Lithos 113 (2009) 318–330

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Structural expression of forearc crust uplift due to subducting asperity

Jean-Marc Fleury a, Manuel Pubellier b,⁎, Marc de Urreiztieta c

a Total E&P-Projects Nouveaux, 2 Place de la Coupole, 92078 Paris-La Défense Cedex, Franceb Laboratoire de Géologie, Ecole Normale Supérieure, C.N.R.S. UMR 8538, 24 rue Lhomond, Paris 75231, Francec Petronas Carigali SDN, BHD, Petronas Twin Towers, 50088 Kuala Lumpur, Malaysia

⁎ Corresponding author.E-mail address: manu_pub@geologie.ens.fr (M. Pube

0024-4937/$ – see front matter © 2009 Elsevier B.V. Adoi:10.1016/j.lithos.2009.07.007

a b s t r a c t

a r t i c l e i n f o

Article history:Received 1 August 2008Accepted 9 July 2009Available online 18 July 2009

Keywords:Subduction blockingSubduction jumpAsperityCrustal upliftErosion

New structural observations and mapping of reefal terraces, carried out both on the field and on satelliteremote sensing data, indicate that Sumba Island is presently undergoing a large amount of extension,associated with a significant regional uplift. This crustal uplift may have been created by a major thrustemerging in the South of the island. The uplift, which partly accommodated the Australian plate–SouthWest Banda Arc convergence, is associated with the general northeastward tilt of the island. Theconsequent anomalous positive topography along the southern coast of the island is being compensatedby significant tectonic erosion along large-scale curvilinear normal faults in the southeastern half of theisland. The most important expression of this gravitational collapse is located at the receding side of anadvancing circular dome, showing striking similarities with accretionary wedges being affected by seamountsubduction.The part of the forearc basin known as the Savu Basin is moderately deformed (mostly in its central part) andappears to act as a rigid buttress in the convergence between the Banda Arc and the Australian plate. As aresult the convergence appears to be transferred northward within the actively-shortening back-arc domain,which goes from the north of the Flores Island to the southwest of the Timor block. The convergent plateboundary shows a transition between a stable domain (West of Sumba) and a tectonized domain (East ofSumba), the latter coinciding with the subduction of the outer Australian passive margin. The subduction ofthe ocean–continent boundary of the Austral-Indian plate below the Banda arc since the Lower Pliocene mayhave incorporated some crustal fragments in the plunging Benioff zone. Most likely, the integration of thestretched continental lithosphere in the subduction zone caused the uplift the entire forearc domain,exhibiting inherited structures of the upper plate.

© 2009 Elsevier B.V. All rights reserved.

1. Introduction

The structure and morphology of subduction zones are condi-tioned by various parameters such as plate convergence rate, dip ofthe subducted slab, interplate coupling, and sediments thickness(Jarrard, 1986; Jordan et al., 1983; Lallemand et al., 2005). In addition,morpho-structures carried by the downgoing plate together with itscrustal nature have proven to control the geometry and the evolutionof the external subduction zone. In contrast, it has not yet beendemonstrated that rheological lateral variations of the upper platehave a strong influence on the subduction geometry.

The eastern termination of the Sunda subduction zone is located infront of the ocean–continent boundary of the Australian Plate and is

llier).

ll rights reserved.

marked by a large bathymetric contrast (Fig. 1). In this region, theSunda Plate shows strong crustal heterogeneities inherited from theTertiary rifting/extensional phases (Rangin et al., 1990, Honthaas et al.,1998). East of 120°E, the subduction zone is outlined by a jog of thetrench and some changes in the structural style, including the unusualpresence of a medium-sized island (Sumba Island) in the center of theforearc basin (Fig. 1). Field observations conducted on the SumbaIsland, combinedwith seismic andmorphological data suggest that thedeformation of this transition zone is primarily marked by a crustaluplift, associatedwith tremendousmasswasting via low-angle normalfaults. Progressive doming of the reefal terraces, radial fractures anddrainage networks are similar to the morpho-structures described inparts of accretionary wedges affected by subduction of volcanicedifices (Cadet et al., 1987, Dominguez et al., 1998, 2000, Fig. 2). Bycontrast, the example of the ongoing subduction process of easternIndonesia leave the upper plate almost undeformed and possiblytriggered the subduction jump, causing this region to rapidly changethe earlier location of the plate boundary. This has been confirmed byrecent geodetic studies (Simmons et al., 1999; Bock et al., 2003).

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Fig. 2. Morphological expressions and deformations of an accretionary wedge createdby the presence of a seamount in the lower plate, as shown by a sandbox modellingexperiment (Dominguez et al., 1998).

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2. Control of deformation patterns at subduction zones; influenceof inherited structures in the deformation processes atsubduction zones

2.1. The lower plate

The impact of structures carried by the downgoing slab on thegeometry and evolution of subduction zones has been demonstratedin many areas of the globe at various scales. Subduction of small-scalefeatures (e.g. volcanic seamounts) affects mainly the structure of theaccretionary wedge (Dominguez et al., 1998; Ranero et al., 2003; vonHuene et al., 2004). In well-known examples such as the Japan Trench(Cadet et al., 1987) or more recently the Sunda Trench in front of theJava Island (Masson et al., 1990; Kopp and Kukowski, 2003), it hasbeen shown that subducted seamounts are responsible for a sharp re-entrant of the trench, a bulge of the sea bottom right above the apex ofthe asperity, and a destabilization of the outer slope at its recedingside (Fig. 2). The evolution of such subduction has been modeled insand-box experiments (Dominguez et al., 2000).

Fig. 1. The lower plate of the subduction system is a composite of (1) an old and dense oceanito the East (i.e. NW Shelf). South of Java the seismicity outlines the present-day plate boundgeological map of Sumba Island showing the extensive coverage of Neogene sediments, and

Although intermediate and shallow seismicity occurs in areas likethe Central America Trench in front of Costa Rica, (Ranero et al., 2003;von Huene et al., 2000), so far no deformation of the crust associatedwith seamount subduction has been reported. In addition, large scalefeatures such as oceanic plateaus or ridges can create shortening inforearc sediments, e.g. the subduction of the Snellius Plateau in frontof the Molucca Sea (Silver et al., 1985; Rangin et al., 1995; Pubellieret al., 1999), or trigger subduction jump or reversal, as observed infront of the Ontong Java Plateau (Mann et al., 1998).

At the longitude of the Sunda Trench termination, the lower plateis composed of, from E toW, the thick Australian continental crust, thethinned margin of the continent, and the Jurassic Argos abyssal plainfloored by oceanic crust (Fig. 1). As it enters the subduction system,the Australian platform undergoes extension characterized by largenormal faults that involve the entire sedimentary pile (e.g Tandonet al., 2000; Londoño and Lorenzo, 2004). The flexure of the lowerplate can be explained by the sharp angle of the old and dense oceanicslab (Lorenzo et al., 1998; Das, 2004) and/or the load of the entireforearc domain.

2.2. The upper plate — structure of the Banda Sea region

Several examples in the literature demonstrate the influence of thestructures carried by the lower plate to control the subduction processand associated structural framework. In the case of the Sunda sub-duction, to thewest of the study area between Sumatra and Java, it hasbeen proposed that the northward bending of the subduction trench isrelated to the change in the crustal nature of the upper plate in theSunda Strait (Kopp et al., 2006). The Banda Sea region is characterizedby a remarkable disparity in the nature and in the geodynamic settingof the upper plate, including continental extension, backarc spreading,and arc-continent collision within less than 10 Ma (Honthaas et al.,1998; Hinschberger et al., 2001). The majority of small continentalblocks presently trapped in the Banda Sea Area are composed of riftedcrust of Australian affinity (Silver et al., 1985). Some others are ofAsian affinity and are considered to be part of the easternmost Sunda(or Argos) continental block (Rangin et al., 1990; Harris, 2006). Thoseblocks are observed as far as the eastern part of Timor Island, in theform of allochthonous series located in a large pop-up structure(Audley-Charles, 2004; Harris et al., 1998; Charlton, 1989).

3. Field evidence for a strongly uplifted crustal block:Sumba Island

Sumba Island, although located in the midst of a forearc basin, doesnot exhibit the typical sedimentary package (i.e. turbidite-bearingseries) expected for a conventional accretionary wedge. Intense defor-mations are not observed either. A specific survey aimed at completingthe mapping with more structural observations, satellite imagery, newradiometric datings and offshore seismic data was conducted (Fig. 1).

3.1. Volcanic activity

The presence on Sumba Island of Late Cretaceous to EarlyOligocene volcanic and plutonic rocks gives evidence of its formerarc origin. It was near a subduction zone situated further north duringthe Cretaceous, probably in the vicinity of the SWarm of Sulawesi, andwhich was certainly located further south in the Oligocene (Abdullahet al., 2000; Soeria-Atmadja et al., 1998). Three volcanic centres havebeen identified. Radiometric datings performed on various sites arein full agreement with the age range of volcanic activity establishedby Abdullah et al., 2000. Further westward K–Ar datings indicate

c lithosphere to theWest (i.e. Indian Ocean) and (2) a stretched continental lithosphereary. To the East the seismic activity is mostly restricted to the back-arc domain. Bottom:localised basement outcrops along the southern coast.

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decreasing ages, ranging from 85–75 Ma for the westernmost centreto 43–31 Ma for the earliest volcanic centre (Fig. 3). Interestingly, theoldest volcanic activity recorded on Flores Island (26Ma; Hendaryono,1998), representing the present-day subduction-related arc, coincideswith the timing of the volcanic activity recorded in Sumba. Never-theless it remains that it has been demonstrated (e.g Soeria-Atmadjaet al., 1998, Honthaas et al., 1998) that the subduction shifted to thesouth following a brutal change in subduction geometry (probablydue to Australian plate entering the system) and therefore trapped theremnant volcanic arc into the forearc.

3.2. Stratigraphy

Dykes and basaltic vents intrude the thick marly/silty CretaceousLasipu Fm (Fig. 3). The Upper Eocene sandy/marly intervalsunconformably overlie the Cretaceous. Intense thrusting and foldingof the Cretaceous series mainly are locally observed. Almost noOligocene sediments were deposited. The only outcrop we founddisplays a sharp angular unconformity between the almost tabularcalcareous reefal Oligocene series and the underlying tilted beds.

On Sumba Island, the cessation of volcanism around 30Mamarkedthe beginning of the basin subsidencewhich culminated in theMiddleMiocene. The thick calcareous series of the Upper Miocene KananggarFormationwas deposited in the East. Continuous sections of this seriesof more than 300 m thick are observed along the southern coast, dueto the general N-NE tilting of the Island. Syn-sedimentary extensionalfeatures are observed in the entire section. To the West, the trans-gression is less dramatic and is represented by reefal clastic series ofthe Waikabubak Fm of Middle to Upper Miocene age according to thecollected samples. Litho-stratigraphic correlations indicate a clearMiocene paleogeography, characterized by platform deposits to thewest and basinal deposits to the east (Fig. 1). Active Quaternary reefalterraces are observed on the Northern coast of the Island, demon-strating a marked Quaternary uplift, the oldest one (1 Ma; Pirazzoliet al., 1993) reaching 600m of magnitude. To the east of the island, the

Fig. 3. Representation on a geological time scale of radiometric K/Ar dating of samples collecwith the beginning of basaltic flows on Flores. Migration of the volcanic centers, especially tbetween 30 and 15 Ma. It is worth to notice that rotations may have occurred since the Ear

reefal terraces fringe a 40 km-wide symmetric dome, along with aradial drainage network suggest a local, although significant, uplift(Deffontaines and Chorowics, 1991).

3.3. Structural patterns — field observations and remotesensing interpretation

Microstructural measurements performed on Miocene rocksindicate a purely extensional tectonic regime; (Fig. 4). A homogeneousradial distribution of horizontal σ3 and a vertical σ1 is inferred,suggesting an isotropic gravitational stress field with no preferentialextensional axis.

During the Miocene evolution of Sumba in the forearc domain,extension (or tilting) resulted in numerous syn-sedimentary exten-sional features such as centimetric to hectometric slumps and intra-formation fault scarps. During the Pleistocene, the sequences ofterraces reveal gradual process of uplift, as shown by topographicmeasurements (Fig. 5). The six observed main sequences could becorrelated to catastrophic seismic events that abruptly uplifted thewhole island which acted as a rigid block. Bard et al. (1996) tried todiscriminate the effect of sea levels changes on terrace formation fromthe tectonic process. It can be observed that the isostatic changes arefar below600m in 1Ma, the significant role of the tectonic componentin the successive development of the terraces can be concluded.

The morpho-structures observed on radar-derived DEM andsatellite images are comparable to those observed in the field. Theyconsist mainly of a series of large SW-dipping tilted blocks, boundedby south-facing fault scarps (Fig. 5). To the east, a large dome isassociated with gravitational collapse which created large ubiquitouslandslide scars. A decrease of the dip of the topographic envelope-surface of the terraces from the highest (and older) to the lowest (andyounger) ones is also observed (Fig. 6). Field evidence of tremendousmass wasting at all scales includes debris flows and rotational slides.At a larger scale, kilometric curvilinear traces of normal faults can beobserved (Fig. 5). However, these represent second-order extensional

ted on Flores and Sumba islands. Cessation of the volcanic activity on Sumba coincideshe northward shift, could be the result of a sudden change in the subduction geometryly Miocene.

Fig. 4. a) Microstructural measurements on Miocene outcrops (mainly) indicate a purely extensional tectonic regime. b) Homogeneous radial distribution of horizontal σ3 (i.e. blue dots on stereograms) is consistent with a vertical σ1,indicating a gravitational stress field. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

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Fig. 5. The extensive tectonic regime is evidenced by spectacular morphological features, as observed on topographic data: a) west of Sumba: the island as awhole is affected by large south-facing normal faults. b) East of Sumba: the Southerncoast is occupied by landslides, large fault scarps and a regional doming, while the northern coast is fringed by reefal terraces. c) To the North the Kananggar Fm is overlain by thin spectacular coral terraces. Hectometric steps in the topographymight be related to brutal seismic events that occurred during the last My.

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Fig. 6. SRTM topographic data on the northern coast of Sumba reveals awestward thinning of the terrace sequences, implying a hingeline between East andWest. A preferential upliftof the eastern part of the island could be related to the plateform-basin paleogeography or to the subduction of a crustal asperity in the lower plate (e.g. tilted block).

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features, resulting from the doming and general uplift of the island.Other indicators of uplift have been found all along the Banda forearc,especially in Timor (e.g Audley-Charles, 1986; De Smet et al., 1990;Merritts et al., 1998).

From the structural pattern and tectono-stratigraphic relationsobservations, it appears that the general uplift began before the LateMiocene, as outlined by Fortuin et al. (1994) and is still ongoing.

4. The Sumba–Savu block: a buttress for the subduction

Due to the lack of intense deformations and the absence of clearcompressional structures, coupled with the identification of aMiocene basin/platform paleogeography on Sumba Island, it ispossible to link the eastern part of the island with the adjacent Savubasin; both domains forming the SWmargin of the basin. This straightconnection is evidenced by the continuity of seismic reflectors on anE–W industrial regional seismic line, from the North of Sumba tothe easternmost part of the Savu basin (Fig. 7). In addition, it hasbeen shown that the Cenozoic series of Sumba and the Cenozoicallochthonous series of Timor Island are quite similar (Audley-Charles,1985; Harris et al., 1998; Milsom, 2000). This allows us to have a goodconfidence in the connection between Sumba Island and the SavuBasin and also to consider that the allochthonous series of Timorbelong to the same unit.

Due to its main characteristics, i.e. its exceptional width (200 km),its shallow depth (3500 m, compared to the adjacent basins: S.

Lombok, 4500 m and Flores, 5500 m) and its lack of internal defor-mation, the Savu Basin is not a classical forearc basin. The intactinternal structure of the Savu Basin suggests that it represents a rigidbuttress for the subduction of the Australian margin beneath theBanda forearc, allowing the transfer of the horizontal motion from thesouthern front to the back-arc area (Fig. 8). The rigidity of the basincould be related to the “oceanic” nature of its basement (Harris, 2006).

5. Discussion and conclusions

As stated bymany authors (Soeria-Atmadja et al., 1998; Keep et al.,2003) Sumba occupies an unusual position, i.e. within a forearcdomain where a deep basin should be expected. Despite the area islocated in a domain of rapid convergence, there is no indication ofcompressional deformation on the island. The strong and localizeduplift is characterised on land by a current active extensional regimepreferentially developed at the eastern edge of the island and is clearlydated with a minimum age of circa. 1 Ma, as recorded by the oldestand highest reefal terrace (Pirazzoli et al., 1993). Identical indicators ofsubstantial uplift have been found along the southern Banda forearcon the Savu and Timor islands (Audley-Charles, 1986; De Smet et al.,1990; Merritts et al., 1998). The observed uplift rates are of the sameorder of magnitude. The rapid building of the Timor Island around3.5–4 Ma is also followed by a rapid uplift, as outlined by deep marinedeposits becoming progressively shallower and sealing phases ofintense thrusting. The subsequent growth of exposed reefal terraces in

Fig. 7. The lateral continuity of the Kananggar Fm. from the East of Sumba to the easternmost part of the Savu Basin is evidenced by a) a N–S structural cross-section and b) the interpretation of a N–S seismic line East of Sumba island. Reflectorgeometry indicates a regional northward tilt similar to what has been observed on Sumba Island. Differentiation of tectono-stratigraphic units is based on field analogues.

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Fig.

7(con

tinu

ed).

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Fig. 8. Incorporation of the northern edge of the Australian rifted margin in the subduction zone 8 to 5 Ma ago caused (1) the deformation of the backstop and (2) the uplift of theentire forearc domain, isolating a Sumba–Savu–Timor rigid block that permits the transfer of all the convergence to the back-arc domain (subduction flip).

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Timor and the topography of Flores (Hendaryono, 1998), Wetar andadjacent islands also corroborate this uplift. Late uplifts are commonin mountain ranges. They generally result from the dislocation ofsubducted slabs at depth and are often characterized by a gap in theintermediate seismicity (Nur and Ben Avraham, 1981). In EasternIndonesia, notably at Wetar and Flores, an active slab rupture is verymuch supported by seismicity data (McCaffrey et al., 1985; Das et al.,2000; Sandiford, 2008). The large wavelength of the uplift iscompatible with a slab detachment and indicates that the Sumba–Savu block forms a single unit. The localized uplift of Sumba is notevidenced by seismicity but suggests instead that a major feature (e.g.old tilted block) in the lower plate has recently entered the subductionzone (Fig. 9). It is therefore difficult to discriminate the origin of the

Fig. 9. South-facing normal faults to the South and reefal terraces to the north indicate anlocalize a zone of high basal friction at the interface. This is believed to reveal the presence

uplift–subduction of a topographic high in the lower plate or slabbreak-off–especially in a system that has evolved from simplesubduction to perturbed subduction and finally collision.

5.1. Nature and rheology of the crust below Sumba

Keep et al. (2003) proposed that the Sumba uplift results from thesubduction of a small cordillera fringing the northern Australian plate,since 8 Ma. Shallow seismicity, especially seismic activity (Engdahlet al., 1998) near the plate boundary displays a clear limit at longitude120°E. A dense patch of seismic events is located in the vicinity of thesouthern edge of the Sumba Island. The catalogue of focal solutionsfrom Harvard (CMTcatalogue) gives a minimum of six plane solutions

asymmetric uplift of the Sumba Island. Seismic activity and focal mechanisms help toa positive topographic anomaly and help to understand the uplift mechanism.

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that can be attributed to the interplate coupling between thedowngoing slab or the northern edge of the Australian margin, andthe overriding plate. Since we believe the Sumba basement to be ofcontinental nature, we consider a deeper crustal root underneath theSumba Island, which in turn could increase the frictional strength atthe plate interface. The presence of a topographic high in the lowerplate below the Sumba Island, corresponding to a distal tilted block ofthe Australian rifted margin would have mechanically pushed theforearc domain upward, causing the uplift of the whole Sumba–Savublock.

Cessation of the seismic activity further east supports theobservation that no recent sediment is currently incorporated intothe accretionary wedge in the Timor Trough (Karig et al., 1987). This isconsistent with geodetic measurements on the southern Banda arcarea which show that Timor is having a northward vector motionsimilar to the Australian craton; both in direction and magnitude(Genrich et al., 1996; Simmons et al., 1999; Bock et al., 2003). Present-day movement is accommodated by a subduction reversal in thebackarc domain, along the Flores backthrust (Silver et al., 1986).

5.2. Continuum in the evolution of the subduction, and sharp variation instructural changes

The onset of the subduction of the Australianmargin about 7–8Ma(Audley-Charles, 2004) drastically changed the forearc basin config-uration. Because of a discrepancy in age, the evolution of thesubduction blocking process can be observed along-strike, from theactive Java subduction in the West to the termination of the sub-duction in the Timor Trough in the East. The present-day “subductionhingeline” coincides with the continent/ocean boundary carried bythe lower plate and with the emergence in the forearc domain, ofSumba Island carried by the overriding plate.

The morphological expression of the active forearc uplift, asobserved on the Sumba Island, is believed to result from a combina-tion of (1) a global uplift due to a possible slab rupture and (2) a localuplift above a positive topographic feature present in the lower plate.Since the entire forearc domain is undergoing vertical movements, the

Fig. 10. Schematic section parallel to the subduction trench outlining the evolution of theinvolvement of a composite lower plate in the subduction zone caused the rapid uplift of th

sharp morphological contrast is best explained by the modification ofthe nature of the downgoing slab at the continent–ocean boundary.However, the upper plate is also heterogeneous in crustal nature.Considering a continental nature for the basement of the SumbaIsland, which implies a thicker and less deformable crust, such lateralchanges have to be taken into account to explain the varying responseto the subduction (Fig. 10).

The global evolution of the southern Banda results from a contin-uous process of deformation (Fig.11).We speculate that in response tothe integration of the Australian craton into the subduction zone, theregion underwent a short period of intense thrusting (but not activetoday) which can be observed on Timor Island. The associatedaccretionary wedge incorporated fragments of the backstop andthick back-thrusted series of the Australian shelf (e.g. Harris et al.,1998). However, the earlier tectonic changes cannot be observed onTimor, because of the intensity of the deformation. In order to give anage to the onset of the deformation, we have to look at the ongoingtectonic process in the forearc domain at the longitude of Sumba. Inthis area, the incorporation of the abrupt continent–ocean boundaryinto the subduction zone, combined with the rigid behaviour of theSumba–Savu block, have made the subduction shift to the back-arcdomain (Fig. 10).

The present-day tectonic framework at the extremity of the SundaTrench, where a sharp uplift is observed in the forearc basin, is deeplycontrolled by Late Cretaceous to Miocene structures, which have beenreactivated in response to the recent subduction of the continentalcrust near the continent–ocean boundary, enhanced by the rupture ofthe slab at depth (Fig. 11). The ongoing geodynamic evolution of thisregion gives us the opportunity to depict a succession of tectonicevents that is generally difficult to unravel in older orogens.

Acknowledgements

This paper results from the cooperation between the EcoleNormale Supérieure, the CNRS, and Total E&P Indonesia, through aPhD program. We gratefully thank Total E&P Indonesia for financialsupport, the GRDC of Indonesia, Chalid Abdullah (ITB) for his precious

upper plate topography. Lateral variations in the nature of the backstop as well as thee forearc domain and isolate the Sumba–Savu–Timor block.

Fig. 11. Geodynamic evolution of the eastern Sunda subduction system since 5–10 Ma. Involvement of the Australian platform in the subduction zone since the Late Miocene hasdrastically modified the subduction pattern, with (1) reactivation of normal faults in the lower plate, (2) uplift of the entire forearc domain with emersion of Sumba/Timor islandsand (3) subduction reversal. The present-day active subduction is located north of Flores Island, in the back-arc domain.

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help on the field. We thank Kara and Yann Philippe for improving theEnglish. We are grateful to the editor and to the two anonymousreviewers for their constructive comments. We benefited discus-

sions with E. Silver and M. Audley-Charles during the initial PhDMemoir. M.P. belongs to Centre National de la Recherche Scientifique(CNRS-UMR 8538).

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