Three-dimensional visualization of a near-vertical slab tearbeneath the southern Mariana arc

Meghan S. MillerResearch School of Earth Sciences, Australian National University, Mills Road, Building 61, Canberra, ACT 0200,Australia (meghan.miller@anu.edu.au)

Alexei GorbatovGeoscience Australia, G.P.O. Box 378, Canberra, ACT 2601, Australia

Brian L. N. KennettResearch School of Earth Sciences, Australian National University, Mills Road, Building 61, Canberra, ACT 0200,Australia

[1] The use of a three-dimensional ray-tracing inversion algorithm has greatly enhanced the resolution ofgradients and strong variations in wave speeds to create improved P wave tomographic images of theMariana arc region. The images obtained from the Mariana arc region show relatively low amplitudes ofheterogeneity due to the limited number of seismic stations in the area. Despite these limitations, detailedinterpretations of the three-dimensional geometry and morphology of the Pacific Plate subducting beneaththe Philippine Sea Plate have provided a three-dimensional model of the steep dip of the Pacific plate andthe curvature of the slab beneath the Mariana arc in unprecedented detail. The new P wave tomographyand seismicity depict a previously unidentified E-W trending near-vertical tear in the subducting plate atthe southern end of the Mariana arc that divides the arc into two distinct segments: a steeply dippingcurved slab penetrating the lower mantle and a short (�250 km depth) slab along the Challenger Deepsegment of the arc. The slab tear is likely to be the result of the need to accommodate the reduced volumethe slab must occupy as it is subducted beneath the Philippine Sea plate along a curved arc.

Components: 4265 words, 4 figures, 2 animations.

Keywords: Mariana arc; slab morphology; subduction zone.

Index Terms: 8199 Tectonophysics: General or miscellaneous; 8120 Tectonophysics: Dynamics of lithosphere and mantle:

general (1213); 8170 Tectonophysics: Subduction zone processes (1031, 3060, 3613, 8413).

Received 7 August 2005; Revised 13 January 2006; Accepted 8 March 2006; Published 14 June 2006.

Miller, M. S., A. Gorbatov, and B. L. N. Kennett (2006), Three-dimensional visualization of a near-vertical slab tear beneath

the southern Mariana arc, Geochem. Geophys. Geosyst., 7, Q06012, doi:10.1029/2005GC001110.

1. Introduction

[2] Tearing or breaking of subducting slabs hasbeen an important focus of geoscience researchin the past decade, as the morphology of sub-ducted slabs has been recognized to be a factorin understanding tectonics of the region [Worteland Spakman, 1992, 2000; Protti et al., 1994;Lallemand et al., 1997; Millen and Hamburger,

1998; Gvirtzman and Nur, 1999; Mazzotti etal., 1999; Bautista et al., 2001; Green, 2001;Levin et al., 2002; Fryer et al., 2003; Gvirtzmanand Stern, 2004]. Slab tears have been imagedusing tomography and seismicity patterns at con-vergent margins around the Earth, primarily whereplate boundary geometry dramatically changes,such as at the ends of arcs or where seafloorspreading ridges are being subducted. Location of

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tears beneath Tonga, Kamchatka, the Mediterra-nean, and Central Mexico [Millen and Hamburger,1998; Wortel and Spakman, 2000; Gvirtzman andNur, 2001; Levin et al., 2002; Ferrari, 2004] andlocations of similar features in subducting slabshave been proposed using other methods such asgeochemical traces, seismic anisotropy to detectmantle flow direction, and analogue models tosimulate slab dynamics. We use tomography andseismicity to interpret the morphology of thesubducting Pacific plate and locate a newly iden-tified near-vertical slab tear beneath the Marianaarc. Our results show the importance of slabmorphology in understanding the tectonic historyof the Mariana arc.

[3] The Mariana arc has a highly arcuate shape incontrast to the roughly linear shape of the Izu-Boninarc to the north (Figure 1). The contrasting plateboundary geometry and proposed evolution of thecurved Mariana arc have involved back arc spread-ing and ‘‘pinning’’ of the cusps of the arc byaseismic ridges [Vogt, 1973; Vogt et al., 1976;McCabe and Uyeda, 1983; Hsui and Youngquist,1985; Carlson and Mortera-Gutierrez, 1990;Taylor, 1992; Miller et al., 2005]. The MarianaTrough is the expression of an active back arc riftthat has formed between the West Mariana Ridgeand the active Mariana volcanic arc [Karig, 1971;Hussong and Uyeda, 1981; Martı́nez et al., 2000].Beneath the Mariana arc the morphology of thesteeply dipping descending Pacific plate has been

Figure 1. Map of the Mariana arc with the major features of the region, ETOPO5 bathymetry [National GeophysicsData Center, 1988] in km depth, hypocenters of earthquakes events with magnitudes of 5.0 and greater from theEngdahl et al. [1998] catalog, location of the new slab tear and previously identified tear [Gvirtzman and Stern,2004], and locations of the cross sections used in Figure 2. PA, Pacific Plate; PH, Philippine Plate.

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imaged with seismicity and seismic tomography,and is well established [Katsumata and Sykes,1969; Creager and Jordon, 1986; Chiu et al.,1991; van derHilst et al., 1991, 1993;Gudmundssonand Sambridge, 1998]. Unlike the plate beneath theMariana arc, the slab beneath the Izu-Bonin arcappears to be stagnant in the transition zone [Zhouand Clayton, 1990; van der Hilst et al., 1991, 1993;Fukao et al., 1992; Castle and Creager, 1998;Widiyantoro et al., 1999]. The differences betweenthe Izu-Bonin arc and the Mariana arc are striking,yet limited research has so far only involved deter-mining the structural evolution of the structure of thePacific plate as it subducts beneath the PhilippineSea plate along the curved Mariana arc. The curva-ture of the arc results in a volumetric problem as theamount of space for the downgoing slab decreaseswith depth. One possibility of accommodating thevolume of the subducting lithosphere is by tearingthe slab. A N-S trending tear in the slab at thesouthern end of the Mariana arc, known asthe Challenger Deep, has been suggested fromseismicity and bathymetry data [Fryer et al., 2003;Gvirtzman and Stern, 2004]. The cause of theunusual depth (nearly 11,000 m) of the ChallengerDeep has been suggested to be partly due to thesubducting lithosphere beneath the southern seg-ment of the Mariana arc tearing away from thenorthern part of the slab [Fujioka et al., 2002; Fryeret al., 2003; Gvirtzman and Stern, 2004] (Figure 1).We have interpreted P wave tomography andseismicity to image and define the location ofanother tear at striking approximately E-W at

14.5�N in three dimensions and discuss therelated tectonic implications.

2. Seismicity

[4] Hypocenters of events between 1995 and 2003catalogued by Engdahl et al. [1998] are plotted inFigures 1–3, along with ETOPO5 bathymetry[National Geophysics Data Center, 1988] inFigures 1 and 2. The earthquake locations mark awell-defined Wadati-Benioff zone along the north-ern and central Mariana arc where events occur todepths of approximately 650 km. In contrast, onlyshallow (to moderate) depth earthquakes are pres-ent (<�250 km) in the southern Mariana arc. Thepresence of a shallow seismogenic zone in thesouthern portion of the Mariana arc has beenpreviously explained by a short (200 km deep)subducting slab south of Guam, which is differentfrom the shape of the slab in the central Mariana arcthat is flat at shallow depth and then near vertical atgreater depth [Gudmundsson and Sambridge, 1998;Fryer et al., 2003; Gvirtzman and Stern, 2004]. Thevisualization of the hypocenters in C-C0 of Figure 2have a more steeply dipping orientation than theevents in B-B0, which is in agreement with [Fryer etal., 2003], who suggested that the slab is torn justwest of Guam.

3. Tomographic Images

[5] The amplitude of seismic anomalies in tomo-graphic images is relatively low in the Marianas

Figure 2. Cross sections of the P wave velocity perturbation model referenced to ak135 [Kennett et al., 1995] alongtraverses illustrated in Figure 1 with bathymetry profiles in km depth [National Geophysics Data Center, 1988] andthe distribution of seismicity plotted from the Engdahl et al. [1998] catalog. Cross section A-A0 illustrates a steeplydipping slab that penetrates into the lower mantle in the central Mariana arc, B-B0 shows the torn region of thesubducting plate, and C-C0 illustrates the continuous short slab along the southern Challenger Deep segment.

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Figure 3. Interpreted three-dimensional model of the morphology and geometry of the subducting slab beneath theMariana arc. The absent block or slice illustrates the location of the newly identified tear between the short segmentof the slab and the steeply dipping portion of the slab that penetrates into the lower mantle. The projection of the EWand NS [Gvirtzman and Stern, 2004] trending tears are identified by blue and green lines, respectively. The plateboundary is illustrated with a black dashed line, and the position of the Ogasawara Plateau is represented by a purpleline. The curvature and morphology of the Mariana arc are illustrated (a) as a solid and (b) as a transparent surface toshow the distribution of seismicity [Engdahl et al., 1998] within the subducting slab and the shallow extent of eventsin the Challenger Deep segment.

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compared to areas such as Japan or North Americabecause there are only a few isolated seismicstations in the region. Through the adaptation ofa 3D ray tracing technique developed by Gorbatovet al. [2000] tomographic images are greatly en-hanced. Using an updated seismicity catalogue[Engdahl et al., 1998] the P wave inversion wasgenerated from events only with both P and Sarrival times recorded and referenced to the ak135model [Kennett et al., 1995]. Single rays with bothP and S readings were picked for events andstations within the study area to optimize datacoverage and to ensure that the final images couldbe directly compared. This criterion improved thelocation accuracy, and in turn, improved the reso-lution of the structures and noise reduction. The raypath data set and methodology is the same asemployed in the bulk sound and shear wave speedinversions [Gorbatov and Kennett, 2003], whichhas yielded the new P wave inversion [Miller et al.,2005]. Verification of sufficient ray path coveragein the data set for the Mariana arc region isexpressed with a ray path density in Figure 2 ofMiller et al. [2005], and it should be noted that thebulk-sound and shear wave speed models alsoshow a similar morphology and tear geometry asdescribed here in the P wave inversion.

[6] Figure 2 illustrates a set of cross sections thatwere sliced perpendicular to the trench positionthrough the P wave three-dimensional tomographicimage. The vertical profiles illustrate the positionand shape of the interpreted subducting plate withfast velocity perturbations (cyan) and slow velocityperturbations (brown). Profiles A-A0 illustrates asteeply dipping slab that can be traced into thelower mantle with earthquakes occurring to almost600 km depth, profile B-B0 shows a discontinuousslab that is interpreted as the position of the newlyidentified slab tear, and finally C-C0 illustrates acontinuous but short steeply dipping slab. Theshape of the subducting slab changes dip directionand length along the southern portion of theMariana arc.

4. Three-Dimensional Slab Morphology

[7] The extent and morphology of the subductedslab were interpreted by picking the maximumcontrast in P wave velocity contours, which werethen used to create a three-dimensional visualmodel. The top and bottom of the subductingPacific plate was picked along maximum gradientin the tomographic images with the hypocentersfrom the Engdahl et al. [1998] catalogue as a

constraint on the slab width and extent. The distinctcurvature of the arc evident in the mapped trenchlocation (Figure 1) and the north-south trendingportion of the steeply dipping, lower mantle pen-etrating, slab as represented with Wadati-Benioffzone seismicity is clear in the 3D interpreted model(Figure 3 and Animation 1). The visualization ofthe slab resulted in detecting a distinct change inmorphology of the slab south of 14.5�N, where thedip and length of the slab changes from penetratingvertically into the lower mantle to a short andWSW trending structure. The seismic velocityanomaly between the two dissimilar morphologiesis interpreted as a newly identified slab tear orientedapproximately E-W, ranging between 75–200 kmdeep, between 143.5�E and approximately 146.5�E.The position of the tear occurs at the junctionbetween the steeply dipping slab, penetratingthrough the transition zone into the lower mantle,and a short slab in which only shallow earthquakesoccur (Figure 3 and Animation 1). The short, steepgeometry of theWSW slab segment is remarkable incomparison to the deep, continuous slab in thecentral and northern portions of the arc.

5. Discussion

[8] A near-vertical tear in the slab subductingbeneath the Mariana arc is clearly imaged usingP wave tomography and the interpreted three-dimensional slab morphology using earthVisionTM,a three-dimensional visualization software package(Figures 2 and 3 and Animation 1). The slab has anarcuate shape in-depth, similar to the surface ex-pression of the trench axis (Figure 1), and plungessteeply into the lower mantle in the northern andcentral portion, similar to the Wadati-Benioff zone.A markedly different geometry exists south ofGuam (14.5�N) where the slab continues to havea steep dip, but is only �250 km in length. The twodistinct morphologies are divided by a near-verticaltear in the slab striking approximately E-W. Theorientation of this tear is different from the N-Strending tear at 144�E (Figure 1) proposed byGvirtzman and Stern [2004], but the new tear ispositioned at the junction between the WSW trend-ing Challenger Deep segment and the N-S trendingportion of the Mariana arc. The two slab tears maybe the result of the reduced volume that thesubducting Pacific plate must occupy at depth. Itis also possible that the origin of the tears could bedue to the more rapid southward rollback of theshort slab fragment or be the effect of the buoyantCaroline Island Ridge colliding with the trench.

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[9] Evidence of fast rollback for the ChallengerDeep segment slab is observed in the distributionof N-S extensional faulting, deformation in theoverriding plate, an uplifted outer fore arc, andthe location and orientation of earthquakes[Gvirtzman and Nur, 1999; Fryer et al., 2003;Gvirtzman and Stern, 2004]. This set of obser-vations and the extreme depth of the trench canbe explained by rapid rollback of the short slab[Fryer et al., 2003; Gvirtzman and Stern, 2004],which produced the N-S trending tear identifiedby Gvirtzman and Stern [2004].

[10] Another possible cause of the north-south andeast-west trending tears could be a result of sub-duction of topographic highs like the CarolineIsland Ridge, which appears to be colliding at thetrench at approximately 145�E (Figure 4, top left).The topographic highs, shown in cross section inFigure 2, illustrate the short slab segment along theChallenger Deep corresponds to the bathymetrichighs in C-C0. The convex shape of the Mariana archas suggested to be caused by the pinning of thearc as aseismic ridges (Caroline Island Ridge and

Marcus-Necker ridge) collided at the trench andresisted subduction [Vogt, 1973; Vogt et al., 1976;McCabe and Uyeda, 1983; Hsui and Youngquist,1985; Miller et al., 2006]. Figure 4 and Animation2, which are a subset of Figure 5 of Miller et al.[2006], illustrate the evolution of the Mariana arccurvature as the cusps are pinned by the topo-graphic highs (aseismic ridges) being subducted(Figures 1 and 2). As the ridges collide with thetrench, causing the curvature of the arc, the shapeof the subducted slab must be accommodated bybending and tearing. The position of the tear(Figures 1, 3, and 4 and Animation 1) and thelocation at which the ridge converges with thetrench axis provides support for this model andcould explain the presence of multiple tears in thisarea. Focal mechanisms from the Harvard CMTcatalog are plotted with the interpreted slab mor-phology in Animation 1. The solutions illustratethe majority of the events near the E-W trendingtear are predominately dip-slip, which suggests thatthe descending Pacific plate is extending andtearing as it becomes increasingly arcuate in shape(concave to the west). The extensional events

Figure 4. Paleogeographic reconstruction of the Izu-Bonin-Mariana arc system from the mid-Miocene [Miller etal., 2006]. Pink lines are magnetic anomalies, black dashed lines are current plate boundaries, blue lines areinterpreted paleo-plate boundaries, green objects are aseismic ridges/oceanic plateaus, the dotted green line is theprojected extension of the Marcus-Necker ridge, and orange lines represent the current position of the slab tearimaged in this study and one proposed by Gvirtzman and Stern [2004].

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support the presence of several tears in the down-going plate.

6. Conclusions

[11] The position of the slab tear and the remark-able disparity in the geometry and morphology ofthe Challenger Deep segment and the entireMariana arc are clearly imaged in three dimen-sions. The steeply dipping portion of the slabbeneath the northern and central Mariana arcappears to be crumpled as it penetrates into thelower mantle. The slab beneath the southern por-tion of the arc is relatively short (�250 km depth)and is trending west-southwest. The interpretedarcuate shape of the Pacific plate at depth, similarto the surface expression of the Mariana Trench,indicates a volumetric problem as the slab descendsinto the mantle. The east-west oriented tear, incombination with the north-south tear proposedby Gvirtzman and Stern [2004] and Fryer et al.[2003], could be a mechanism that would allow thesubducted oceanic lithosphere to continue its arcu-ate shape at depth. Further refinement of the modelwill improve the understanding of the relationshipbetween the morphology and the geodynamics ofthe southern Mariana region.

Acknowledgments

[12] We would like to thank Robert J. Stern and Zohar

Gvirtzman for their contributions to early versions of the

manuscript and discussions on the related geochemistry. We

would also like to thank the two reviewers, Haijiang Zhang

and Patricia Fryer, for their detailed comments and suggestions

that greatly improved the manuscript and to Christian Noll for

his continuing support.

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