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 deforma- tion 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 varia- ble 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. 18 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/98/1/18/3445133/i0016-7606-98-1-18.pdf by guest on 02 October 2021 DEFORMATION ACROSS BANDA ARC-AUSTRALIAN COLLISION ZONE 19 121° 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 se- quence, 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 indus- trial 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 col- lected 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 colli- sion 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. Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/98/1/18/3445133/i0016-7606-98-1-18.pdf by guest on 02 October 2021 20 KARIG AND OTHERS 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 proba- ble 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.