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Tectonophysics 389 (2004) 65–79 www.elsevier.com/locate/tecto

Ocean blocked and obliterated by Banda collision with Australian proximal continental slope

M.G. Audley-Charles*

Department of Sciences, University College London, Gower Street, London WC1E 6BT, UK SE Asia Research Group, Department of Geology, Royal Holloway, University of London, Egham, Surrey TW20 0EX, UK Received 19 November 2003; accepted 9 July 2004 Available online 27 August 2004

Abstract

The and abrupt changes in seismicity around the eastern end of the Java Trench suggest it is now blocked south–east of Sumba by the Australian, Jurassic-rifted, forming the largely submarine Roti– Ridge. Plate reconstructions have demonstrated that from at least 45 Ma the Java Trench continued far to the east of Sumba. From about 12 Ma the eastern part of the Java Trench (called Banda Trench) continued as the active plate boundary, located between what was to become , then part of the Australian proximal continental slope, and the Banda . This Banda Trench began to be obliterated by continental margin-arc collision between about 3.5 and 2 Ma. The present position of the defunct Banda Trench can be located by use of plate reconstructions, earthquake seismology, deep reflection seismology, DSDP 262 results and geological mapping as being buried under the para-autochthon below the foothills of southern Timor. Locating the former trench guides the location of the apparently missing large southern part of the Banda forearc that was carried over the Australian continental margin during the final stage of the period of of that continental margin that lasted from about 12 Ma to about 3.5 Ma. Tectonic collision is defined and distinguished from subduction and rollback. Collision in the southern part of the was initiated when the overriding forearc basement of the upper plate reached the proximal part of the Australian continental slope of the lower plate, and subduction stopped. Collision is characterised by fold and thrust deformation associated with the development of structurally high decollements. This collision deformed the basement and cover of the forearc accretionary prism of the upper plate with part of the unsubducted Australian cover rock sequences from the lower plate. Together with parts of the forearc basement they now form the exposed Banda orogen. The conversion of the northern flank of the from being the distal part of the Banda forearc accretionary prism, carried over the Australian continental margin, into a was initiated by the cessation of subduction and simultaneous onset of collisional tectonics. This reinterpretation of the locked eastern end of the Java Trench proposes that, from its termination south of Sumba to at least as far east as Timor, and probably far beyond, the Java-Banda Trench and forearc overrode the subducting Australian proximal continental slope, locally to within 60 km of the shelf break. Part of the proximal forearc’s accretionary prism together with part of the proximal continental slope cover sequence were detached and thrust northwards over the Java-Banda Trench and forearc by up to 80 km along the southwards dipping Savu Thrust and Suture. These reinterpretations explain the

* La Serre, St. Pantale´on, 46800 Montcuq, France. Tel.: +33 5 65 31 80 67; fax: +44 709 231 7196. E-mail address: [email protected].

0040-1951/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.tecto.2004.07.048 66 M.G. Audley-Charles / Tectonophysics 389 (2004) 65–79 present absence of any discernible subduction trench in the southern Banda Arc and the narrowness of the forearc, reduced to 30 km at Atauro, north of . D 2004 Elsevier B.V. All rights reserved.

Keywords: Banda trench; Banda arc; Java trench; Sumba; Timor trough

1. Introduction This paper presents no new data and does not attempt an overall review of Banda Arc evolution. Its Geological and geophysical observations, analyses principal objectives are focused on illuminating the and interpretations during the last dozen years have post-15 Ma evolution of the Java Trench eastward of advanced knowledge and understanding of the region Sumba with the formation of the Banda subduction to a stage that encourages reconsideration of some of ocean trench. This trench was intimately associated the outstanding unresolved problems of the Banda Arc with the initiation of the Banda Volcanic Arc. The (Engdahl et al., 1998; Fortuin et al., 1994, 1997; Banda Trench was then destroyed at the collision Genrich et al., 1994; Hall, 2002; Hall and Wilson, stage of subduction-rollback, as part of the evolution 2000; Hall and Spakman, 2002; Hamilton, 1995; of the Banda orogen in the region from Sumba to Harris, 1991; Harris and Long, 2000; Harris et al., Timor. The main nine objectives of this paper may be 2000; Hughes et al., 1996; Kreemer et al., 2000; summarised as: McCaffrey, 1996; Richardson and Blundell, 1996; Snyder et al., 1996). (a) To locate and define the active continuation of the One of the enigmatic geological features of SE Java Trench eastward of Sumba at 5 Ma (Fig. 3). Asia (Fig. 1) is the sudden termination at 1208E of the (b) To explain the absence now of any oceanic 3000 km long, more than 6 km deep Sunda–Java subduction trench in the Banda Arc (Figs. 1 Trench (Fitch, 1970; Audley-Charles, 1975; Chama- and 4). laun et al., 1982; McCaffrey, 1988; Engdahl et al., (c) To show that the Timor Trough could never have 1998; Kreemer et al., 2000; Hall, 2002). A linked acted as a subduction ocean trench. problem is the absence of any clearly recognisable (d) To explain the exceptionally small size of the subduction ocean trench (Fig. 2) that could have been Banda forearc now having a width of only 30 to responsible for the Banda Volcanic Arc and the related 50 km between Atauro and Wetar (Figs. Neogene orogen represented by the islands of the 1, 3 and 4). Outer Banda Arc and their linking submarine ridge (e) To define and distinguish tectonic collision from whose geology is best known from the largest islands subduction-rollback in passive continental mar- of Timor and Seram. A related geological mystery is gin-volcanic collision zones, as the whereabouts within the Banda orogen of the exemplified by the southern part of the Banda fundamental decollement that originally separated the Arc. now folded and thrust para-autochthonous cover rocks (f) To explain the presence now of the distal part of from their autochthonous Australian continental base- the Banda forearc accretionary prism about 150 ment of the lower plate. This key objective was not km south of the Australian continental margin attained by seismic reflection surveys (Richardson exposed in northern Timor (Figs. 1 and 4). and Blundell, 1996; Snyder et al., 1996). Another (g) To explain the contribution of overthrusting of anomaly of the Banda Arc is the locally extreme the distal part of the Banda forearc accre- narrowness of the Banda forearc between Atauro and tionary prism to the evolution of the Timor Wetar, reduced to 30–50 km. These riddles are Trough as subduction-rollback ceased and considered as part of the discussion of the mode of collision commenced. blocking and of burial of the Banda subduction (h) To indicate the possible role of gravity and of trench, and burial of a large part of the forearc pore-pressure loss in slowing and stopping the basement during the Neogene–Quaternary collision. subduction process as the upper plate rode up the M.G. Audley-Charles / Tectonophysics 389 (2004) 65–79 67

Fig. 1. Physiographic and tectonic sketch map of the Timor–Sumba region to show: present-day northward dipping tectonic plate suture in the Java Trench, and the much younger southward dipping suture now relocated north of the Banda volcanic arc; the southward dipping Savu Thrust (Silver et al., 1983) and the Wetar Suture (Audley-Charles, 1981) now locked; selected transcurrent faults; and location of former, now obliterated Banda subduction Trench. This assumes the Banda Arc to the north wall of Banda Trench separation dimension (Fig. 3) has not changed by more than 10% since 5 Ma. Australian continental margin is now locked in collision with the Banda forearc from Wetar to the Sumba Ridge. The east end of the Java Trench now blocked against the Australian margin has been overridden by the Banda forearc, which itself has been overridden by the continental margin para-autochthon forming the Roti-Savu Ridge moving on the Savu Thrust. Bathymetry in metres from Gebco database (IOC, IHO, BODC, 1997).

lower plate ramp to within 60 km of the 2. The Timor Trough Australian shelf break (Fig. 4). 2.1. The Timor Trough initiated from tectonic Many of these objectives can be advanced by emplacement of the Banda forearc’s distal part of recognising the geological significance of the deep the accretionary prism onto the Australian continental reflectors in the two profiles across the Banda forearc margin in the BIRPS data (Richardson and Blundell, 1996; Snyder et al., 1996) which can be shown to The Timor Trough (Figs. 1 and 4) is a correspond with the postulated southward dipping 700 km long and between about 30 and 75 km wide. Wetar Suture, and the northward overthrusting of part Its depth varies between 2 and 3.2 km. It is located of the Australian parautochthon together with part of entirely within what is now the Australian continen- the proximal Banda forearc (Figs. 1 and 4). tal slope and is underlain by Australian continental 68 M.G. Audley-Charles / Tectonophysics 389 (2004) 65–79

Fig. 2. Seismicity in the Sumba to Timor region of the Banda Arc showing hypocentres below 75 km based on the dataset of Engdahl et al. (1998). Hypocentres shallower that 75 km are omitted for clarity because they are so widely distributed they obscure deeper seismicity. The seismic data indicate the presence below the Banda Volcanic Arc of the northward-dipping Australian lithospheric . (Richardson and Blundell, 1996; Snyder by Hughes et al. (1996) in the upper part of the et al., 1996). At 5 Ma the Banda Trench, located cover sequence in the northern flank of the Timor between the Banda Volcanic Arc and proto-Timor, Trough. They interpreted this as indicating that these was continuing to subduct the northward-dipping imbricated Cenozoic deposits represent part of the Australian continental margin (Fig. 3). That active Banda forearc accretionary prism; and by implica- plate boundary (Hall, 2002) was then about 250 km tion, but not stated, of the distal part of the prism. north of what was to become the present axis of the The southerly limit of the prism has been long Timor Trough about 4 million years later. recognised as the Late Cenozoic deformation front The Timor Trough was formed partly as a (Barber, 1981). The presence of the apparently consequence of loading (Jordan, 1981)ofthe undisturbed Mesozoic–Upper Palaeozoic cover subducting Australian continental lithospheric plate sequence below the Mid-Base Tertiary decollement, by the Banda forearc accretionary prism and part of that marks the base of the imbricate section mapped its basement (Figs. 1, 3 and 4), and partly from by Hughes et al. (1996), indicates the sub-decolle- inelastic failure at the beginning of continental ment section is very likely autochthonous. How subduction (Tandon et al., 2000). Veevers et al. much of the structural deformation in this accre- (1978) with reference to DSDP 262 showed how the tionary prism resulted from the pre-collision sub- Timor Trough axis migrated away from Timor duction and how much from post-subduction towards the Australian as the northern collision stresses is difficult to resolve. Hughes et edge of the former shelf subsided into the trough on al. (1996) remarked on the difficulties of determining its southern flank (Figs. 3 and 4). An imbricate the finer structural resolution even after reprocessing structural pattern was mapped by seismic reflection of the seismic data. M.G. Audley-Charles / Tectonophysics 389 (2004) 65–79 69

Fig. 3. Palaeogeographical sketch map of the eastern part of the Java Trench and its continuation as the Banda Trench during active subduction at 5 Ma. Positions of the named present-day islands follows Hall (2002) but their shapes are for reference only. Timor, , Moa, Sermata, Babar, Roti, Savu and Sumba were entirely submarine at 5 Ma. Volcanic islands and their associated bathymetry sketched in present-day configuration. Sumba region bathymetry partly after Fortuin et al. (1992, 1994, 1997). Schematic bathymetry in meters used to locate Banda Trench constrained by assuming 5 km for the depth of the Banda Trench, and 50 km for its width at that depth; and by the positions of the Banda volcanic arc islands (Hall, 2002); the Australian 5 Ma shelf edge position estimated from DSDP 262 (Veevers et al., 1978), and micropalaeontology of Neogene part of para-autochthonous Timor Kolbano sequence (Audley-Charles, 1986b) in what was to become Timor Island after collision at 3.5–2 Ma.

2.2. The Timor Trough as a perisutural foreland basin thrusting in foreland basins particularly as one approaches their related orogen. They distinguished A case for the Timor Trough being a foreland basin Ampferer-or A-zone subduction, which they defined has already been made (Audley-Charles, 1986a; as exclusively continent-based, and which they Tandon et al., 2000). The regional geological and characterised as typical of perisutural foreland basins. tectonic setting of Neogene-Quaternary perisutural They contrasted such thrust zones as fundamentally foreland basins in Eastern and Papua New different in location, function and scale from Wadati– Guinea was discussed by Audley-Charles (1991).Itis Benioff subduction zones that are always associated widely recognised (Dickinson, 1974) that thrusting is with an . The imbricate structural usually developed within foreland basins. Bally and pattern in the northern flank of the Timor Trough Snelson (1980) demonstrated one can expect to find (Hughes et al., 1996) resembles that found in forearc 70 M.G. Audley-Charles / Tectonophysics 389 (2004) 65–79

Fig. 4. Schematic cross section to illustrate present-day relationships between the Banda forearc, the former Neogene Banda Trench and the detached and overriding Australian para-autochthon. The geological section partly after Harris et al. (2000) and Hughes et al. (1996). Tectonic structure partly after Richardson and Blundell (1996), Hall and Wilson (2000) and Hall (2002). Location of the top of the lower plate that functioned during subduction as the basal decollement on or near the top of the Australian autochthonous crystalline basement; and the overriding forearc that formed the upper plate during subduction, both now hidden by burial in the Timor orogen, indicated schematically. The 5 Ma shelf edge represents its southernmost position estimated from DSDP 262. Position of buried trench linked with southern limit of overriding forearc. Basis for locating Banda Trench as in Figs. 1 and 3. Line of section A–AVsee Fig. 1. accretionary prisms and in many foreland basins recent (Quaternary) deformation in the trough axis is worldwide. Moreover, the decollement mapped by indicative of a continuation of deformation and Hughes et al. (1996) at the base of the imbricate episodic propagation of the thrust front in the Timor section is a relatively superficial structure, which they Trough (Masson et al., 1991) corresponding to the correlated with the Mid-Base Tertiary reflector. Below evidence of deformation among the uplifted Quater- this decollement in the Timor Trough is the well nary coral-algal reefs onshore Timor (Merritts et al., stratified autochthonous Mesozoic–Upper Palaeozoic 1998). section that, in a highly deformed state, forms the Timor orogen together with allochthonous nappes of the Banda Terrane derived from the forearc basement 3. The Wetar Strait and Wetar Suture (Harris, 1991; Harris et al., 2000). The dominating presence of these deformed para-autochthonous strata Timor Island is characterised by fold and thrust and forearc basement nappes in the Timor orogen raised to almost 3 km above level, results from their having been detached from very with a total uplift of about 4 km of deep marine much deeper decollements close to their fundamental deposits, which may have been an episodic process basements by subduction followed by collision. It initiated between about 3.5 and 2 Ma (Kenyon, appears that the Timor Trough began to evolve from 1974; Audley-Charles, 1986b; De Smet et al., 1990). the distal part of an overthrust forearc accretionary The island is bounded to both the north and south by prism, that had overridden the Australian proximal marine depressions (Fig. 1). To the south of the continental slope, into a foreland basin when colli- island is the 3 km deep Timor Trough discussed sional deformation began as subduction-rollback above. North of Timor is a very different deep ceased. The indications from reflection seismicity of marine trough, the Wetar Strait, that separates East M.G. Audley-Charles / Tectonophysics 389 (2004) 65–79 71

Timor from the islands of Wetar and Atauro. The 4. Identification of the former Banda Trench Strait drops precipitously to more than 3 km depth from both the north coast of Timor and from its 4.1. Implications of earthquake seismicity northern margin along the Banda Volcanic Arc. It varies in width between 30 km (Atauro) and 50 km The southern part of the Banda Arc is still (Wetar). In both these islands of the Banda Arc most characterised by earthquake epicentres (McCaffrey, of the volcanic activity ceased in the Pliocene 1989; Engdahl et al., 1998) indicating the northward- (Abbot and Chamalaun, 1981). The Wetar Strait dipping, Australian lithospheric slab (Figs. 2 and 4) opens westward into a 175-km wide forearc basin that descends steeply from below northern Timor, that also descends steeply from the north coast of continuing at depth below the Wetar Strait and the Timor and from the submarine margin of the Roti– Banda Arc inactive volcanic islands to below 300 km. Savu Ridge and Sumba Ridge to a depth of 3 km. A But across Timor and south of it earthquake records key factor in the search for the now defunct Banda offer no support for subduction now being active south Trench is that the active Banda Trench must have of the Banda Volcanic Arc (McCaffrey, 1996; Kreemer always been located south of the Banda Volcanic Arc et al., 2000). McCaffrey (1996) also emphasised that because it formed as the Australian continental earthquake seismology indicates that the movements margin was subducted northwards into the Banda between the lower and upper plates of the subduction Trench. Ocean less than 7 km deep have a system in the Timor region, that had been responsible commonly observed width at 5 km depth of about 50 for the strong Neogene deformation including much km. The fact that the Wetar Strait now separates the thrust stacking mapped in the Timor orogenic moun- former Banda Volcanic Arc from the northern edge tains (Barber, 1981; Harris, 1991; Harris et al., 2000, of the exposed Australian continental margin, whose Fig 4), have locked. Moreover, McCaffrey (1996), deformed cover rocks crop out on the north coast of using preliminary GPS results of Genrich et al. (1994), Timor, by only 30–50 km indicates (Audley-Charles, added that the geophysical evidence indicated that 1981, 1986a; Harris, 1991) that the cover rocks of dTimor appears to be moving northward relative to the the Australian continental margin, exposed in north- Sunda Shelf at about the same rate as AustraliaT.He ern Timor, have overridden the southern part of the added that the dnorthward motion of the extinct Banda forearc (Figs. 1 and 4). volcanic islands north of Timor is revealed with GPS The elevation difference between the bottom of the to be similar to that of southern Timor (Genrich et al., Wetar Strait and the north Timor coastal zone, 1994); hence, little of the northward motion of approaches 4 km in places; and the exceptional is now accommodated by the internal steepness of the gradient between them suggests deformation of the Banda arc structure.T He also faulting on, what in this paper is interpreted as, the observed that dThe island structure, bounded by the Pliocene Wetar Suture (Fig. 4). Masson et al. (1991) Timor Trough and the Wetar backarc thrustT (also did not find normal marginal faults with their known as the Wetar Thrust) d is rigidly thrusting over reflection seismic survey. But Richardson and Blun- the backarc basinT (also known as the South Banda dell (1996) using BIRPS seismic reflection mapped Sea). Kreemer et al. (2000) on the basis of GPS and multiple south-dipping thrusts in the Banda forearc earthquake data concluded that active, southerly immediately east of Wetar and Kisar that correspond directed thrusting on the Wetar and backarc closely with the postulated position (Audley-Charles, thrusts suggest a jump in the locus of convergence 1986a) of the Wetar Suture. Moreover, Masson et al. from the Java Trench to the backarc east of 1188E. The (1991) and Snyder et al. (1996) found a south-dipping entire region, comprising East Timor, Wetar and part of thrust near the north coast of Kisar, but they also did the intervening Wetar Strait (Fig. 2), is without active not notice any connection with the postulated funda- at depths greater than 75 km (Engdahl et mental Wetar Suture; while Breen et al. (1989), Harris al., 1998). This extraordinary aseismic feature may (1991) and later Prasetyadi and Harris (1996) also accord with the suggestion that part of the Australian found south-dipping thrusts close to the north coast of lower plate in this region has ruptured (Price and East Timor. Audley-Charles, 1983; Tandon et al., 2000). 72 M.G. Audley-Charles / Tectonophysics 389 (2004) 65–79

4.2. Wetar Strait as part of the foundered forearc Carter et al. (1976) who had described the uncon- formable relationship of the Cablac Limestone to the Geological mapping in northern Timor has recog- underlying Lolotoi Complex, an observation widely nised that the deformation suture there includes the reported from throughout the island of Timor (Tap- roots of the thrust sheets belonging to the Banda penbeck, 1940; De Waard, 1957; Audley-Charles and allochthon derived from the Banda forearc, and which Carter, 1972). appear to continue offshore (Harris, 1991; Richardson and Blundell, 1996; Harris et al., 2000). Moreover, Masson et al. (1991) dredged samples from the Wetar 6. Java Trench–Banda Trench evolution Strait of forearc volcanics analysed by Harris (1992). These data indicate that the present Wetar Strait is a 6.1. Termination of Java Trench at 1208E: bathymet- subsided part of the Banda forearc, implying that the ric, earthquake and lower plate characteristics former Banda Trench must now be south of the Strait (Figs. 1 and 4). The abrupt termination of the 5–6 km deep and 50 km wide Java Trench at 1208E(Fig. 1) is reinforced by the observations and conclusions by geophysicists 5. Significance of the allochthonous Banda terrane (Fitch, 1970; Chamalaun et al., 1982; McCaffrey, 1988; Engdahl et al., 1998; Kreemer et al., 2000)of The metamorphic Lolotoi and Mutis Complexes the dramatic change in Java Trench earthquake are rightly regarded by Harris (1991) as derived from seismicity east of 1188E, especially the abrupt the basement of the Banda forearc on the evidence of decrease eastwards of 0–75 km depth earthquakes in the rocks that rest on these metamorphic complexes. the context of a lack of indications of subduction. The basement is unconformably overlain in different Moreover, east of what is now 1208E the Australian places by volcanic and sedimentary rocks of the continental lithosphere had begun before 5 Ma to Upper Cretaceous to Paleocene Palelo Group, neritic replace the lithosphere as the lower Eocene limestones, and neritic Lower Miocene Cablac plate entered the subduction trench (Figs. 3 and 4). Limestone. These are entirely allochthonous elements Today, between 1208Eand1218E the Australian of the Banda Terrane (Audley-Charles and Harris, continental slope shows no evidence of the passage 1990; Harris, 1992; Harris and Long, 2000) occupy- of the Java oceanic trench, and the oblique strike ing a very high structural position. They are vastly difference between the existing, well observed Java different from any age-equivalent deposits found in Trench at 1208E, and the western edge of the the Australian sequences. They appear Australian continental slope is notable, making an to have been derived from the basement of the Banda angle of 708. Harris et al. (1998) attributed the small forearc, and transferred from the base of the upper re-entrant in the 4000 m bathymetric contour to it plate into the developing orogen (Fig. 4). This may representing the southern edge of the westernmost have occurred during the collision phase when the part of the Timor Trough, it being the distal part of the northward thrusting Wetar Suture was active. The now Banda forearc accretionary prism. It is notable that, high level, relatively -lying thrusts of the Banda while the location of the Java Trench with respect to Terrane together with the structurally underlying the Sunda volcanic arc west of 1208E has been folded and thrust para-autochthonous Permian to relatively fixed since the early Cenozoic (Hall, 2002), Neogene strata are exposed widely throughout Timor. the nearby Australian continental slope, now abruptly Charlton (2002) suggested the metamorphic Lolo- cutting across the strike of the Java Trench, has only toi and Mutis Complexes were derived from the recently arrived at its present position. At about 10 Australian plate, but he arrived at this view by Ma, the Australian continental margin, now immedi- ignoring their stratigraphic and structural relations ately east of the Java Trench, was located about 600 with the overlying volcanic and sedimentary Palelo km further south (Hall, 2002). Thus, details of the Group as well as with the Eocene Limestones, and his bathymetry, such as the re-entrant revealed in the interpretation depends on misreporting of work by 4000 m contour (Fig. 1) on the present Australian M.G. Audley-Charles / Tectonophysics 389 (2004) 65–79 73 slope next to the end of the Java Trench, must be a corresponds closely with the commonly observed relatively recent feature. It would have developed width at 5 km depth of ocean trenches that are less after the slope occupied its present position with than 7 km deep. Hence, 50 km is the postulated width respect to the Java Trench. Furthermore, the Austral- of the Banda Trench at about 5 Ma (Fig. 3). The Java ian continental lithosphere, cutting obliquely across Trench has existed since at least the Eocene (Hall, the end of the Java Trench, continues north of the 2002) whereas the Banda Trench cannot be much 4000 m re-entrant to its tectonic junction with the older than the earliest volcanics in the Banda Arc volcanic Sumba Ridge, a distance that varies from 100 dated as 12 Ma (Abbot and Chamalaun, 1981; Barberi to 150 km, marked by the contact indicated by the et al., 1987). On the basis of biostratigraphy the Savu Thrust (Fig. 1). This accords with the suggestion youngest deformed rocks in the Timor para-autoch- that this submarine morphology developed after the thon are N18 pelagic lutites in the Kolbano sequence, Australian slope arrived at the Java Trench. and the oldest autochthonous rocks resting uncon- formably on the deformed para-autochthon belong to 6.2. Plate reconstruction palaeogeographical the lowest part of the Batu Putih Limestone (Kenyon, implications 1974) determined as N18–19 which is between 5.0 and 3.5 Ma in age (Early to Mid-Pliocene). Timor Plate reconstructions (Hall, 2002) show the Java studies reveal a Late Pliocene–Quaternary regional Trench continuing east of Sumba into the west Pacific uplift that amounts to about 4 km; such a long-lived from at least 45 Ma, supported by interpretation of and substantial regional uplift requires a fundamental (Hall and Spakman, 2002). This tectonic cause. Detailed studies have recognised both older part of the Java Trench was eliminated by local -related uplift and subsidence (Kenyon, collision in eastern Indonesia during the early 1974; De Smet et al., 1990). Uplift began with Miocene. Between 15 and 10 Ma the Java Trench Kenyon (1974) Fatu Laob uplift phase whose revised developed eastward in a new position where it date (Audley-Charles, 1986b) is Late Pliocene (N21) functioned as the Banda Trench (not named as such ranging from 3.5 to 2.0 Ma. Beginning from between by Hall, 2002) in association with the beginning of the ~3.5 and 2.0 Ma, the regional uplift has continued, Banda Volcanic Arc. This subduction trench was with some local subsidence. The unconformable located, from at least 10 Ma to at least 5 Ma, between relationships described above suggest that subduction the Banda volcanic arc to the north and what was to may have ceased in this part of the Banda Trench at become the island of Timor to the south of this trench. 3.5 Ma. This implies that the active life of the trench Hall (2002) reconstruction is used as the basis for the here was from ~12 to ~3.5 Ma, but if the onset of palaeogeographical sketch map (Fig. 3). Timor is uplift in this region was as late as 2.0 Ma and if this shown as always on the northern edge of the uplift, which has continued throughout the Quater- Australian continental slope to the south of the Banda nary, is an isostatic response to cessation of sub- Trench. Because of later northeastward movement of duction then Banda Trench subduction may have the Australian continental margin, at 12 Ma the stopped and collision began as late as 2 Ma. Timor–Savu region lay some hundreds of kilometres to the south of the position it occupied at 5 Ma. The 6.4. Locating the present position of the defunct Australian continental margin continued its conver- Banda trench gence on the Banda Arc during the Neogene and Quaternary. The defunct Banda Trench has been located in this paper (Figs. 1 and 4) by considering the consequences 6.3. Reconstruction of the Banda Trench at 5 Ma, and of convergence between the northward-subducting dating cessation of subduction Australian plate and the Banda Volcanic Arc from 5 Ma (Fig. 3) to the date of onset of arc–continent The width of the Java Trench, where it is least collision. Geological mapping indicates collision affected by incoming from the ocean floor of began between about 3.5 and 2 Ma. As explained the India–Australia plate, is about 50 km. This above, collision is correlated here with the locking of 74 M.G. Audley-Charles / Tectonophysics 389 (2004) 65–79 the subduction trench. The important assumption is upper plate reached the proximal part of the Austral- that from 5 Ma to the time the trench was locked the ian slope whose basement was comprised of con- Banda forearc basement did not grow southwards, and tinental lithosphere, subduction probably began to the distance from the volcanic arc to the north wall of slow and stopped when the distal edge of the shelf the Banda trench did not change significantly. Note was at a distance of about 60 km to the south (Figs. 3 that in Fig. 3 the distance from the arc (at Wetar) to the and 4). Where subduction or rollback of all the Indian north wall of the Banda Trench was 100 km at 5 Ma. In Ocean lithosphere below the Banda Arc was com- Figs. 1 and 4, representing present-day reconstruc- pleted in the Sumba-Timor region the oceanic Banda tions, this distance has shortened by about 10%. The Trench began to form part of the orogenic northern consequences of the convergence of the Australian margin of the foreland basin evolving to become the margin with the Banda Volcanic Arc are revealed by present Timor Trough. plotting the position of proto-Timor and the Australian The now missing, defunct Banda Trench can only continental margin at 5 Ma in Fig. 3 in their relative have been located, when it was actively subducting, present-day positions as shown in Fig. 1. This indicates where the seismicity (Engdahl et al., 1998) implies its that the present position of the Banda Trench (now former presence between the northwards subducting defunct) and its closely linked upper plate, comprising Australian plate and the SE Asian Banda Volcanic the southern part of the Banda forearc basement, must Arc. The now invisible, obliterated Banda Trench, that both be below Timor. The only important assumption probably ceased to subduct in the Timor region has been that the distance from the Banda Arc to the between 3.5 and 2 Ma, is now located below the north wall of the Banda Trench has not increased since islands of Timor and Roti (Figs. 1 and 4). Here it is 5Ma(Figs. 1 and 4 imply a 10% shortening which obscured by the thick pile of folded and thrust para- might well be expected to result from the tectonic autochthonous cover rocks from the Australian con- compression). The case for the location of the Banda tinental rise and slope now thrust northwards by the Trench at 5 Ma has already been set out above. It is Wetar Suture over the overriding forearc. clear that there is little room for manoeuvring the These now para-autochthonous rocks first began to geographical positions of any feature once the width of form the Banda forearc accretionary prism from the the Banda Trench at 5 km depth is taken to have the time the distal Australian value of 50 km, and that, following Hall (2002), the began to arrive in the Banda Trench, when the Banda position of the Java Trench in the region of Sumba has Volcanic Arc began to appear at about 12 Ma. This not altered significantly during the last 5 Ma. Banda forearc accretionary prism grew until the subduction process was halted probably after slowing as the Australian continental slope, with its crystalline 7. Distinguishing subduction from tectonic collision basement and great thickness of overlying Lower Permian to Mid-Jurassic Gondwana sequences, and When a passive continental margin converging on post-rift Upper Jurassic to Pliocene Australian rifted a volcanic island arc system, whether by subduction continental margin sequences, entered the subduction or by rollback, reaches the stage that all of the oceanic trench between about 5 and 2 Ma. lithosphere in any sector has passed below the inner The locking of the subduction process is associated forearc wall of the trench so that the continental with the gradual onset of tectonic continental margin– lithosphere of the converging continental slope begins volcanic arc collision. It is interpreted here as having to pass below this inner forearc trench wall the begun between 3.5 and 2 Ma. Thickness of the lower condition of continental margin-arc collision may be plate probably increased as the slope entered the said to have commenced. Given that the shape of the trench. A steepening of the subduction angle may strike of the converging continental margins may be have impeded the downward motion of the lower irregular there maybe varying degrees of diachronism plate into the trench. Subduction may then have been in the tectonic collision. halted by the increasing gravitational resistance to the Collision in the southern part of the Banda Arc was upward movement of the forearc over the Australian initiated when the overriding forearc basement of the margin on the incline rising from the floor of the M.G. Audley-Charles / Tectonophysics 389 (2004) 65–79 75

Banda Trench southwards up to arrive about 60 km simpler structures than those of the main collision from the shelf break (Fig. 4), where the sea floor could phase of structural deformation so evident in the have been at less than 1000 m depth. Another braking exposures on Timor (Fig. 4). Following the transfer of effect might have been the loss of pore-fluid pressure, the Australian cover rocks by subduction from the from below the overriding forearc basement moving lower plate into the forearc to form the accretionary on a decollement of Jurassic shale and escaping into wedge of the upper plate, the main collision deforma- the more permeable formations of the Australian tion involved at least three recognisable stages (i) cover sequence. Continuing or reactivated crustal severe shortening of the forearc to shortening after the cessation of subduction consti- form part of the Australian para-autochthon; (ii) tutes tectonic collision. The collision in the southern detachment of part of the underlying forearc basement part of the Banda Arc resulted from compression of eventually to form the nappes of the allochthonous the forearc accretionary wedge which consequently Banda Terrane intimately associated with (iii) trans- became the Australian para-autochthon on both upper port northwards by the Wetar Suture and associated and lower plates, with part of its forearc basement uplift of the para-autochthon and slices of the forearc becoming nappes of the allochthonous Banda Terrane. basement to become the Timor orogen. In contrast, the This upper plate tectonic flake, in response to Timor Trough accretionary prism structures appear to continuing compressive stresses, produced north- be mainly the result of some probably reduced ward-verging structures concentrated along the north convergent forces (McCaffrey, 1996) towards the coast of Timor as mapped by Harris (1991) and end of subduction and especially after that ceased in Prasetyadi and Harris (1996). This northward thrust- the Timor region of the Banda Trench between 3.5 ing over the forearc helped close the forearc to only 30 and 2.0 Ma. A major structural feature developed km in the region of Atauro. Harris (1991) and towards the end of subduction and during collision Prasetyadi and Harris (1996) also identified various was the Late Pliocene Wetar Suture, now locked. The thrust slices of the Banda Terrane incorporated into crustal shortening resulted in the deformation that this northward-verging thrust wedge (Fig. 4) carried characterises that part of the para-autochthon detached on the Wetar Suture (Audley-Charles, 1986a). from part of the unsubducted Australian lower plate Another characteristic of subduction and roll-back causing the detached para-autochthon to overthrust related processes that distinguishes them from colli- the forearc tectonic flake (Fig. 4) on the Wetar Suture. sion related processes is the structural level of the The compressional strain continued into the Pleisto- principal decollement. In subduction and roll-back the cene when the subduction polarity reversed and the main decollement is the upper and lower plate Wetar Thrust began to develop a southward-dipping boundary, whereas in collision the principal decolle- subducting plate margin (McCaffrey, 1996; Hall and ments develop at a higher level even if they occur as Wilson, 2000). The gentle folding of the Viqueque structurally low as to be within the cover rock Group basins (Kenyon, 1974) that affected some of sequence on the autochthonous crystalline basement the reef terraces (Merritts et al., 1998) also indicates of the lower plate, as in the Timor Trough (Hughes et that some crustal shortening continued into the late al., 1996). Both subduction and roll-back (Hamilton, Quaternary. 1988; 1995) appear to transfer much of the cover rock sequence from the descending lower plate to the leading edge of the upper plate as the lower plate 8. Sumba Ridge–Banda Trench relations passes below the forearc basement that forms the inner (arc) wall of the trench. Fig. 1 shows the ENE-striking submarine Roti– Savu Ridge has a southward-dipping thrust junction, 7.1. Main and later phases of collision the Savu Thrust (Silver et al., 1983), with the southern part of the south–east striking submarine Sumba The recognition by Hughes et al. (1996) of Ridge. Fortuin et al. (1994), Rutherford et al. (2001) accretionary prism-type deformation in the northern and Lytwyn et al. (2001) interpret the Sumba Ridge as flank of the Timor Trough indicates they are much part of the Banda forearc. Comparison was made by 76 M.G. Audley-Charles / Tectonophysics 389 (2004) 65–79

Audley-Charles (1985) of the allochthonous Timor volcanic forearc, occurred at about 8 Ma. These early Banda Terrane, thought to have been derived from the dates for Banda Arc collision in the Sumba–Timor Banda forearc, with some sections in West Sumba region cannot be reliably based on the radiometric described by Van Bemmelen (1949), Chamalaun et al. ages for metamorphism in the Permian Aileu Meta- (1982) and Von der Borch et al. (1983). For example, morphic Complex of Timor. Harris et al. (2000) on the both Timor (Tappenbeck, 1940) and West Sumba basis of vitrinite reflectance, conodont alteration and sections have Lower Paleogene ignimbrites overlain illite crystallinity and structural mapping questioned by neritic limestones with Middle and Upper Eocene the interpretations of pre-Late Miocene tectonic burial large foraminifera, and both have Lower Miocene and pre-Pliocene exhumation of the metamorphism of neritic limestones. Both have an angular unconformity the Aileu Complex Australian continental margin in at the base of the Paleogene and an unconformity at Timor. They pointed out that dthe only possible the base of the Miocene. evidence of such deep burial is a thin Barrovian Rutherford et al. (2001) have interpreted Sumba as metamorphic sequence along the north coast in the having been part of the Sunda volcanic arc from Late Aileu ComplexT (Berry and Grady, 1981). They went Cretaceous to Early Miocene. If the tentative corre- on to emphasise that dIt is also possible that the Banda lation of the Palelo Group and associated sequences Volcanic Arc has thermally disturbed the Aileu unconformable on the allochthonous metamorphic Complex, since it is as close as 30–40 km in placesT. complexes in Timor with the Cretaceous–Lower Furthermore, Hall (2002) and Hall and Wilson (2000) Miocene volcanics in Sumba is shown to be valid have argued that such metamorphic dates in the then this would strengthen the correlation of the Permian Aileu Complex (that is part of the Gondwana Lolotoi and Mutis metamorphic complexes and their cratonic basin sequence and belongs structurally with unconformable Palelo cover (Banda Terrane) with the the Australian continental basement to the post-Late Banda forearc basement and with the igneous base- Jurassic rift sequence) are more easily attributable to ment of Sumba. an extension episode beginning about 12 Ma and The Pliocene–Quaternary uplift in East Sumba associated with Banda Arc volcanism. There is no (Fortuin et al., 1994, 1997) suggests comparison with independent evidence for this early collision date, and the Late Pliocene–Quaternary uplift of 4 km in Timor. there is much in the Timor and Sumba islands to The Timor uplift has been tentatively attributed (Price contradict it, as indicated above. and Audley-Charles, 1987) to isostatic rebound of the The Banda forearc is made exceptionally wide by blocked subduction zone of the defunct Banda Trench the Sumba ridge which projects about 250 km south no longer able to subduct the Australian continental of the existing narrow Banda forearc (Figs. 1 and 3), lower plate. so that a collision about 5 Ma earlier than the collision The suggestion by Rutherford et al. (2001) for the in the Timor region between the Australian continen- initial collision (undefined and not distinguished from tal margin forming the Roti–Savu Ridge and the subduction) between the Australian continental mar- Sumba Ridge of Banda forearc seems superficially gin and Banda Arc having been as early as Early possible. However, their suggestion (Keep et al., Miocene, 16–18 Ma, conflicts with the results from 2003, Fig. 10a) that the Roti–Savu Ridge at 8 Ma numerous field studies, including apatite FT ages for formed a narrow peninsula that protruded orthogo- dating of exhumation in Timor (Harris et al., 2000), nally from the regional strike of the Australian geological mapping across Timor and other islands in margin, the ridge having a rectilinear outline and the southern Banda Arc (Audley-Charles, 1986a, northwest strike, is difficult to reconcile with the 1986b; Harris, 1991), offshore geophysics around geological strike of the Australian margin in the the Banda Arc (Bowin et al., 1980) and the review by southern Banda Arc. Keep et al. (2003) appear to be Hall (2002). Keep et al. (2003) suggest that the making a speculative exaggeration of the shape and collision (undefined and not distinguished from size of their projecting ridge (that they described as ba subduction) of what is here described as the sub- fingerQ to indicate its length/width ratio) of the step- marine Roti–Savu Ridge with the submarine Sumba rifted margin in the Sumba region. Their attribution of Ridge, that they agree represents a part of the Banda the stepped morphology of the present rifted con- M.G. Audley-Charles / Tectonophysics 389 (2004) 65–79 77 tinental margin to a pre-subduction age of pre-12 Ma Nappe Tectonics. Geological Society of London, Spec. Pub., ignores the effects of subduction and collision-related vol. 9, pp. 407–416. Audley-Charles, M.G., 1985. The Sumba Enigma: is Sumba a NNE transcurrent faulting that has affected the shape diapiric nappe in process of formation? Tectonophysics 119, of the present northern margin of the Australian 435–449. continent in the Banda orogen (Fig. 1). But even more Audley-Charles, M.G., 1986a. Timor-Tanimbar Trough: the fore- importantly it is impossible to reconcile the age of an land basin to the evolving Banda orogen. Foreland Basins. Spec. interpreted Late Miocene collision with the record of Publs. Int. Ass. Sediment, pp. 91–102. Audley-Charles, M.G., 1986b. Rates of neogene and quaternary uninterrupted pelagic deposition, perturbed only by tectonic movements in the southern Banda Arc based on slumping in a deep water environment, at the micropalaeontology. Journal of Geological Society (London) supposed site of the collision on Sumba itself (Fortuin 143, 161–175. et al., 1992, 1994). The youngest pelagic sediments on Audley-Charles, M.G., 1991. Tectonics of the Area. Sumba are late Early Pliocene (N19/20) in age (Roep Annual Review of Earth and Planetary Sciences 19, 17–41. Audley-Charles, M.G., Carter, D.J., 1972. Palaeogeographical and Fortuin, 1996; Fortuin et al., 1997) suggest significance of some aspects of Palaeogene and Early Neogene emergence of Sumba did not occur before 3 Ma, an stratigraphy and tectonics of the Region. Palae- age very close to that inferred from Timor for the ogeography, Palaeoclimatology, Palaeoecology 11, 247–264. beginning of uplift. Thus, the evidence from Audley-Charles, M.G., Harris, R.A., 1990. Allochthonous terranes extremely well-exposed sections on Sumba support of the South-west Pacific and Indonesia. Philosophical Trans- actions of the Royal Society of London 331, 571–587. the timing for the end of subduction and onset of Bally, A.W., Snelson, S., 1980. Facts and principles of World collision suggested in this paper. petroleum occurrence: realms of subsidence. Memoir-Canadian Society of Petroleum Geologists 6, 9–94. Barber, A.J., 1981. 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