Journal of Asian Earth Sciences 76 (2013) 428–438

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Journal of Asian Earth Sciences

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The origin of mélanges: Cautionary tales from Indonesia

1367-9120/$ - see front matter � 2013 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.jseaes.2012.12.021

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A.J. Barber ⇑Department of Earth Sciences, Royal Holloway, University of London, Egham, Surrey TW20 0EX, UK

a r t i c l e i n f o a b s t r a c t

Article history:Available online 8 January 2013

Keywords:MélangeMud diapirsMud volcanoesTimorSumbaNias

The origin of block-in-matrix mélanges has been the subject of intense speculation by structural and tec-tonic geologists working in accretionary complexes since their first recognition in the early twentiethcentury. Because of their enigmatic nature, a number of important international meetings and a largenumber of publications have been devoted to the problem of the origin of mélanges. As mélanges showthe effects of the disruption of lithological units to form separate blocks, and also apparently show theeffects shearing in the scaly fabric of the matrix, a tectonic origin has often been preferred. Then itwas suggested that the disruption to form the blocks in mélanges could also occur in a sedimentary envi-ronment due to the collapse of submarine fault scarps to form olistostromes, upon which deformationcould be superimposed tectonically. Subsequently it has proposed that some mélanges have originatedby overpressured clays rising buoyantly towards the surface, incorporating blocks of the overlying rocksin mud or shale diapirs and mud volcanoes.

Two well-known examples of mélanges from the Banda and Sunda arcs are described, to which tectonicand sedimentary origins were confidently ascribed, which proved on subsequent examination to havebeen formed due to mud diapirism, in a dynamically active environment, as the result of tectonism onlyindirectly. Evidence from the Australian continental Shelf to the south of Sumba shows that large quan-tities of diapiric mélange were generated before the diapirs were incorporated in the accretionary com-plex. Comparable diapirs can be recognised in Timor accreted at an earlier stage. Evidence from bothTimor and Nias shows that diapiric mélange can be generated well after the initial accretion processwas completed.

The problem is: Why, when diapirism is so abundantly found in present convergent margins, is it sorarely reported from older orogenic belts? Many occurrences of mélanges throughout the world to whichtectonic and/or sedimentary and origins have been ascribed, may in future investigations prove to havehad a diapiric origin.

It is emphasised that although the examples of diapiric mélange described here may contain ophioliticblocks, they were developed in shelf or continental margin environments, and do not contain blocks ofhigh grade metamorphic rocks in a serpentinous matrix; such mélanges originate diapirically during sub-duction in a mantle environment, as previous authors have suggested.

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1. Introduction

When Greenly (1919) described a chaotic unit in the GwnaGroup of Anglesey as ‘mélange’, containing a variety of blocks ofocean floor and continental shelf materials, such as spilitic pillowlava and tuff, red jasper chert, manganiferous limestone, shale,quartzite and grey limestone in a fine-grained matrix, it seemedobvious that this mélange had been disrupted and sheared duringa process of tectonic deformation. Later Beneo (1955) describedextensive deposits of mélange in Sicily in the western Mediterra-nean, which he interpreted as having been formed in a sedimen-tary environment as the product of large-scale submarine

landslides. This interpretation has been used frequently to accountfor the large-scale mélanges of the Argille Scagliose throughoutItaly. In the discussion following Beneo’s (1955) paper, Floresintroduced the terms olistostrome for these deposits, and olistolithsfor the included blocks. Although Hsü (1968, 1974) recognised tec-tonic and olistostromal mélanges he suggested that the term‘mélange’ should be restricted to those formed tectonically, inpractice, due to the difficulty of determining their origins, the termhas been used to describe all ‘block in matrix’ mélange depositsworldwide, however they may have originated.

Subsequently both tectonic and sedimentary interpretations,sometimes in combination, have been offered for many otheroccurrences, especially where mélanges are found in associationwith accretionary complexes, formed in subduction trenches fromoceanic material scraped off downgoing oceanic plates (e.g. Aalto,

A.J. Barber / Journal of Asian Earth Sciences 76 (2013) 428–438 429

1981; Cowan, 1985; Collins and Robertson, 1997; Ogawa, 1998;Robertson and Pickett, 2000; Robertson and Ustaömer, 2011).Meanwhile, geologists using seismic reflection profiling and dril-ling for oil in deltaic deposits, frequently encountered salt diapirsand shale diapirs containing block-in-matrix deposits in deltas onpassive margins, attributed to the mobilisation of salt and shaleunits due to overpressure generated in areas with very high sedi-mentation rates, e.g. Nile Delta (Loncke et al., 2006), Niger Delta(Morley and Guerin, 1996) Baram Delta (Morley et al., 1998; Mor-ley, 2003). Shale diapirs are also encountered on active continentalmargins, where the overpressuring and mobilisation of shales isattributed to the stacking of thrust slices, e.g. Barbados (Brownand Westbrook, 1988; Stride et al., 1982; Westbrook and Smith,1983), the Makran (Stöcklin, 1990; Grando and McClay, 2007), Ara-kan, Burma (Maurin and Rangin, 2009). Much effort has been ex-pended in defining criteria by which tectonic, olistostromal anddiapiric mélanges might be distinguished (e.g. Barber et al., 1986;Camerlenghi and Pini, 2009; Codegone et al., 2012b; Cowan, 1985;Dilek et al., 2012; Festa et al., 2010; Festa, 2011; Lash, 1987; Or-ange, 1985; Pini, 1999; Raymond, 1984; Silver and Beutner,1980; Sengör, 2003; Wakabayashi, 2011).

Experience gained during study of mélanges regions of presentday active tectonics in the Outer Banda Arc and the Outer SundaArc in Indonesia, to which both tectonic and olistostromal originshave been attributed, but for which substantive evidence of diapir-ism has been subsequently obtained, suggests that diapiricmélange is much more common than many structural and tectonicgeologists have appreciated: Diapirism should be considered as anorigin for mélanges, before tectonism or submarine slumping areautomatically assumed.

2. Mélange in the Outer Banda Arc

The island of Timor in eastern Indonesia, just to north of Austra-lia, forms part of the Outer Banda Arc (Fig. 1). The island was

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Fig. 1. The Banda Arcs Eastern Indonesia, with the island of Timor formed by the colldistribution of active mud diapirs and mud volcanoes around the Outer Arc (Barber and

formed from Australian Precambrian basement and continentalmargin sediments, ranging in age from the Permian to the Pliocene,imbricated in accretionary complexes and thrust beneath an ophi-olite nappe, during the collision between the northern continentalmargin of Australia and the Banda Volcanic Arc (Barber et al., 1977;Charlton et al., 1991). The eastern part of Timor (Timor Leste) wasmapped geologically by Audley-Charles (1968), and the strati-graphic units he established were later adopted by the IndonesianGeological Survey when they mapped the western part of the is-land (Rosidi et al., 1981) (Fig. 2). During the mapping Audley-Charles (1965, 1968) recognised an extensive mélange deposit,the Bobonaro Scaly Clay, containing a variety of blocks in a scalyclay matrix (Fig. 3) which, following Beneo (1955), he interpretedas olistostromes, the product of large-scale submarine slumping.This interpretation was accepted by Hutchison (2007) in his ac-count of the geology of Timor in the textbook Geological Evolutionof South-east Asia (p. 58).

Audley-Charles (1968) and Carter et al. (1976) regarded theBobonaro Scaly Clay as a distinct stratigraphic unit, formed at aspecific period of time, Early Pliocene, N19-20 in Blow’s (1969)foraminiferal zonal scheme, although Pleistocene forams wereidentified in the scaly clay matrix, these were interpreted as beingincorporated by soil creep from overlying raised coral reefs. Theyvisualised that slumping had occurred by the collapse of submar-ine fault scarps developed in the accretionary complex, with thedeposition of the blocks into adjacent troughs, where deep-waterclays, which now form the scaly clay matrix, were being deposited.These escarpments were formed as the result of thrusting duringthe subduction/collision between the northern margin of the Aus-tralian continent and the Banda Arcs.

Later, Hamilton (1979) visited West Timor and described andillustrated the Bobonaro Scaly Clay Mélange. In the outcrops thathe visited, Hamilton observed that the fabric of the scaly clay ma-trix had a predominantly northerly dip and suggested that themélange had been generated tectonically by southerly-directed

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Fig. 2. Geological map of Timor, Eastern Indonesia, showing the extent of the Bobonaro Scaly Clay Mélange: East Timor (Timor Leste) from Audley-Charles (1968); WestTimor from Rosidi et al. (1981).

Fig. 3. Landscape in West Timor with scattered blocks of a variety of lithologies,some the size of houses, due to the erosion of the clay matrix from an outcrop ofBobonaro Scaly Clay Mélange.

430 A.J. Barber / Journal of Asian Earth Sciences 76 (2013) 428–438

overthrusting, during the collision between the northern Austra-lian margin and the Banda Arc.

Subsequently, in a geophysical cruise in Eastern Indonesia bythe Hawaiian Institute of Geophysics, the Research Vessel KanaKeoki surveyed the Timor accretionary complex and the AustralianContinental Shelf to the south of Timor. The survey mapped thebathymetry, recorded seismic profiles and used the side-scan sonarSeaMarc II system (Blackington et al., 1983) to image the Timorslope, the Timor Trough and the northern part of the Australianshelf. The seismic section showed that sediments from the TimorTrough and the Australian margin had been uplifted into the accre-tionary complex along bedding parallel thrust planes which stee-pen toward the surface and generate fault-bend folds in theoverlying sediments (Karig et al., 1987) (Fig. 4). Earlier, Upper Pli-ocene to Recent sediments had been drilled in DSPD 262 on thefloor of the Timor Trough and showed that sediments on the floorof the trough, to a depth of 400+ m, consist of unlithified radiolar-ian and clay-rich nanno ooze (Heirtzler et al., 1973). The SeaMARCII side-scan imagery showed fault scarps, both on the Australian

margin and on the Timor slope, together with slumps, débris andmud flows and sediment-filled slope basins (Fig. 5), as postulatedby Audley-Charles (1965), but the seismic section shows that thesurface of the Australian slope and the accretionary complex iscomposed only of recently deposited soft sediments of Plioceneto Recent age, with no major fault scarps uplifting the underlyingcontinental shelf sequence from which blocks of lithified sedi-ments could be derived, to form olistostromes, as postulated byAudley-Charles (1965, 1968) to account for the Bobonaro ScalyClay Mélange on Timor.

A similar survey was conducted by the R.V. Kana Keoki furtherwest, to the south of the island of Sumba, across the accretionarycomplex, the trench and the Australian margin (Breen et al.,1986). SeaMARC II side-scan sonar imagery, together with bathym-etry and seismic reflection profiling, showed that the surface of thesea floor on the Australian continental shelf, immediately to thesouth of the deformation front of the accretionary complex, is cov-ered by mounds and ridges (Fig. 6). Neither the side-scan sonarimagery nor the seismic profiles show any large-scale fault scarpsin the accretionary complex; the upper slopes of the complex arecovered by a thin drape of undisturbed Recent sediments, andare devoid of structural features (Breen et al., 1986, Fig. 7). Seismicsections obtained south of the deformation front show that atintervals of 10 km or so, the regular horizontal reflections in thebedded sedimentary sequence are disrupted by transparent zones,devoid of reflections, capped by the mounds on the sea floor. Thesetransparent zones are interpreted as vertical mud diapirs and themounds are interpreted as mud volcanoes (Fig. 6).

Breen et al. (1986) correlated the sequence represented in theprofiles with the undisturbed Australian continental marginal se-quence encountered in boreholes on the Scott Reef (Rad and Exon,1982). The deepest reflection (E) is identified as the top of the clas-tic sequence, which marks the break-up unconformity in the pas-sive margin of the NW Australian Shelf. A transparent zonebetween (E) and (C) is identified as Lower Cretaceous hemipelagicmarine shale deposited on the subsided ocean floor. Overlyingreflectors between (E) and (A) are identified as lithified Upper Cre-taceous to Miocene pelagic carbonates, with interbedded marls,deposited on a prograding carbonate shelf, and the zone from (A)to the surface, as Pliocene to Recent pelagic marls. The vertical

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Fig. 4. Interpreted seismic section from the Australian continental shelf across the Timor Trough and the Timor accretionary complex showing that only the most recentsediments are exposed; the shelf sequence is not exposed and cannot provide blocks to form olistostromes (Karig et al., 1987).

sediment lobes

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Fig. 5. Interpretation of SeaMARC II imagery of the accretionary complex south of Timor shows slumping of soft sediment from the most recent deposits from the TimorTrough, with no olistostromes containing blocks derived from the underlying continental margin sequence (reinterpreted after Charlton (1987) and personal communication2012). N.B. The subduction system in the Timor Trough is no longer active. GPS measurements show that Timor Island is accreted to the Australian Plate and is moving NE-wards at 7.7 cm/yr together with Australia (Genrich et al., 1996).

A.J. Barber / Journal of Asian Earth Sciences 76 (2013) 428–438 431

transparent zones seen in the seismic sections rise from the level ofthe Lower Cretaceous shale at a depth of 1.5 km, and cut across thebedded reflectors to the base of the surface mounds (Fig. 6). Inpassing through the overlying bedded sequence the mobilised clayis likely to have incorporated blocks of the lithified carbonate unitsto form a mélange of Upper Cretaceous to Miocene limestoneblocks in a hemipelagic clay matrix. Scattered occasional reflec-tions in the transparent zones may from larger blocks. The largestof the ridges on the sea floor is 200–300 m high, 18 km long with awidth of 2.5 km and extends to a depth of �1.5 km, representing avolume of �70 km3 of mélange.

A subsequent survey by the R.V. Charles Darwin using the GLO-RIA side-scan sonar system showed that the diapir field extendsover an area of 100 km � 50 km on the Australian continental shelf(Masson et al., 1991). The seismic sections show that continentalmargin sediments incorporating the diapirs and the mud volca-noes, including many hundred cubic kilometres of mélange, arein the process of incorporation into the toe of the accretionarycomplex (Fig. 6).

Westbrook and Smith (1983), in a comparable situation inBarbados, where mud diapirs and mud volcanoes occur in OrinocoFan sediments on the oceanward side of the deformation front,

ACCRETION OF DIAPIR COMPLEX SOUTH OF SUMBA(after Breen et al. 1986)

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Line drawing from S-N shipboard seismic reflection profile across the Java Trench south of Sumbafrom HIG R.V. ‘Kana Keoki’. The profile shows mud volcanoes on the sea-floor and mud-diapirsrising from C-E through reflectors in consolidated Australian marginal sediments.

The deformation front (Df ) at the toe of the accretionary complex has been replaced recently by Dfincorporating the sediments and the diapirs into the complex.

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Fig. 6. Line drawing from a seismic section across the mud volcano and diapir field in Australian continental margin sediments to the south of Sumba (after Breen et al.(1986)) Transparent zones not capped by mud volcanoes evidently mark diapirs which have not yet reached the surface. Scattered reflections within these zones may be fromlarger blocks. The sediments, together with the diapirs are being accreted as the deformation front of the accretionary complex moves southwards.

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Fig. 7. Areas covered by the SeaMARC II and GLORIA side scan sonar surveys showing the 50 � 100 km extent of the diapir field (Breen et al., 1986; Masson et al., 1991). Theline of section in Fig. 6 is shown within the SeaMARC II coverage.

432 A.J. Barber / Journal of Asian Earth Sciences 76 (2013) 428–438

suggested that stacking of thrust slices in the accretionary complexgenerated overpressure in fluids (water, oil and gas) in the under-lying sediments. The overpressured fluids were projected alongpermeable horizons in the incoming sedimentary sequence tomobilise overlying impermeable clays, which rose buoyantlythrough the sequence to form clay diapirs and mud volcanoes onthe surface. This model is applicable to the Australian continentalmargin sediments south of Sumba, where Jurassic sands at the

break-up unconformity form a permeable horizon within the se-quence, while the overlying Lower Cretaceous hemipelagic claysform the impervious seal which was mobilised by overpressuredfluids to form the diapirs and mud volcanoes (Fig. 6). This combi-nation of circumstances are likely to occur whenever a passive con-tinental margin enters a subduction system. Interestingly, theLower Cretaceous hemipelagic clays had remained in a stable con-dition in the continental shelf sequence for �100 million years

Fig. 9. Cross-section of a mud diapir containing lensiod and irregular blocks oflimestone and sandstone in a scaly clay matrix in roadside quarry, near Takari, WestTimor. The matrix is intruded along fractures in the blocks, indicating highoverpressures, and the foliar structure of the scaly clay matrix is attributed to flowrather that to shearing.

A.J. Barber / Journal of Asian Earth Sciences 76 (2013) 428–438 433

before they were activated by the overpressure generated by theaccretion process.

During mapping in West Timor the Geological Survey of Indone-sia identified twenty-one mud volcano fields and cross-cuttingmud diapirs, intruded through the other rocks, as the BobonaroScaly Clay Mélange (Rosidi et al., 1981) (Fig. 2). In the Kolbano area,and in other areas in the southern part of the island, imbricatedand folded Late Cretaceous to Miocene carbonates, form a Late Ter-tiary accretionary complex and are cut by mud diapirs composed ofred scaly clay containing manganese nodules and blocks of LateCretaceous to Miocene limestones, forming a mélange (Barberet al., 1986, Fig. 2). Hydrofracturing of the blocks, with films ofthe matrix injected along joints and fractures indicate that themud matrix in the diapirs was overpressured (Barber et al., 1986;Barber and Brown, 1988). Yassir (1987) has demonstrated experi-mentally that the fabric in scaly clay matrix in mélanges developsin overpressured clays, without the addition of superimposedshearing.

The Cretaceous to Miocene limestones and the accompanyingmélange in Kolbano and other areas in the southern part of Timor,correspond to the Australian marginal sediments and diapirs southof Sumba, which were incorporated at an earlier stage in the islandarc-continental collision, along a décollement at a level near thebreak-up unconformity. There are no active mud volcanoes in thispart of Timor, evidently the diapiric system is now inactive. Thecollision commenced in the region of East Timor, and has since ex-tended to both east and west.

In the island of Semau (Barber et al., 1986) (Fig. 8) and in otherareas in the northern part of Timor (Fig. 2), active mud volcano ex-trude fluid mud and bubbles of methane and have an oily scum onthe surface of mud pools. Mud in the volcanoes and scaly clay inthe diapirs were derived from Permo-Triassic red shale horizonsand carry blocks of Permo-Triassic sandstone and limestone (Figs. 9and 10). Evidently, the Permo-Triassic rocks were uplifted into theaccretionary complex from a décollement surface near the base-ment unconformity with the Australian basement in a process ofunderplating (Charlton et al., 1991). Hutchison (2007, p. 320), fol-lowing Audley-Charles (1968), noted that ‘ultramafic’ rocks arealso included in the mélange, showing that some of the diapirshad intruded the overlying ophiolite nappe. Other diapirs andmud volcanoes contain blocks of reef material and were intruded

Fig. 8. The mud volcano field showing small-scale mud volcanoes up to 3 m high,with fluid mud flows and scattered blocks, in the crater of the 1 km diameter largemud volcano forming Palau Kambing, SW of Kupang, West Timor. The crater wall inthe background is littered with blocks. The soft clays are eroded during the rainyseason and washed out through a gap in the crater wall (see Barber et al., 1986,Fig. 3).

Fig. 10. Bedded block of Permian sandstone in varicoloured scaly clay matrixderived from Permo-Triassic red marls in a mud diapir, exposed by river erosion inthe Noil Toko, near Nenas, West Timor. Patrick Bird provides scale. The location isshown in Fig. 4 of Barber et al. (1986). (For interpretation of the references to colourin this figure legend, the reader is referred to the web version of this article.)

through Pleistocene coral reefs (Barber et al., 1986). The evidencefrom the incorporation of the coral blocks and active mud volca-nism, indicates that compression, overthrusting and the generationof fluid overpressures continues within the collision complex in thenorthern part of Timor to the present day.

Diapirs and active mud volcanoes occur extensively on all theBanda Outer Arc islands between Timor and Seram (Fig. 1). Theprocess of the formation of mélanges must be occurring on a verylarge scale today as the result of the collision between Australiaand the Banda Arcs.

434 A.J. Barber / Journal of Asian Earth Sciences 76 (2013) 428–438

3. Mélange in the Outer Sunda Arc

Outcrops of mélange were mapped in the outer arc islands ofNias, Simeulue, Siberut, Sipora and Pagai, associated with subduc-tion of the Indian Ocean floor beneath Sumatra. (Andi Mangga andBurhan, 1994; Budhitrisna and Andi Mangga, 1990; Endharto andSukido, 1994). The mélange contains blocks of serpentinite, gabbro,basalt, chert, calcilutite, Eocene limestone with foraminifera, gra-nitic and metamorphic rocks together with abundant greywackeshale and claystone in a scaly clay matrix.

In the 1970s the Scripps Institution of Oceanography (SIO) ob-tained seismic sections from the Indian Ocean floor, the SundaTrench and the Outer Arc Ridge to the SW of Sumatra (Mooreand Curray, 1980) (Fig. 11A). Karig et al. (1980) made a detailedstudy of the trench and the accretionary complex from the SIOseismic profiles, and then completed a series of traverses acrossthe Outer Arc Ridge where it is exposed on the island of Nias(Moore et al., 1980b; Moore and Karig, 1980). On Nias the mélangedeposits, termed the Oyo Complex, occur as a series of NW–SEtrending linear belts parallel to the length of the island, severalhundred metres wide and kilometres in length, alternating withbedded turbidites of Early Miocene to Early Pliocene age, termedthe Nias Beds (Fig. 12).

In the early 1990s Nias Island was mapped in detail by Samuel(1994; Samuel et al., 1997) (Fig. 12) with the ages of the rocks verytightly controlled by detailed microfossil analysis, supported bypetrographic, mineralogical and geochemical analyses. In particu-lar a systematic study was made of the Oyo Mélange and the rela-tionships to the bedded units.

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Fig. 11. (A) NE–SW line drawing from seismic profile to the south of Nias, from the IndiaRidge to the Sumatran Forearc Basin (redrawn from Moore et al. (1980b)). (B) NE–SW linKopp and Kukowski (2003).

From their study of the seismic sections, Moore and Karig(1976) and Karig et al. (1979) developed a model for the evolutionof the accretionary complex, the development of the forearc ridgeand the geology and structure of Nias. They speculated that theOyo Mélange was developed as a trench-fill deposit composed offragments of ocean crust and turbidites which slumped down theface of the accretionary complex as debris flows (olistostromes)and were subsequently accreted at the toe of the complex (Mooreand Karig, 1980). Further debris flows were derived from faultscarps developed on the NE side of slope basins formed on top ofthe accretionary complex. The resolution of the 1970s seismicreflection imaging was not sufficiently clear to either confirm orcontradict these interpretations (Fig. 11). Neither debris flowsnor diapirs are seen in the seismic profiles across the Sunda Trench(Fig. 11A and B). It may require swath mapping or 3D seismic sur-veys across the trench and the accretionary complex to identifysuch flows (e.g. Gee et al., 2007). Karig et al. (1979) further specu-lated that the chaotic and sheared appearance of the mélange wasdue to the dynamic tectonic environment within the developingaccretionary complex, in which original thrust surfaces were con-tinually reactivated, with the development of new thrust planes,disrupting the accreted oceanic material and breaking it intoblocks. They were unable to determine the age of the mélange di-rectly, but inferred that the age was indicated by the youngestdateable included blocks, the Eocene limestones.

Although no depositional contacts were observed, Moore et al.(1980a) and Moore and Karig (1980) suggested that the OyoMélange was the oldest unit on Nias, on which the Nias Beds weredeposited unconformably. They suggested that the Nias Beds had

NEOuter Arc Ridge Mentawai

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n Ocean Floor across the Sunda Trench, the accretionary complex and the Outer Arce drawing from a seismic section offshore South Sumatra. The original section is in

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SUMATRAN FOREARC

Fig.11B

Fig. 12. Geological and structural map of the Island of Nias, Outer Arc, showing outcrops of the Oyo Mélange and the location of mud volcano fields. Locations marked ‘B’ areoutcrops of coherent ophiolitic basement, in contrast to ophiolitic blocks in the mélange (based on Djamal et al. (1994) and Samuel (1994)). The thrusts shown on the SW sideof the mélange outcrops may not be correct, see text. Inset map shows location of Nias in the Sumatran Forearc and the position of the seismic lines in Fig. 11A and B.

A.J. Barber / Journal of Asian Earth Sciences 76 (2013) 428–438 435

been deposited in slope basins on the accretionary complex andwere uplifted and compressed during thrusting, with the growthof the complex, until they were exposed in Nias.

On the other hand Samuel et al. (1995) found that the beddedsediments on Nias had been deposited in basins developed ontop of the accretionary complex, bounded to the NE by syn-depo-sitional extensional faults. They also found that the Upper Palaeo-gene and Neogene stratigraphic sequences on Nias correspondwith the same sequences in the Banyak and Batu islands, whichlie within the forearc basin to the northeast of Nias, and also inboreholes in the forearc basin itself. These sequences resemble instratigraphy and lithology the units on Nias. Samuel et al. (1995)therefore concluded from the occurrence of well-rounded quartzpebbles and metamorphic clasts, in the Oligocene and Lower

Miocene sandstones and conglomerates, that the sediments onNias were derived from a mature continental provenance, eitherbasement uplifts in the forearc or the mainland of Sumatra, andthat prior to the Pleistocene, sedimentation extended continuouslyfrom Sumatra across the present forearc basin onto the outer arcislands, and do not correspond to the deposits in the Sunda Trench.

Samuel (1994) found that in addition to ophiolitic material, ser-pentinite, gabbro, basalt, calcilutite and chert of ocean floor prove-nance, at least 50%, and commonly 90% of the blocks in themélange had been derived from the bedded Oligocene and LowerMiocene units, while some outcrops contained clasts of Upper Mio-cene and even Pleistocene units. He also found that the mud matrixcontained the same microfossils, had the same mineral composi-tion and the same thermal history, yielding the same range of

Fig. 13. The margin of a mud diapir exposed in the bank of a tributary stream of theOyo River, showing the grey scaly clay matrix, with a fabric parallel to the margin,cutting and injected along joint fractures in a greywacke sandstone bed, OyoMélange, Nias, Sumatran Outer Arc. Notebook 110 � 150 mm.

436 A.J. Barber / Journal of Asian Earth Sciences 76 (2013) 428–438

vitrinite values, as mudstones in the Oligocene to Lower Miocenesuccession.

Samuel (1994) found that the scaly fabric that pervades themélange matrix is commonly vertical, but may be folded andwrapped around the enclosed clasts, and at the contacts, the fabricis parallel to the margins of the mélange outcrop (Fig. 13). Kariget al. (1980) observed that as mélange units are traced along strikethey are in contact with different units of the Nias Beds, and aresheared and mixed into the mélange. But as we have seen already,a shearing process is not necessary to impart the scaly fabric to theclay matrix of the mélanges (Yassir, 1987). Karig et al. (1980) inter-preted these contacts as high angle reverse faults. However, Sam-uel et al. (1997) observed that the contacts between the mélangeand the adjacent bedded units are always intrusive, with the ma-trix penetrating along the bedding planes and fractures in the bed-ded units. If these observations are correct, the thrusts shownalong the SW margins of the mélange outcrops in geological map(Fig. 12) are probably erroneous. The mélange was observed tocut units of all ages from Oligocene to Recent. It appears that themain period of mélange formation occurred during the Pliocene,but mélange production continues to the present day, as shownby the eruption of mud volcano fields (Fig. 12), extruding blocksin a grey mud slurry, identical in composition and ages to the ma-trix of the mélange.

Similar mélanges, have been mapped on the other SumatranOuter Islands of Simeulue, Siberut, Sipora and Pagai, commonlywith circular outcrops, and associated with active mud volcanoes(Endharto and Sukido, 1994; Andi Mangga and Burhan, 1994;Budhitrisna and Andi Mangga, 1990). Evidently as in Timor andthe other island of the Outer Banda Arc System, the outer arc ridgein the Sunda Arc system continues to be compressed, causing over-pressure and the mobilisation of clays to form mud diapirs andmud volcanoes.

4. Conclusions

Contrary to the hypothesis of Audley-Charles (1965), evidencefrom Sumba and Timor indicates that there are no débris flowscontaining continental shelf blocks, associated with the develop-ment of the accretionary complex forming at the present day,which could account for the Bobonaro Scaly Clay Mélange on Ti-mor. Evidence from Sumba indicates that large volumes ofmélange, in the form of diapirs, were generated by overpressuredfluids in the Australian continental marginal sediments, beforethey were incorporated into the accretionary complex. The

experimental work of Yassir (1987) shows that the presence of ascaly clay fabric does not necessarily indicate tectonic deformation,as Hamilton (1979) assumed. However, it is possible for deforma-tion to be superimposed on mélanges after accretion has ceased,e.g. Barber et al. (1986) describe mélange sheared along strike-slipfaults in West Timor, and Harris et al. (1998) has suggested that themélanges occur along the base of major thrusts developed duringthe progress of the collision. It may be that the diapiric mélangeshave provided weak zones along which the thrust planes weredeveloped. Evidence from Timor and the islands to the east, indi-cates that compression and thrusting to produce overpressure,with the formation of mud diapirs and mud volcanoes in the OuterBanda Arc, due to the collision with Australia, continues to thepresent day.

The seismic sections across the Sunda Trench, the accretionarycomplex and the Outer Arc Ridge offshore Sumatra show no evi-dence of mud diapirism from the Nicobar Fan sediments on the In-dian Ocean floor (Fig. 11A and B), in contrast to the NorthwestAustralian Shelf south of Sumba, where the sequence contains apermeable clastic sequence overlain by impermeable clays whichcould be mobilised by overpressures generated by the loading ofthe accretionary complex. Although the presence of débris flowsin the Sunda Trench or in slope basins on the developing accretion-ary complex in the Sunda Arc system, as proposed by Karig et al.(1979), nor the disruption and break-up of units by thrusts withinthe accretionary complex, cannot definitely be ruled out, Samuel’s(1994; Samuel et al., 1995, 1997) study of the geology of the islandof Nias has shown conclusively that these processes were notresponsible for the formation of the mélanges on Nias. Samuel(1994) found that the Oyo Mélange Complex was formed as muddiapirs within extensional sedimentary basins, formed on top ofthe accretionary complex. Nor were the mélanges deformed asthe result of the continual reactivation of the accretionary com-plex, causing the development of thrust planes with further dis-ruption and the formation of the scaly fabric, as Moore and Karig(1980) had suggested. As we have seen the occurrence of mud vol-cano fields on Nias and the other Sunda Outer Arc islands indicatesthat compression-generated overpressures forming diapiricmélange is still in the process of formation at the present day.

Nevertheless, the olistostromal and tectonic models for the for-mation of mélanges proposed by Karig et al. (1979) has been so im-mensely persuasive and influential that it has been invoked as anexplanation for the occurrence of mélanges all over the world;e.g. Turkey and the Himalayas (Collins and Robertson, 1997; Rob-ertson and Pickett, 2000; Robertson and Ustaömer, 2011). Hamil-ton (1979), as a Californian geologist, would have been wellaware of the work at Scripps, which led him to the interpretationof the Bobonaro Scaly Mélange of West Timor as of tectonic origin,formed by the process visualised by Karig et al. (1979).

As far as the author is aware no clasts of blueschist or eclogitehave been recorded from the mélanges on Timor or on the OuterSunda Arc islands, which as has been explained originated as dia-pirs on a continental shelf or in an active continental margin envi-ronment. Clasts of ophiolitic materials are found in the mélangeson both Timor and Nias but these have been incorporated whendiapirs have penetrated overlying accreted slices of ocean crust.Mélanges that contain clasts of blueschists, eclogites and otherhigh grade metamorphic rocks, often in a serpentinous matrix suchas those in the Franciscan Mélange of California and the PeleruMélange Complex of Central Sulawesi have risen diapirically upthe subduction zone from the mantle, as earlier authors have sug-gested (Cloos, 1984, 1985; Parkinson, 1996).

In a landmark paper, Kopf (2002, Fig. 1) building on an earlierreview by Higgins and Saunders (1967) catalogued the occurrenceof mud volcanism and mud diapirism throughout the world; henoted that they are especially developed on convergent plate

A.J. Barber / Journal of Asian Earth Sciences 76 (2013) 428–438 437

margins. Mud diapirs and mud volcanoes are particularly abundantin the collision zone between Africa and Eurasia and in the subduc-tion and collision zones between the Indian and Australian platesand Southeast Asia. The floor of the Mediterranean Sea has beenparticularly well studied by marine geophysical surveys in the pasttwo decades (see Camerlenghi and Pini, 2009, Fig. 4). Kopf (2002)claims that is ‘‘arguably the region with the highest abundanceof MVs (mud volcanoes) and diapirs on Earth’’. Several hundredmud volcanoes and mud diapirs have been discovered on Mediter-ranean Ridge accretionary complex to the south of Crete, formedby the subduction of the African Plate beneath the European Plate.The mud volcanoes have been imaged by seismic surveys andswath mapping and have been sampled by coring, deep drillingand the use of submersibles, with study of the blocks and the claymatrix. It is a remarkable anomaly that although mélanges occurcommonly on the adjacent land areas of Greece, Crete, Cyprusand Southern Turkey they are invariably attributed to the pro-cesses of submarine slumping and tectonism, not to mud diapirism(Camerlenghi and Pini, 2009, Fig. 3). Although mud diapirs may bedifficult to recognise in older mélanges on land because diagnosticfeatures such as mud volcanoes have long since been eroded away,and the cross-cutting relationships have been modified or de-stroyed by later tectonism. However, examples are known wheremélanges are incorporated into metamorphic complexes and havebeen deformed and recrystallised to form schists, in which blocksflattened in the foliation planes are injected by the matrix, showingevidence of overpressure, as seen in diapiric mélange. Structuraland tectonic geologists working on mélanges on land should con-sider more seriously the possibility that mud diapirism may haveplayed a role in their formation. In the recent Special Issue of Tec-tonophysics (Dilek et al., 2012) there is evidence in the papers byFesta et al. (2012), Codegone et al. (2012a), Codegone et al.(2012b) and Wakita (2012) that this possibility is beginning tobe appreciated.

Acknowledgements

The author is indebted to Dr. Tim Charlton for his companion-ship and support over many years of work on the geology of Timor.He is also thanked for reinterpreting the side-scan sonar imageryacross the Timor Trough, shown as Fig. 5 in this paper, to clarifysome ambiguities in the original interpretation prepared for hisPh.D. Thesis in 1987.

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