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Bivergent thrust wedges surrounding oceanic island arcs: Insight from observations and sandbox models of the northeastern Caribbean plate

Uri S. ten Brink1,†, Stephen Marshak2, and José-Luis Granja Bruña3 1U.S. Geological Survey, Woods Hole, Massachusetts 02543, USA 2Department of Geological Sciences, University of Illinois, Urbana, Illinois 61801, USA 3Geodynamics Department, Universidad Complutense de Madrid, Madrid 28040, Spain

ABSTRACT modated entirely within the prowedge and 1993; Beaumont and Quinlan, 1994; Fig. 1A). the arc—the retrowedge hosts only dip-slip The material incorporated in bivergent orogens At several localities around the world, faulting (“frontal thrusting”). The existence is primarily crustal, therefore such orogens are thrust belts have developed on both sides of of large retrowedges and the distribution of also called “bivergent crustal wedges.” oceanic island arcs (e.g., Java-Timor, Pan- faulting in an island arc may, therefore, be There are several examples of island arcs in ama, Vanuatu, and the northeastern Carib- evidence that the arc is relatively rigid. The which a thrust belt with vergence opposite to that bean). In these localities, the overall vergence rigidity of an island arc may arise from its of the accretionary prism develops in the back- of the backarc thrust belt is opposite to that mafi c composition and has implications for arc region, so that bivergent thrusting involves of the forearc thrust belt. For example, in seismic-hazard analysis. the entire island arc (e.g., Banda, Vanuatu, the northeastern Caribbean, a north-verging and Panama arcs; Fig. 2). The Eastern Greater accretionary prism lies to the north of the INTRODUCTION Antilles arc (Hispaniola and Puerto Rico) of the Eastern Greater Antilles arc (Hispaniola and northeastern Caribbean illustrates this geometry. Puerto Rico), whereas a south-verging thrust It has long been recognized that conver- A north-verging accretionary prism lies between belt called the Muertos thrust belt lies to gence in continent-continent collisions and the Puerto Rico Trench in the forearc region, the south. Researchers have attributed such in continental subduction zones produces while a south-verging thrust belt, called the bivergent geometry to several processes, bivergent (or doubly vergent) orogens (e.g., Muertos thrust belt, lies between the arc and including: reversal of subduction polarity; Burchfi el and Davis, 1975; Willett et al., 1993). the Muertos Trough in the backarc region (Fig. 3). subduction-driven mantle fl ow; stress trans- Such orogens are so named because the overall The issue of whether such island arcs are biver- mission across the arc; gravitational spread- vergence, or transport direction, of the thrust gent crustal wedges, similar to those developed ing of the arc; and magmatic infl ation within belt on one side of the orogen is opposite to during Alpine-type continental collisions, has the arc. New observations of deformational that of the thrust belt on the other side. In a proven to be problematic. The literature attri- features in the Muertos thrust belt and of general sense, bivergent orogens consist of butes the existence of a backarc thrust belt to fault geometries produced in sandbox kine- three zones: (1) the prowedge, consisting of a different phenomena, such as: subduction rever- matic models, along with examination of thrust system building on the downgoing plate; sal (Dewey and Bird, 1970; Byrne et al., 1985); published studies of island arcs, lead to the (2) a retrowedge, consisting of a thrust system converging mantle fl ow beneath the arc (Conrad conclusion that the bivergence of thrusting propagating into the foreland of the overrid- et al., 2004); gravitational spreading of the thick- in island arcs can develop without reversal of ing plate; and (3) a central uplift, consisting ened crust in the arc (Silver et al., 1983; Breen subduction polarity, without subarc mantle of the wedge’s internal zone that rises between et al., 1989); infl ation of the arc by magmatic in- fl ow, and without magmatic infl ation. We the prowedge and retrowedge (Willett et al., trusion (Hamilton, 1979); and stress propagation suggest that the Eastern Greater Antilles arc and comparable arcs are simply crustal- Uplifted core scale bivergent (or “doubly vergent”) thrust A wedges formed during unidirectional subduc- Retrowedge Prowedge tion. Sandbox kinematic modeling suggests, Figure 1. (A) Shape of bivergent in addition, that a broad retrowedge contain- wedge and material trajectory ing an imbricate fan of thrusts develops only from numerical models (after where the arc behaves relatively rigidly. In Willett et al., 1999). S—Sin- such cases, the arc acts as a backstop that gularity point. (B) Geometry transmits compressive stress into the backarc B of bivergent wedge simplifi ed Uplifted core region. Further, modeling shows that when Retrowedge Prowedge from results of oblique conver- arcs behave as rigid blocks, the strike-slip gence in sandbox models (modi- component of oblique convergence is accom- fi ed from McClay et al., 2004).

†E-mail: [email protected] Subduction slot

GSA Bulletin; November/December 2009; v. 121; no. 11/12; p. 1522–1536; doi: 10.1130/B26512.1; 10 fi gures; Data Repository item 2009115.

1522 For permission to copy, contact [email protected] © 2009 Geological Society of America Downloaded from gsabulletin.gsapubs.org on February 8, 2010 Bivergent thrust wedges surrounding oceanic island arcs

110°E 115°E120°E 125°E 130°E 135°E –5°S A

1992 M7.9 Continental arc Oceanic arc –10°S

Smooth seafloor

–15°S

B Caribbean C

14°N 166°E 167°E 168°E 169°E

bac

kar NPDB c 1991 1882 15°S 10°N M7.7 S. Blas thrust Limon Panama 55 mm/a belt N85 30–40 mm/a N70 16°S 6°N 1999 M7.5 10°S

17°S 2°N 30°S 90 mm/a 100 km N74 150°E 170°E 170°W

86°W 82°W 78°W

Figure 2. Location maps of bivergent wedges around island arcs. (A) Java-Timor Trench (modifi ed from Silver et al., 1983). (B) Panama and North Panama deformation belt (NPDB) (modifi ed from Suárez et al., 1995). PFZ—Panama fracture zone. (C) New Hebrides Trench near Vanuatu (modifi ed from LaGabrielle et al., 2003). PNG—Papua New Guinea; NZ—New Zealand, AU —Australia.

across the arc and into the backarc in response to (or extinct arc) serve during the development This paper begins by presenting an analysis collision in the forearc (Silver et al., 1983). of a backarc thrust belt? (3) How does oblique of the Eastern Greater Antilles arc based on The apparent lack of consensus concerning convergence affect the kinematics of backarc multibeam and seismic-refl ection studies. We the origin of backarc thrust systems led us to thrusting? To address these questions, we uti- then provide a brief review of published obser- address the following three questions: (1) Are lized analysis of multibeam bathymetric maps vations from comparable island arcs elsewhere. backarc thrust systems the retrowedge part of and seismic-refl ection profi les in the vicinity Finally, we describe the results of simple sand- a bivergent crustal wedge, formed in response of the northeast Caribbean arc, we produced box models designed to illustrate the map-view to transmission of compressive stress across the simple sandbox kinematic models simulating kinematics of bivergent thrusting. Our observa- crust of the arc, or are they a manifestation of relative motions of crustal components in the tions lead us to suggest that the Eastern Greater subduction reversal or of upper-mantle fl ow? northeast Caribbean, and we examined pub- Antilles arc is a bivergent crustal wedge formed (2) What mechanical role does the volcanic arc lished studies of island arcs worldwide. due to stress transmission across the arc from

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Bahamas Platform NORTH AMERICAN PLATE

Hispaniola Trench Puerto Rico Trench

SFZ BF Cordillera Central Gonave microplate HISPANIOLA MRS PUERTO RICO VIRGIN ISLANDS EFZ GSPRFZ SPB YB Anegada BP Passage Muertos thrust belt IFZ St. Croix SB Haiti Muertos Subbasin Trough

Venezuela Basin CARIBBEAN PLATE Aves Ridge Beata Ridge

16°N72°W 18°N70°W 20°N 68°W 66°W 64°W Fig. 10B Fig. 10A

Figure 3. Regional tectonic setting of northeastern Caribbean plate. Contours show the satellite bathymetry data gridded at 2 min interval (Smith and Sandwell, 1997). Thick arrows—relative convergence direction between North American and Caribbean plates. Oligocene- Pliocene carbonate platform extension is outlined by dotted line (van Gestel et al., 1998). Star corresponds with the location of the Ms = 6.7 event calculated by Byrne et al. (1985). BF—Bunce fault, BP—Bahoruco Peninsula; EFZ—Enriquillo fault zone, SFZ—Septentrional fault zone, GSPRFZ—Great Southern Puerto Rico fault zone, IFZ—Investigator fault zone, MRS—Mona Rift system, SB—Saba Bank, SPB— San Pedro Basin, YB—Yuma Basin.

the subduction zone, that the existence of back- OBSERVATIONS FROM THE Virgin Islands at a rate of 18–20 mm/a (e.g., arc thrusting in the Muertos thrust belt does not NORTHEASTERN CARIBBEAN Mann et al., 2002). Adjacent to the Hispaniola require subduction of the Caribbean plate be- Trench, the Bahamas Platform block of sig- neath the Eastern Greater Antilles arc, and that Background: Geologic Setting of the nifi cantly thicker crust with continental affi nity the kinematics of the belt argue against its asso- Northeastern Caribbean Plate has moved into the trench (Freeman-Lynde and ciation with mantle fl ow. We argue that a simi- Ryan, 1987). Thrust earthquakes occur along a lar interpretation also applies to several other The Greater Antilles volcanic arc, which shallowly dipping (~20°) zone under northern oceanic island-arc systems around the world extends from Cuba to the Virgin Islands, was Hispaniola (Dolan and Wald, 1998), indicating (Fig. 2). Our sandbox modeling also empha- formed during the Cretaceous and Early Tertiary that convergence still takes place across this sizes that a broad backarc thrust belt contain- as the North American plate subducted south- boundary, so that Hispaniola is currently thrust- ing an imbricate fan of thrusts only develops westerly beneath the Caribbean plate (Pindell ing over the Bahamas Platform. where an oceanic island arc behaves as a rela- and Barrett, 1990). Beginning at 49 Ma, rela- The on-land geology of Hispaniola and tively rigid block and can transmit compressive tive plate motion changed to a more easterly di- Puerto Rico in the eastern Greater Antilles is stress to the backarc region. If the arc behaves rection (~250°), resulting in a cessation of arc complex (see reviews by Draper et al., 1994; rigidly, the strike-slip component of oblique volcanism, and in highly oblique subduction Larue, 1994). A Cretaceous to early Eocene arc convergence is accommodated in the forearc; with a large component of left-lateral strike- assemblage consisting of accretionary prism relatively few thrust faults develop within the slip. The direction of plate convergence has sediments, arc volcanics and intrusives, and arc itself; and only perpendicular convergence been fairly constant since 49 Ma, as indicated ophiolites forms the basement of Hispaniola. occurs in the backarc. Notably, the bivergent by the length and trend of transform faults Eocene and younger clastics and carbonates crustal wedge model of island arcs has impli- bordering Cayman Trough between Cuba and have accumulated over this arc-assemblage cations for seismic-hazard assessment of arcs Honduras (e.g., Draper et al., 1994). Presently, basement. The last arc-building phase recorded because it provides a basis for predicting the normal-thickness, 85–125 Ma oceanic crust of in Puerto Rico occurred in the early Tertiary. distribution of faulting. the Atlantic subducts under Puerto Rico and the In the late Eocene and Oligocene, several

1524 Geological Society of America Bulletin, November/December 2009 Downloaded from gsabulletin.gsapubs.org on February 8, 2010 Bivergent thrust wedges surrounding oceanic island arcs kilometers of uplift, as well as rotation around locally, it is over twice as thick as that of the of the Muertos Trough and Muertos thrust a vertical axis, affected the island (Larue, 1994). Atlantic Ocean crust north of Puerto Rico. belt, reveal the detailed structure of this area In post-Oligocene time, a carbonate platform Formation of thick Caribbean crust probably (Fig. 4). Vertical cross sections through this re- was formed over the arc complex. Recent ex- refl ects production of fl ood basalts in a large ig- gion are provided by 10 newly acquired high- humation of mid- or lower-crustal rocks has not neous province (e.g., Burke et al., 1978; Driscoll resolution multichannel profi les, 8 reprocessed occurred in the eastern Greater Antilles. and Diebold, 1998). In the region south of profi les (obtained from the seismic-data library The Muertos thrust belt forms the southern Hispaniola, an asymmetric NNE-trending high, of the University of Texas), and 7 scanned ana- margin of Hispaniola and Puerto Rico and is known as the Beata Ridge, brings Caribbean log single-channel seismic lines, supplied by ~650 km long (Fig. 3). The boundary between crust up to shallower depths. The Beata Ridge A. Mauffret. These data, together with published the thrust system and the exposed fl oor of the appears to be underlain by somewhat thicker core and dredge data (Nagle et al., 1978), allow Caribbean plate is a linear depression known crust than that of the adjacent Venezuelan Basin interpretation of the Muertos thrust system and as the Muertos Trough (Fig. 4), in which water to the east, and it appears to be bounded on help distinguish among proposed mechanisms depths reach 5580 m. The age of initial conver- its southeastern edge by faults (Driscoll and for its formation. gence across the Muertos Trough is unknown. Diebold , 1998; Mauffret and Leroy, 1999). The Muertos thrust belt can be divided into Early Miocene and older sedimentary rocks, three provinces. In the Lower Slope Province, which cover the Caribbean plate, appear to be in Observations from the Muertos Trough and active imbricate thrusts break the seafl oor sur- the footwall, beneath the thrust belt’s basal de- Muertos Thrust Belt face. This thrusting, along with thrust-related tachment (Fig. 5), whereas younger sediments folding, has yielded distinct, <20-km-long and may be incorporated into the thrust belt itself. Bathymetric digital elevation models (DEM) <5-km-wide ridges that trend roughly paral- The Caribbean plate’s interior includes re- with a spatial resolution of 50 m, produced lel to contours of the slope (Fig. 5). The Lower gions of anomalously thick oceanic crust— during recent multibeam bathymetry surveys Slope Province widens signifi cantly from 4 to

71°W 70°W 69°W 68°W 67°W 66°W 65°W

A DOMINICAN REPUBLIC Santo Domingo San Pedro de Macoris Arecibo SanJuan

Mayaguez PUERTO RICO San Pedro Basin Ponce

18°N 1000 2000

3000 5000

4000 17°N Venezuela Basin 020 40 80 120 160 km

B DOMINICAN REPUBLIC Santo Domingo San Pedro de Macoris Arecibo SanJuan

Mayaguez PUERTO RICO San Pedro Basin Fig. 6 Ponce 18°N

LL66

6

6 n

1 --1n

b

vvb

Deformation front Thrust fault Older thrust fault Sand waves Normal fault Beata Ridge lineaments 17°N Extent of compressional features Venezuela Basin 020 40 80 120 160 Seismic profiles km

71°W 70°W 69°W 68°W 67°W 66°W 65°W Figure 4. (A) Shaded relief bathymetry map illuminated from northwest, gridded at 150 m grid size. Sources of multibeam bathymetry data are SeaCarib (Mauffret and Leroy, 1999), Geoprico-Do (Carbó et al., 2005), and U.S. Geological Survey (USGS) 2006 Tsucar and USGS 2007 Mona Passage cruises. (B) Same as A, overlain by interpretation. Solid lines—locations of seismic profi les in Figure 5. Rectangle— location of Figure 6.

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10 km south of Puerto Rico to ~25 km south of longitude 68.5°W, these faults curve north- elevations reach 3 km, lies inboard of the Beata of the Dominican Republic (Figs. 4 and 5). In ward, and between longitude 68°W and 67.7°W, Ridge and why thrust faults on the island itself the Middle Slope Province, slope-surface ridges they terminate at the thrust front. Similar normal curve adjacent to this highland (Hernaiz Huerta are subdued, suggesting that imbricate thrust- faults have been observed on many subducting and Pérez-Estaún, 2002; Heubeck and Mann, ing is less active. Some of the subdued ridges plates to a distance of 50–60 km oceanward of 1991; Mercier de Lepinay et al., 1988). trend ~20° more northwesterly than those of the trench, and they are interpreted to be a defor- In detail, thrust-fault traces of the Muertos the Lower Slope Province (purple lines in mational response to tensile fl exural stresses re- thrust belt are segmented along strike into Fig. 4), and thus they may indicate that the trans- sulting from the bending of the subducting plate 50–90-km-long segments that are slightly con- port direction of thrusting changed as thrusting (Bodine and Watts, 1979; Ranero et al., 2005). vex to the south. These curves are symmetrical propagated southward with time. The combined Notably, the normal faults south of the Muertos salients, the presence of which may refl ect inter- Lower Slope and Middle Slope Provinces, south Trough are best developed where the Lower and actions between the thrust system and basement of Puerto Rico, are 18–22 km wide and increase Middle Slopes Provinces of the Muertos thrust highs on the underlying Caribbean plate. The in width to 45 km south of the Domini can Re- belt are particularly wide—the faults gradually fact that the salients are symmetrical suggests public (dashed blue line in Fig. 4). The Upper terminate along strike to the east, where the that the transport direction within the Muertos Slope Province of the Muertos thrust belt is retrowedge is narrower. This relation suggests thrust system is southward perpendicular to the underlain by island-arc basement and accreted that the normal faulting may have been ampli- regional trend of the belt. sedimentary rocks buried by carbonate-platform fi ed by loading when retrowedge thrust sheets Active strike-slip faulting has not been and slope sediments. In general, thrusting in the loaded the Caribbean plate. identifi ed within the southern insular slopes Upper Slope Province appears to have become At the western end of the Muertos Trough, of Puerto Rico and Hispaniola, despite highly inactive. However, in the vicinity of longitude fault traces and the thrust front of the Muertos oblique motion between the Caribbean and 69.2°W, high-angle north-dipping thrust faults thrust belt change trend, curving from east- North American plates (Manaker et al., 2008). (as seen in reprocessed seismic lines from Ladd west to north-south over a distance of ~30 km This statement derives from the following et al., 1981) cut the surface of the Upper Slope (Fig. 4). This change occurs where the thrust three observations. First, the Enriquillo fault,

Province. These faults lie above the 1984 Ms 6.7 belt terminates against three north-south– the only fault with documented strike-slip dis- earthquake (star in Fig. 3), and their presence trending ridges that lie offshore and to the east placement in the southern part of the eastern may indicate that active faulting delineates the of the Bahoruco Peninsula of southern Hispan- Greater Antilles , terminates west of the Muer- southern edge of the San Pedro Basin, 75 km iola. The Bahoruco Peninsula and the ridges tos Trough and thus does not cut the Muertos north of Muertos Trough. represent the northernmost end of the Beata thrust system. Second, the Investigator fault Normal faults subparallel to the deformation Ridge (Fig. 3; Mauffret and Leroy, 1999). (Garrison, 1969), which is manifested on front are exposed on the Caribbean plate seafl oor Thus, the curvature of the Muertos thrust belts bathymetric maps as a system of several east- within Muertos Trough. These faults extend to a here suggests that the Beata Ridge has acted west–trending subparallel lineaments crossing distance of ~30 km south of the frontal thrust of as an indenter that pushed into Hispaniola the Upper Slope Province south of Puerto Rico the Muertos thrust belt, and they have produced (cf. Marshak, 2004). Indentation by the Beata (Fig. 4), does not offset submarine canyons and an axial low reaching a depth of 5580 m in the Ridge may also explain why the highest por- thus cannot currently accommodate strike-slip region west of longitude 68.5°W (Fig. 4). East tion of Hispaniola’s Cordillera Central, where motion (Fig. 6). In fact, seismic profi les suggest

A ′ Profile L66 A 0 N S 0 km

Profile vb-1n 1 INVESTIGATOR 1 0 5 10 B FAULT B′ 0 0 NE 2 Upper Slope 2 SW TALUS 1 3 3 1 TWO-WAY TIME (s) TWO-WAY TERRACES 2 4 2

Middle Slope (s) 3 3 TERRACES Lower 5

Slope TIME (s) TWO-WAY 4 TERRACES BURIED LARGELY MUERTOS 6 4

INACTIVE THRUSTING TROUGH TIME -WAY 5 7 5 O ACTIVE EARLY MIOCENE

″ TW

TWO-WAY TIME (s) TWO-WAY A 6 THRUSTING B″ 6 VENEZUELA BASIN 7 MUERTOS TROUGH 7 EARLY MIOCENE A″ B″ 8 8 Seafloor Accretionary wedge reflectors Thrust fault VENEZUELA BASIN REFLECTORS Venezuelan basin reflectors Possible thrust fault Wedge trough reflectors Normal fault Unconformity Decollement

Figure 5. Comparison between line drawing interpretations of the seismic profi le L66 (R/V Pelican, 2006) across the narrow thrust belt south of Puerto Rico and the reprocessed IG1503 seismic profi le vb-1n (Ladd and Watkins, 1979) across the wide thrust belt south of Mona Passage.

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036912 Subducting seafl oor of the Cocos and Nazca PUERTO RICO km Guayanilla plates has signifi cant relief because it is dis- Ponce rupted by fracture zones as well as by the small 18°N Santa Isabel spreading ridge that separates the plates. Arc volcanism extends southeastward into western Caja de Muertos Island –200 Panama. Subduction may not take place east of

–400 –200 longitude 80°W (Westbrook et al., 1995). The Northern Panama deformed belt is an arcuate thrust belt off the northern shore of Panama and eastern Costa Rica (Fig. 2B; Silver et al., 1995). The trace of folds and faults in the N ′ belt generally follows the convex-to-the-north curve of the Panama arc, except at its western 17°45 end, where it extends on land (Silver et al., 1995; Silver et al., 1990). Overall, the Northern Panama deformed belt verges north, but locally it includes south-verging thrust faults (Reed 66°45′W 66°30′W and Silver, 1995). Earthquakes occur along the length of the belt, indicating that the entire belt Figure 6. Shaded relief bathymetry, illuminated from northwest, of is actively deforming. Examples of seismicity

the Investigator fault area. See Figure 4 for location. Arrows show include the Mw 7.7 1991 Limon earthquake at fault traces. Note the lack of lateral offset of canyon valleys across the western end of the belt (Suárez et al., 1995), the fault traces, indicating that the fault system likely accommo- and a devastating tsunamigenic earthquake that dates north-south extensional motion, not strike-slip motion. took place in the eastern part of the Northern Panama deformed belt in 1882 (Mendoza and Nishenko, 1989). The Limon earthquake oc- that the fault instead accommodates a small et al., 1986). In this 500-km-long Flores thrust curred at a depth of 24 km, and its p-axis was amount of north-south extension. Third, traces belt, traces of individual thrusts are relatively oriented perpendicular to thrust traces in the of the Great Southern Puerto Rico fault, in the short (20–30 km long). The 1992 M 7.9 earth- Northern Panama deformed belt. southern coastal plain of Puerto Rico (Fig. 3), quake occurred at the eastern end of this thrust GPS measurements and seismic monitoring do not cut post-Eocene sediments and thus are belt, and it had a hypocentral depth of 16 km indicate that the Panama arc is quite rigid and is not active (Erikson et al., 1990; Glover, 1971). (Beckers and Lay, 1995). A 300-km-long gap in undergoing relatively little internal deformation backarc thrusting occurs to the east of the Flores (Trenkamp et al., 2002). Kellogg et al. (1995) OBSERVATIONS FROM OTHER belt. Backarc thrusting is evident again in the suggested that the relative motion between the ISLAND ARCS north-verging Alor-Wetar thrust belt. Notably , Nazca and Caribbean plates may be fully ab- the Alor-Wetar thrust belt developed where sorbed by deformation within the Northern The northeastern Caribbean is not unique the continental part of the Australian plate has Panama deformed belt. However, despite the in having bivergent thrust geometry. Here, we started to move into the subduction zone and high strain, there is no indication for a south- describe briefl y other examples of bivergent where volcanic activity has ceased. Global po- dipping subduction zone under Central America thrust belts associated with island arcs. Obser- sitioning system (GPS) measurements indicate (Suárez et al., 1995). Also, strike-slip faults are vations from these examples provide additional that most convergence across the plate boundary not abundant within the Northern Panama de- constraints on models to explain bivergent here currently takes place in the backarc thrust formed belt, despite oblique convergence across thrusting. belt (Genrich et al., 1996). Faults of the Alor- the Central American arc. Wetar belt are discontinuous and locally trend Java-Timor perpendicular to the slope of the islands rather New Hebrides than perpendicular to the general plate conver- Subduction of the Australia plate occurs gence vector (Breen et al., 1989). Arc-parallel The 400-km-long segment of the New along a trench that curves from a north-south strike-slip faults have not been found within Hebrides subduction zone that includes the trend west of Sumatra to an east-west trend the backarc thrust belt, but a strike-slip fault Vanuatu Island group exhibits bivergent defor- along Banda arc in southeastern Indonesia. may accommodate some motion between indi- mation (Fig. 2C). Here, seismically active Backarc thrusting occurs intermittently along vidually moving arc blocks (Breen et al., 1989). east-verging thrust faults occur east of Vanuatu the 1200-km-long segment of this convergent The p-axes of earthquakes in the backarc are on the opposite side of the arc from the west- margin east of Java, i.e., north of the eastern perpendicular to the strike of the trench and verging accretionary prism that lies between Java trench and the Timor Trough (Fig. 2A; are confi ned to the crust (McCaffrey, 1988). the island arc and the New Hebrides Trench. Silver et al., 1983). Notably, the backarc thrust Notably, the backarc thrusting has developed belt does not occur north of Java, a crustal block Northern Panama where the d’Entrecasteaux Ridge is converging with continental affi nity. with the trench (Geist et al., 1993; Lagabrielle An example of backarc thrusting occurs to the Subduction of the Cocos and Nazca plates et al., 2003), although the d’Etrecasteaux Ridge north of a line of active volcanoes on the islands takes place south of Central America, along a is only 100 km wide. Rapid uplift of the east- of Sumbawa and Flores in the Banda arc (Silver north-dipping zone (Moore and Sender, 1995). ernmost islands in the Vanuatu group shows

Geological Society of America Bulletin, November/December 2009 1527 Downloaded from gsabulletin.gsapubs.org on February 8, 2010 ten Brink et al. that the backarc wedge is propagating eastward the crust is represented by a layer of noncohe- 2004). The most advanced models use servo- with time and that most of the convergence be- sive Coulomb material spread uniformly over motors to provide constant strain rates, deform tween the Australian and Pacifi c plates of the converging rigid plates. During convergence, a sand layer in which alternating layers of region is currently taking place in the backarc, the downgoing plate slides beneath the overrid- different-colored sand represent bedding in the not the forearc. The focal mechanism and co- ing plate at a singularity (S). As displacement upper crust, and/or place the sand on a substrate seismic uplift of the Mw 7.5 1999 earthquake progresses , a central uplift rises over the sin- of silicon putty to provide an appropriately indicate that faults in the backarc thrust belt gularity, and two thrust belts develop, one on scaled analog for plastically deforming litho- extend at least down to a depth of 16–18 km each side of the central uplift (Fig. 1A). On the sphere (e.g., Davy and Cobbold, 1991; Pinet (Lagabrielle et al., 2003). prowedge side of S, the velocity of the rigid plate and Cobbold, 1992; Pubellier and Cobbold, is constant (moving toward S), whereas on the 1996; Marques and Cobbold, 2002; McClay SANDBOX MODELING OF THE retrowedge side of S, the velocity is zero. These et al., 2004). Our study utilizes the simplest NORTHEASTERN CARIBBEAN boundary conditions give rise to an asymmetry possible model confi guration—we deformed an of the bivergent wedge—the prowedge becomes unstratifi ed sand layer without a substrate of The aforementioned observational data from signifi cantly wider than the retrowedge. This plastic material and without a servo-controlled the northeastern Caribbean, and from other asymmetry is also observed in sandbox models driver. In map-view, our model is approxi- island arcs, demonstrate that thrusting occurs on (e.g., Fig. 1B). Modeling of fl ow trajectories in mately six orders of magnitude smaller than both sides of some island arcs, that the vergence the wedge indicates that material fi rst moves to- the northeastern Caribbean. However, the of backarc thrusting is, overall, opposite to that ward S and then up into the axial uplift zone and thickness of the sand in our model is 2–3 cm, of forearc thrusting, and that backarc thrust belts the retrowedge. In real continental collisional and if scaled up by six orders of magnitudes, tend to contain only frontal thrusts, even where orogens, this motion contributes to transporta- will represent the entire crust, not just the sedi- relative plate motions across the volcanic arc are tion of metamorphic rocks, formed at depth, mentary layer, which is rheologically inappro- highly oblique. Seismic and geodetic data from up to Earth’s surface (e.g., Platt, 1986). Beau- priate. Thus, we do not intend for our models the Panama arc also indicate a rigid arc behavior. mont and Quinlan (1994, their p. 760) com- to be viewed as “scale models” in the sense that To gain insight into the factors that may produce pared sandbox model results (e.g., Malavielle, the sand can be considered to be a rheologi- such characteristics, we produced simple sand- 1984) to the results of their numerical model cal proxy for the crust. Rather, we consider our box kinematic models designed to simulate rela- and stated that it “shows the same deformation models to be simply kinematic representations, tive plate motions at a convergent plate boundary style.” Thus, analog models, which employ sand the sole purpose of which is to provide visual with and without a relatively rigid volcanic arc. as a laboratory-scale proxy for the crust, can map-view insight into the relative movements Next, we provide background information on provide insight into the kinematics and geometry among different components of the modeled analog studies of thrust wedges, and then we de- of thrust-wedge development. system, the overall distribution of faults, and scribe the setup of our models and several geo- Most subduction zones are associated with the overall slip direction on faults. The simplic- logic situations simulated by our models. oblique convergence, and thus study of their ity of our model design made it possible to set kinematics requires analysis of transpression in up experimental runs very quickly, and it al- Background: Previous Modeling of three dimensions (3-D). Oblique convergence lowed us to examine the consequences of many Bivergent Thrusting in orogens may be accommodated by strain different kinematic confi gurations. Despite its partitioning, in which the regional motion di- simplicity, our apparatus yields similar results, Sand and numerical models, which assign vides into a component of subperpendicular geometrically, to those produced using a so- a noncohesive Coulomb yield criteria to the slip (thrust faulting) at the trench and a com- phisticated driving apparatus and rigorously rheol ogy of the deformed layer, have been used ponent of strike-slip faulting and/or block ro- scaled materials (cf. Pinet and Cobbold, 1992; for many years to describe the process of oro- tation in the forearc and arc (e.g., Beck, 1982; McClay et al., 2004). gen formation (e.g., Davis et al., 1983; Koyi, Pinet and Cobbold, 1992; Tikoff and Teyssier, Our sandbox apparatus consists of a wooden 1997; Malavieille, 1984; Willett et al., 1993). 1994; McCaffrey, 1994). Oblique convergence base (120 cm × 90 cm) surrounded by Plexiglas Such models assume that the crust, or part of has been addressed in computer models (Braun walls that are 25 cm high (Fig. 7A). At a dis- the crust, participates in the collision, while the and Beaumont, 1995; Vernant and Chéry, 2006) tance of 46 cm from one end of the box, we cut mantle lithosphere subducts (e.g., Beaumont and and sandbox models (e.g., Pinet and Cobbold, a 60-cm-long slit through the fl oor of the box. Quinlan, 1994). In these models, the orogen is a 1992; McClay et al., 2004). Although some of This “exit slit,” which represents the kinematic thickened ridge, or “thrust wedge,” that forms by the sand in the bivergent wedge produced in singularity of Willett et al. (1993), is parallel to faulting and folding of material that lies above their model moves toward the retrowedge, the the ends of the box. For ease of comparison with a nondeforming substrate. Critical-taper theory faults across the entire deformed region (includ- features of the northeast Caribbean region, we shows that the wedge’s geometry refl ects both ing the retrowedge) verge to the prowedge side will henceforth refer to the end closer to the exit the strength of the material in the wedge and (McClay et al., 2004; Fig. 1B). With increasing slit as the “south end” of the box and the other the strength of the detachment at the base of the obliquity of convergence, strike-slip faults form end as the “north end” (Fig. 7A). We placed a wedge (Davis et al., 1983). Willett et al. (1993) at the center of the sand wedge, and the wedge 3-cm-wide by 65-cm-long Mylar fl ap over the emphasized that wedge shape also depends on becomes more symmetric. exit slit and taped it on the south side to keep isostasy, erosion rates, and the possible presence sand from spilling through the slit. Next, we of an underlying ductile root. Model Setup and Strategy ran a Mylar transport sheet along the fl oor of Willett et al. (1993) and Beaumont et al. the box, beneath the fl ap, and through the exit (1996) produced two-dimensional (2-D) numeri- Sandbox models can be built with varying slit. We buried the transport sheet with a layer cal models that simulated the formation of biver- levels of sophistication (e.g., Malavieille, 1984; of sieved sand (18 mesh) smoothed to a uniform gent wedges in cross section. In such models, Marques and Cobbold, 2002; McClay et al., thickness (2 cm to 3 cm thick). In order to make

1528 Geological Society of America Bulletin, November/December 2009 Downloaded from gsabulletin.gsapubs.org on February 8, 2010 Bivergent thrust wedges surrounding oceanic island arcs

Transport sheet displaced a rigid block toward the foreland to original exit slit. This phenomenon does not generate a monovergent thrust wedge. Next, we happen on the retrowedge side because a sig- describe our experimental runs. nifi cant strike-slip component of displacement Exit slit North does not transfer across the slit. At a very low Case 1: Frontal Wedge, Without a angle of convergence, the sand wedge develops Rigid Block into a positive fl ower structure (Fig. A1 [see footnote 1]), as was also noted by Richard and sand layer To simulate a frontal wedge, we pulled the Cobbold (1990) and McClay et al. (2004). transport sheet slowly to the south, through the Frontal Oblique South displacement exit slit. The nonmoving sand forms an effec- Case 3: Frontal Displacement, With a displacement tive backstop that prevents the sand on the trans- Rigid Block port sheet from moving past the slit, because as the wedge thickens, gravitational loading To simulate the existence of a rigid island arc, Bivergent wedge Mylar North South strengthens the sand in the wedge as required by we placed a sand-fi lled cardboard box on top roll Prowedgeg Retrowedge Byerlee’s law of friction. Thus, as the wedge of of the Mylar fl ap above the subduction slit and Table Exit slit thickened sand gets wider, the effective position surrounded the box with sand. Upon pulling the Transport sheet of the backstop moves to the foreland (Fig. 8A; transport sheet through the slit, the prowedge Figure 7. Schematic diagram of the simple Fig. A11). As observed by previous researchers develops in front of the rigid block, with a sandbox modeling apparatus. Top: Oblique (e.g., Malavieille, 1984; McClay et al., 2004; shape and taper identical to that which develops view, showing the position of the exit slit, Storti et al., 2001), the resulting bivergent wedge without the block (Fig. 8B). When the block is and the confi guration of Mylar transport is asymmetric. Continued convergence builds a present, the forearc face of the block serves the sheets for simulating frontal convergence succession of slit-parallel thrusts on the north role of a backstop. Notably, in contrast to runs and oblique convergence. Bottom: Cross- side of the slit, resulting in a prowedge that with no backstop (case 1), progressive defor- section view, showing the position of the tapers gently to the north. On the south slide of mation with the backstop present leads to the prowedge and retrowedge relative to the exit the slit, a single obvious thrust trace develops, development of a wider retrowedge consisting slit. “North” and “south” are used for ease and the retrowedge is relatively narrow. This of distinct imbricate thrust slices (cf. Figs. 8B of reference. trace bounds a steep escarpment that undergoes and Fig. 8A). This happens because the backarc constant slumping during the experiment. face of the block also acts as a rigid backstop, which pushes toward the backarc. Neverthe- Case 2: Oblique Wedge, Without a less, the retrowedge remains narrower than the Rigid Block prowedge. thrust traces stand out better during experimen- tal runs, we moistened the sand very slightly to If the convergence vector trends at N35°E Case 4: Oblique Displacement, With a produce a slight amount of cohesion. With the relative to the slit, displacement of the Mylar Rigid Block edge of a ruler, we made spaced linear inden- transport sheet results in transpression with a tations on the surface of the sand, providing a sinistral strike-slip component (Figs. 8C and The setup for this experiment was identical visible grid that allowed us to track the amount 8E). Signifi cantly, the initial bivergent wedge to that of case 3, except that the transport sheet of shortening in the direction perpendicular to that develops trends parallel to the exit slit, as was moved obliquely, at a moderate angle (30°) the slit and the amount of strike-slip parallel was observed by McClay et al. (2004). On the to the subduction slit. In this case, a compo- to the exit slit. north side of the slit, left-lateral shear develops nent of left-lateral shear affects the prowedge, To generate deformation of the sand layer, penetratively across the wedge, as indicated by and sand is transported to the west edge of the we slowly pulled the sheet southward, through distortion of the surface grid lines. As a con- block (Figs. 8D and 8F). As in case 2, succes- the exit slit from beneath the box. For perpen- sequence, sand moves progressively toward sive thrust sheets are wider at the west end of the dicular convergence models, we used a 200-cm- the west end of the slit. The excess volume of prowedge than at the east end. As the prowedge long transport sheet. For oblique-convergence sand causes the thrust-wedge width to increase grows, the strike of the leading thrust rotates models , we used a narrower Mylar transport along the west edge of the slit, relative to the clockwise until it becomes perpendicular to sheet (30 cm wide) and oriented it at an angle to east edge, so the leading edge of the prowedge the transport direction. In the backarc region, a the exit slit before passing it through the slit; the rotates progressively clockwise with the growth retrowedge containing imbricate thrusts grows. narrower width was necessary to fi t the width of of each successive thrust until the leading edge Transport direction in the retrowedge is gener- the slit. In a number of experiments, we simu- of the wedge becomes perpendicular to the ally almost perpendicular to the face of the rigid lated the consequences of having a relatively convergence direction (Fig. 8C; Fig. A1 [see block. In runs where the block rotates counter- rigid crustal block within the bivergent wedge. footnote 1]). At this point, further growth of clockwise slightly (presumably due to the push Our blocks, which symbolized Puerto Rico and the prowedge ignores the orientation of the of excess sand on the western end of the eastern Hispaniola, consisted of 16 × 6 × 5 cm prowedge), the retrowedge is wider to the west and 17 × 8 × 5 cm sand-fi lled cardboard boxes. (Figs. 8D and 8F). There is no shear zone in The boxes were placed on top of the fl ap cover- 1GSA Data Repository item 2009115, Additional the retrowedge. This observation suggests that ing the exit slit and were surrounded by the sand sandbox models of oblique convergence show the the strike-slip component of deformation in this development of sedimentary wedges above a subduc- layer. Finally, to investigate the consequences tion zone with time and with changing convergence run was taken up entirely in prowedge deforma- of differential displacement of the backstop and angles, is available at http://www.geosociety.org/pubs/ tion. Retrowedge deformation was due solely to of interaction with obstacles in the foreland, we ft2009.htm or by request to [email protected]. the push of the rigid block toward the backarc.

Geological Society of America Bulletin, November/December 2009 1529 Downloaded from gsabulletin.gsapubs.org on February 8, 2010 ten Brink et al.

A B Puerto Rico), above the Mylar fl ap over the sub- Forearc prowedge duction slit, and we placed a broad rigid block (representing the Bahamas Platform) on the transport sheet. When the transport sheet was Forearc prowedge pulled, the “Bahamas block” collided with the western arc block (Figs. 9A and 9B). Rigid block In cases where the transport sheet moves Backarc retrowedge perpendicular to the subduction slit (i.e., frontal Backarc retrowedge convergence), the collision leads to a distinct along-strike change in the character of the retro- wedge. Specifi cally, the retrowedge is wider and thicker on the south side of the arc block C impacted by the “Bahamas block” than on the North Convergence south side of the block that is not involved in D North Late leading direction Convergence the collision. Notably, the southernmost thrust edge direction of the retrowedge grows eastward in front of of mylar the block that is not involved in the collision. Edge Prowedge In experimental runs that utilized an obliquely Edge of mylar converging wedge, the collision resulted in Early leading rotation of the western block, and this caused edge even more widening of the retrowedge behind the rotating block. Prowedge Rigid arc Rigid arc Case 6: Presence of an Obstacle in Front of the Retrowedge Retrowedge 2 cm Retrowedge 2 cm The Beata Ridge of the northeastern Carib- E F bean region acts as a basement high in the foreland of the retrowedge. We simulated the Forearc Backarc Convergence Forearc Backarc impingement of the Muertos thrust system with Convergence Prowedge Retrowedge direction direction Prowedge Retrowedge the Beata Ridge by placing a rigid block in front

Ed of a thrust wedge produced by pushing another ge of my rigid block to the foreland through a sand layer. When the thrust wedge reaches the obstacle, lar preexisting thrust traces are oroclinally bent, and Rigid new thrusts initiate with a distinct curve toward block the point of collision (Fig. 9C). The result is a syntaxial curve in the thrust wedge (cf. Paulsen and Marshak, 1999). 2 cm 2 cm DISCUSSION Figure 8. Comparison between bivergent wedge deformation in models with and without a rigid block. (A) Map view of a frontal bivergent wedge consisting of sand only. Arrow indi- What causes the growth of thrust belts on cates convergence direction. The retrowedge does not contain visible imbricates. (B) Map both sides of an island arc, such as occurs in view of a bivergent wedge formed in the presence of a rigid block (yellow rectangle). Distinct the northeastern Caribbean? Next, we fi rst imbricates have developed in the retrowedge. (C) Map view of a bivergent wedge forming review previous hypotheses used to explain during oblique convergence. The retrowedge does not contain visible imbricates. (D) Map island-arc thrusting geometries, and then we use view of a bivergent wedge during oblique convergence in the presence of a rigid block. our observations and our modeling results to Because of mass accumulation at the west end of the block, there is a slight rotation of the evaluate these hypotheses and suggest our own block. Distinct imbricates have formed on the retrowedge side, and the width of the retro- interpretation. wedge decreases slightly to the east. E and F are same as C and D but in oblique view. Previous Explanations for Bivergent Thrusting along Island Arcs Case 5: Colliding Rigid Blocks paniola) into a collisional boundary in the west (Frontal and Oblique) (from central Hispaniola westward). The colli- Several mechanisms have been proposed sional boundary results from the impingement as explanations for bivergent thrusting in is- The northeastern Caribbean plate bound- of the Greater Antilles arc against the Bahamas land arcs. We describe some of these here. ary changes from being an oblique subduction Platform. To simulate this situation, we placed Of these, two attribute backarc thrusting to zone in the east (from the Virgin Islands, across two narrow rigid blocks, representing compo- phenomena happening in the asthenospheric Puerto Rico, and across the front of eastern His- nents of the Greater Antilles arc (Hispaniola and mantle, and three attribute backarc thrusting

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Figure 9. Collision of a rigid block with the cal models suggesting that a retrowedge only prowedge. (A) In this model, block H and develops when a buoyant slab moves into the Prowedge (accretionary block P represent rigid blocks within the subduction system. Colliding prism) bivergent wedge. Collision of a rigid block rigid block (block B) with block H causes block H to Inferences from Observations of the move toward the backarc relative to block Northeastern Caribbean P. As a result, the retrowedge is wider along B block H than it is along block P. (B) An Byrne et al. (1985) suggested that backarc oblique view of the image in A, emphasizing thrusting in the northeastern Caribbean is due the along-strike widening of the retrowedge to active northward subduction of the Caribbean H P due to the partial collision in the prowedge. plate beneath the eastern Greater Antilles. Their Rigid block Rigid block (C) In a sandbox model, the Beata Ridge can proposal was based on the location of an Ms be represented as an obstacle or an indenter 6.7 earthquake at a depth of 32 km on a gently that lies in the foreland of the Muertos northward-dipping fault south of the Dominican thrust belt. As a result, thrust traces of the North Republic (star in Fig. 3), on the location of two belt curve into the edge of the ridge. deeper events associated with similar or more Retrowedge steeply dipping fault planes, and on the observa- 2 cm (backarc thrust belt) tion that a continuous refl ector extends from the A trough for ~40 km under the Muertos thrust belt (Ladd and Watkins, 1979). The interpretation of Forearc Backarc narrow plied to explain backarc thrusting in northern the Muertos thrust belt as a subduction zone, and New Guinea (Cooper and Taylor, 1988). thus the Muertos Trough as a trench, has been (2) Basal Traction: Conrad et al. (2004) sug- explicitly or implicitly accepted in the literature gested that trench-directed asthenospheric fl ow during the past 20 years in papers dealing with the wide develops at the base of the overriding plate northeastern Caribbean (Dillon et al., 1996; Dolan in response to symmetrical suction from the et al., 1998; Driscoll and Diebold, 1998; Manaker downgoing slab in cases where slab-pull forces et al., 2008; Mann et al., 2002; McCann, 2007; are only weakly transmitted to the subducting van Gestel et al., 1998). We argue that this inter- plate at the surface. Backarc thrusting is caused pretation is problematic for two reasons. B in this framework by traction applied to the First, since true subduction is driven by base of the overriding lithosphere by fl owing a slab’s negative buoyancy within the upper Eastern asthenosphere. mantle, there must be suffi cient room in the Hispaniola (3) Magmatic Infl ation: Hamilton (1979) ar- mantle for a plate to sink before the process gued that intrusion of plutons “infl ates” volcanic can begin. The distance between the Puerto arcs, causing displacement of both arc margins Rico and Hispaniola Trenches on the north side sideways, by thrusting on both sides of the arc. of the Eastern Greater Antilles island arc and (4) Gravitational Spreading: Silver et al. the Muertos Trough on the south side is only Muertos thrust belt (1983) and Breen et al. (1989) suggested that 235 km (Fig. 3). Considering the age of the thickening of the crust in the Java-Timor arc subducting North American plate (85–125 Ma; eventually led to gravitational spreading (i.e., Mueller et al., 2008) and that of the subducting extensional collapse), which produced move- Caribbean plate (100–120 Ma; Mueller et al., ment of the sides of the arc outward. According 2008), both slabs should be 90–100 km thick. to this concept, the movement would be accom- Further, seismic observations (Dolan and Wald,

Beata Ridge “Beata Recess” modated by slip on thrust faults that develop at 1998; Ladd and Watkins, 1979; ten Brink, 2005) the base of the slope, where compressive stress show that the North American and Caribbean Caribbean Sea is presumed to be greatest (Dalmayrac and plates dip at 10° to 20° within 30 km inland of C Molnar, 1981). the Puerto Rico Trench and Muertos Trough, (5) Collision with Buoyant Crust: Hamilton respectively. Thus, there is simply too little (1979) and Silver et al. (1983) suggested that room for true simultaneous subduction of two to phenomena happening within the crust and/ a backarc thrust belt develops when buoyant plates with opposite polarities under the Eastern or lithospheric mantle. crust arrives at a subduction zone, as is hap- Greater Antilles arc. Recognizing this problem, (1) Subduction Reversal: Dewey and Bird pening at the Australia-Timor arc boundary. Dillon et al. (1996), Dolan et al. (1998), Mann (1970), Pinet and Cobbold (1992), and Pubellier Geist et al. (1993) explored this concept by us- et al. (2002), and van Gestel et al. (1998) en- and Cobbold (1996) suggested that subduction ing a thin viscous-sheet model to predict the visioned direct contact between the downgoing reversal occurs when a buoyant crustal block stress fi eld within island-arc lithosphere result- North American and Caribbean slabs at sub- collides with a continent after an intervening ing from impingement of a buoyant oblique crustal depth beneath the island arc. However, oceanic basin has been consumed. In this view, ridge. They found that the collision produced it is not clear why Caribbean subduction is sus- the original downgoing slab breaks off, and the transpression in the forearc and predominantly tained if such contact exists. original overriding plate bends down and begins arc-perpendicular compression in the backarc. Second, the kinematics of deformation within to sink into the mantle. This model has been ap- Beaumont et al. (1996) presented numeri- the Muertos thrust belt are not compatible with

Geological Society of America Bulletin, November/December 2009 1531 Downloaded from gsabulletin.gsapubs.org on February 8, 2010 ten Brink et al. true subduction along the boundary. Specifi cally, compressional stress at the base of the thrust belt Muertos Trough are thicker south of Puerto Rico if the Caribbean plate interior were subducting (Silver et al., 1983; Breen et al., 1989). We doubt, than south of the Dominican Republic (Fig. 5). under the Eastern Greater Antilles arc, the con- however, that it serves as the primary driver for Oblique convergence may result in rotation of an vergence direction would be highly oblique to the development of bivergent thrust belts, for if it arc block, leading to additional widening of the the strike of the Muertos Trough, based on GPS did, retrowedges would form behind all arcs and backarc thrust wedge. Changes in width and ori- measurements (Manaker et al., 2008; Mann et al., along their entire lengths, and they do not. entation of the retrowedge along strike are seen 2005). Such oblique convergence would have to in the Wetar, Alor, North Panama, and Muertos be accommodated either by oblique thrusting Inferences from Sandbox Models backarcs. In the Muertos thrust belt, the orienta- within the Muertos belt, or by partitioning be- tion of the partially buried thrust system in the tween frontal thrusting and strike-slip faulting in Our simple sandbox analog models demon- Middle Slope Province may indicate clockwise the Muertos belt and adjacent onshore portions strate the following key points about the devel- rotation since this portion of the thrust belt was of the eastern Greater Antilles. As described opment of thrust wedges involving island arcs: active (purple line in Fig. 4). earlier in this paper, the salients of the Muertos Pulling a sand-laden Mylar sheet through an Collision of the retrowedge with an obstacle deformation front indicate that the belt is a zone exit slit in a sandbox to simulate “subduction” to its foreland may generate map-view curvature of dip-slip thrusting, and that there are no active leads to the development of a bivergent wedge. in the thrust wedge. An example is the northward zones of strike-slip faulting within the Muertos If the deformation involves only sand, the retro- curvature of the western end of Muertos Trough, thrust belt or along the south shore of the arc. wedge is narrow. The addition of a rigid block where the Beata Ridge serves as an obstacle to Finally, the physical conditions and the work above the subduction slit results in a widening of the southward motion of Hispaniola (Fig. 10). necessary for subduction initiation are still not the retrowedge, and the development of upward well constrained, and therefore, even if the sub- imbricate thrusts, with vergence opposite to that Implications of Retrowedge Formation for ducted lithosphere of the North American plate of the subduction direction, in the retrowedge. Arc Rigidity were to break off, it is not clear that loading by This observation implies that signifi cant growth the Muertos thrust belt may succeed in pushing of retrowedges occurs only when the strength of Simple sandbox models simulating the kine- the plate down into the asthenosphere, thus trig- an island arc is suffi cient to transmit stress to matics of a bivergent thrust wedge mimic the gering the process. the backarc region. Application of this concept development of a retrowedge along oceanic is- The tectonic history of the Eastern Greater to the northeastern Caribbean suggests that the land arcs most successfully when a rigid block Antilles arc is also incompatible with the pro- inactive Eastern Greater Antilles arc is stronger is placed over the subduction slit, thus implying posal that magmatic infl ation drives the de- than the sediment being incorporated in its mar- that the existence of a backarc thrust belt may velopment of bivergent thrust belts. Volcanic ginal thrust wedges, so subduction of the North be evidence that the adjacent arc is, indeed, activity has not happened in Puerto Rico and American plate produces compressive stress relatively rigid. Observations of a rigid arc in Hispaniola during the past 30 Ma or more, a pe- that is transmitted through the arc to drive the Panama support this association (Trenkamp riod during which thrusting has continued in the development of thrusts in the Muertos thrust et al., 2002). Growth of the prowedge appears to retrowedge. belt of the backarc (Fig. 10). apply horizontal compression to the block, driv- In sandbox models in which a rigid block lies ing it into the sediment of the backarc, causing Inferences from Observations of Other over the exit slit, the retrowedge consists domi- the backarc side of the block to act effectively as Island Arcs nantly of frontal thrusts and does not accommo- a backstop. Unlike orogenic belt models (e.g., date a strike-slip component of strain. Rather, Willett et al., 1993), material is not translated Observations from other backarc thrust belts strike-slip deformation is concentrated in the from the forearc into the arc and backarc region lead us to conclude that shear tractions beneath forearc. We found a similar distribution of fault- and is not exhumed from deep within the crust. the overlying plate, as suggested by Conrad ing in the island-arc systems that we examined. The rigidity of oceanic island arcs may be et al. (2004), are not responsible for the ob- The observation implies that the backarc thrust attributable to their dominantly mafi c com- served retrowedges. First, if shear traction oc- belt is driven by the displacement of the arc position—oceanic island arcs are composed curred due to subduction, retrowedges should block toward the backarc. primarily of oceanic crust and mafi c and ultra- occur along the entire length of the arc. They The collision of a buoyant block with one part mafi c volcanics and intrusives (Shillington do not. For example, only the 400-km-long of a rigid arc pushes that portion farther toward et al., 2004). Not all island arcs behave rigidly. Vanuatu segment of the convergent boundary in the backarc region. As a result, the retrowedge is For example, the Japan arc contains many ac- the New Hebrides subduction zone includes a wider and thicker behind the colliding portion of tive thrust faults whose vergence is often similar retrowedge (Lagabrielle et al., 2003), and in the the arc than behind the part where only subduc- to the subduction zone, indicating that the arc Banda arc, a 300-km-long gap exists between tion occurs (Figs. 9A and 9B). This model, there- is deforming internally (Okamura et al., 2007). the Flores and Alor-Wetar retrowedges (Silver fore, suggests that the development of a wider The weakness of the crust there may be due et al., 1983). Second, if the formation of the retrowedge south of Hispaniola than south of to the occurrence of intermediate and felsic retrowedge were a consequence of shear trac- Puerto Rico refl ects the collision of the buoyant crust in the arc, because the arc originated as a tions related to subduction, then the kinematics Bahamas Platform with Hispaniola (Fig. 10). continental-crustal sliver. of deformation in the wedge should refl ect the Previous models suggesting that greater sedi- obliquity of the convergence. It does not. For ment thickness leads to widening of a thrust Role of Colliding Buoyant Crust in example, in the Muertos thrust belt, there is no wedge (e.g., Marshak, 2004) cannot explain the Retrowedge Development strike-slip deformation despite the oblique di- difference in the width of the retrowedge along rection of plate convergence. the Muertos Trough—the retrowedge south of The mechanical conditions for the develop- Our work does not rule out the possibility that the Dominican Republic is wider and thicker ment of a bivergent wedge in island arcs are not gravitational spreading could locally add to the than south of Puerto Rico, yet sediments in the entirely clear. Beaumont et al. (1996) suggested

1532 Geological Society of America Bulletin, November/December 2009 Downloaded from gsabulletin.gsapubs.org on February 8, 2010 Bivergent thrust wedges surrounding oceanic island arcs

N S Puerto Rico Trench Muertos thrust system (Retrowedge) Forewedge Puerto Rico arc Muertos Trough

North American plate S Caribbean plate

? A 80 km

N Hispaniola Trench Muertos thrust system (Retrowedge) S Eastern Forewedge Bahamas Hispaniola arc Muertos Trough

S Caribbean plate North American plate

? B V.E. ~ 2:1 80 km

B A C Bahamas Platform NORTH AMERICAN PLATE

Puerto Rico Trench

SFZ Cordillera CentralHISPA Gonave NIOLA microplate rigid V.I. EFZ arc PUERTO RICO Muertos thrust belt Muertos Trough

CARIBBEAN PLATE Beata Ridge

16°N72°W 18°N70°W 20°N 68°W 66°W 64°W Figure 10. Conceptual tectonic cross sections across the North American–Caribbean plate boundary in (A) Puerto Rico and (B) Hispaniola. Location of cross sections and major elements are shown in map view. (C) Minor intra-arc deformation is observed in the area enclosed by dashed grey lines. VI—Virgin Islands, V.E.—vertical exaggeration.

Geological Society of America Bulletin, November/December 2009 1533 Downloaded from gsabulletin.gsapubs.org on February 8, 2010 ten Brink et al. that bivergent wedges in continental orogens and the 1992 M 7.9 Flores earthquakes. These in the absence of arc collision with a buoyant only begin to develop when relative buoyant earthquakes occur at crustal depths beneath the block, observed backarc thrusting around the crust moves into the subduction zone and is retrowedge, and, depending on the width of the globe may be initiated by the impact of a buoy- thrust beneath the edge of the overriding plate. deforming retrowedge, they can be destructive. ant subducting lithosphere with a prowedge. The relatively buoyant crust can be a continen- They can also generate devastating tsunamis, as Notably however, the affected region can ex- tal margin, an aseismic ridge, or an oceanic happened during the Flores 1992, Vanuatu 1999, tend, in the direction parallel to the arc axis, plateau. Some bivergent island arc systems and the San Blas (Panama) 1882 earthquakes. beyond the region of impact, perhaps because have developed adjacent to the subduction of South of Puerto Rico, the belt of active thrust compressive stresses are also distributed later- an aseismic ridge (d’Entrecasteaux Ridge near faults, including occasional buried thrusts ally within the rigid arc. Vanuatu; Fig. 2C), a thick plateau (the Bahamas within the Muertos Trough, is only ~20 km In the case of the northeastern Caribbean, we Platform next to Hispaniola; Fig. 3), or a conti- wide. The narrow retrowedge there suggests consider the Muertos thrust belt south of Puerto nental margin (the Wetar thrust near the Timor either a small amount of compression has been Rico and eastern Hispaniola to be a retrowedge, Trench; Fig. 2A); hence, increased buoyancy of transmitted across the island arc, or that com- formed by cross-arc transmission of stress gen- the subducting lithosphere must play a role in the pression started only recently. The former expla- erated by the subduction of the North American development of bivergent wedges in island arcs nation is in agreement with previous modeling plate, and by the collision of the Bahamas Plat- (Silver et al., 1983). However, some active biver- inferences for low degree of coupling across the form with Hispaniola. In detail, the geometry of gent wedges have developed next to subduction subduction interface of the Puerto Rico Trench this wedge refl ects differential block movement zones where the seafl oor is smooth and deep. (ten Brink and Lin, 2004). Contrary to a recent of the Hispaniola arc relative to the Puerto Rico Examples include the bivergent wedge of Puerto assessment (McCann, 2007), the narrow width arc and the impingement of the Beata Ridge Rico, where the Puerto Rico Trench is unusually of the retrowedge limits the downdip width of against the Hispaniola arc. deep (Fig. 3; ten Brink, 2005), the Flores backarc the potential rupture area and, hence, limits the ACKNOWLEDGMENTS thrust, which forms along a portion of the island magnitude of potential earthquakes. There is arc where a smooth abyssal plain is subducting indeed no historical record of large earthquakes We thank Bob Barton for building the sandbox and into the Java Trench (Fig. 2A), and the Northern south of Puerto Rico. The zone of active thrust- Kathy Scanlon for arranging the collaboration. J.L. Panama deformed belt east of 80°W (Fig. 2B). ing widens signifi cantly south of Hispaniola. Granja Bruña was supported by a Ph.D. grant from the Spanish Department of Education and Science. One explanation for these observations is that Thus, it is more likely that large earthquakes Constructive reviews by Jason Chaytor, Eric Geist, Eli oceanic island compressive stresses are distrib- could happen there because of the wide down- Silver, Peter Cobbold, and Associate Editor An Yin are uted laterally within the rigid arc and therefore dip extent of the detachment fault (Fig. 10). gratefully acknowledged. are transmitted to the backarc region over a much If the entire 170-km-long backarc thrust belt REFERENCES CITED wider region than the region being impacted di- south of the Dominican Republic ruptured, it rectly by a buoyant subducting lithosphere. 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