GeowgicaL Soci£ty of MalayJUz - circum-Pacific Council for Energy a/lu MineraL I!':0urc~ Tectonic Fl'flITUwork and Energy Ruourcu of tbe W(Jtern Margin of the PacifIC Btum November 27 December 2, 1992, Kuo.Ia Lumpur, MalayJUz

Lithosphere structure and dynamics of the Banda Arc collision zone, Eastern

ADRIAN RICHARDSON

Geological Research in Southeast , Department of Geology Royal Holloway and Bedford New College, Egham, Surrey, TW20 O~X, England Present Address: Shellinternationale Petroleum MaatschapplJ B.V. Postbus 162, 2501, AN Den Haag, Oostduinlaan 75, Nederland

Abstract: and adjacent in Eastern Indonesia are the site of a currentl~ active col~ision between the Australian continental shelf and the volcanic islands of the Banda Arc. PreVIously pubhshed sedimentological and micro-paleontological data from the islands and the trench date the start of this collision at 2.5 to 3 Ma. Previously published work on seismicity in the Banda Arc shows the geometry of the subduction zone to be fairly simple and continuous down to below 650 km depth. In this paper, the geometry of both the top and bottom surfaces of the subd~cting sla~ is mapp~d to a hl:gh~r degree of accuracy than before. The edge of , which was preVIously a rifted paSSIve margm, IS shown to be markedly thickened in the collision zone itself and has been subducted to a depth of about 200 km. In addition, it appears that the subduction zone has major lateral and vertical discontinuities at depth. It is proposed that these discontinuities reflect slab separation during a previous micro­ continental collisional event some 10 to 7 million years ago and that this collision was related to the transfer of continental lithosphere from the Indo-Australian plate to the South East Asian plate.

INTRODUCTION into six groups (Audley-Charles, 1968; Barber, 1981; Charlton et al., 1991). The "para-autochthon" forms One of the more fundamental geological the core of the and consists of problems being addressed by current research is to unmetamorphosed to high grade metamorphosed discover how collisional mountain ranges are sedimentary material which has been interpreted constructed. Understanding is hampered by both as being underplated Late Palaeozoic to Early the geographical remoteness and structural Mesozoic Australian-derived rocks. The complexity of some of the areas under study. "allochthon" is made up of ultrabasic rocks with Another approach to the problem is to study a some metamorphosed sedimentary cover. This unit young, active collision zone which has not yet has a "tectonic" relationship with the "para­ developed to the same degree of complication as a autochthon" and was traditionally regarded as being mature mountain range. The Banda Axc in Eastern derived from the Asian plate forearc , (Barber Indonesia is, arguably, the youngest mountain belt and Audley-Charles, 1976). However, in the world. A collision is presently underway Sopaheluwakan (1990) has good evidence that the between the northern edge of the Australian unit was metamorphosed during a period of intra­ and the volcanic islands of the Banda Axc oceanic thrusting near a spreading centre and was (Fig. 1). The collision has progressed to a stage subsequently emplaced against the "para­ such that a "collision complex" has been built up autochthon" in a collisional event around 38 million parallel to and just south of the . The years ago. The rocks of the "autochthon" consist of higher parts of the collision complex have risen unmetamorphosed Mesozoic to Recent sediments above level to form the islands of Sumba, Timor, that have been accreted to the southern side of the Tanimbar and a string of smaller islands. developing orogen. The remaining three groups Timor is the largest island of the chain and it are mud diapir deposits, raised reef terraces and is here that geological research has been historically Recent alluvium. focused since the late 1800s. The age of onset of this collision has been derived by many workers SEISMOLOGICAL DATA using a variety of types of evidence at 2 to 3 million years ago in the region of Timor, (Johnston and Earthquakes are one of the main sources of Bowin, 1981; De Smet et al., 1990; Harris, 1992). information on the large-scale structure and The rocks of Timor have traditionally been split dynamics of tectonically active areas and it is with

Geol. Soc. MalaYJia, Bulletin 33, July 1993; pp. 105-118 --'- o J' (J)

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Figure 1. Location map of Eastern Indonesia. liTHOSPHERE STRUCTURE AND DYNAMICS OF THE BANDA ARC COLLISION ZONE, EAST INDONESIA 107 these data that this paper is largely concerned. INTERPRETATION This part of Indonesia is one of the most seismically active areas in the world. The International The "Top of Envelope" plot (Fig. 2) is interpreted Seismological Centre (ISC) currently records about as showing the geometry of the active part of the 5 earthquakes per week within the area of the map Wadati-Benioff zone since the top of a subduction in Figure 1. A number of papers have already been slab is relatively cold and brittle. This is in keeping published about the seismicity of the Banda Arc with many other works, for instance those referred area (Fitch and Molnar, 1970; Cardwell and Isacks, to above. The shallow portions « 200 km depth) to 1978; Hamilton, 1979; McCaffrey et al., 1985; Eva the west and to the east of Timor are seismically et al., 1988; McCaffrey, 1988, 1989; Ritsema et al., very active and define a fairly smooth surface. The 1989). All of these works regard the geometry of high activity in the eastern region is largely due to the Wadati-Benioff zone to be a simple concave stresses within the plate near the region of greatest bowl shape to the east and a steeply dipping single curvature. Most events for which fault plane surface to the west penetrating to below 600 km solutions have been calculated in this region are depth; see, for instance, Figure 5 of Cardwell and north-east directed thrusts whose orientation is Isacks (1978). The Wadati-Benioff zone contours in parallel to the direction of motion of the Australian all the above works are only rough approximations plate. In contrast, there is little shallow seismic through the data at depth intervals of 100 km. activity under eastern Timor. The probable The data set for this study is an extract from explanation for this observation is that subduction the ISC catalogue from 1 January 1964 to 31 August under eastern Timor has now ceased. Eastern 1988 and the USGS Preliminary Determination of Timor is the location of the highest mountains and Epicenters Bulletin from 1 September 1988 to 31 has the smallest arc-orogen cross-strike width and December 1991. Also included are earthquakes is therefore the most developed part of the collision recorded by temporary regional networks zone. Inactivity of this segment of the subduction (McCaffrey et al., 1985), giving a total of about zone is in accord with the work of Johnston and 8500 events. Undefined depth events that had Bowin (1981) on sediments in the trench to the been assigned a default depth of 33 km were south of eastern Timor which indicates that discarded but all other events down to the smallest subduction in this region has been inactive for the recorded magnitudes were retained. last 600 000 years. Continuing deformation in this The aim of this work was to create a three­ segment ofthe collision zone is being accommodated dimensional model of the structure of the by a high rate of uplift (Johnston and Bowin, 1981; lithospheric components of the Banda Arc region. Milsom and Audley-Charles 1986; De Smet et al. Locations of earthquake hypocentres were initially 1989; De Smet et al.; 1990) and back-arc thrusting plotted on maps. The plotted earthquakes were north of Alor and Islands (Nabelek and then grouped into horizontal slices 10 km in McCaffrey, 1983; Silver et al., 1983; McCaffrey and thickness. The surface curvature of the was Nabelek, 1984, 1986; McCaffrey, 1989). ignored. The depth error for any given The deeper portion of the subduction zone is teleseismicallY located earthquake is of the order of more difficult to contour confidently because of the 10-20 km (Dziewonski et al. 1981). The sample relative scarcity of events. Nevertheless, it is clear group is large enough, however, for the statistical that to the west of Timor, the seismic zone descends errors to have no impact on the overall distribution vertically to below 650 km depth. To the west of of events. With the aid of maps and sections, the Timor and to the west of Sumba, there appear to be outer surfaces of the envelope of active seismicity large gaps in the vertical portion of the seismic were contoured by hand with a contour interval of zone. 10 km. The resulting contour maps are rather There is still great debate about the actual complicated since, in many , the contoured mechanism of deep earthquakes (Davies, 1980; Goto surfaces are almost vertical and the contours lie on et al., 1985, 1987; Frohlich, 1987). Nevertheless, it top of each other. The results are therefore appears from hypocentre plots that earthquakes at displayed in isometric view (Figs. 2, 4 and 5). The or near the base of the Australian lithosphere do main assumption implicit for interpretation of these occur. It is unlikely therefore that they are all data is that anything within the envelope of active spuriously located shallow events. Although the seismicity is lithospheric material while anything base of a lithospheric slab is hot, its viscosity is still outside may be either seismically quiescent generally considered to be much higher than the lithosphere or asthenosphere material. convecting asthenosphere underneath. Perhaps o (Xl

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Figure 2. Banda Arc seismic zones: isometric view of the top of the envelope of activity. 118' 119' 120 ' 121' 122' 123' 124 ' 125' 126' 127' 128' 129' 130' 131' 132' 133' 134' 135 ' 2 • , ./ t. _i , 2' Depth (km) 3' - - 100 110 120 r 4 • 130 1o C/l-u 140 I m 150 :n m 5 • 160 5 ' C/l -I:n 170 c (") -I 180 C 190 :n 6 ' 6' m :l>z 200 o Lines with ticks represent near-surface faults. o 210 -< 220 Black dots represent the locations of earthquakes ~ 7" s:: 7" 230 used to generate the contour map. o C/l 240 o "T1 -I 250 I 8' m 8 • co ~ o .------... ----~ ...... --' :l> /' » // -_._--- 9' 9' :n(") (") o:= ./ U5 10 ~ tl snz m 11 11 6; -I Z o o rnz 12 5>

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Figure 4. BanJa Arc t;eit;mic zones: it;umetric view of the base of the envelope of activity, liTHOSPHERE STRUCTURE AND DYNAMICS OF THE BANDA ARC COLLISION ZONE, EAST INDONESIA 111 these deep events relate to the "plate drag" force have, however, investigated rocks from a few often mentioned when plate driving forces are locations and have found some to be anomalous in considered (Forsyth and Uyeda, 1975; Chapple and a number of ways. For example, the high 87SrjBSSr Tullis, 1977). The "Base of Envelope" plot was ratios in Recent calk-alkaline lavas from the Banda drawn using the deepest earthquakes only and Arc investigated by Whitford and Jezek (1979) are ignoring shallower ones. Where the subducting interpreted to result from mixing of a sialic slab plunges vertically the "Base of Envelope" component with a mantle derived component. The contours were drawn using the southernmost events sialic component must come from continental of the seismic zone although they were few and far material that has been subducted to a depth of over between. In spite of being derived from far fewer 100 km. Morris and Hart (1980) looked at lead events than the "Top of Envelope" plot, the "Base of isotopes in volcanic rocks from the Banda Arc and Envelope" plot shows some interesting features. concluded that they were the most radiogenic of Firstly, the surface is, to the fIrst order, parallel any due to entrainment of continental to the surface that represents the top of the material. Morris et al. (1984) reported the recovery lithosphere. Secondly, the surface is much shallower of 400 000 year old basalt dredged from north of the in the south-west corner of the map (Fig. 3) than Wetar thrust to the north of . These the south-east. This is consistent with the transition samples also showed high 87SrjBSSr ratios, LIL westwards from Archean continental lithosphere enrichment and other trace element concentrations to oceanic lithosphere. Thirdly, an L-shaped arch similar to those from the southern Banda Arc is evident to the south of Timor (Figs. 3 and 4). The (Schwartz et al., 1984). Thus it appears that eastern leg of this arch is parallel to the pre-existing subducted continental material does underlie rifting structures on the Australian North-West eastern Timor but that actual subduction in this Shelf (Mollan et al., 1970). The northern part of segment of the collision zone is presently inactive. this arch strikes normal to the motion of the In contrast, the 87SrjBSSr ratios in lavas presently Australian continent i.e. NNE. Also, the northern erupting from the volcanoes Rinj ani on end of the arch is located under central Timor and Tambora on are similar to the lowest where the subduction zone is inactive. Fourthly, derived from the where purely oceanic the deep gaps in seismicity are more-or-Iess in the lithosphere is currently subducting (Foden and same place as those for the "Top of Envelope" plot Varne, 1980). Hutchison (1981) noted that the (Fig. 5). For these reasons the "Base of Envelope" decrease in 87SrjBSSr ratio in volcanic rocks from plot is interpreted tentatively as representing the West Java to Bali suggested an upper plate base of the lithosphere. basement transition from continental to oceanic material. DISCUSSION Foden and Varne (1980, 1981) also studied the geochemistry of volcanic rocks from extinct Miocene The existence of gaps in seismicity creates a to Pliocene rocks of Lombok and Sumbawa. At the problem and begs the question: are they real gaps time, the only work on the geometry of the Wadati­ in the subducting slab or merely a seismic portions Benioff zone was that of Fitch and Molnar (1970) of a continuous slab? Uncertainty in the density of and Cardwell and Isacks (1978) showing simple a slab at this depth makes the errors inherent in continuous slabs to great depth. In spite of this, gravity studies larger than the actual effect under Foden and Varne (1980) concluded that "We must investigation and therefore unresolvable. The invoke inhomogeneities in the source region to seismic tomography technique could be a future explain mineralogy and chemical composition ... possibility for investigating this type of phenomenon We suggest that LIL-rich (K, Rb, Sr) component is but current global three-dimensional tomographic progressively added to the source regions ... If this models do not show structures to the necessary component occurs deep within the mantle, it might resolution (Dziewonski and Woodhouse, 1987). gain passage to shallower regions either by Geochemical techniques are the only other way percolating up the down-going slab to yield the of looking at material from deep in the mantle. The familiar arc magma zonation, or up substantial Sunda and Banda volcanic arcs have been bringing cross-arc fractures." The "substantial cross-arc this material to the surface for millions of years. fractures" referred to by Foden and Varne could be Because of their geographical remoteness, there the large gap in the vertical portion of the are rather few geochemical data or accurate isotopic subducting slab. dates already acquired from these volcanic islands; Several authors (Goto et al., 1985) have pointed certainly not enough to monitor the change in out that a minimum of seismic activity can be geochemistry of volcanic rocks over the course of observed between 300 and 350 km depth in "normal" the last few million years. A number of studies oceanic slabs as they subduct. This is because the ~

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Figure 5. Banda Arc seismic zones: isumetric view of the both the top and base of the envelope of activity. liTHOSPHERE STRUCTURE AND DYNAMICS OF THE BANDA ARC COLLISION ZONE, EAST INDONESIA 113 down-dip tension in the upper part of the slab is unsmoothed data are displayed to minimize bias. cancelled out by down-dip compression in the lower Note that the top surface of the descending slab part of the slab forming a mid -depth zone of neutral appears much smoother in the 125.5°E section than stress. In fact, the slab section at 120.5°E in Figure the others. This is because the shallow portion of 6 and other slab sections across the oceanic slab the Wadati-Benioff zone is inactive in this area and subducting under Java appear (Eva et al., 1988) to its geometry has been derived by extrapolation. be consistent with this. Nevertheless, the seismic The true geometry is probably more like that at zone to the east of Timor is continuous down to 650 127.5°E than that at 123.5°E but since there is so kID and was, presumably, part of the same slab little seismic activity, there are no real data to before subduction. Also, to the west of the gap, at substantiate this claim. 117°E, the seismic zone is also continuous down to Figure 6 shows marked variations in the about 650 kID depth (Eva et al., 1988). In vfew of thickness of the slabs. The sections clearly show the above evidence, I argue that the gaps in the that the Australian continental lithosphere is over seismicity genuinely represent asthenosphere filling 200 km thick under the Archean part of the breaks in the lithospheric slab. The lower portion continent which is consistent with other work of the lithospheric slab, below the gap, must have (Lerner-Lam and Jordan, 1987). The previously become detached at some time in the past as has rifted margin ofthe Australian lithosphere becomes been previously proposed by others (Nur and Ben­ thinner towards the north, to the south of the point Avraham, 1982; Price and Audley-Charles, 1987). of inflection where it plunges down the subduction Figure 6 shows four vertical sections (located in zone. The sections show the lithosphere becoming Fig. 1) drawn across the seismicity envelope contour thicker towards the north but a sharp decrease in maps. The surfaces of the deeper portions of the thickness at a depth of about 200 kID. These sections slabs appear rather jagged because they are less cross East Timor which is the site of the highest well defined. Deviations in the surface with mountains, the narrowest cross-arc width and is wavelengths less than about 20 kID, the average therefore the most mature segment of the collision error in teleseismic earthquake depth locations, zone. The sharp decrease in thickness probably could justifiably be smoothed out. However, the denotes the boundary between sinking oceanic

IBanda Arc Seismic Zones - Slab Sections

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Figure 6. Banda Arc seismic zones: sections through the lithosphere located in Figure 1. 114 ADRIAN RICHARDSON material below and continental material above, Is there any other evidence for a short-lived which thickens, being too buoyant to be subducted collision preceding the current one? An extremely (McKenzie, 1969). detailed study was made by Berry and Grady In Figure 6, the numbers in circles represent (1981) of the deformation and metamorphism of the time in millions of years since the portion ofthe the rocks near Dili in Timor (124.5°E). These rocks slab adjacent to the number entered the subduction are traditionally described as the "para-autochthon" zone. The values were derived by calculating the and form the core of the island. Using radiometric constant plate motion velocity inferred by the dating, they concluded that there were two periods magnetic lineations between Australia and of prograde metamorphism; the first probably Antarctica. Retaining a constant velocity for the relating to Mesozoic rifting and the second occurring slab during and after detachment is undoubtedly a after 11 Ma ago but which was finished before 6 simplification but the values are probably of the Ma. . These ages fall neatly around the dates right order. proposed above for a collisional event in Table 1. It is presumed that slab detachment results How does this event tie in with the tectonic from an event that inhibits subduction such as scenario of the surrounding region? The most recent continental collision where the continental material interpretations of magnetic anomaly data in the is too buoyant to subduct (McKenzie, 1969). The Banda Sea and Argo Abyssal Plain (powell and oceanic lithosphere that has already subducted is Luyendik, 1982; Silver et al., 1985; Fullerton et al., too dense to remain static in the mantle. During 1989; Prasetyo, 1989, pers. comm.; Hartono, 1990) the course of a continental collision, therefore, the tend to support the proposal that the Banda Sea oceanic slab ahead of the continent continues to was originally continuous with the Argo Abyssal sink into the asthenosphere. Thrusting of Plain and part of the Indian on the Australian continental material along the shallow portion of plate. If this was the case, the Banda Sea must the subduction zone first slows and then disrupts have become detached from the Australian plate by subduction completely. Continued compressive some sort of tectonic event involving a change in deformation is accommodated by thickening of the location of the subduction zone. Figure 8 is a continental lithosphere, uplift at the surface and reconstruction of the position of the Australian plate strike-slip expulsion. As it continues to sink, the 11 Ma ago. It is apparent that the magnetic descending oceanic slab first thins to a narrow neck anomalies in the Argo Abyssal Plain come into (Agnon and Eidelman, 1991) and then parts alignment with those in the Banda Sea, suggesting company. An upper mantle section derived by continuity at about this time. In addition, the Sula 'seismic tomography appears to show this same Platform and Buton continental blocks had been process of thinning and necking of the African plate colliding with eastern since around 15 as it descends under Southern (Spakman, Ma and collisional and volcanic activity there were 1990, Fig. 2b). If the "subduction age" of the top of at a maximum at 11 Ma (parkinson, 1991). It was a separated slab can be calculated (that is the time probably the collision at eastern Sulawesi that led that part of the slab entered the subduction zone) to the transfer of the Banda Sea from the Australian it is possible to obtain an approximate indication of plate to the South East Asian plate and the jump of the age of onset of continental collision. the subduction zone to the south of the Banda Sea. In order to produce the geometry in Figure 6, Intrusive volcanics were present in Wetar 12 subduction would have had to begin again after the Ma ago (Abbott and Chamalaun, 1981) indicating collision. Mueller and Phillips (1991) concluded that subduction had begun in this area by this time that the only forces large enough to initiate and that there existed an arc against which Timor subduction were those generated during continental could collide. The gaps in the lithospheric slab collision. So if after a collisional event subduction opened by the collision remained geographically then resumes, the "subduction age" of the leading linked to the Timor-Sumba region and hence identify edge of the subducting slab indicates the age of the the former collision zone. It is unclear whether the end of the preceding continental collisional episode. Thus the deep gaps in the subducted slab shown in Table 1: Proposed dates for a collision event. Figure 6 may be interpreted as in Table 1. That the geometry of the gaps in the lithospheric slabs in Onset of End of this part of the world is therefore envisaged as Section Collision (Ma) Collision (Ma) evidence for a continental collision which occurred in the recent geological past. A short period of 120.5°E 9.0 7.5 subduction resumed before the onset of the current 123.5°E 10.0 8.0 collision which began 2.5 to 3 Ma ago. Figure 7 summarizes this evolution in cartoon form. 125.5°E 8.0 5.5 liTHOSPHERE STRUCTURE AND DYNAMICS OF THE BANDA ARC COLLISION ZONE, EAST INDONESIA 115 rest of the late Miocene arc underlies the present a micro-continental collision took place between 10 day arc or was incorporated into the collision zone. and 7 Ma preceding the current collision event. The present day arc consists of only younger rocks This was the second accretion event of a young at the surface but outcrops of late Miocene volcanics mountain belt. The first was the emplacement have been noted on both Timor (Abbott and against the micro-continent of the ultrabasic rocks Chamalaun, 1981) and Sumba (Chamalaun et al., at 38 Ma in the Eocene. These two units together 1981). Subduction subsequently resumed along formed a composite micro-continent trapped within the southern edge of the Banda Sea. Perhaps it is a few hundred km north of the significant that all the volcanic islands to the east Australian . It appears that of Timor are less than 5 Ma old (Bowin et al., 1980), "untidy" rifted continental margins may be very decrease in size eastwards and were probably only common (Nur and Ben-Avraham, 1982). Similar built following the change in location of subduction. situations have been documented many times when a continental margin with many inherent CONCLUSIONS weaknesses has rifted to leave a collage of rifts, failed rifts and isolated continental fragments. Evidence presented in this paper suggests that Analogues in the Circum-Pacific region are the

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Figure 8. Tectonic reconstruction of Eastern Indonesia about 10 Ma ago, Present day coast-lines are shown for reference only, Magnetic lineations from Hartono (1990), LITHOSPHERE STRUCTURE AND DYNAMICS OF THE BANDA ARC COLLISION ZONE, EAST INDONESIA 117

Yamato bank in the (Tamaki, 1988) or M.P. and Ries, AC. (Eds.), Collision Tectonics. Geol. Soc. the Macclesfield Bank in the London Spec. Pub., 19, 203-220. (Taylor and Hayes, 1980). When continental DAVIES, G.F., 1980. Mechanics of Subducted Lithosphere. J. fragments such as these arrive at a subduction Geophy. Res., 85 B11, 6304-6318. DE SMET, M.E.M., FORTUIN, AR, TJOKROSAPOETRO, S. AND VAN zone, collisions take place in discrete steps. HINTE, J.E., 1989. Late Cenozoic Vertical Movements of Eventually, this ends up as an extremely Non-Volcanic Islands in the Banda Arc Area. Neth. J. Sea complicated region of geology such as Alaska (Silver Res., 24(2/3), 1263-275. and Smith, 1983) or the (Coward et aZ., DE SMET, M.E.M., FORTUIN, AR, TROELSTRA, S.R, VAN MARLE, 1986). Subduction has now ceased at the narrowest L.J., KARMINI, M., TJOKROSAPOETRO, S. AND BADIWASASTRA, and most uplifted part of the collision zone at eastern 5.,1990. Detection of collision-related vertical movements Timor. 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