forearc deformation bader pubellier2000

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Thematic Article

Forearc deformation and tectonic significance of the ultramafic Moluccacentral ridge, Talaud islands (Indonesia)

A NNE G AËLLE B ADER1 AND M ANUEL PUBELLIER2*1 Institut Francais du Petrole, 1 & 4 Av de Bois Preau, 92852 Rueil Malmaison Cedex, now at (2).

2CNRS, UMR 8538, Laboratoire de Géologie, Ecole Normale Supérieure, 24 rue Lhomond 75231 Paris,

Cedex 05 France (email: [email protected])

Abstract The Molucca Sea is a narrow basin located south of Mindanao (Philippines) andunderlined by a north-south ophiolitic ridge. This ridge represents the outer ridge of theSangihe subduction zone and emerges by uplift in the central part of the basin, in theTalaud Islands. Field studies indicate that forearc sediments rest uncomformably on (i) a

dismembered ophiolitic series and (ii) thick melanges. Structural analysis indicates twodeformation events, one of which is oriented east-west coaxial with the present state of strain. We interpret the earlier (N20°E) direction as a thrusting event that affected anophiolitic basement associated with the edge of the Celebes Sea. Thrusting within theoceanic crust and sediments also generated olistostromes (melanges). The style of defor-mation is characterized by flattened rhombs of peridotites which exists in situ in the uppersection of the crustal sequence and were also found inside the melange. Incipient Sangihesubduction around 15 Ma uplifted the deformed crust and buried the melanges beneaththe forearc sediments. Recent east-west shortening during subduction of the SnelliusPlateau reactivated the melanges within thrusts cutting the forearc series.

Key words: forearc deformation, kinematic reconstruction, melanges, ophiolite, Talaud

Island.

(the Sangihe basin, west of the Central Ridge) anda contracted accretionary wedge, located east of the central ridge (Fig.2). At this latitude, thetectonic significance of the ophiolitic ridge is stillunclear.

We take into account marine geophysical datacollected during the Molugues Decrochement Col-lision (MODEC) cruise, which gives the presentsetting of the ridge (Rangin et al. 1996; Pubellier

et al. 1999; Bader et al. 1999) and fieldwork on theTalaud Islands, in order to explore in this paperthe evolution of the deformation that resulted inophiolite deformation prior to its emplacement.

STRUCTURAL UNITS

The Talaud Islands, located in the central part of the Molucca Sea, represent an emergent pointof the north-south Molucca Central Ridge (Fig. 1).The tectonic significance of this ridge has recently

INTRODUCTION

The Molucca Sea (Fig.1) is a narrow basin locatedsouth of Mindanao (Philippines), at the junctionbetween three major converging plates: Eurasian,

Australian and Philippine Sea Plate (PSP).Dismembered ophiolites crop out in a chain

of islands that occupies a narrow strip bisectingthe Molucca Sea. The largest outcrops are found

in the Talaud Islands, but smaller occurrenceshave also been reported in Tifore Island (Evanset al. 1983; Sukamto & Suwarna 1986). Theseislands are underlain by a ridge which presentlyoccupies the central part of the Molucca Sea(Fig.1).

The ophiolitic rocks which outcrop in Talaud arelocated between a slightly deformed forearc basin

*Correspondence.

Accepted for publication 7 July 2000.

© 2000 Blackwell Science Asia Pty Ltd.

The Island Arc (2000) 9, 653–663



654 A. G. Bader and M. Pubellier

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Fig.1 Bathymetric (Etopo5 database) and gravity (FAA from Smith & Sandwell 1995) map of the Molucca Sea. Talaud corresponds to the emergentpoint of the Central Ridge. Latitudes in degrees north, longitudes in degrees east.



Forearc deformation and tectonic significance of the Talaud islands 655

been reviewed after the MODEC cruise ( R/V l’Atalante, Rangin et al. 1996; Pubellier et al. 1999;Bader et al. 1999).

Marine geophysical data have shown that theCentral Ridge serves as a backstop of the west-

verging Sangihe subduction zone and thus repre-sents the substratum of the Sangihe forearcbasin (Bader et al. 1999). The positive free airanomaly centered over the Central Ridge (Smith

& Sandwell 1995) indicates it is composed of high-density material. This observation is supported bythe presence, on the Pujada peninsula and on theTalaud Islands, of a dismembered, but complete,ophiolitic sequence. Indeed, the Talaud Islands arecomposed of two major units (Fig. 3): a crustalsequence of Eocene age and constituted by peri-dotites, gabbros and basalts, and a sedimentarysection represented by the Awit formation

Fig.2 Structural map of the CentralMolucca Sea and Talaud Islands. Bathyme-try from the MODEC cruise, contouredevery 100 m. Latitudes in degrees north,longitudes in degrees east. Lithology of thetwo areas located around ophiolitic bodies(a) Kabaruang island, (b) central part of

Karakelang.




(Rothaan 1925; Moore et al. 1981; Sukamto andSuwarna 1986), that we correlate to the forearcseries of the Sangihe arc.

However, the three principal islands of Talaud(Karakelang, Kabaruang and Salebabu) are sur-

rounded by well-developed reefs. These reefalterraces correspond to the youngest formationpresent on Talaud. They are estimated from thePleistocene to Present and are slightly deformed.Locally, they are tilted and uplifted. In the north-

western tip of Kabaruang Island, these terracesare found from 0 to 200m.

THE AWIT FORMATION

The Awit Formation is represented by a successionof middle Miocene to Pleistocene sedimentary

units. Several nannofaunal samples indicate amiddle Miocene (NN5 to NN6; NN, nanno fossil)age for this formation.

The series was deposited above the ophioliticsequence. Along the Pampini River, the Awit For-mation lies directly on a monogenic basalt breccia,as observed at the Pampini Cape. The base of the

Awit Formation is represented by graywackeand coarse sandstone with lignite-rich layers andchannel structures (Fig.4a) indicating a shallow tosubterrestrial sedimentary environment. Thislithology grades upward to deeper sediments, typ-

ically centimetric layers of calcareous clay andcalcarenite (Fig. 4b). The Awit Formation is wellexposed in the Pampini River. It is composed ofa succession of centimetric marly layers, finesandstones and calcarenite. The whole Formationis affected by north-south trending folds and east

verging thrusts. Competent beds are affected byconjugate N30°E and N150°E joints (Fig.4c).Deformation is concentrated near the fault zonesbut is quickly attenuated away from them. It alsooccurs notably within shaly layers.

ULTRAMAFICS AND MELANGE

Three main ophiolitic bodies have been recognizedon Talaud (Moore et al. 1981; Evans et al. 1983).Two of them are located in the eastern part of Karakelang Island and the third one constitutes a

major part of Kabaruang Island. All the ophioliticmembers are present but the sequence is dis-rupted. Cherts present on these ophiolitic rockshave been previously dated as middle Eocene(Evans et al. 1983).

On Karakelang Island, the mantle section of theophiolite is found near Pampini Cape and formsthe highest point of the island. Several east-westsections have been performed in the Pulutan,Toaduraman and Pampini Rivers (see location onFig. 2).

The sections along the Pulutan and Toaduraman

River are similar (Fig.5a). They are located,respectively, at 2 and 5 km south of Pulutan village.Going west from the coastline along these sections,

we observe that the graywackes and coarse sand-stones, which are affected by thrust faults, areoverlain by melange-like lithologies. These arecomposed of siltstones that engulf blocks of ophi-calcite and weathered gabbros. The blocks areelongated and flattened. However, the lithology of the matrix is at places difficult to distinguish fromamphibolite clay resulting from the alteration of gabbros. In the upper part of the section, clay dis-

appears and massive gabbros are found immedi-ately above more typical melange facies.Peridotites are found as a thrusted unit overly-

ing the gabbros. They are separated by a largefault zone in which blocks of both peridotites andgabbros are present. Massive peridotites form acliff on which our section stopped.

The Pampini section (Fig. 5b, loc. on Fig. 2)begins at the Pampini Cape. The cape is a tabularoutcrop of monogenic breccia (Fig. 4d) composedof centimetric to decimetric blocks of basaltsembedded within a basaltic sandy matrix. We were

Fig.3 Stratigraphic column of the Talaud Islands. The two stages ofdeformation are reported and affect, respectively, the ophiolitic rocks(north-south compression, P1) and the whole series (east-west shorten-ing, P2).




Channel print

Lignite richlayer

Fig.4 (a) Coarse sandstones located at the base of the forearc series (called Awit Formation). (b) The main sedimentation of the Awit Formation isrepresented by alternation of marl and calcarenite, representing a deep basin environment. (c) Conjugate N30°E and N150°E joints affecting the basalconglomerates of the Awit Formation. Location: Niampak River on the west coast of Karakelang. (d) Pampini Cape. Monogenic breccia of basalt. The tableof basalt is clearly affected by the active east-west compression materialized by east verging thrust and conjugated N50°E/N130°E fractures.

not able to find the volcanics described by Mooreet al. (1981), except some blocks of rhyolites andbasalts in a polygenic conglomerate at the base of the Awit Formation. White tuffs exist southwardalong the road, just south of the Pampini River, butthe position of this bed within the stratigraphicseries is not established with confidence.

On the basis of our observations, the basaltic

breccia and the polygenic congolomerate seemsto be part of the base of the overlying AwitFormation, immediately beneath the subcontinen-tal layers described above. We thus interpret thebasaltic breccia to be the substratum of the dis-membered basin that was redeposited in situduring a short stage of tilting and emersion of theforearc basement. This was closely followed by thedeposition of the forearc basin.

The section stops at the foot of a cliff composedof massive gabbros, probably thrusted eastwardonto the Awit Formation. The contact, underlined

by a deformed zone, is not clear but may include very weathered amphibolite.

Melanges correspond to a heterogeneous for-mation composed of blocks, essentially of peri-dotitic nature although some sandstone andcalcareous mudstone come from the Awit Forma-tion. Blocks are embedded in a highly deformedscaly clay matrix and are lens-shaped. They do not

seem to have a coherent orientation and are sys-tematically striated on all faces. According to our observations, melanges are

systematically associated with fault zones and areoccasionally thrust over the younger Awit Forma-tion, together with ophiolitic silvers.

STYLE OF DEFORMATION

Although the Central Ridge is trending north-south, the shape of the islands is controlled by




N20°E and N150°E directions. These directionscorrespond to a conjugate set of faults with minordisplacement, except the one which underlinesthe channel between Salebabu and KarakelangIslands. These faults are compatible with the foldaxes of Karakelang which trend north-south andare the result of east-west compression. Fault slipdata analyses within the Awit Formation, as wellas the basement, indicate east-west to eastnorth-east–westsouthwest maximal principal directionof stress (Fig. 6, top). The faults are both low angle

and high angle reverse faults. Conjugate sets of strike–slip faults and joints are also presentmostly within the sandstone of the Awit Forma-tion. The east-west compression, being present inall units, is interpreted as the most recent direc-tion. It is also compatible with the active state of stress.

The north-south (N20°E) compression is, on thecontrary, present only in the basement rocks(Fig. 6, bottom). The major part of the KabaruangIsland (Fig. 2) being composed of a massive ultra-mafic body, most of the structural observations

within the ophiolite were made along three paral-lel sections located on the southern coast (syn-thetic cross-section of the whole island, Fig. 5c).

In the river located directly south of Pangerang village, the section begins with graywacke, marlsand calcarenite of the Awit Formation. Upwardin this section, a thick zone of sheared material

with serpentine and scaly clay bearing elongatedrhombs of serpentinized peridotite marks thetransition between the Awit sedimentary seriesand the massive peridotitic body. The X (long)

maximum strain axes of these lens-shaped (or pha-coids) blocks are systematically oriented north-south to N20°E (Fig. 7). Massive peridotite is itself affected by this north-south deformation. It corre-sponds to the lowest structural level (Fig. 7,Level 1). There, deformation seems to respond tohorizontal stress. The conjugate faults thus definerhombshape microlithons. In the upper section of this layer, some thrusts seem to branch on hori-zontal shear zones where the rhombs becomesomewhat rounder (phacoids) with the long axisoriented parallel to the movement observed along

Fig.5 (a) Schematic cross-section ofthe Pulutan and Toaduraman Rivers.Location on Fig. 2. (b) Pampini sectionfrom the Pampini Cape to the gabbro. (c)Schematic cross-section of the Kabaru-ang Island.




the detachment (Fig.7, Level 3). Occasional stria-tions also occur on the phacoids, implying a rela-tive displacement along the low angle faults.

On the higher structural level (Fig. 7, Level 2),the gross rock is sheared and only relics of the pha-

coids are present within a scaly clay and serpen-tine matrix. They form melanges sensu sticto.

We do not know at this point if the clast whichoriginated in the highest structural level wereresedimented as olistostrome inside the local

Fig.6 Stereographic diagrams com-puted from field observation. Observationsites are reported on the map. (a), Present

east-west compression. (b), Pre-middleMiocene N20°E compression. Latitudesin degrees north, longitudes in degreeseast.




basins prior to the deposition of the forearc basin,or if they were left in place. However, the melanges

of Talaud Islands crop out within thrust zones andare associated with sediments reworked from theforearc basin series (Fig. 7, Level 4).

Locally, in the Pangerang River (Fig. 6, see loca-tion on Fig. 3), the section begins in an ultramaficseries. Near the shoreline, gabbros and amphibo-lites are affected by horizontal shear zones. Wehave observed striations oriented N20°E thatindicated brittle low temperature conditions.There was no indication of ductile shear within thegabbros. Massive peridotites are found directlyabove gabbros and are affected by the same style

of deformation (north-south horizontal shears).Pillow basalts also occur on the Pangerang section.

The crustal section is also disrupted and thebasalts are present along a fracture zone justbeneath the Awit Formation.

DISCUSSION

ACTIVE SETTING

The Central Ridge corresponds to the outer ridgeof the Sangihe subduction zone (Bader et al. 1999).This subduction is abnormal because the subduct-

Fig.7 Schematic sketch representing the formation of the melanges.




ing plate is carrying the Snellius Plateau (Fig. 1).The convergence at this latitude is approximatelyeast-west.

The global seismicity as well as the microseis-micity recorded during 3 weeks by 10 OceanBottom Seismometer (IRD, Paris, France) duringthe MODEC cruise showed the presence of a westdipping plane below the Central Ridge. Focalmechanisms indicate both active east-west com-pression on this fault plane and sinistral strike–slip movement along northwest–southeast faults(Bader et al., in press). These faults seem to offsetthe Central Ridge on the bathymetry (Fig. 2). TheSnellius Plateau, located immediately east of theSangihe subduction zone is a large and flat shallow plateau, representing the northern equivalent of the Halmahera arc (Fig. 1). At the latitude of Talaud Islands (near 4°N), west verging thrusts

affect the forearc basin as well as the ridge whichis back-thrusted westward on the Sangihe basin.East of the ridge, at this latitude, the accretionary

wedge is severely contracted and probably forcedinto the subduction zone (Pubellier et al. 1999).Therefore the active setting is dominated by east-

west convergence along north-south low anglefaults.

FORMATION AND SIGNIFICANCE OF THE MELANGES

The formation of melanges has been a matter of

debate since the early works on the Franciscanmelange (Hsü 1968; Gansser 1974; Cowan 1981)and were related to subduction complexes. Sincethen, the concept of melanges was extensivelyused to trace paleo-accretionary wedges (Hamil-ton 1979). Melanges were described by Clennel(1991) in Sabah and by Moore and Karig (1980) onNais Island. There, Pubellier et al. (1992) consid-ered these melanges as older than subduction/ collision material injected into recent thrusts.In Talaud, melanges were described by Sukamto(1980) and Moore et al. (1981).

More recently, results from Ocean DrillingProgram (ODP) drillings (Fryer et al. 1995)described large serpentine diapirs developed inthe Marianas forearc basin at the edge of the outerridge. The authors suggested that the occurrenceof the ultramafic materials has been conditioned bythe existence of large-scale faults.

The materials present in Talaud, being associ-ated with blocks of the forearc sediments and base-ment, similarly indicate recent reactivation bylarge faults. We cannot clearly state if the Talaudmelanges directly came from the materials

scrapped off the oceanic crust by large thrustsassociated with recent deformations or if they

were affected by thrusts in the mid-Tertiary, re-deposited prior to the deposition of the forearc andfinally remobilized by the recent faults. The struc-tural position of the melange above the massivecrustal section favors the second hypothesis.

ORIGIN OF THE NORTH-SOUTH DEFORMATION

The north-south compression has been observedon the ophiolitic rocks and apparently does notaffect younger units. The Awit Formation wasdated from the middle Miocene to Pleistocene(NN5 to NN15; C. Müller, pers. comm., 1996), thenorth-south deformation is thus older than 15 Ma(P1 on Fig.3). A second event occurred after theforearc development. This later east-west com-

pression induced thrusts and folds affecting boththe ophiolitic basement and the Awit sediments(P2 on Fig. 3).

In order to understand the origin of this com-pression, we performed kinematic reconstructionon a sphere tangent to the WGS84 ellipsoid at thelocation of the Euler pole. The rotation pole of thePSP corresponds to that of Seno et al. (1993) forthe first 5 Ma and the pole of Rangin et al. (1990)for older positions. We also tested poles of Ali andHall (1995) which do not significantly differ at thescale discussed here.

From 0 to 15 Ma, the PSP subducts under theeastern Eurasian margin. This subduction fits thekinematics observed presently. The late stages of this motion ended up with the east-west compres-sion observed on Talaud Island, when the Halma-hera arc entered the Sangihe Trench (Pubellieret al. 1999).

Prior to the initiation of the westward subduc-tion, the 20 Ma stage (Fig. 8) shows that the first

Australian continental blocks, rifted from the Australian margin, collided against the Eurasianmargin. Figure 8 illustrates the small circles of the

PSP and Australian blocks motion with respect toEurasia. At the latitude of Talaud Island, the Australian movement was approximately N20°Eand was therefore compatible with the expecteddeformation of the Eurasian margin at this lati-tude. Such north-south movement has been alsoobserved in the central part of Sulawesi Island(Monnier 1996) and in Mindanao, in the PujadaPeninsula, which corresponds to the northern tipof the Central Ridge. This deformation probably

was followed by incipient subduction of theMolucca Sea under the newly formed Talaud ridge.




If so, one may think that the north-south trend of

the Central Ridge was acquired at this time, whena north-south rupture dipping toward the westuplifted a portion of the basin. We propose alsothat the ridge emerged at this time prior to thedeposition of the forearc sediments, within conti-nental and shallow water environment.

ACKNOWLEDGEMENTS

This work was possible due to the cooperation pro-grams between France and Indonesia. Dating of

nannofossils has been done by Carla Müller. We

thank R. Soeria Atmadja and A. Najihal Amalfrom the University of Bandung for help in thelogistics. Laboratory facilities were supplied bythe Ecole Normale Supérieure in Paris. Oneauthor (M.P.) belongs to the Center National de laRecherche Scientifique (CNRS).

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