, because strain associatedwith northwest-trending sinistral faults favorsdevelopment of northeast-trending extensionalstructures. Figure 10(b) presents a further sim-pli®cation of forearc structures, with a broadzone of forearc extension shown extending from� 15°N to 24°N.

The aforementioned three causes may mutuallyamplify effects and so contribute to localizing theESP±WSP boundary as the sur®cial expression ofa subhorizontal rheological boundary in the fore-arc lithosphere. Shallower lithosphere above thisboundary dominates the response of forearclithosphere beneath the ESP, causing the forearcto fracture in response to along-strike extensionand seamount subduction. This provides channelsfor ¯uids which serve to localize serpentinizationof the lithosphere along fractures beneath theESP. The end result is a close association offorearc faulting and fracturing, ¯uid ¯ow, and lo-calization of serpentinization, the sur®cial ex-pression of which is the ESP. The thickerlithosphere beneath the WSP allows ductile de-formation at depth, and the lack of pervasivefracturing coupled with a diminished ¯ow of ¯uidfrom the subducted slab leads to the greater sta-bility of the inner forearc and the relatively sub-dued sur®cial expression of the WSP.

The ®nal cause of geological complexity in theMariana forearc manifests as the progressivetransition from nearly normal convergence in thesouth to highly oblique convergence in the north.Figure 10(b) shows the relative motion betweenthe Paci®c and Philippine plates, after Seno(1989). We have previously pointed out the un-certainties in precise determination of relativemotion between these two plates, but in spite ofthese problems it is still apparent that conver-gence is nearly orthogonal to the trench in thesouth and becomes progressively more oblique to


the north. We cannot prove that the highly linearstructures that dominate the outer forearc northof 20° are strike±slip faults, but the fact thatthese structures become more important to thenorth (as plate convergence becomes increas-ingly oblique) is most readily explained by such amodel. The southernmost part of the inferredstrike±slip faults extends almost as far as 18°N,where the relative convergence according toSeno (1989) lies at an angle of 30° to the trend ofthe trench. These strike±slip faults become in-creasingly important northward while exten-sional forearc structures become progressivelyless well-developed in the same direction.

Two ®nal points should be made regarding theMariana forearc. First, the frontal arc of theMarianas lies along that portion of the forearcwhere orthogonal convergence is occurring. Wedo not know what is the relationship betweenorthogonal convergence and uplift of the frontalarc, but this behavior may contain importantclues about how stresses are transmitted acrossthe forearc; second, there is little evidence offorearc canyons analogous to those identi®edfurther north in the Izu forearc (Klaus & Taylor1991; Smoot 1997; Taylor & Smoot 1984). Thismay re¯ect relatively low sediment supply in thesouth; alternatively, this may re¯ect continueddeformation of the forearc. Forearc submarinecanyons initially develop along faults (Smoot1997), but to grow in size they must be able tomaintain a channel for sediment dispersion.Tectonic activity especially in the outer portionsof the forearc can disrupt these channels and haltcanyon development. All of the processes thatare identi®ed here (hydration of forearc litho-sphere, along-strike extension, and strike±slipfaulting north of 18°N) should still be active inthe southern IBM arc system, with the resultthat the Mariana forearc is still being deformed.It may be that the formation of submarine can-yons is related to how long the forearc has beenstable. The abundance of mature forearc canyonsin the northern IBM forearc and the absence ofthese features in the Mariana forearc thus mayresult from the relative tectonic stability of theformer and continued faulting and uplift of thelatter. The hallmark of the northern IBM forearc(canyons) and that of the Marianas (serpentinemud volcanoes) thus may simply signify thestability of the former and instability of the lat-ter.

CONCLUSION

The Mariana forearc is a wonderfully complexand dynamic tectonic system that also serves asone end-member of the forearc continuum: thenon-accretionary forearc. The bathymetricoverview presented here allows us to sketch inbroad strokes the general processes responsiblefor its present con®guration and opens the doorto a wide range of potential investigations intoits evolution. Several kinds of studies are calledfor. Global Positioning Satellite (GPS) campaignsare needed to constrain relative motion betweenthe Philippine and Paci®c plates and the Marianaplatelet. Such studies will also help constraindeformation within the Mariana platelet. Weneed better coverage of the forearc using sea¯oor imaging systems such as Izanagi and HA-WAII MR-1 in order to identify areas of activefaulting, serpentinite diapirs, and other tectoni-cally active areas. We need a better seismicnetwork in order to resolve where the deforma-tion is presently concentrated within the forearc,and fault-plane solutions are needed to test theidea that strike±slip faulting is important in thenorthern Mariana forearc. We need better seis-mic re¯ection and refraction studies to de®ne thegeometry of sedimentary basins and constrainforearc crustal and mantle lithospheric structure.Especially, we need multichannel seismic workto better de®ne the complex interrelationshipsbetween forearc faulting and the location ofserpentine mud volcanoes. Finally, we need fur-ther deep ocean drilling to resolve forearcstructures in detail and their relationship tovariations in structural provenance of the fore-arc. We hope that this essay serves to stimulatethese and other studies of this paradigmaticforearc.

ACKNOWLEDGEMENTS

The views in the present article are those of theauthors and do not re¯ect the of®cial policy orposition of the Department of the Navy, De-partment of Defense, nor the US Government.We appreciate the improvements to our struc-tural interpretations suggested by MohamedGamal Abdelsalam. The comments of refereesA. Taira and P. Fryer are greatly appreciated.This is UTD Geosciences contribution 873.


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thematic article a bathymetric overview of the mariana forearcrjstern/pdfs/sternsmoot98.pdfthematic...

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