Barber 2013 Indonesi

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 ⇑ A.J. Barber

Department of Earth Sciences, Royal Holloway, University of London, Egham, Surrey TW20 0EX, UK article info abstract

Article history: The origin of block-in-matrix mélanges has been the subject of intense speculation by structural and tec- Available online 8 January 2013 tonic geologists working in accretionary complexes since their ﬁrst recognition in the early twentieth century. Because of their enigmatic nature, a number of important international meetings and a large Keywords: number of publications have been devoted to the problem of the origin of mélanges. As mélanges show Mélange the effects of the disruption of lithological units to form separate blocks, and also apparently show the Mud diapirs effects shearing in the scaly fabric of the matrix, a tectonic origin has often been preferred. Then it Mud volcanoes was suggested that the disruption to form the blocks in mélanges could also occur in a sedimentary envi- Timor ronment due to the collapse of submarine fault scarps to form olistostromes, upon which deformation Sumba Nias could be superimposed tectonically. Subsequently it has proposed that some mélanges have originated by overpressured clays rising buoyantly towards the surface, incorporating blocks of the overlying rocks in mud or shale diapirs and mud volcanoes. Two well-known examples of mélanges from the Banda and Sunda arcs are described, to which tectonic and sedimentary origins were conﬁdently ascribed, which proved on subsequent examination to have been formed due to mud diapirism, in a dynamically active environment, as the result of tectonism only indirectly. 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 complex. Comparable diapirs can be recognised in Timor accreted at an earlier stage. Evidence from both Timor and Nias shows that diapiric mélange can be generated well after the initial accretion process was completed. The problem is: Why, when diapirism is so abundantly found in present convergent margins, is it so rarely reported from older orogenic belts? Many occurrences of mélanges throughout the world to which tectonic and/or sedimentary and origins have been ascribed, may in future investigations prove to have had a diapiric origin. It is emphasised that although the examples of diapiric mélange described here may contain ophiolitic blocks, they were developed in shelf or continental margin environments, and do not contain blocks of high grade metamorphic rocks in a serpentinous matrix; such mélanges originate diapirically during subduction in a mantle environment, as previous authors have suggested. Ó 2013 Elsevier Ltd. All rights reserved.

1. Introduction landslides. This interpretation has been used frequently to account for the large-scale mélanges of the Argille Scagliose throughout When Greenly (1919) described a chaotic unit in the Gwna Italy. In the discussion following Beneo’s (1955) paper, Flores Group of Anglesey as ‘mélange’, containing a variety of blocks of introduced the terms olistostrome for these deposits, and olistoliths ocean floor and continental shelf materials, such as spilitic pillow for the included blocks. Although Hsü (1968, 1974) recognised tec- lava and tuff, red jasper chert, manganiferous limestone, shale, tonic and olistostromal mélanges he suggested that the term quartzite and grey limestone in a fine-grained matrix, it seemed ‘mélange’ should be restricted to those formed tectonically, in obvious that this mélange had been disrupted and sheared during practice, due to the difficulty of determining their origins, the term a process of tectonic deformation. Later Beneo (1955) described has been used to describe all ‘block in matrix’ mélange deposits extensive deposits of mélange in Sicily in the western Mediterra- worldwide, however they may have originated. nean, which he interpreted as having been formed in a sedimen- Subsequently both tectonic and sedimentary interpretations, tary environment as the product of large-scale submarine sometimes in combination, have been offered for many other occurrences, especially where mélanges are found in association

⇑ Tel.: +44 020 7228 1897. with accretionary complexes, formed in subduction trenches from E-mail address: [email protected] oceanic material scraped off downgoing oceanic plates (e.g. Aalto,

1981; Cowan, 1985; Collins and Robertson, 1997; Ogawa, 1998; formed from Australian Precambrian basement and continental Robertson and Pickett, 2000; Robertson and Ustaömer, 2011). margin sediments, ranging in age from the Permian to the Pliocene, Meanwhile, geologists using seismic reflection profiling and dril- imbricated in accretionary complexes and thrust beneath an ophi- ling for oil in deltaic deposits, frequently encountered salt diapirs olite nappe, during the collision between the northern continental and shale diapirs containing block-in-matrix deposits in deltas on margin of Australia and the Banda Volcanic Arc (Barber et al., 1977; passive margins, attributed to the mobilisation of salt and shale Charlton et al., 1991). The eastern part of Timor (Timor Leste) was units due to overpressure generated in areas with very high sedi- mapped geologically by Audley-Charles (1968), and the strati- mentation rates, e.g. Nile Delta (Loncke et al., 2006), Niger Delta graphic units he established were later adopted by the Indonesian (Morley and Guerin, 1996) Baram Delta (Morley et al., 1998; Mor- Geological Survey when they mapped the western part of the is- ley, 2003). Shale diapirs are also encountered on active continental land (Rosidi et al., 1981)(Fig. 2). During the mapping Audley- margins, where the overpressuring and mobilisation of shales is Charles (1965, 1968) recognised an extensive mélange deposit, attributed to the stacking of thrust slices, e.g. Barbados (Brown the Bobonaro Scaly Clay, containing a variety of blocks in a scaly and Westbrook, 1988; Stride et al., 1982; Westbrook and Smith, clay matrix (Fig. 3) which, following Beneo (1955), he interpreted 1983), the Makran (Stöcklin, 1990; Grando and McClay, 2007), Ara- as olistostromes, the product of large-scale submarine slumping. kan, Burma (Maurin and Rangin, 2009). Much effort has been ex- This interpretation was accepted by Hutchison (2007) in his ac- pended in defining criteria by which tectonic, olistostromal and count of the geology of Timor in the textbook Geological Evolution diapiric mélanges might be distinguished (e.g. Barber et al., 1986; of South-east Asia (p. 58). Camerlenghi and Pini, 2009; Codegone et al., 2012b; Cowan, 1985; Audley-Charles (1968) and Carter et al. (1976) regarded the Dilek et al., 2012; Festa et al., 2010; Festa, 2011; Lash, 1987; Or- Bobonaro Scaly Clay as a distinct stratigraphic unit, formed at a ange, 1985; Pini, 1999; Raymond, 1984; Silver and Beutner, specific period of time, Early Pliocene, N19-20 in Blow’s (1969) 1980; Sengör, 2003; Wakabayashi, 2011). foraminiferal zonal scheme, although Pleistocene forams were Experience gained during study of mélanges regions of present identified in the scaly clay matrix, these were interpreted as being day active tectonics in the Outer Banda Arc and the Outer Sunda incorporated by soil creep from overlying raised coral reefs. They Arc in Indonesia, to which both tectonic and olistostromal origins visualised that slumping had occurred by the collapse of submar- have been attributed, but for which substantive evidence of diapir- ine fault scarps developed in the accretionary complex, with the ism has been subsequently obtained, suggests that diapiric deposition of the blocks into adjacent troughs, where deep-water mélange is much more common than many structural and tectonic clays, which now form the scaly clay matrix, were being deposited. geologists have appreciated: Diapirism should be considered as an These escarpments were formed as the result of thrusting during origin for mélanges, before tectonism or submarine slumping are the subduction/collision between the northern margin of the Aus- automatically assumed. tralian continent and the Banda Arcs. Later, Hamilton (1979) visited West Timor and described and illustrated the Bobonaro Scaly Clay Mélange. In the outcrops that 2. Mélange in the Outer Banda Arc he visited, Hamilton observed that the fabric of the scaly clay matrix had a predominantly northerly dip and suggested that the The island of Timor in eastern Indonesia, just to north of Austra- mélange had been generated tectonically by southerly-directed lia, forms part of the Outer Banda Arc (Fig. 1). The island was

118°E 120° 122° 124° 126° 128° 130° 132° 134° Banda Accretionary and Collisional Complex Kai Active mud volcano fields BANDA SEA 6° 6°S Aru

Arc nic FLORES SEA Volca Tanimbar 8° Volcanic Arc 8°

ust Timor thr k 1000m c t Sumba a n b fro 10° on 10° Savu ati orm LF def HE Semau L S NTA NE NTI CO AN ALI 12° TR ° US 12 A Darwin INDIAN AUSTRALIAN OCEAN MARGINAL FLOOR SEDIMENTS AUSTRALIA 14° 14°

5000m

1000m 118° 120° 122° 124° 126° 128° 130° 132° 134°

Fig. 1. The Banda Arcs Eastern Indonesia, with the island of Timor formed by the collision between the northern margin of Australia and the volcanic arc showing the distribution of active mud diapirs and mud volcanoes around the Outer Arc (Barber and Brown, 1988). 430 A.J. Barber / Journal of Asian Earth Sciences 76 (2013) 428–438

124°E 125° 126° 127°

0 10 20 30 40 50km Dili

WEST TIMOR 5°S 5°

EAST TIMOR

Melange & Bobonaro Scaly Clay mud volcanoes

Late Miocene- Post-collision Units Recent ° 10° Kolbano Permian- 10 Kupang Pre-Collision Units Late Miocene

124°E 125° 126° 127°

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); West Timor from Rosidi et al. (1981).

margin and on the Timor slope, together with slumps, débris and mud flows and sediment-filled slope basins (Fig. 5), as postulated by Audley-Charles (1965), but the seismic section shows that the surface of the Australian slope and the accretionary complex is composed only of recently deposited soft sediments of Pliocene to Recent age, with no major fault scarps uplifting the underlying continental shelf sequence from which blocks of lithified sediments could be derived, to form olistostromes, as postulated by Audley-Charles (1965, 1968) to account for the Bobonaro Scaly Clay Mélange on Timor. A similar survey was conducted by the R.V. Kana Keoki further west, to the south of the island of Sumba, across the accretionary complex, the trench and the Australian margin (Breen et al., 1986). SeaMARC II side-scan sonar imagery, together with bathymetry and seismic reflection profiling, showed that the surface of the sea floor on the Australian continental shelf, immediately to the Fig. 3. Landscape in West Timor with scattered blocks of a variety of lithologies, south of the deformation front of the accretionary complex, is cov- some the size of houses, due to the erosion of the clay matrix from an outcrop of ered by mounds and ridges (Fig. 6). Neither the side-scan sonar Bobonaro Scaly Clay Mélange. imagery nor the seismic profiles show any large-scale fault scarps in the accretionary complex; the upper slopes of the complex are covered by a thin drape of undisturbed Recent sediments, and overthrusting, during the collision between the northern Austra- are devoid of structural features (Breen et al., 1986, Fig. 7). Seismic lian margin and the Banda Arc. sections obtained south of the deformation front show that at Subsequently, in a geophysical cruise in Eastern Indonesia by intervals of 10 km or so, the regular horizontal reflections in the the Hawaiian Institute of Geophysics, the Research Vessel Kana bedded sedimentary sequence are disrupted by transparent zones, Keoki surveyed the Timor accretionary complex and the Australian devoid of reflections, capped by the mounds on the sea floor. These Continental Shelf to the south of Timor. The survey mapped the transparent zones are interpreted as vertical mud diapirs and the bathymetry, recorded seismic profiles and used the side-scan sonar mounds are interpreted as mud volcanoes (Fig. 6). SeaMarc II system (Blackington et al., 1983) to image the Timor Breen et al. (1986) correlated the sequence represented in the slope, the Timor Trough and the northern part of the Australian profiles with the undisturbed Australian continental marginal se- shelf. The seismic section showed that sediments from the Timor quence encountered in boreholes on the Scott Reef (Rad and Exon, Trough and the Australian margin had been uplifted into the accre- 1982). The deepest reflection (E) is identified as the top of the clas- tionary complex along bedding parallel thrust planes which stee- tic sequence, which marks the break-up unconformity in the pas- pen toward the surface and generate fault-bend folds in the sive margin of the NW Australian Shelf. A transparent zone overlying sediments (Karig et al., 1987)(Fig. 4). Earlier, Upper Pli- between (E) and (C) is identified as Lower Cretaceous hemipelagic ocene to Recent sediments had been drilled in DSPD 262 on the marine shale deposited on the subsided ocean floor. Overlying floor of the Timor Trough and showed that sediments on the floor reflectors between (E) and (A) are identified as lithified Upper Cre- of the trough, to a depth of 400+ m, consist of unlithified radiolar- taceous to Miocene pelagic carbonates, with interbedded marls, ian and clay-rich nanno ooze (Heirtzler et al., 1973). The SeaMARC deposited on a prograding carbonate shelf, and the zone from (A) II side-scan imagery showed fault scarps, both on the Australian to the surface, as Pliocene to Recent pelagic marls. The vertical A.J. Barber / Journal of Asian Earth Sciences 76 (2013) 428–438 431

AUSTRALIA Australian slope Timor TroughTimor slope TIMOR SE NW 2.0 Pliocene-Recent 2.0 slope sediments deformation front seconds two-way time Upper Jurassic trough fill - Miocene 3.0 3.0

4.0 passive margin 4.0 Permian - sediments Upper Jurassic brea k-up unconformity 5.0 5.0 syn-rift sediments seconds two-way time

6.0 6.0

0 10 20km

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

° 124°00’ ° 123 45’E 124 15’ 1000m back thrust slope basins normal 1700 1700 10°30S faults 10°30’ deformation front (inactive) 2300 TIMOR SLOPE sediment slump scars lobes 2300

2000

600 1000 1500 1600 1800 10°45’ 10°45’ 1800 normal anticlines 2000 faults deformation front (inactive) 2300 floor of Timor Trough 2300 1600 0 10 20km 2000 turbidite 11°00’ 123°45’ 124°00’ channels AUSTRALIAN SLOPE 11°00’

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 Timor Trough, with no olistostromes containing blocks derived from the underlying continental margin sequence (reinterpreted after Charlton (1987) and personal communication 2012). 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). transparent zones seen in the seismic sections rise from the level of A subsequent survey by the R.V. Charles Darwin using the GLO- the Lower Cretaceous shale at a depth of 1.5 km, and cut across the RIA side-scan sonar system showed that the diapir field extends bedded reflectors to the base of the surface mounds (Fig. 6). In over an area of 100 km Â 50 km on the Australian continental shelf passing through the overlying bedded sequence the mobilised clay (Masson et al., 1991). The seismic sections show that continental is likely to have incorporated blocks of the lithified carbonate units margin sediments incorporating the diapirs and the mud volca- to form a mélange of Upper Cretaceous to Miocene limestone noes, including many hundred cubic kilometres of mélange, are blocks in a hemipelagic clay matrix. Scattered occasional reflec- in the process of incorporation into the toe of the accretionary tions in the transparent zones may from larger blocks. The largest complex (Fig. 6). of the ridges on the sea floor is 200–300 m high, 18 km long with a Westbrook and Smith (1983), in a comparable situation in width of 2.5 km and extends to a depth of 1.5 km, representing a Barbados, where mud diapirs and mud volcanoes occur in Orinoco volume of 70 km3 of mélange. Fan sediments on the oceanward side of the deformation front, 432 A.J. Barber / Journal of Asian Earth Sciences 76 (2013) 428–438

ACCRETION OF DIAPIR COMPLEX SOUTH OF SUMBA (after Breen et al. 1986) S N sec/twt Australian Margin sediments Accretionary Complex Df 2 Df1 5 Miocene-Recent O pelagic carbonates A Late-Cretaceous-Miocene lithified pelagic carbonates & marls C Early Cretaceous 6 hemipelagic clays E Jurassic deltaic sandstones

5km vertical exaggeration = 6.2 7 E = BREAK_UP UNCONFORMITY Line drawing from S-N shipboard seismic reflection profile across the Java Trench south of Sumba from HIG R.V. ‘Kana Keoki’. The profile shows mud volcanoes on the sea-floor and mud-diapirs rising from C-E through reflectors in consolidated Australian marginal sediments.

The deformation front (Df 1 ) at the toe of the accretionary complex has been replaced recently by Df2 incorporating the sediments and the diapirs into the complex.

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 from larger blocks. The sediments, together with the diapirs are being accreted as the deformation front of the accretionary complex moves southwards.

120°E 121° 120° 10°S SUMBA 10°

SAVU

ROTI 11° Limit of GLORIA Survey 11° A c c r e t i o n a r y C o m p l e x Fig. 6 Back thrust Diapir Field SeaMARC II Deformation 0 50 100km front Survey 12° 12° Mud diapirs 120° 121° 122°

Fig. 7. Areas covered by the SeaMARC II and GLORIA side scan sonar surveys showing the 50 Â 100 km extent of the diapir field (Breen et al., 1986; Masson et al., 1991). The line of section in Fig. 6 is shown within the SeaMARC II coverage. suggested that stacking of thrust slices in the accretionary complex break-up unconformity form a permeable horizon within the se- generated overpressure in fluids (water, oil and gas) in the under- quence, while the overlying Lower Cretaceous hemipelagic clays lying sediments. The overpressured fluids were projected along form the impervious seal which was mobilised by overpressured permeable horizons in the incoming sedimentary sequence to fluids to form the diapirs and mud volcanoes (Fig. 6). This combi- mobilise overlying impermeable clays, which rose buoyantly nation of circumstances are likely to occur whenever a passive con- through the sequence to form clay diapirs and mud volcanoes on tinental margin enters a subduction system. Interestingly, the the surface. This model is applicable to the Australian continental Lower Cretaceous hemipelagic clays had remained in a stable con- margin sediments south of Sumba, where Jurassic sands at the dition in the continental shelf sequence for 100 million years A.J. Barber / Journal of Asian Earth Sciences 76 (2013) 428–438 433 before they were activated by the overpressure generated by the accretion process. During mapping in West Timor the Geological Survey of Indone- sia identified twenty-one mud volcano fields and cross-cutting mud diapirs, intruded through the other rocks, as the Bobonaro Scaly Clay Mélange (Rosidi et al., 1981)(Fig. 2). In the Kolbano area, and in other areas in the southern part of the island, imbricated and folded Late Cretaceous to Miocene carbonates, form a Late Ter- tiary accretionary complex and are cut by mud diapirs composed of red scaly clay containing manganese nodules and blocks of Late Cretaceous to Miocene limestones, forming a mélange (Barber et al., 1986, Fig. 2). Hydrofracturing of the blocks, with films of the matrix injected along joints and fractures indicate that the mud 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 develops in overpressured clays, without the addition of superimposed shearing. The Cretaceous to Miocene limestones and the accompanying mélange in Kolbano and other areas in the southern part of Timor, correspond to the Australian marginal sediments and diapirs south of Sumba, which were incorporated at an earlier stage in the island arc-continental collision, along a décollement at a level near the break-up unconformity. There are no active mud volcanoes in this part of Timor, evidently the diapiric system is now inactive. The Fig. 9. Cross-section of a mud diapir containing lensiod and irregular blocks of collision commenced in the region of East Timor, and has since ex- limestone and sandstone in a scaly clay matrix in roadside quarry, near Takari, West tended to both east and west. Timor. The matrix is intruded along fractures in the blocks, indicating high In the island of Semau (Barber et al., 1986)(Fig. 8) and in other overpressures, and the foliar structure of the scaly clay matrix is attributed to flow areas in the northern part of Timor (Fig. 2), active mud volcano ex- rather that to shearing. trude fluid mud and bubbles of methane and have an oily scum on the surface of mud pools. Mud in the volcanoes and scaly clay in the diapirs were derived from Permo-Triassic red shale horizons and carry blocks of Permo-Triassic sandstone and limestone (Figs. 9 and 10). Evidently, the Permo-Triassic rocks were uplifted into the accretionary complex from a décollement surface near the basement unconformity with the Australian basement in a process of underplating (Charlton et al., 1991). Hutchison (2007, p. 320), following Audley-Charles (1968), noted that ‘ultramafic’ rocks are also included in the mélange, showing that some of the diapirs had intruded the overlying ophiolite nappe. Other diapirs and mud volcanoes contain blocks of reef material and were intruded

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

through Pleistocene coral reefs (Barber et al., 1986). The evidence from the incorporation of the coral blocks and active mud volcanism, indicates that compression, overthrusting and the generation of fluid overpressures continues within the collision complex in the northern part of Timor to the present day. Fig. 8. The mud volcano field showing small-scale mud volcanoes up to 3 m high, Diapirs and active mud volcanoes occur extensively on all the with fluid mud flows and scattered blocks, in the crater of the 1 km diameter large Banda Outer Arc islands between Timor and Seram (Fig. 1). The mud volcano forming Palau Kambing, SW of Kupang, West Timor. The crater wall in process of the formation of mélanges must be occurring on a very the background is littered with blocks. The soft clays are eroded during the rainy season and washed out through a gap in the crater wall (see Barber et al., 1986, large scale today as the result of the collision between Australia Fig. 3). and the Banda Arcs. 434 A.J. Barber / Journal of Asian Earth Sciences 76 (2013) 428–438

3. Mélange in the Outer Sunda Arc From their study of the seismic sections, Moore and Karig (1976) and Karig et al. (1979) developed a model for the evolution Outcrops of mélange were mapped in the outer arc islands of of the accretionary complex, the development of the forearc ridge Nias, Simeulue, Siberut, Sipora and Pagai, associated with subduc- and the geology and structure of Nias. They speculated that the tion of the Indian Ocean floor beneath Sumatra. (Andi Mangga and Oyo Mélange was developed as a trench-fill deposit composed of Burhan, 1994; Budhitrisna and Andi Mangga, 1990; Endharto and fragments of ocean crust and turbidites which slumped down the Sukido, 1994). The mélange contains blocks of serpentinite, gabbro, face of the accretionary complex as debris flows (olistostromes) basalt, chert, calcilutite, Eocene limestone with foraminifera, gra- and were subsequently accreted at the toe of the complex (Moore nitic and metamorphic rocks together with abundant greywacke and Karig, 1980). Further debris flows were derived from fault shale and claystone in a scaly clay matrix. scarps developed on the NE side of slope basins formed on top of In the 1970s the Scripps Institution of Oceanography (SIO) ob- the accretionary complex. The resolution of the 1970s seismic tained seismic sections from the Indian Ocean floor, the Sunda reflection imaging was not sufficiently clear to either confirm or Trench and the Outer Arc Ridge to the SW of Sumatra (Moore contradict these interpretations (Fig. 11). Neither debris flows and Curray, 1980)(Fig. 11A). Karig et al. (1980) made a detailed nor diapirs are seen in the seismic profiles across the Sunda Trench study of the trench and the accretionary complex from the SIO (Fig. 11A and B). It may require swath mapping or 3D seismic sur- seismic profiles, and then completed a series of traverses across veys across the trench and the accretionary complex to identify the Outer Arc Ridge where it is exposed on the island of Nias such flows (e.g. Gee et al., 2007). Karig et al. (1979) further specu- (Moore et al., 1980b; Moore and Karig, 1980). On Nias the mélange lated that the chaotic and sheared appearance of the mélange was deposits, termed the Oyo Complex, occur as a series of NW–SE due to the dynamic tectonic environment within the developing trending linear belts parallel to the length of the island, several accretionary complex, in which original thrust surfaces were con- hundred metres wide and kilometres in length, alternating with tinually reactivated, with the development of new thrust planes, bedded turbidites of Early Miocene to Early Pliocene age, termed disrupting the accreted oceanic material and breaking it into the Nias Beds (Fig. 12). blocks. They were unable to determine the age of the mélange di- In the early 1990s Nias Island was mapped in detail by Samuel rectly, but inferred that the age was indicated by the youngest (1994; Samuel et al., 1997)(Fig. 12) with the ages of the rocks very dateable included blocks, the Eocene limestones. tightly controlled by detailed microfossil analysis, supported by Although no depositional contacts were observed, Moore et al. petrographic, mineralogical and geochemical analyses. In particu- (1980a) and Moore and Karig (1980) suggested that the Oyo lar a systematic study was made of the Oyo Mélange and the rela- Mélange was the oldest unit on Nias, on which the Nias Beds were tionships to the bedded units. deposited unconformably. They suggested that the Nias Beds had

SW NE seconds Mentawai two-way-time Outer Arc Ridge Fault Forearc Basin 0

1 (A) Nias Transect Accretionary Complex 2 SUNDA TRENCH 3 Deformation front 4

6 0 10 20km Indian Ocean Crust Vertical exaggeration 6.1 X SW NE 2000 (B) South Sumatra Transect Outer Arc Ridge 3000 Accretionary Complex SUNDA TRENCH 4000 slope basins Deformation 5000 front

6000

Depth in m 7000

8000

9000 Indian Ocean Crust 10000 Vertical exaggeration 1.5 X 11000 26 28 30 32 34 36 38 40 42 44 46 48 50 52 54 56 58 60 62 64 66 68 Distance along profile in km

Fig. 11. (A) NE–SW line drawing from seismic proﬁle to the south of Nias, from the Indian Ocean Floor across the Sunda Trench, the accretionary complex and the Outer Arc Ridge to the Sumatran Forearc Basin (redrawn from Moore et al. (1980b)). (B) NE–SW line drawing from a seismic section offshore South Sumatra. The original section is in Kopp and Kukowski (2003). A.J. Barber / Journal of Asian Earth Sciences 76 (2013) 428–438 435

100°E 6°N 6° 97°15'E 97°30' SUMATRAN FOREARC Simeulue

B 1°30'N NIAS SUMATRA 0° 0°

Fig.11A * INDIAN OCEANSiberut Pagai Sunda Trench

* GUNUNG- ° SITOLI 6 S 6° 97°45' ° 1B ° 1 15' 1 15' Fig.1

100° 200km

Fault Lineament Thrust Anticline Syncline B 1°00 1°00'

Mud volcanoes B * Alluvium C B Oyo Mélange (blocks in scaly clay) Gunungsitoli Formation (Plio-Pleistocene) B Gomo Formation ) ) 'Nias Beds' (Early Miocene-Early Pliocene) B Lelematua Formation ) B C Conglomerate Member (Late Oligocene-Early Miocene) 0°45' B Ophiolitic basement (B)

reefs

0 10 20km reefs TELUKDALAM 97°15'E 97°30' 97°45' reefs

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’ are outcrops 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 side of 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.

been deposited in slope basins on the accretionary complex and Miocene sandstones and conglomerates, that the sediments on were uplifted and compressed during thrusting, with the growth Nias were derived from a mature continental provenance, either of the complex, until they were exposed in Nias. basement uplifts in the forearc or the mainland of Sumatra, and On the other hand Samuel et al. (1995) found that the bedded that prior to the Pleistocene, sedimentation extended continuously sediments on Nias had been deposited in basins developed on from Sumatra across the present forearc basin onto the outer arc top of the accretionary complex, bounded to the NE by syn-depo- islands, and do not correspond to the deposits in the Sunda Trench. sitional extensional faults. They also found that the Upper Palaeo- Samuel (1994) found that in addition to ophiolitic material, ser- gene and Neogene stratigraphic sequences on Nias correspond pentinite, gabbro, basalt, calcilutite and chert of ocean ﬂoor prove- with the same sequences in the Banyak and Batu islands, which nance, at least 50%, and commonly 90% of the blocks in the lie within the forearc basin to the northeast of Nias, and also in mélange had been derived from the bedded Oligocene and Lower boreholes in the forearc basin itself. These sequences resemble in Miocene units, while some outcrops contained clasts of Upper Mio- stratigraphy and lithology the units on Nias. Samuel et al. (1995) cene and even Pleistocene units. He also found that the mud matrix therefore concluded from the occurrence of well-rounded quartz contained the same microfossils, had the same mineral composi- pebbles and metamorphic clasts, in the Oligocene and Lower tion and the same thermal history, yielding the same range of 436 A.J. Barber / Journal of Asian Earth Sciences 76 (2013) 428–438

experimental work of Yassir (1987) shows that the presence of a scaly clay fabric does not necessarily indicate tectonic deformation, as Hamilton (1979) assumed. However, it is possible for deformation to be superimposed on mélanges after accretion has ceased, e.g. Barber et al. (1986) describe mélange sheared along strike-slip faults in West Timor, and Harris et al. (1998) has suggested that the mélanges occur along the base of major thrusts developed during the progress of the collision. It may be that the diapiric mélanges have provided weak zones along which the thrust planes were developed. Evidence from Timor and the islands to the east, indicates that compression and thrusting to produce overpressure, with the formation of mud diapirs and mud volcanoes in the Outer Banda Arc, due to the collision with Australia, continues to the present day. The seismic sections across the Sunda Trench, the accretionary complex and the Outer Arc Ridge offshore Sumatra show no evi- Fig. 13. The margin of a mud diapir exposed in the bank of a tributary stream of the Oyo River, showing the grey scaly clay matrix, with a fabric parallel to the margin, dence of mud diapirism from the Nicobar Fan sediments on the In- cutting and injected along joint fractures in a greywacke sandstone bed, Oyo dian Ocean floor (Fig. 11A and B), in contrast to the Northwest Mélange, Nias, Sumatran Outer Arc. Notebook 110 Â 150 mm. Australian Shelf south of Sumba, where the sequence contains a permeable clastic sequence overlain by impermeable clays which could be mobilised by overpressures generated by the loading of vitrinite values, as mudstones in the Oligocene to Lower Miocene the accretionary complex. Although the presence of débris flows succession. in the Sunda Trench or in slope basins on the developing accretion- Samuel (1994) found that the scaly fabric that pervades the ary complex in the Sunda Arc system, as proposed by Karig et al. mélange matrix is commonly vertical, but may be folded and (1979), nor the disruption and break-up of units by thrusts within wrapped around the enclosed clasts, and at the contacts, the fabric the accretionary complex, cannot definitely be ruled out, Samuel’s is parallel to the margins of the mélange outcrop (Fig. 13). Karig (1994; Samuel et al., 1995, 1997) study of the geology of the island et al. (1980) observed that as mélange units are traced along strike of Nias has shown conclusively that these processes were not they are in contact with different units of the Nias Beds, and are responsible for the formation of the mélanges on Nias. Samuel sheared and mixed into the mélange. But as we have seen already, (1994) found that the Oyo Mélange Complex was formed as mud a shearing process is not necessary to impart the scaly fabric to the diapirs within extensional sedimentary basins, formed on top of clay matrix of the mélanges (Yassir, 1987). Karig et al. (1980) inter- the accretionary complex. Nor were the mélanges deformed as preted these contacts as high angle reverse faults. However, Sam- the result of the continual reactivation of the accretionary com- uel et al. (1997) observed that the contacts between the mélange plex, causing the development of thrust planes with further dis- and the adjacent bedded units are always intrusive, with the ma- ruption and the formation of the scaly fabric, as Moore and Karig trix penetrating along the bedding planes and fractures in the bed- (1980) had suggested. As we have seen the occurrence of mud vol- ded units. If these observations are correct, the thrusts shown cano fields on Nias and the other Sunda Outer Arc islands indicates along the SW margins of the mélange outcrops in geological map that compression-generated overpressures forming diapiric (Fig. 12) are probably erroneous. The mélange was observed to mélange is still in the process of formation at the present day. cut units of all ages from Oligocene to Recent. It appears that the Nevertheless, the olistostromal and tectonic models for the for- main period of mélange formation occurred during the Pliocene, mation of mélanges proposed by Karig et al. (1979) has been so im- but mélange production continues to the present day, as shown mensely persuasive and influential that it has been invoked as an by the eruption of mud volcano fields (Fig. 12), extruding blocks explanation for the occurrence of mélanges all over the world; in a grey mud slurry, identical in composition and ages to the ma- e.g. Turkey and the Himalayas (Collins and Robertson, 1997; Rob- trix of the mélange. ertson and Pickett, 2000; Robertson and Ustaömer, 2011). Hamil- Similar mélanges, have been mapped on the other Sumatran ton (1979), as a Californian geologist, would have been well Outer Islands of Simeulue, Siberut, Sipora and Pagai, commonly aware of the work at Scripps, which led him to the interpretation with circular outcrops, and associated with active mud volcanoes of the Bobonaro Scaly Mélange of West Timor as of tectonic origin, (Endharto and Sukido, 1994; Andi Mangga and Burhan, 1994; formed by the process visualised by Karig et al. (1979). Budhitrisna and Andi Mangga, 1990). Evidently as in Timor and As far as the author is aware no clasts of blueschist or eclogite the other island of the Outer Banda Arc System, the outer arc ridge have been recorded from the mélanges on Timor or on the Outer in the Sunda Arc system continues to be compressed, causing over- Sunda Arc islands, which as has been explained originated as dia- pressure and the mobilisation of clays to form mud diapirs and pirs on a continental shelf or in an active continental margin envi- mud volcanoes. ronment. Clasts of ophiolitic materials are found in the mélanges on both Timor and Nias but these have been incorporated when 4. Conclusions diapirs have penetrated overlying accreted slices of ocean crust. Mélanges that contain clasts of blueschists, eclogites and other Contrary to the hypothesis of Audley-Charles (1965), evidence high grade metamorphic rocks, often in a serpentinous matrix such from Sumba and Timor indicates that there are no débris flows as those in the Franciscan Mélange of California and the Peleru containing continental shelf blocks, associated with the develop- Mélange Complex of Central Sulawesi have risen diapirically up ment of the accretionary complex forming at the present day, the subduction zone from the mantle, as earlier authors have sug- which could account for the Bobonaro Scaly Clay Mélange on Ti- gested (Cloos, 1984, 1985; Parkinson, 1996). mor. Evidence from Sumba indicates that large volumes of In a landmark paper, Kopf (2002, Fig. 1) building on an earlier mélange, in the form of diapirs, were generated by overpressured review by Higgins and Saunders (1967) catalogued the occurrence fluids in the Australian continental marginal sediments, before of mud volcanism and mud diapirism throughout the world; he they were incorporated into the accretionary complex. The noted 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 abundant Breen, N.A., Silver, A.E., Hussong, D.M., 1986. Structural styles of accretionary wedge in the collision zone between Africa and Eurasia and in the subduc- south of Sumba, Indonesia, revealed by SeaMARC II side-scan sonar. Bulletin of the Geological Society of America 98, 1250–1261. tion and collision zones between the Indian and Australian plates Brown, K.M., Westbrook, G.K., 1988. Mud diapirism and subcretion in the Barbados and Southeast Asia. The floor of the Mediterranean Sea has been Ridge Complex. Tectonics 7, 613–640. particularly well studied by marine geophysical surveys in the past Budhitrisna, T., Andi Mangga, S., 1990. The Geology of the Pagai and Sipora Quadrangles, Sumatra, Scale 1:250,000. Geological Research and Development two decades (see Camerlenghi and Pini, 2009, Fig. 4). Kopf (2002) Centre, Bandung. claims that is ‘‘arguably the region with the highest abundance Camerlenghi, A., Pini, G.A., 2009. Mud volcanoes, olistostromes and Argille Scagliose of MVs (mud volcanoes) and diapirs on Earth’’. Several hundred in the Mediterranean region. Sedimentology 56, 319–365. Carter, D.J., Audley-Charles, M.G., Barber, A.J., 1976. Stratigraphic analyses of island mud volcanoes and mud diapirs have been discovered on Mediter- arc continental margin collision in eastern Indonesia. Journal of the Geological ranean Ridge accretionary complex to the south of Crete, formed Society, London 132, 179–198. by the subduction of the African Plate beneath the European Plate. Charlton, T.R., 1987. The Tectonic Evolution of the Kolbano-Timor Trough Accretionary Complex, Southern West Timor. Unpublished Ph. D. Thesis, The mud volcanoes have been imaged by seismic surveys and University of London. swath mapping and have been sampled by coring, deep drilling Charlton, T.R., Barber, A.J., Barkham, S.T., 1991. The structural evolution of the Timor and the use of submersibles, with study of the blocks and the clay Collision Complex, Eastern Indonesia. Journal of Structural Geology 13, 489– matrix. It is a remarkable anomaly that although mélanges occur 500. Cloos, M., 1984. Flow mélanges and the structural evolution of accretionary wedges. commonly on the adjacent land areas of Greece, Crete, Cyprus Geological Society of America Special Paper 198, 71–80. and Southern Turkey they are invariably attributed to the pro- Cloos, M., 1985. Thermal evolution of convergent plate margins: thermal modelling cesses of submarine slumping and tectonism, not to mud diapirism and re-evaluation of isotopic Ar-ages for blueschists in the Franciscan Complex of California. Tectonics 4, 421–433. (Camerlenghi and Pini, 2009, Fig. 3). Although mud diapirs may be Codegone, G., Festa, A., Dilek, Y., 2012a. Formation of Taconic mélanges and broken difficult to recognise in older mélanges on land because diagnostic formations in the Hamburg Klippe, Central Appalachian Orogenic Belt. features such as mud volcanoes have long since been eroded away, Tectonophysics 268–269, 215–229. Codegone, G., Festa, G., Dilek, Y., Pini, G.A., 2012b. Small-scale polygenetic mélanges and the cross-cutting relationships have been modified or de- in the Ligurian accretionary complex, Northern Apennines, Italy, and the role of stroyed by later tectonism. However, examples are known where shale diapirism in superposed mélange evolution in orogenic belts. mélanges are incorporated into metamorphic complexes and have Tectonophisics 568–569, 170–184. Collins, A.S., Robertson, A.H.F., 1997. Lycian mélange, southwest Turkey: an been deformed and recrystallised to form schists, in which blocks emplaced Late Cretaceous accretionary complex. Geology 25, 255–258. flattened in the foliation planes are injected by the matrix, showing Cowan, D.S., 1985. Structural styles in Mesozoic and Cenozoic mélanges in the evidence of overpressure, as seen in diapiric mélange. Structural Western Cordillera of North America. Geological Society of America Bulletin 96, 451–452. and tectonic geologists working on mélanges on land should con- Dilek, Y., Festa, A., Ogawa, Y., Pini, G.A. (Eds.), 2012. Chaos & Geodynamics: sider more seriously the possibility that mud diapirism may have Mélanges, Mélange forming Processes and their Significance in the Geological played a role in their formation. In the recent Special Issue of Tec- Record. Tectonophysics 568–569, 1–388. tonophysics (Dilek et al., 2012) there is evidence in the papers by Endharto, M., Sukido, 1994. Geological Map of the Sinabang Quadrangle, Sumatra, Scale 1:250,000. Geological Research and Development Centre, Bandung. Festa et al. (2012), Codegone et al. (2012a), Codegone et al. Festa, A., 2011. Tectonic, sedimentary and diapiric formation of the Messinian (2012b) and Wakita (2012) that this possibility is beginning to mélange: Tertiary Piedmont Basin (northwestern Italy). In Wakabashi, J., Dilek, be appreciated. Y. (Eds.), Mélanges: Processes of Formation and Societal Significance. Geological Society of America Special Papers, 480. doi: http://dx.doi.org/10.1130/ 20112480(10). Festa, A., Pini, G.A., Dilek, Y., Codegone, G., 2010. Mélanges and mélange-forming Acknowledgements processes: a historical overview and new concepts. In: Dilek, Y. (Ed.), Alpine Concept in Geology. International Geology Review, 52, 1040–1105. doi: http:// dx.doi.org/10.1080/00206810903557704. The author is indebted to Dr. Tim Charlton for his companion- Festa, A., Dilek, Y., Pini, G.A., Codegone, G., Ogata, K., 2012. Mechanisms and ship and support over many years of work on the geology of Timor. processes of stratal disruption and mixing in the development of mélanges and He is also thanked for reinterpreting the side-scan sonar imagery broken formations: redefining and classifying mélanges. 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