Nature and Distribution of Deformation Across the Banda Arc-Australian Collision Zone at Timor

Nature and distribution of deformation across the Banda Arc-Australian collision zone at Timor

DANIEL E. KARIXj Department of Geological Sciences, Cornell University, Ithaca, New York 14853 A J BARBER \ T R CHARLTON J Geology Department, Royal Holloway and Bedford New College, University of London, Egham TW20 OEX, Great Britain

SIMON KLEMPERER* Department of Geological Sciences, Cornell University, Ithaca, New York 14853 DONALD M. HUSSONG Hawaii Institute of Geophysics, 2525 Correa Road, Honolulu, Hawaii 96822

ABSTRACT ing of this example (for example, Hamilton, distribution of collision-related deformation, 1979; Audley-Charles, 1983). The locus, style, whether concentrated near the trough or distrib- Recently acquired seismic-reflection and and distribution of deformation in the Banda uted across the arc system. These differences of SeaMARC II (side-scan and swath bathym- collision have long been major areas of dis- opinion result largely from different interpreta- etry) profiles near Timor show that the agreement. For various reasons, some workers tions of the sediments on Timor and from the Banda Arc-Austnilia collision zone has a locate the surface trace of the plate boundary correlation of these sediments with those on the tectonic framework similar to that of a typical north of Timor and interpret the deformation to Australian margin. oceanic subduction system. Deformation is the south as intraplate collision effects (for ex- The resolution of these questions, particularly occurring, at present, most intensely at the ample, Chamalaun and Grady, 1978; Crostella, those concerning the location and distribution of foot of the inner slope of the Timor Trough. 1977). Other workers place the boundary along deformation, is important for the application of This deformation front is discontinuously ad- the Timor Trough and stress the similarities to the Banda Arc as a collision model as well as for vancing southward as new thrust slices de- normal oceanic subduction systems (for ex- the general understanding of collision-zone velop within the sulxlucted Australian margin ample, von der Borch, 1979; Hamilton, 1979; processes. It is particularly important to under- strata. In contrast, present deformation is ap- Cardwell and Isacks, 1978; Johnston and stand how these collision settings differ from parently negligible in the Savu Basin, the Bowin, 1981). Even among this latter group, normal subduction zones on one hand and from complex fore-arc basin north of Timor. A there is a wide range of opinion concerning the foreland thrust belts on the other. possible significant exception is a postulated right-lateral, northeast-trending fault zone offsetting the outer-arc high between Savu and Roti. Although back-arc thrusting has been documented north of the volcanic arc, this component of convergence is minor compared with the scale of ongoing deformation in the Timor Tirough. The detailed nature of these surveys hsis also led to the recogni- tion of along-strike variations in deformation in the Timor Trough and in the Savu Basin. These variations may be related to the variable degree of involvement of the Australian continental margin along the arc.

INTRODUCTION

The collision between the Banda Arc and the northern margin of Australia (Fig. 1) has been repeatedly cited as a:.i actualistic model for fossil arc-continent collision zones (for example, Row- ley, 1980; Shanmugam and Lash, 1982), yet serious controversy still clouds the understand- Figure 1. Major tectonic features of the Banda Arc-Australia collision zone in the region of the study transect (ruled box). The crest of the outer-arc high is shown by the anticlinal •Present address: Department of Earth Sciences, University of Cambridge, Cambridge CB3 OE2, Great symbol; back-arc thrusting (Silver and others, 1983), by thrust symbols; and late Quaternary Britain. volcanic centers, by stars.

Geological Society of America Bulletin, v. 98, p. 18-32, 13 figs., January 1987.

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121° E 122° E 123° E I24°E ment (Powell and Mills, 1978). These strata can ~~'T" be divided into four sequences representing dif- Lomblen ferent stages of a long and varied tectonic history (Figs. 3 and 4). The first sequence, of Paleozoic strata, was deposited in northwest-trending cratonic basins (for example, Veevers, 1982). In the area south of Timor, these strata are not well delineated by reflection profiles or by drilling. A second sequence, from about Late Permian to mid- Jurassic in age, constitutes a thick section of clastic sediments that represent syn-rift deposits in basins that developed along the east-northeast trend of the future continental separation IO°S (Veevers, 1982). Final separation in the mid- Jurassic led to formation of oceanic crust in the ! / KEY Argo Basin and to the creation of a pronounced

! -LDGO + breakup unconformity (for example, Powell and y Mobil Mills, 1978). /) Shell ^ I Gulf In the transect area, strata of the pre- and x BOCAL I syn-rift sequences cannot be separated seismi- IPS I25°E cally. Together, they are characterized by mod- Figure 2. Seismic-reflection coverage available for this study. The heavy line is the track of erately reflective and moderately well layered R/V Kana Keoki during Leg 2 of cruise 83-01-16. Lighter lines show industrial single- and sediments, separated from the overlying section multichannel profiles as keyed in the figure. Also shown is the generalized region of Kolbano by the breakup unconformity (Fig. 3), and easily Complex on Timor. defined in some areas by an angular unconform-

To approach these problems, a swath across NW SE the Banda Arc from the descending Australian slope of the Timor Trough to the volcanic arc, ~ 100 km wide, was studied using data collected by the authors during a recent marine survey and using mapping on Timor as well as industrial seismic profiles released by the Indonesian authorities. Our marine survey data (Fig. 2) were collected aboard the R/V Kana Keoki of the Hawaii Institute of Geophysics in February 1983. Two areas were surveyed using the Sea- MARCII system, which produces rectified side- scan images and a 10-km-wide swath of digitally recorded bathymetry at 100-m contour intervals (Blackinton and others, 1983). Single-channel seismic-reflection, gravity, and 3.5-kHz profiles were collected simultaneously along the Sea- MARC profiles as well as along additional tracks in the region. Magnetic data were collected intermittently as weather conditions permitted. Industrial data from the larger region between Sumba and Timor (Fig. 2) were also kilometers reviewed to develop the evolution of the collision zone. Figure 3. Seismic profile and interpretive section of the Timor Trough and outer slope from Mobil line 246 (see Fig. 2 for location). This profile illustrates the division of the sedimentary OUTER SLOPE OF TIMOR TROUGH cover of the downgoing plate into pre-rift strata (A), passive-margin strata (B), and outer-slope strata (C). Note that the profile crosses the trough axis where there is no undeformed trough The outer slope of the Timor Trough is under- fill. The syn-rift origin of the normal faults on the outer slope (labeled as in Figs. 5 and 6) is also lain by a thick (>5 km) sequence of shelf and clearly shown, especially in that area where the breakup unconformity (D) is marked by stratal slope strata that is interpreted to overlie Pre- angularity. Reflector X is a prominent horizon slightly above the breakup unconformity that cambrian rocks of the Australian cratonic base- was used to correlate between lines and across faults.

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on the upthrown sides of large normal faults (for example, Fig. 3). The entire suite of available profiles also shows an irregular southward transgression of the facies change from shelf to slope strata. At DSDP site 262 in the Timor Trough, the transition occurred during the early Pliocene (3 m.y. B.P.; Johnston and Bowin, 1981; Veevers, Heirtzler, and others, 1974), but it should become irregularly younger southward toward the present shelf edge. In comparison to the outer slopes of most oceanic trenches, which have dips of 4° to 8° (for example, Karig and others, 1976), that of the Timor Trough is very gently dipping, reach- ing only 2° at the trough axis. In contrast to other miloly flexed outer trench slopes, in which few or no normal faults occur (for example, Nankai Trough; Karig, 1986), however, the Timor outer slope displays numerous, large- displacement normal faults, predominantly down- thrown to the northwest. Displacement de- creases upward from several hundred metres to near zero at the surface on most of these faults (Figs. 3,4, 5, and 6). It is therefore most probable that these faults were associated with rifting of the Australian margin and, being subparallel with the Timor Trough (Fig. 6), some have been reactivated during the collision phase. Some faults that are close to the base of the outer slope (for example, faults A and B of Fig. 5) are marked by a graben in the uppermost slope sequence, by erosion of the upslope block, kilometers and by widespread slumping on the downslope VE 6 4x side (Figs. 5 and 7). This slumping appears to be Figure 4. Timor Trough and outer slope from Gulf line AU-38F (see Fig. 2 for location). This very superficial in some cases but seems to in- multichannel profile images at least S km of strata beneath the trough axis. The major stratal clude sliding of the entire slope sequence in oth- units and normal faults are keyed as in Figure 3; the relatively large trough wedge is labeled ers (Fig. 7). This latter process is documented by (unit F). Note that the frontal thrust can be traced downward to the breakup unconformity. backward rotation of slope strata along the The upward decrease in displacement of normal faults is also clear on this profile. Part of this downslope side of the graben and by anoma- profile is Figure 10 of Montecchi (1976). lously large and variable thicknesses of this section farther downslope. The surficial manifesta- ity and elsewhere by the better seismic layer- of the passive margin. In the area of our transect, tion of these faults is thus in large part a result of ing in the overlying ¡sequence (Fig. 4). A possible this third sequence is -1.5 km thick near the gravitational movement of shallow slope strata basement reflector, noted at ~6.4 sec beneath Timor Trough, as determined from Figures 3 rather than displacement on the fault surface. the trough fill on profile AU-38F (Fig. 4), to- and 4 and velocities from Veevers and others These observations emphasize that extensive, if gether with a velocity structure suggested by (1974) and Stagg and Exon (1981). only small-scale, slumping occurs on outer Veevers, Heirtzler, and others (1974) and The upward shallowing sharply reversed in trench slopes. Jacobson and others (1979), would define a the Pliocene to Recent, with a rapid facies pre-breakup section between 3.5 and 4 km change to the fourth sequence, of hemipelagic TROUGH FILL AND thick. slope calcilutites, recording progressively deeper DEFORMATION FRONT A third sequence of strata, representing the sites of deposition (for example, Veevers and evolution of the northwest Australian passive others, 1974; Johnston and Bowin, 1981). This margin, overlies the breakup unconformity. This change in facies represents the downward deflec- The axis of the Timor Trough is the site of a sequence, which h;as initial deposits of deep- tion of the Australian margin in response to col- flat-topped wedge of strata composed of trench water lutites followed by shelfal carbonates, lision. Our seismic profiles near the Timor or trough fill. This feature represents sediment spans the time range from late Early Cretaceous Trough show this outer-slope sequence to aver- trapped between the outer slope and the defor- to late Cenozoic and represents a shallowing- age 250 m in thickness but also outline areas of mation front or southernmost folds and thrusts upward sequence following the early subsidence reduced deposition and even removal by erosion associated with collision. As in typical trenches,

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Figure 5. Stacked seismic profiles across the axis of the Timor Trough in the SeaMARC B survey area (see Fig. 6 for location). These profiles show the rapid change in trough geometry along trend, resulting from the sequential initiation of widely spaced thrust faults. Correlated fault/fold ridges are numbered along trend and show a rapid decrease in separation as deformation proceeds. Several large normal faults on the outer slope, identified by letter, show some reactivation during collision. The arrow over fold 2 on profile G-H points to a possible diapiric structure.

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Figure 6. Structural framework of the inner slope of the Timor Trough in the SeaMARC II survey area. Control for structural trends was derived not only from seismic profiles but also from SeaMARC bathymetry and side-scan data. Structural features are keyed as in Figure 5. The steep slope marks the division between the ridged lower segment and the acoustically opaque, convex-upward upper slope segment. Lineations on this front represent a large slump (between lines K-L and M-N) and a channel system (crossing line D-E). The buried folds beneath the shelf outline the Bena Basin. One of the more striking observations to be drawn from this map is the conti- J K E Y nuity of structures, particularly Normoi fault slope basins, along the trend of the >- x (Buried) arc. The termination along trend of Tfirusf fault Anticlines and structures near the deformation Structural hrghs front, which leads to the very irreg- /.(Buried) ular pattern of the trough floor, is Basinal areas and also quite obvious. Structural lows . Contemporary trough floor ~7

the subsurface geometry of this wedge is ex- ments show a progressive increase in deforma- ward vergence, the clearest of which is the large pressed on seismic profiles by a sharp upward tion and incorporation into the inner slope, until north-vergent structure, well shown on profiles change from slope-parallel, nearly transparent on profile C-D (Fig. 5), there is almost no unde- E-F to I-J (fold 1) at the base of the inner slope. slope sediments to horizontal, well-layered, and formed wedge remaining. Farther to the north- The imbricate thrusts are seen to displace reflective strata representing mass-flow deposits east on profile A-B, a new, very small wedge is most of the passive margin sequence but on of the wedge (Figs. 4 and 5). presently forming southeast of and adjacent to several of the most favorably oriented profiles, The size of the wedge of sediment filling the the deformed older wedge. On these profiles, do not cut strata beneath the breakup uncon- Timor Trough is highly variable along the trend sedimentation and deformation in the wedge formity, suggesting a basal décollement at or of the collision zone. Even within the 40-km- progress contemporaneously, but sharp stratal near the lithologie contrast represented by the wide detailed survey area, the present flat- angularities within the wedge (for example, Fig. breakup unconformity. floored depocenter varies from 0 to 10 km in 5) show that development of individual folds/ In addition to the deformation associated width, and the thickness of the trough fill varies thrusts began relatively suddenly. with the larger thrust sheets, stratal disruption similarly, from 0 to > 1 km (Fig. 5). Integration This severe disruption of the trough wedge resulting from slumping and from diapirism of the seismic data into a structural map reveals differs from the more continuous migration of might also be expected in this plate-convergent that these variation are caused by growing the deformation front observed in most accret- setting. SeaMARC side-scan images show the linear, but laterally discontinuous, folds and/or ing oceanic trenches (Karig, 1986) and reflects flanks of the rising thrust ridges to be orna- thrusts. These structures are developing within the larger thrust or fold units involved in the mented by patterns that do not appear to affect the previously accumulated trench fill and are Timor Trough. In the survey area, these units are the deeper strata as imaged on reflection profiles. causing present deposition to be restricted to initially 7-10 km in breadth and at least several These patterns are therefore interpreted as surfi- structural lows. times as long. Our nonmigrated seismic sections cial slump scars. On the other hand, the Sea- The sequence of closely spaced, high- do not image the geometry of these structures MARC images reveal no larger "slump apron" resolution seismic and 3.5-kHz profiles crossing very well, but particularly where profiles cross as has been noted at the foot of the frontal thrust the trough wedge in the survey area shows a less highly deformed examples obliquely, it is in the Sunda, Nankai, and Makran Trenches progression of sedimentation and deformation obvious that these structures are basically thrust (Karig, 1983). Neither did the SeaMARC sur- that extends to the surface, indicating continuing sheets, with folding caused by movement up vey show unambiguous large diapiric structures deformation in this setting. From southwest to thrust ramps that dip northward from 15° to as have been identified in the Lesser Antilles Arc northeast across the survey area, trough sedi- 25°. There are several exceptions to this south- (Biju-Duval and others, 1982). Several sub-

Figure 7. Seismic profile (top), 3.5-kHz profile (middle), and SeaMARC II side-scan image (bottom), showing the surficial Sk?M~2.5 sec slumping on the outer slope. Feature B is a graben-like scar caused by sliding of the slope cover from the area of fault zone B. Feature S is a slump of the slope cover that is not related to a deeper structure. Both profiles are along the side-scan track of line E-F indicated by arrows. The 0500 Z position is shown "2.25 along this track by a heavy transverse bar; the fault zone, by the letter "B."

2.50

as 10 km beneath the inner slope on several profiles (for example, Fig. 5 of Crostella and Powell, 1976). The strata accreted to the simple convex prism toe are predominantly trough fill and slope sediments, whereas between 123°30'E and 124°45'E, the ridged lower slope is underlain by thrust and fold slices largely of the Australian passive-margin sequence. On the lowermost slope in the survey area, these accreted passive-margin strata have steep northerly dips and form linear ridges that can be traced along strike for >30 km (Figs. 6 and 8). The ridges are usually parallel, but several cases of convergence between ridges occur, in which the lower, southeasterly ridge is truncated against the upper. No cross-trending faults can be identified that displace ridges on the lower slope. circular surficial disturbances near the northern ridge that carries Roti and Timor, displays two The closer spacing of ridges on the lower edge of the trough (for example, on the flank of distinctly different aspects along the trend of the slope in comparison to that in the trough, the fold 2 along profile G-H) could be smaller dia- arc. Between ~ 123°30'E and 124°45'E and best truncation of ridges noted above, and a crude pirs but alternatively, could be surficial slumps. delineated in the survey corridor, this inner slope mass-balance calculation (see Karig, 1986) sug- The sediment fill of the trough wedge at is divided into distinctly different upper and gest that at least 10 km of shortening has taken DSDP site 262 is composed of very fine- lower sections (Figs. 8 and 9). The lower sec- place over the 20-km-wide lower slope. Struc- grained clastic and biogenic carbonate sediment tion, which shows a fair degree of seismic coher- tural lows between thrust ridges contain slope (Veevers, Heirtzler, and others, 1974), indicating ence, is strongly ridged, has a low average slope, basins with sediment fills that show pronounced a limited source terrane and the trapping of and is only crudely convex upward, whereas the downward increase in dip. Individual slope coarse clastic material on the inner trough slope. upper section has a strongly convex-upward basins are laterally continuous for at least 30 to Because there are no submarine canyons that profile and is separated from the lower section 50 km (Fig. 6) but can show marked variations traverse the entire inner slope, the source for the by a pronounced steeper section. Both our seis- in structural character along trend, from simple, trough wedge is predominantly reworking of mic profiles and industrial multichannel profiles arcward-rotated sediment wedges to complexly sediments from both slopes of the trough. De- show that the upper slope is underlain by acous- deformed, almost seismically unresolvable spite the clear difference in seismic character tically opaque material. structures. between the fill of the trough wedge and the West of 123°30'E and also east of 124°45'E, The ridge-trough pattern and semi-coherent slope sediments, their lithologic differences are the available industrial profiles show a simple, seismic structure of the lower slope section ends thus minor. This is reflected in the difficulty in strongly convex-upward inner slope with no abruptly where the slope rises steeply to the differentiating trough fill from slope strata in ridged lower slope section. Published examples upper section (Figs. 5 and 8). SeaMARC side- DSDP hole 262 and is shown subsequently to that illustrate this geometry include Figure 7 of scan images of the steep slope show numerous be an important point in the interpretation of the Montecchi (1976) and the upper profile in Fig- small slide-like patterns and one very large present distribution of deformation. ure 24 of Beck and Lehner (1974). These simple slump complex (Figs. 6, 8, and 10). This semi- convex slopes are associated with the absence of curcular complex, ~10 km wide, has partially INNER TROUGH SLOPE acoustic coherence within the accretionary buried the ridge-trough system of the lower The inner trough slope, defined as that area prism, although subducted Australian margin slope. Upslope, the slump complex shows sug- between the Timor Trough and the outer-arc strata remain acoustically coherent for as much gestions of channels and may have a source

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10° 10'

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123° 50' 124° 00' 124° 10' 124° 20' Figure 8. Detailed bathymetry in the survey area from smoothed SeaMARC II data, augmented by 3.3-kHz profiles. The contours (100-m intervals) outline Ihe linear pattern of ridges on the lower slope as well as the large slump on the steep slope (Fig. 6). The labeled lines have 3.5-kH2 profiles only; SeaMARC lines are labeled on Fig. 6.

region in an adjacent bathymétrie depression ment," (2) the slump mass overlies slope basins tionary prisms. Nor can this slump serve as a (Fig. 8). This slump complex has almost and their fill, and (3) although areally extensive model for the Bobonaro scaly clay of Timor, certainly produced olistostromal deposits, but (200 km2), the unit is no more than 200 m thick. which reportedly underlies all of the slope strata several important implications should be noted. These characteristics do not permit this slump (Kenyon, 1974). (1) The probable source area consists of young, mass to be a model for sheared olistostromal The structure of the upper slope differs slope sediments rather than accreted "base- deposits that comprise elements within accre- markedly from that of the lower slope. Rela-

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20 -20 V.E. ~ 6x "Beno Basin" Figure 9. High-resolution (water-gun) seismic profiles across the inner slope of the Timor Trough (see Fig. 8 for locations). Profile Y-Z demonstrates the striking difference in nature of the lower- and upper-slope segments, more fully discussed in the text. Profile V-W crosses the Bena Basin (Crostella and Powell, 1976), a large slope-basin complex on the upper slope.

tively little-deformed sediments, generally only and 9). In this basin, termed the "Bena Basin" by lain unconformably by thin surficial accumula- as much as a few hundred metres thick, have Crostella and Powell (1976), the structures all tions (Fig. 9). draped and smoothed most of this slope (Fig. 9), appear to trend east-northeasterly, approxi- Seismic profiles across the Bena Basin (for but in the eastern half of the survey corridor, mately parallel to the lower slope structures, example, Fig. 9) show that the basin fill deposi- seismic profiles outline a series of troughs sepa- This basin cannot be traced west of ~ 124°05'E, tionally overlies acoustic basement, which rises rated by deeply buried basement ridges (Figs. 6 where an acoustically opaque substrate is over- sharply landward to exposures of highly deformed Kolbano Complex (T. R. Charlton, unpub. data). The concentration of deformation at depths greater than -500 m in the basinal strata and the lack of surface expression of the basin indicate that this feature formed earlier, probably as a slope basin. Several similar basins have been identified during the search for petroleum along the south coast of Timor (Crostella and Powell, 1976; Crostella, 1977). Where exposed or drilled, these basins show a sequence of shallowing-upward strata, from uppermost Mi- ocene and lower Pliocene bathyal calcilutites and pelagic limestone to coarser terrigenous clastics (Crostella and Powell, 1976; Kenyon, 1974).

DISCUSSION OF THE AREA SOUTH OF TIMOR

The new data presented from the Timor Trough region allow us to discuss the present, or instantaneous distribution of deformation across the inner slope. They also suggest an interpretation for the different over-all character of this slope both along trend and between the lower Figure 10. SeaMARC side-scan images over the large slump on the steep slope between lines and upper sections in the survey area. K-L and M-N. Note the convex frontal lobes of the slump and their superposition over the Evidence has been presented for deformation linear ridges. at the foot of the inner slope as recently as can

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Figure II. Interpretive tectonic map of the Savu Sea region between Timor and Savu; seismic coverage is used for control in this region. Profiles are identified on Figure 2. There is i i ; „\ no evidence for crustal shortening north of Timor that can be related to collision. On the con- trary, all basement ridges trend northeast parallel to Timor (outer-arc high) and are covered by the undeformed South Savu and North Savu sedimentary se-

^ X / a \ \ /r quences. The sharp change in character of the structures along the south flank of the Savu Sea A-, Arch occurs where the South Savu Basin is crossed by a zone /// of stratal disruption, bounded by y / the double, questioned lines. The Timor ' » ^ V ¥7 dotted pattern delimits the iden- / ^ t" tifiable extent of South Savu Basin strata. The open-circle pattern shows the edges of the present North Savu Basin. be resolved by 3.5-kHz profiles. The instantane- strata of the passive-margin sequence are in- ous rate of deformation in this setting in compar- volved in this thrusting. Beneath these steep, ison to that farther upslope can be qualitatively smooth slopes, the internal structure is seis- corroborated by the predominance of highly determined by the relative intensity of deforma- mically incoherent, suggesting more distributed folded and thrusted, fine-grained continental-rise tion of the youngest strata. Despite effects such deformation and smaller individual structural sediments in the Kolbano Complex of south as different rates of sedimentation that compli- units. Timor (Barber and others, 1977), which com- cate this analysis, all of our 3.5-kHz and high- In the area south of western Timor, we would prises much of the basement beneath the upper frequency seismic data show quite clearly that interpret the two-part slope as resulting from a trough slope. By this logic, the eastward lateral displacements in the youngest strata are greatest temporal change from a convex-upward, seismi- change to the thrust ridges near 123°30'E would at the inner edge of the Timor Trough front and cally incoherent style to a ridged, structurally reflect the westward migration of the oblique are nearly absent on the upper slope. This inter- more coherent style. It then remains to be asked intersection between the deformation front and pretation, coupled with the interpretation that why this change occurred through time in the the Australian margin. A reversion to the cumulative deformation of the inner slope in- survey area and why it now occurs laterally steeper, diffusely deformed slope east of 125°E creases northwestward, leads us to conclude along the arc. We attribute the difference to a would support the speculation by Johnston and that, at present, deformation south of Timor is contrast in mechanical properties of the sedi- Bowin (1981) that the collision process was also continuing and is most intense at the foot of the ment section involved in thrusting. Relatively not as far advanced in that area. inner trough slope. weak sediments, without lithologie units that The change in st ructural character across the could act as a basal décollement, lead to steeper OUTER-ARC HIGH, TIMOR inner trough slope in the survey corridor can be slopes (for example, Davis and others, 1983) interpreted as a result of changing structural and to smaller structural units. Thin, uniform, The most recent geologic studies of Timor processes during the growth of this deformed and clay-rich sections would tend to respond suggest that the island can be divided into two mass. The differences between the upper- and this way because of the correlation of strength major basement units, representing very differ- lower-slope sections closely resemble the lateral with lithology on one hand and with porosity on ent geologic histories. These units, roughly un- changes in characte r of the lower slope along the the other. derlying the northern and southern halves of arc and are felt to represent similar differences in Such conditions should be more nearly Timor, are further interpreted to have been jux- processes. The strongly ridged aspect of the reached in the thin, calcilutite-rich continental taposed during the Banda-Australian collision. lower slope occurs in cases in which thrusting at rise and slope strata off northwest Australia than The region south of the central basin is under- the deformation front involves most or all of the in the shelf strata that constitute much of the lain by the Kolbano Complex and related rocks, passive-margin sequence as well as the trough landward parts of the passive-margin prism. If which represent an uplifted earlier segment of fill. Structural units are large and moderately this assumed mechanical contrast were valid, the the accre*ionary complex developed during the coherent on a seismic scale, and the resultant southerly migration of the deformation front collision. Best exposed in the area just east of the deformed wedge lias a low average surface across the Australian margin should lead to the study corridor (Fig. 2), the Kolbano Complex slope. In contrast, the smoother, steeper, and transition from steeper and more diffusely de- consists of Jurassic sandstones, Lower Creta- strongly convex-upward sections of the lower formed slope sequences to ridged sequences of ceous marls with radiolarites, and Upper Creta- slope occur where only the trough fill and slope more coherent thrust slices. This assumption is ceous to Pliocene calcilutites, interpreted as

200 Km

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200 Km

Figure 12. Mobil seismic-reflection profiles across the Savu Sea. Profile 254 crosses the basin between Sumba and Savu (Fig. 2) and shows the configuration that typifies the western section of the basin. There, two major basement ridges can be traced between profiles on the basis of their relationship to the South Savu sedimentary sequence. Large-scale arcward rotation of the basin has resulted both from regional tilting and displacement on the Savu thrust. Profile 250 crosses the eastern section of the basin (Fig. 2) where no thrusting occurs along the southern flank and basement ridges trend northeasterly.

lower slope sediments forming part of the Aus- the section. The clasts in the lower section are has a triangular map pattern between Sumba tralian continental-margin sequence. This as- derived from the units to the north, derivation and Timor, as defined by the 3 km contour semblage is folded, thrusted, and imbricated by from the Kolbano area not occurring until high (Fig. 1). Seismic profiles, however, reveal a steep faults (Barber and others, 1977). Locally, in the section. The older strata, such as the Batu series of buried basement ridges that trend east- Permian-Triassic strata of the Australian margin, Putih Formation, show evidence of deformation west from Sumba to Roti but swing northeast- from beneath the breakup unconformity, are in- that dies out upward. erly north of Timor (Figs. 11 and 12). The volved in the thrusting. These older sediments, These units are overlain by uplifted coral largest of these ridges, herein termed the "North stratigraphically overlain by deep-water deposits, reefs, which form extensive elevated plateaus at Sumba Ridge," divides the Savu Sea into north suggest that extremely thin continental crust heights of 700-800 m in the central part of West and south structural basins of quite different underlay this section after the Jurassic rifting Timor, rising to 1,200 m on the northern margin character. and before late Neogene collision. of the central basin and to nearly 1,300 m near To the north, there is a group of allochtho- the border with East Timor. Continued uplift of South Savu Basin and North Sumba Ridge nous units, quite different among themselves but the central basin to the present is indicated by all unrelated to the present margin of Australia. stepped alluvial terraces along all of the major The South Savu Basin is a paleo-depocenter These allochthonous units are interpreted as rep- rivers in this region. If those reefs are Pleistocene between the North Sumba Ridge and the outer- resenting older parts of the Banda Arc. They in age (Tjokrosapoetro, 1978), average uplift arc high. It is now tilted and deformed to vary- show Asiatic affinities in their stratigraphic and rates must reach 1 mm/yr or even more. Recent ing degrees and is being overlapped by strata of structural development and were probably subsidence of the north coast of West Timor is the North Savu Basin, which itself overlies derived from the Asian margin and incorporated indicated by the total absence of raised coral another earlier depocenter. The South Savu into the Banda Arc by Eocene time, in part as reefs. Chappell and Veeh (1978), using Basin is defined by a sequence of parallel- microcontinental fragments and in part as earlier 230Th/234U dating methods on reefs along the layered strata that have a maximum strati- accretion of sea floor. north coast of East Timor, found major varia- graphic thickness of >2 km (Fig. 12). This Overlying the Kolbano Complex and the tions in the rate of uplift from 0.03 to 0.55 sequence dips gently northward and appears to south edge of the northern units, there are Plio- mm/yr. downlap on an acoustic basement along a cene and younger sediments that record uplift sharply defined unconformity. The persistent associated with collision. These deposits, con- Area North of the Outer-Arc High and uniform stratification of the South Savu centrated in the central basin, commence with Basin strata, as well as their projection to out- the lower Pliocene Batu Putih Limestone, a If the chain of islands including Savu, Roti, crops in eastern Sumba (von der Borch and oth- bathyal calcilutite, and pass upward into terrig- and Timor is considered to define an outer-arc ers, 1983), identifies them as basinal turbidites, enous turbidite deposits of the Viqueque Group high, the Savu Basin to the north would be clas- which leads to the interpretation that they were (Kenyon, 1974). These deposits become of in- sified as a complex fore-arc basin. This basin is originally horizontal and that the downlap is ac- creasingly shallow-water type upward through bounded on the north by the volcanic arc and tually a northward-tilted onlap. A northward

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sense of onlap can be traced on profiles between The thrust fault, which has been termed the but also there is a broad (>40 km) zone where Sumba and Roti from the North Sumba Ridge "Savu thrust" (Silver and others, 1983), brings the South Savu sequence shows marked seismic southward for -30 km, at which point onlap acoustically opaque material northward over degradation (Figs. 11 and 13). Basement and ceases and the sense may even reverse. From this most or all of the basin fill. Where emergent on major refl<;ctors can be traced through this zone, basin axis to the crest of the North Sumba Savu, this opaque material is identified as de- but there aire many hyperbolic diffractions, both Ridge, there is structural relief of >4 km. formed Permian and Mesozoic sediments along the basement reflector and within the sed- The North Sumba Ridge itself was not a (Hamilton, 1979). East of Savu, the Savu thrust iment sequence. major sediment source for the fill of the South swings sharply southeastward (Fig. 11) and This zone of relatively recent disruption ap- Savu Basin because the basinal strata show no cannot be observed on profiles east of pears to trend north-south and to be approxi- change in seismic fa.ties toward the ridge flank ~122°20'E. Although this thrust post-dates Pli- mately colinear with the change in nature of the (Fig. 12). The ridge, however, was the source for ocene basin strata, its trace is overlain on several south flank of the South Savu Basin. The zone a wedge of older sediments that can be observed profiles by as much as several hundred metres of would extrapolate farther southward to the to underlie the basin fill and to thin away from undeformed sediment, constraining its age as major break in the outer-arc high between Roti the ridge on at least several of the seismic pro- late Pliocene to early Quaternary. and Savu I Fig. 1). The nature and significance of files (for example, Mobil profile 250, Fig. 12). East of 122°30'E, the South Savu Basin se- this zone are unclear. It could be a zone of slip That the North Sumba Ridge was a submarine quence of well-stratified sediments is also ob- line displacements resulting from collision feature for much or all of its length during the served to onlap northward against a large (Burke arid Sengor, 1986), with right-lateral filling of the South Savu Basin is indicated by a basement ridge (Fig. 12), but here, the basinal displacement of the outer-arc high. Offsets of thin cap of slope-conforming transparent (prob- sequence is narrower and more complex than to ridges in the fore-arc basin (Savu Sea), however, ably pelagic) sediment over the ridge crest and the west. Moreover, individual features cannot are not obvious. beneath the uppermost basin fill. These upper- be interpolated across an unfortunate 50-km- most basin strata overlap the North Sumba wide gap in profile coverage across which the North Savu Basin Ridge and merge with older strata of the North structural aspect changes abruptly (Fig. 11). Savu Basin. The most striking difference in the eastern sec- Between the North Sumba Ridge and the Where the North Sumba Ridge and the fill of tion is that the North Sumba Ridge and several presently defined volcanic arc is another basin, the South Savu Basin emerge on the island of other basement ridges to the south, as well as the which is now the active depocenter of the Savu Sumba (Fig. 1), a sequence of mid(?)-Miocene outer-arc high itself, trend 040° to 050° rather Sea. This depocenter, the North Savu Basin, has to Pliocene deep-water carbonates and volcani- than east-west as in the western section a sedimentary fill in excess of 2 km, and acoustic clastic turbidites (Cliamalaun and others, 1982; (Fig. 11). This northeasterly trend continues at basement reaches depths greater than can be re- von der Borch and ethers, 1983) unconformably least to 123°45'E, where our data coverage solved on the available seismic profiles. The overlies lower Miocene and upper Tertiary ig- ends. A second difference is the absence of deepest strata beneath the south flank of the neous rock, volcanogenic sediments, and reefal thrusting along the southern flank of the basin basin dov/nlap northward onto a rough base- sediments. These, in turn, overlie Cretaceous (Fig. 12). Instead, the strata of the South Savu ment at a very low angle (Fig. 12). This se- mass-flow deposits that are only moderately Basin lap sharply against the rapidly rising quence can be approximately correlated with tilted and faulted but are extensively intruded by acoustic basement of the outer-arc high. Minor the middle part of the South Savu Basin se- mafic to intermediate igneous rocks. high-angle faulting occurs along sections of this quence on Mobil profile 250, which would sug- The structure of the southern flank of the flank, but the sense of displacement is uncertain gest a late Miocene age. Overlying this gently South Savu Basin has two distinctly different and, possibly, even variable along strike. northward-dipping sequence is a thick, flat-lying aspects along trend with a sharp change near There is a progressive eastward narrowing of basin fill. On some of the profiles (for example, 122°30'E (Figs. 11 and 12). This division is also the South Savu Basin and disappearance of Mobil 250), this sequence shows a distinct angu- reflected in the strike and continuity of all fea- basement ridges, as these features are intersected lar onlap relationship across the south flank of tures within the basin. West of 122°30'E from by the north-northeasterly trending insular slope the basin, whereas on other profiles (for exam- Savu at least as far ¡is eastern Sumba, the South of westernmost Timor (Figs. 1 and 11). This ple, Mobil 251), the change in dip is more con- Savu sequence is deformed into a large base- truncation might reflect some shortening be- tinuous. In both cases, northward rotation of the ment arch, which in turn is flanked to the south tween the outer-arc high and the North Sumba basement is implied, but the areas that have dis- by a thrust fault and/or deformation front. This Ridge, or it might instead represent rapid uplift crete onlap indicate that most of the rotation basement arch, illustrated in Figure 72A of along trend and the linear continuation of the occurred early in the present basin-filling phase. Hamilton (1979) aid Figure 18 of Silver and basement structures into the broader part of On the northern flank of the basin, multiple others (1983), is herein termed the "South Timor. Support for the second alternative is of- onlap and tilting of the upper sequence, here Sumba Arch" to differentiate it from the geo- fered by the continuation of the southernmost only poorly layered, probably represent differen- graphically distinct North Sumba Ridge basement ridge into the northern mountain tial vertical displacement between the basin and (Figs. 11 and 12). The South Sumba Arch prob- range of Timor as well as the parallelism of that the volcaniclastic sediments forming the mid- ably had an early precursor in that the range with the North Sumba Ridge. Miocene and younger volcanic arc. Seismic continuity of this basin fill with the uppermost part northward basal onlap of basin strata reverses to The interpretation of the discontinuity be- of the South Savu sequence suggests that the southerly onlap over the north flank of the arch, tween eastern and western sections of the Savu North Savu Basin became a major depocenter in to be replaced by northerly onlap again on its Basin is hampered by the very poor seismic cov- the Pliocene. A small, older depocenter beneath southern flank (Fig. 72A of Hamilton, 1979). erage in the critical area. All four profiles that the central and northern parts of the basin is Most arching, however, affects and thus post- cross this zone, however, clearly show a zone of suggested on some profiles (Fig. 12). dates all of the basin fill. At present, there is faulting in the South Savu Basin sequence. Not significant erosion of strata over the arch (for only are there several discrete faults, across During the deposition of the basin fill, and example, Fig. 18 of Silver and others, 1983). which there is little total vertical displacement, even during deposition of the older tilted se-

of the swell through Sumba, that for the eastern constriction is quite different. In the east, the cause is the sharp change in trend of the outer- arc high and all basement ridges at least as far north as the North Sumba Ridge. This trend must pre-date the undeformed Pliocene to Re- cent fill of the North Savu Basin. If the discor- dance of the basement trends with that of the volcanic arc is a result of crustal shortening, the major locus of this shortening would have to be in the North Savu Basin, because all of the ridges from the North Sumba Ridge to Timor are parallel. Such shortening would have to occur be- 0 V.E.-3* (Okm neath the deepest part of the North Savu Basin, Figure 13. A portion of Gulf line IBA-28 (for location, see Fig. 2) and interpretive section, the base of which was not imaged in the availa- crossing the zone of disturbed South Savu Basin strata north of the Roti-Savu discontinuity ble seismic data, and would have been active (Fig. 11). The distribution of stratal disturbance on four profiles indicates that the zone trends during the mid-Miocene or earlier. Alterna- north-south, but the individual diffraction trains and offsets cannot be correlated between lines. tively, the northeasterly trends might have de- Offsets involve basement, but there is no consistent sense of vertical displacement within the veloped before the construction of the mid-Mio- zone, suggesting that there may be large cumulative lateral offset. cene to Recent volcanic arc, which would then overlap and bury the northeastern continuation of those ridges. Additional seismic coverage might be able to aid discrimination between quence, there appears to have been very little North Savu Basin of several kilometres is con- these alternatives. deformation within the North Savu Basin. sistent with the seismic data. Strike-slip faults may exist but have not been The pattern of vertical displacement described detected on our data. The only structure clearly above is modified toward the west by rapid Pli- GENERAL INTERPRETATIONS AND observed is a gentle flexure of basin fill that ocene to Recent uplift along a broad northwest- IMPLICATIONS FOR trends northeasterly and parallel to the North trending swell, defined by the long axis of COLLISION TECTONICS Sumba Ridge near 123°30'E, 9°S (Fig. 11). Sumba (for example, von der Borch and others, This flexure raises the southern block several 1983). Because this swell obliquely intersects the Despite its collision with Australia, the Banda hundred metres and may coincide with a base- older basement features, the North Sumba Arc near Timor resembles a normal island arc in ment ridge. East of 123°40'E, there appear to be Ridge can be characterized using subaerial expo- many respects. This similarity is particularly true additional structural complexities, but our data sures on Sumba. Silicic to mafic, intrusive and of the Timor Trough and its inner slope. Thrust- are insufficient to define the structural geometry. extrusive igneous rocks of early Miocene and ing, producing accretion, is continuing, with Together, the data sets from the North and Paleocene age crop out widely on Sumba and maximum instantaneous shortening at the de- South Savu Basins outline a post-mid-Miocene have tentatively been identified as arc related formation front. This distribution of deforma- history of large-scale vertical displacements, but (von der Borch and others, 1983). We thus iden- tion is also responsible for the convex-upward with remarkably little evidence for horizontal tify the North Sumba Ridge as an early Tertiary profile of the inner slope, which is particularly displacements. Uplift of the outer-arc high, rela- volcanic arc. Upper Cretaceous igneous rocks marked where a thinner section is accreted. Sed- tive to sea level and to the South Savu Basin, might also represent arc rocks, although Chama- imentation on the ridged inner slope leads to the occurred primarily by regional northward rota- laun and others (1982) and von der Borch and development of superimposed slope basins, tion rather than by faulting. Even along the others (1983) favored a rift origin. Sumba may which record continued deformation but at an western part of the basin flank, the Savu thrust be a continental fragment (Hamilton, 1979), but arcward-decreasing rate. accounts for only a minor fraction of the differ- the present data from that island demonstrate Convergence and accretion are also the most ential uplift. that it could as easily have developed as a sec- probable causes for the sequence of mildly de- The lower Miocene shallow-water strata on tion of the fore arc. In either case, Sumba must formed strata overlying much more intensely de- Sumba, as well as the older wedges of sediment have become an element within the Banda Arc formed sediments that constitute the southern flanking the North Sumba Ridge, point to a before the development of the early Tertiary section of Timor. The more deformed Kolbano large-scale subsidence of the North Sumba volcanic arc. Complex represents continental-rise or lower- Ridge since the early Miocene. Some of this Geological and seismic data suggest that the most continental-slope strata offscraped during subsidence probably accompanied the deposi- volcanic arc represented by the North Sumba the early Pliocene. The overlying strata have tion of the middle to upper Miocene bathyal Ridge ceased to be active after the early Mio- often been termed "post-orogenic," but they dis- strata of the South Savu Basin, but some subsi- cene and that a new volcanic arc developed by play the characteristics of trench slope strata, dence of the North Sumba Ridge, as well as that the mid-Miocene near the position of the present including an upward shallowing and decrease in of the North Savu Basin, accompanied the re- arc. If so, the North Sumba Ridge and asso- deformation and a southward decrease in age of gional rotation that clearly post-dated deposition ciated basement ridges would have assumed the equivalent facies. of the South Savu sequence. During this same role of a mid-Miocene and younger frontal arc, Local geologic relations on Timor have led Miocene to Recent interval, the volcanic arc has the South Savu depocenter representing a fore- Crostella and Powell (1976), Audley-Charles been uplifted relative to sea level (for example, arc basin. (1968), and others to describe the collision in Brouwer, 1942; Sudradjat, 1975). Differential Although the kinematic cause for the constric- terms of discrete orogenic events, although a displacement along the northern flank of the tion of the west end of the Savu Sea is the uplift southerly migration of deformation is also rec-

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ognized by those workers. We are led by our structures, which parallel the West Timor trend, rates, as approximated by Holocene and late data to emphasize this southerly migration and also appear to have developed before the Mio- Quaternary values, are available for the study to interpret the local orogenic relations as but a cene to Quaternary volcanic arc developed. The area, but a qualitative sense of the recent dis- single spatial section in a southwardly migrating possibility that the volcanic arc has migrated placement pattern across the arc can be obtained spectrum of collision processes. This migration is northward during the Neogene in one or several from Quaternary depositional and structural analogous to the outbuilding of the accretionary discrete jumps and that the post-mid-Miocene data. Rapid subsidence of the outer trough slope prism in a normal arc system. Structures homol- east-west volcanic arc trend was superimposed has been well documented (Veevers and others, ogous to those generated during the Pliocene on older northeasterly trending structures should 1974; Johnston and Bowin, 1981) and is inter- and early Pleistocene in south Timor are thus be explored. Yet another northward jump may preted here as the vertical component of conver- now being generated close to the Timor Trough. be underway in the inactive section of the vol- gence between Australia and the Banda Arc. In Given the seismic data available for this canic arc between 124°30'E and 126°E, where any model, the subsidence rate should increase study, we can now place very strong constraints volcanism occurs >100 km north of the Mio- northward to the trough; if due to convergence, on the recent distribution of deformation across cene to Pliocene trend. the subsidence rate would equal the convergence the collision zone near western Timor. There is Johnston and Bowin (1981) proposed a very rate times the tangent of the local structural effectively no horizontal shortening now occur- different distribution of recent deformation on slope. ring between the ou ter-arc high (Timor) and the the basis of the rates of facies migration deduced Subsidence is replaced by uplift at the defor- volcanic arc. Shortening is presently occurring from DSDP hole 262 and from associated seis- mation front as can be seen by the absolute up- from Timor south to the Timor Trough and also mic profiles. In this modification of the steady- lift of the accreted trough and slope strata. Until north of the volcanic arc (Hamilton, 1979; state trench model of von Huene and Kulm depth control on the surficial sediments becomes Silver and others, 1983). That part of plate con- (1973) and others, the southward rate of migra- available using benthic foraminifera, we can es- vergence which is absorbed between Timor and tion of the facies bands related to subduction in timate uplift rates at the deformation front only the trough is a maximum at the deformation the Timor Trough, with respect to the Austra- from the structural geometry. A 200- to 300-m front. The relative fractions of the convergence lian plate, is assumed to equal the convergence uplift of the frontal structures, associated with a rate now absorbed in the fore arc and in the rate between Australia and the Timor deforma- probable recurrence interval of development of back arc are not determinable, but if only sev- tion front. Because their careful analysis showed new thrust sheets of 105 yr or less, produce uplift eral tens of kilometres of backthrusting have oc- a slowing of this migration rate from 1.3 m.y. rates of several millimetres per year. This high curred during the past few million years as B.P. to the present, Johnston and Bowin (1981) uplift rate; should decrease northward if vertical suggested by Silver and others (1983), then concluded that crustal shortening near the and horizontal displacement rates are related back-arc convergeE.ce would be a minor com- Timor Trough has nearly ceased and has been and if the convex profile of the slope accurately ponent of the total. There is, however, the possi- replaced by equivalent shortening elsewhere reflects that distribution of shortening (Karig, bility that this back-arc thrusting is now across the arc system. Our more recent data re- 1986). Subcretion, or underplating, would also replacing that in the fore arc as the major mode veal, however, that the deformation front is not cause uplift and, to the extent that it might be of plate convergence. migrating smoothly but is discontinuously jump- occurring in this collision setting, could tend to West of Savu, where the collision process is in ing southward as subsequent large thrust slices offset this arcward decrease in uplift rate, de- an earlier phase, the pattern is basically similar. develop. In the vicinity of DSDP site 262, pending on the distribution of underplating The Savu thrust has been interpreted as a zone thrusting is clearly very active but is causing across the outer arc. of recent major crustal shortening (for example, internal disruption rather than migration of the Uplift across Timor must have been highly Audley-Charles, 1985) but this feature can be wedge of the trough fill. A contributory problem variable. At least several kilometres of cumula- identified only between Sumba (Silver and oth- is the local rapid narrowing of the wedge of tive uplift; in south Timor is documented by the ers, 1983) and Roti (Figs. 1 and 11). Further- trough fill at site 262 (Fig. 8). This observation bathyal sediments overlying the Kolbano Com- more, this thrust could not serve as the structure and a high-resolution seismic profile along line plex, but it is not clear how rapidly that region is along which the northeasterly trends of the Y-Z lead us to conclude that the hole did not rising at present. The greatest elevation and outer-arc high and Savu Sea converge with the penetrate as much of the trough fill as Johnston highest reef terraces are in north-central Timor, volcanic arc because basement ridges north of and Bowin estimated, and the thinner the section but recent vertical displacement rates along the the projected thrust also converge with the vol- identified as trough fill, the higher would be the north coast are quite variable and low. apparent rate of convergence. canic arc. Because the sector of the arc where the The lack of consistent uplift along the north Savu thrust can be identified coincides with the Vertical displacements associated with colli- coast of Timor, major northward tilting of the northeasterly swing of the outer-arc high, we sion are far less in magnitude than the horizontal southern part of the Savu Sea floor, and a very suggest, alternatively, that the thrust merely ac- displacements, but possibly even more impor- thick, rapidly deposited section in the North commodates the local differential northerly dis- tant in that they are more clearly inscribed in the Savu Basin suggest that uplift switches to subsi- placement of the outer-arc high between Sumba rock record. To a great extent, the pattern of dence near the south flank of the South Savu and Savu. vertical displacement during collision is similar Basin. To the west, in the vicinity of Sumba, the Basement structures in the Savu Sea must be to the pattern of relief across the collision sys- entire fore-arc basin is rising along the Sumba investigated further, but even the present data tem. A major exception is the inner trough Island trend, as shown by uplifted reef terraces outline a more complex evolution of this region slope, where sediments deposited at 2- to 3-km on Sumba (for example, von der Borch and oth- than had been anticipated. The structural water depths are being uplifted. As in the case of ers, 1983) and by the relative uplift and dissec- framework, with the North Sumba Ridge divid- horizontal displacement, there is also the need to tion of the North Savu Basin fill seen on several ing the major basin into two sub-basins, seems differentiate between instantaneous and cumula- industrial seismic profiles. to have developed by the mid-Miocene. Even tive uplift. Cumulative vertical motions since the mid- more surprising, the northeasterly trends of these Very few instantaneous vertical displacement Miocene 'differ only slightly in distribution from

the recent motions. The intensely deformed Pliocene would indicate that the leading edge of ridden by the outer-arc high and greatly nar- Kolbano Complex leads us to presume that the continental crust was subducted ~3 m.y. ago. rowed (Chi and others, 1981), a process that is Timor Trough was located in southern Timor Given plate convergence at 7 cm/yr (for exam- proposed to occur in many collision zones (Mit- during the late Miocene and early Pliocene. Al- ple, Johnston and Bowin, 1981), almost all of chell, 1984). though water depths would have been in excess which has been absorbed south of the Savu Sea, The Taiwan collision is approximately the of 2 km, this zone should have been rapidly well over 150 km of continental crust should same age (3-4 m.y.) as that in West Timor and rising. In fact, this response is recorded by the have been subducted. The stratigraphic superpo- has been proceeding at nearly the same conver- overlying slope strata. The lack of northerly de- sition of deep-water Kolbano strata on shallow- gence rate (5-7 cm/yr). The contrast in behav- rived terrigenous clastics in the basal sections of water Australian shelf units indicates that a ior, therefore, cannot be simply a function of the these slope strata has been cited as evidence that highly thinned continental crust was being sub- amount of crust consumed. The differences in the outer-arc high (northern Timor) was not yet ducted during most of this time, represented by trough fill and nature of accreting units probably emergent in the early Pliocene (for example, the opaque part of the inner slope. Deep subduc- reflect the much greater sediment input to the Kenyon, 1974). This is a reasonable interpretation of continental crust is also suggested by trough from the downgoing Asian plate, but tion, although such sediments could also have seismologic data (McCaffrey and others, 1985) such differences as the response of the fore-arc been trapped higher on the slope. The appear- and corroborated by analysis of gravity profiles basin are not yet understood. ance of coarse, arc-derived clastics in the mid- (J. McBride and D. E. Karig, unpub. data). The Pliocene "post-orogenic" strata requires an thin continental crust quite probably represents CONCLUSIONS emergent source to the north, but this source a marginal plateau, as occurs along much of the must have been local and in northern Timor, north and west Australian margin. Our data, which span the fore arc in the West rather than in the region now occupied by the We are now in a position to suggest behav- Timor area, indicate that the Banda collision Savu Sea. Not only has the South Savu Basin ioral patterns for the West Timor section of the zone, in its present stage of evolution, is very remained lower than northern Timor, as shown Banda collision zone, but we stress that these similar in most respects to a normally subduct- by its onlap-flanking relations, but it was also a patterns undoubtedly vary along the arc trend as ing arc system. Deformation, particularly as site of deposition throughout much of the well as among collision zones. Several character- manifested by horizontal shortening, is concen- Pliocene. istics suggest that this collision segment is still trated beneath the inner slope of the Timor Onlap and facies relationships define the functionally in an early stage of evolution. Trough. There is no evidence for a zone of South Savu Basin as a topographic low relative First, the Timor Trough is still relatively deep crustal shortening between the outer-arc high to the Sumba Ridge and neighboring highs as for collision zones, primarily because it has only and the volcanic arc, certainly not at present well as to the outer-arc high from the early Mio- a thin trough fill. The material accreted varies and, apparently, not since some time before the cene until the presumed late Pliocene initiation from structurally complex fold-thrust units inception of collision. Crustal shortening behind of tilting. This regional tilting, which has ac- composed largely of trough fill to 5- to 10-km- the volcanic arc may possibly be rapid at present counted for well over 2 km of differential dis- broad, more coherent thrust sheets of both but has not accounted for more than several tens placement between the Sumba Ridge and trough fill and continental-margin strata. This of kilometres of displacement. Timor, is interpreted to be a major cause of the contrasts with more mature collision zones such South of western Timor, crustal shortening is present arc geometry, in which the fore-arc basin as Taiwan and the southeastern Zagros, where resulting in the accretion of fairly large thrust is significantly deeper than is the Timor Trough. thick orogenic sediments (molasse) fill the slices of trough fill as well as Australian This anomaly occurs in an even more exagger- trough, now a foredeep, to nearly sea level, and continental-margin strata. The increase in size ated form east of Timor where the 7-km-deep where thrust or fold units are several tens of and in internal coherence of the accreted units is Weber Trough is the fore-arc basin to a shallow kilometres wide (for example, Suppe, 1980; interpreted to reflect the increase in mechanical subduction trough. Colman-Sadd, 1978). The rapid deposition of strength and heterogeneity of the accreted mate- Several explanations for this inversion of typ- coarse debris from the outer-arc high and frontal rial as collision progressed, but it also might be ical fore-arc basin depth relationships can be arc is not observed in the Timor Trough, be- viewed as an early stage in the transition of the suggested. Perhaps most appealing is incremen- cause uplift of that source is recent and still only Timor Trough to a foredeep. If the Taiwan colli- tal flexure of the overriding lithospheric plate. If modest. Debris now being eroded from the sion zone or the far more advanced Canadian thicker, higher standing continental lithosphere outer-arc high is still being trapped in slope ba- Cordilleran foreland thrust belt (Price, 1981) is subducted, the mechanical leading edge of the sins and on the insular shelf. Moreover, in can be used as examples, the Timor Trough will upper plate should be uplifted, but the area be- contrast to the voluminous sediment supplied to become filled with orogenic sediment (molasse) hind should be flexed down, similar in cause but the Taiwan collision zone from Asia (for exam- from the greatly uplifted fore arc, and thrust opposite in direction from the swell on the outer ple, Niino and Emery, 1961), the continent sub- units will become much larger as the collision trench slope. This flexure might be intensified by ducted in the Timor Trough (Australia) supplies proceeds. increased compressional stress induced during very little sediment. In many respects, the Banda Arc-Australia collision as well as by large-scale underplating. Second, the fore-arc basin along most of the collision zone is a good model for ancient colli- The uplift of the outer-arc high that has been Banda collision zone shows large relative, and sion sutures. It must be recognized, however, induced by collision is estimated at 3 to 5 km, probably absolute, subsidence. This response is that this collision zone is in an early or immature producing almost 3 km of subaerial relief. This is similar to that of large fore arcs in such normal phase and that it may evolve significantly before to be contrasted with the subduction of the lead- arc systems as the Sunda and eastern Aleutians convergence ceases and it becomes frozen in the ing edge of the Australian margin to depths of at but to an exaggerated extent. Horizontal short- geologic record. The Banda Arc seems a particu- least 20 km and quite probably several times this ening within the Banda fore arc, however, has larly good model for at least the early phases of amount. If the Kolbano Complex represents been minor. In contrast, the fore-arc basin in the the Taconic collision of eastern North America continental-rise strata, its accretion in the early Taiwan collision zone has apparently been over- (Rowley, 1980), where a relatively deep bathy-

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metric trough was preserved until nearly the end Blackinton, J. G., Hussong, D. M., and Kosalos, J., 1983, First results from a Montecchi, P. A., 1976, Some shallow tectonic consequences of subduction and combination side-scan sonar and seafloor mapping system (SeaMARC their meaning to the hydrocarbon explorationist, in Circum-Pacific of convergence (Cisne and others, 1982). II): Offchore Technology Conference, OTC 4478, p. 207-311. energy and mineral resources: American Association of Petroleum Brouwer, H. A., 1942, Summary of the geological results of the expedition, in Geologists Memoir 25, p. 189-202. Clearly, the Banda model is not applicable to Geological expedition to the Lesser Sunda Islands: Amsterdam, the Niino, H., and Emery, K. O., 1961, Sediments of shallow portions of East other systems typified by a sediment-choked Netherlands, N.V. Noord-Hollandsche Vitgevers Mij., v. 4, p. 245-402. China and South China Seas: Geological Society of America Bulletin, Burke, K., and Sengor, A.M.C., 1986, Tectonic escape in the evolution of the v. 72, p. 731-762. trough and by large thrust sheets. Neither is continental crust, in Barazangi, M., and Brown, L., eds., Reflection Powell, D. E„ and Mills, S. J., 1978, Geological evolution and hydrocarbon seismology: The continental crust: American Geophysical Union Geo- prospccts of contrasting continental margin types, southwest Australia, there evidence for closing and imbrication of the dynamics Series, v. 14, p. 41-54. in Wiryosujono, S., and Sudradjat, A., eds., Regional Conference on fore-arc basin in the West Timor section of the Cardwell, R. K., and Isacks, B. L., 1978, Geometry of the subducted lithosphere Geology and Mineral Resources of SE Asia, Proceedings: Jakarta, In- beneath the Banda Sea in eastern Indonesia from seismicity and fault- donesia, Association of Indonesian Geologists, p. 77-100. Banda collision. Nevertheless, if the variability plane solutions: Journal of Geophysical Research, v. 83, p. 2825-2838. Price, R. A., 1981, The Cordilleran foreland thrust and fold belt in the southern Chamaiaun, F. H., and Grady, A. E., 1978, The tectonic development of Cansxlian Rocky Mountains, in McClay, K., and Price, N. J., eds., of collision zones and of collision processes is Timor: A new model and its implications for petroleum exploration: Thrust and nappe tectonics: Geological Society of London Special Pub- recognized, the Banda collision serves as a well- Australian Petroleum Exploration Association Journal, v. 18, lication 9, p. 427-488. p. 102-108. Rowley, D. B., 1980, Timor-Australian collision: Analog for Taconic allocb- displayed example of one collision variant. Chamaiaun, F. H., Grady, A. E„ von der Borch, C. G, and Hartono, H.M.S., thon emplacement: Geological Society of America Abstracts with Pro- 1982, Banda Arc tectonics: The significance of the Sumba Island (In- grams, v. 12, p. 79. donesia): American Association of Petroleum Geologists Memoir 34, Shanmugair., G., and Lash, G. G., 1982, Analogous tectonic evolution of the p. 261-375. Ordovician foredeeps, southern and central Appalachians: Geology, ACKNOWLEDGMENTS Chi, W., Namson, J., and Suppe, J., 1981, Stratigraphie record of plate interac- v. 10, p. 562-566. tions in Coastal Range, eastern Taiwan: Geological Society of China Silver, E. A., Reed, D., McCaffrey, R., and Joyodiwiryo, Y., 1983, Back arc Memoir 4, p. 491-530. thrusting in the eastern Sunda Arc, Indonesia: A consequence of arc- The SeaMARC II data were acquired in Cisne, J. L., Karig, D. E., Rabe, B. D., and Hay, B. J., 1982, Topography and continent collision: Journal of Geophysical Research, v. 88, tectonics of the Taconic outer trench slope as revealed through gradient p. 7429-7448. cooperation with the staff at the Hawaii Institute analysis of fossil assemblages: Lethaia, v. 15, p. 229-246. Stagg, H.N J , and Exon, N. F., 1981, Geology of the Scott Plateau and Rowley Colman-Sadd, S. P., 1978, Fold development in Zagros simply folded belt, Terrace: Australian Bureau of Mineral Resources, Geology and Geo- of Geophysics, who also did the initial process- southwest Iran: American Association of Petroleum Geologists Bulletin, physics Bulletin 213. ing. We owe particular thanks to Karen Mans- v. 62, p. 984-1003. Sudradjat, T., 1985, Reconnaissance geologic map of Sumbawa, west Nusa Crostella, A., 1977, Geosynclines and plate tectonics in Banda Arcs, eastern Tenmpra, with description: Bandung, Indonesia, Geological Survey of field, who tolerated our persistent requests. We Indonesia: American Association of Petroleum Geologists Bulletin, Indonesia, scale 1:250,000. are also grateful 1:0 the Indonesian authorities, v. 61, p. 2063-2081. Tjokrosapoe ;ro, S., 1978, Holocene tectonics on Timor Island, Indonesia: Geo- Crostella, A., and Powell, D. E., 1976, Geology and hydrocarbon prospects of logical Survey of Indonesia Bulletin, v. 4, p. 49-63. who released the industrial geophysical data for the Timor area: Indonesian Petroleum Association Proceedings, v. 4, Veevers, J. J., 1982, Western and northwestern margin of Australia, in Nairn, p. 149-171. A.E.M., and Stehli, F,, eds., Ocean basins and margins: v. 6, our use and who facilitated this study in numer- Davis, D. M., Suppe, J., and Dahlen, F. A., 1983, Mechanics of fold and thrust p. 513-544. ous ways. Studies of aspects of these data during belts and accretionary wedges: Journal of Geophysical Research, v. 88, Veevers, J. J., Heirtzler, J. R., and others, 1974, Initial reports of the Deep Sea p. 1153-1172. Drilling Project, Volume 27: Washington, D.C., U.S. Government a marine tectonics course by Carol Lee, Fred Hamilton, W., 1979, Tectonics of the Indonesian region: U.S. Geological Sur- Print ng Office, 1060 p. vey Professional Paper 1078, 345 p. Veevers, J. J., Falvey, D. A., Hawkins, L V., and Ludwig, W. J., 1974, Seismic Zanoria, William Sanford, and John McBride Jacobson, R. S., Shor, G. G., Jr., Kieckhefer, R. M., and Purdy, G. M., 1979, reflection measurements of northwest Australian margin and adjacent added considerably to the paper. Critical re- Seismic refraction and reflection studies in the Timor Arc-Trough sys- deep:»: American Association of Petroleum Geologists Bulletin, v. 58, tem and Australian continental shelf, in Geological and geophysical p. 1731-1750. views by Greg Moore, Vern Kulm, and an un- investigations of continental margins: American Association of Petro- von der Borch, C. 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