Tectonophysics 389 (2004) 65 – 79
www.elsevier.com/locate/tecto
Ocean trench blocked and obliterated by Banda forearc collision
with Australian proximal continental slope
M.G. Audley-Charles*
Department of Earth Sciences, University College London, Gower Street, London WC1E 6BT, UK
SE Asia Research Group, Department of Geology, Royal Holloway, University of London, Egham, Surrey TW20 0EX, UK
Received 19 November 2003; accepted 9 July 2004
Available online 27 August 2004
Abstract
The bathymetry and abrupt changes in earthquake seismicity around the eastern end of the Java Trench suggest it is now
blocked south–east of Sumba by the Australian, Jurassic-rifted, continental margin forming the largely submarine Roti–Savu
Ridge. Plate reconstructions have demonstrated that from at least 45 Ma the Java Trench continued far to the east of Sumba.
From about 12 Ma the eastern part of the Java Trench (called Banda Trench) continued as the active plate boundary, located
between what was to become Timor Island, then part of the Australian proximal continental slope, and the Banda Volcanic Arc.
This Banda Trench began to be obliterated by continental margin-arc collision between about 3.5 and 2 Ma.
The present position of the defunct Banda Trench can be located by use of plate reconstructions, earthquake seismology,
deep reflection seismology, DSDP 262 results and geological mapping as being buried under the para-autochthon below the
foothills of southern Timor. Locating the former trench guides the location of the apparently missing large southern part of the
Banda forearc that was carried over the Australian continental margin during the final stage of the period of subduction of that
continental margin that lasted from about 12 Ma to about 3.5 Ma.
Tectonic collision is defined and distinguished from subduction and rollback. Collision in the southern part of the Banda Arc
was initiated when the overriding forearc basement of the upper plate reached the proximal part of the Australian continental
slope of the lower plate, and subduction stopped. Collision is characterised by fold and thrust deformation associated with the
development of structurally high decollements. This collision deformed the basement and cover of the forearc accretionary
prism of the upper plate with part of the unsubducted Australian cover rock sequences from the lower plate. Together with parts
of the forearc basement they now form the exposed Banda orogen. The conversion of the northern flank of the Timor Trough
from being the distal part of the Banda forearc accretionary prism, carried over the Australian continental margin, into a
foreland basin was initiated by the cessation of subduction and simultaneous onset of collisional tectonics.
This reinterpretation of the locked eastern end of the Java Trench proposes that, from its termination south of Sumba to at
least as far east as Timor, and probably far beyond, the Java-Banda Trench and forearc overrode the subducting Australian
proximal continental slope, locally to within 60 km of the shelf break. Part of the proximal forearc’s accretionary prism together
with part of the proximal continental slope cover sequence were detached and thrust northwards over the Java-Banda Trench
and forearc by up to 80 km along the southwards dipping Savu Thrust and Wetar Suture. These reinterpretations explain the
* La Serre, St. Pantaléon, 46800 Montcuq, France. Tel.: +33 5 65 31 80 67; fax: +44 709 231 7196.
E-mail address: [email protected].
0040-1951/$ - see front matter D 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.tecto.2004.07.048
66
M.G. Audley-Charles / Tectonophysics 389 (2004) 65–79
present absence of any discernible subduction ocean trench in the southern Banda Arc and the narrowness of the forearc,
reduced to 30 km at Atauro, north of East Timor.
D 2004 Elsevier B.V. All rights reserved.
Keywords: Banda trench; Banda arc; Java trench; Sumba; Timor trough
1. Introduction
Geological and geophysical observations, analyses
and interpretations during the last dozen years have
advanced knowledge and understanding of the region
to a stage that encourages reconsideration of some of
the outstanding unresolved problems of the Banda Arc
(Engdahl et al., 1998; Fortuin et al., 1994, 1997;
Genrich et al., 1994; Hall, 2002; Hall and Wilson,
2000; Hall and Spakman, 2002; Hamilton, 1995;
Harris, 1991; Harris and Long, 2000; Harris et al.,
2000; Hughes et al., 1996; Kreemer et al., 2000;
McCaffrey, 1996; Richardson and Blundell, 1996;
Snyder et al., 1996).
One of the enigmatic geological features of SE
Asia (Fig. 1) is the sudden termination at 1208E of the
3000 km long, more than 6 km deep Sunda–Java
Trench (Fitch, 1970; Audley-Charles, 1975; Chamalaun et al., 1982; McCaffrey, 1988; Engdahl et al.,
1998; Kreemer et al., 2000; Hall, 2002). A linked
problem is the absence of any clearly recognisable
subduction ocean trench (Fig. 2) that could have been
responsible for the Banda Volcanic Arc and the related
Neogene orogen represented by the islands of the
Outer Banda Arc and their linking submarine ridge
whose geology is best known from the largest islands
of Timor and Seram. A related geological mystery is
the whereabouts within the Banda orogen of the
fundamental decollement that originally separated the
now folded and thrust para-autochthonous cover rocks
from their autochthonous Australian continental basement of the lower plate. This key objective was not
attained by seismic reflection surveys (Richardson
and Blundell, 1996; Snyder et al., 1996). Another
anomaly of the Banda Arc is the locally extreme
narrowness of the Banda forearc between Atauro and
Wetar, reduced to 30–50 km. These riddles are
considered as part of the discussion of the mode of
blocking and of burial of the Banda subduction
trench, and burial of a large part of the forearc
basement during the Neogene–Quaternary collision.
This paper presents no new data and does not
attempt an overall review of Banda Arc evolution. Its
principal objectives are focused on illuminating the
post-15 Ma evolution of the Java Trench eastward of
Sumba with the formation of the Banda subduction
ocean trench. This trench was intimately associated
with the initiation of the Banda Volcanic Arc. The
Banda Trench was then destroyed at the collision
stage of subduction-rollback, as part of the evolution
of the Banda orogen in the region from Sumba to
Timor. The main nine objectives of this paper may be
summarised as:
(a)
(b)
(c)
(d)
(e)
(f)
(g)
(h)
To locate and define the active continuation of the
Java Trench eastward of Sumba at 5 Ma (Fig. 3).
To explain the absence now of any oceanic
subduction trench in the Banda Arc (Figs. 1
and 4).
To show that the Timor Trough could never have
acted as a subduction ocean trench.
To explain the exceptionally small size of the
Banda forearc now having a width of only 30 to
50 km between Atauro and Wetar islands (Figs.
1, 3 and 4).
To define and distinguish tectonic collision from
subduction-rollback in passive continental margin-volcanic island arc collision zones, as
exemplified by the southern part of the Banda
Arc.
To explain the presence now of the distal part of
the Banda forearc accretionary prism about 150
km south of the Australian continental margin
exposed in northern Timor (Figs. 1 and 4).
To explain the contribution of overthrusting of
the distal part of the Banda forearc accretionary prism to the evolution of the Timor
Trough as subduction-rollback ceased and
collision commenced.
To indicate the possible role of gravity and of
pore-pressure loss in slowing and stopping the
subduction process as the upper plate rode up the
M.G. Audley-Charles / Tectonophysics 389 (2004) 65–79
67
Fig. 1. Physiographic and tectonic sketch map of the Timor–Sumba region to show: present-day northward dipping tectonic plate suture in the
Java Trench, and the much younger southward dipping suture now relocated north of the Banda volcanic arc; the southward dipping Savu
Thrust (Silver et al., 1983) and the Wetar Suture (Audley-Charles, 1981) now locked; selected transcurrent faults; and location of former, now
obliterated Banda subduction Trench. This assumes the Banda Arc to the north wall of Banda Trench separation dimension (Fig. 3) has not
changed by more than 10% since 5 Ma. Australian continental margin is now locked in collision with the Banda forearc from Wetar to the
Sumba Ridge. The east end of the Java Trench now blocked against the Australian margin has been overridden by the Banda forearc, which
itself has been overridden by the continental margin para-autochthon forming the Roti-Savu Ridge moving on the Savu Thrust. Bathymetry in
metres from Gebco database (IOC, IHO, BODC, 1997).
lower plate ramp to within 60 km of the
Australian shelf break (Fig. 4).
Many of these objectives can be advanced by
recognising the geological significance of the deep
reflectors in the two profiles across the Banda forearc
in the BIRPS data (Richardson and Blundell, 1996;
Snyder et al., 1996) which can be shown to
correspond with the postulated southward dipping
Wetar Suture, and the northward overthrusting of part
of the Australian parautochthon together with part of
the proximal Banda forearc (Figs. 1 and 4).
2. The Timor Trough
2.1. The Timor Trough initiated from tectonic
emplacement of the Banda forearc’s distal part of
the accretionary prism onto the Australian continental
margin
The Timor Trough (Figs. 1 and 4) is a depression
700 km long and between about 30 and 75 km wide.
Its depth varies between 2 and 3.2 km. It is located
entirely within what is now the Australian continental slope and is underlain by Australian continental
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M.G. Audley-Charles / Tectonophysics 389 (2004) 65–79
Fig. 2. Seismicity in the Sumba to Timor region of the Banda Arc showing hypocentres below 75 km based on the dataset of Engdahl et al.
(1998). Hypocentres shallower that 75 km are omitted for clarity because they are so widely distributed they obscure deeper seismicity. The
seismic data indicate the presence below the Banda Volcanic Arc of the northward-dipping Australian lithospheric slab.
lithosphere (Richardson and Blundell, 1996; Snyder
et al., 1996). At 5 Ma the Banda Trench, located
between the Banda Volcanic Arc and proto-Timor,
was continuing to subduct the northward-dipping
Australian continental margin (Fig. 3). That active
plate boundary (Hall, 2002) was then about 250 km
north of what was to become the present axis of the
Timor Trough about 4 million years later.
The Timor Trough was formed partly as a
consequence of loading (Jordan, 1981) of the
subducting Australian continental lithospheric plate
by the Banda forearc accretionary prism and part of
its basement (Figs. 1, 3 and 4), and partly from
inelastic failure at the beginning of continental
subduction (Tandon et al., 2000). Veevers et al.
(1978) with reference to DSDP 262 showed how the
Timor Trough axis migrated away from Timor
towards the Australian continent as the northern
edge of the former shelf subsided into the trough on
its southern flank (Figs. 3 and 4). An imbricate
structural pattern was mapped by seismic reflection
by Hughes et al. (1996) in the upper part of the
cover sequence in the northern flank of the Timor
Trough. They interpreted this as indicating that these
imbricated Cenozoic deposits represent part of the
Banda forearc accretionary prism; and by implication, but not stated, of the distal part of the prism.
The southerly limit of the prism has been long
recognised as the Late Cenozoic deformation front
(Barber, 1981). The presence of the apparently
undisturbed Mesozoic–Upper Palaeozoic cover
sequence below the Mid-Base Tertiary decollement,
that marks the base of the imbricate section mapped
by Hughes et al. (1996), indicates the sub-decollement section is very likely autochthonous. How
much of the structural deformation in this accretionary prism resulted from the pre-collision subduction and how much from post-subduction
collision stresses is difficult to resolve. Hughes et
al. (1996) remarked on the difficulties of determining
the finer structural resolution even after reprocessing
of the seismic data.
M.G. Audley-Charles / Tectonophysics 389 (2004) 65–79
69
Fig. 3. Palaeogeographical sketch map of the eastern part of the Java Trench and its continuation as the Banda Trench during active subduction
at 5 Ma. Positions of the named present-day islands follows Hall (2002) but their shapes are for reference only. Timor, Kisar, Moa, Sermata,
Babar, Roti, Savu and Sumba were entirely submarine at 5 Ma. Volcanic islands and their associated bathymetry sketched in present-day
configuration. Sumba region bathymetry partly after Fortuin et al. (1992, 1994, 1997). Schematic bathymetry in meters used to locate Banda
Trench constrained by assuming 5 km for the depth of the Banda Trench, and 50 km for its width at that depth; and by the positions of the Banda
volcanic arc islands (Hall, 2002); the Australian 5 Ma shelf edge position estimated from DSDP 262 (Veevers et al., 1978), and
micropalaeontology of Neogene part of para-autochthonous Timor Kolbano sequence (Audley-Charles, 1986b) in what was to become Timor
Island after collision at 3.5–2 Ma.
2.2. The Timor Trough as a perisutural foreland basin
A case for the Timor Trough being a foreland basin
has already been made (Audley-Charles, 1986a;
Tandon et al., 2000). The regional geological and
tectonic setting of Neogene-Quaternary perisutural
foreland basins in Eastern Indonesia and Papua New
Guinea was discussed by Audley-Charles (1991). It is
widely recognised (Dickinson, 1974) that thrusting is
usually developed within foreland basins. Bally and
Snelson (1980) demonstrated one can expect to find
thrusting in foreland basins particularly as one
approaches their related orogen. They distinguished
Ampferer-or A-zone subduction, which they defined
as exclusively continent-based, and which they
characterised as typical of perisutural foreland basins.
They contrasted such thrust zones as fundamentally
different in location, function and scale from Wadati–
Benioff subduction zones that are always associated
with an oceanic trench. The imbricate structural
pattern in the northern flank of the Timor Trough
(Hughes et al., 1996) resembles that found in forearc
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M.G. Audley-Charles / Tectonophysics 389 (2004) 65–79
Fig. 4. Schematic cross section to illustrate present-day relationships between the Banda forearc, the former Neogene Banda Trench and the
detached and overriding Australian para-autochthon. The geological section partly after Harris et al. (2000) and Hughes et al. (1996). Tectonic
structure partly after Richardson and Blundell (1996), Hall and Wilson (2000) and Hall (2002). Location of the top of the lower plate that
functioned during subduction as the basal decollement on or near the top of the Australian autochthonous crystalline basement; and the
overriding forearc that formed the upper plate during subduction, both now hidden by burial in the Timor orogen, indicated schematically. The 5
Ma shelf edge represents its southernmost position estimated from DSDP 262. Position of buried trench linked with southern limit of overriding
forearc. Basis for locating Banda Trench as in Figs. 1 and 3. Line of section A–AVsee Fig. 1.
accretionary prisms and in many foreland basins
worldwide. Moreover, the decollement mapped by
Hughes et al. (1996) at the base of the imbricate
section is a relatively superficial structure, which they
correlated with the Mid-Base Tertiary reflector. Below
this decollement in the Timor Trough is the well
stratified autochthonous Mesozoic–Upper Palaeozoic
section that, in a highly deformed state, forms the
Timor orogen together with allochthonous nappes of
the Banda Terrane derived from the forearc basement
(Harris, 1991; Harris et al., 2000). The dominating
presence of these deformed para-autochthonous strata
and forearc basement nappes in the Timor orogen
results from their having been detached from very
much deeper decollements close to their fundamental
basements by subduction followed by collision. It
appears that the Timor Trough began to evolve from
the distal part of an overthrust forearc accretionary
prism, that had overridden the Australian proximal
continental slope, into a foreland basin when collisional deformation began as subduction-rollback
ceased. The indications from reflection seismicity of
recent (Quaternary) deformation in the trough axis is
indicative of a continuation of deformation and
episodic propagation of the thrust front in the Timor
Trough (Masson et al., 1991) corresponding to the
evidence of deformation among the uplifted Quaternary coral-algal reefs onshore Timor (Merritts et al.,
1998).
3. The Wetar Strait and Wetar Suture
Timor Island is characterised by fold and thrust
mountains raised to almost 3 km above sea level,
with a total uplift of about 4 km of deep marine
deposits, which may have been an episodic process
initiated between about 3.5 and 2 Ma (Kenyon,
1974; Audley-Charles, 1986b; De Smet et al., 1990).
The island is bounded to both the north and south by
marine depressions (Fig. 1). To the south of the
island is the 3 km deep Timor Trough discussed
above. North of Timor is a very different deep
marine trough, the Wetar Strait, that separates East
M.G. Audley-Charles / Tectonophysics 389 (2004) 65–79
Timor from the islands of Wetar and Atauro. The
Strait drops precipitously to more than 3 km depth
from both the north coast of Timor and from its
northern margin along the Banda Volcanic Arc. It
varies in width between 30 km (Atauro) and 50 km
(Wetar). In both these islands of the Banda Arc most
of the volcanic activity ceased in the Pliocene
(Abbot and Chamalaun, 1981). The Wetar Strait
opens westward into a 175-km wide forearc basin
that also descends steeply from the north coast of
Timor and from the submarine margin of the Roti–
Savu Ridge and Sumba Ridge to a depth of 3 km. A
key factor in the search for the now defunct Banda
Trench is that the active Banda Trench must have
always been located south of the Banda Volcanic Arc
because it formed as the Australian continental
margin was subducted northwards into the Banda
Trench. Ocean trenches less than 7 km deep have a
commonly observed width at 5 km depth of about 50
km. The fact that the Wetar Strait now separates the
former Banda Volcanic Arc from the northern edge
of the exposed Australian continental margin, whose
deformed cover rocks crop out on the north coast of
Timor, by only 30–50 km indicates (Audley-Charles,
1981, 1986a; Harris, 1991) that the cover rocks of
the Australian continental margin, exposed in northern Timor, have overridden the southern part of the
Banda forearc (Figs. 1 and 4).
The elevation difference between the bottom of the
Wetar Strait and the north Timor coastal zone,
approaches 4 km in places; and the exceptional
steepness of the gradient between them suggests
faulting on, what in this paper is interpreted as, the
Pliocene Wetar Suture (Fig. 4). Masson et al. (1991)
did not find normal marginal faults with their
reflection seismic survey. But Richardson and Blundell (1996) using BIRPS seismic reflection mapped
multiple south-dipping thrusts in the Banda forearc
immediately east of Wetar and Kisar that correspond
closely with the postulated position (Audley-Charles,
1986a) of the Wetar Suture. Moreover, Masson et al.
(1991) and Snyder et al. (1996) found a south-dipping
thrust near the north coast of Kisar, but they also did
not notice any connection with the postulated fundamental Wetar Suture; while Breen et al. (1989), Harris
(1991) and later Prasetyadi and Harris (1996) also
found south-dipping thrusts close to the north coast of
East Timor.
71
4. Identification of the former Banda Trench
4.1. Implications of earthquake seismicity
The southern part of the Banda Arc is still
characterised by earthquake epicentres (McCaffrey,
1989; Engdahl et al., 1998) indicating the northwarddipping, Australian lithospheric slab (Figs. 2 and 4)
that descends steeply from below northern Timor,
continuing at depth below the Wetar Strait and the
Banda Arc inactive volcanic islands to below 300 km.
But across Timor and south of it earthquake records
offer no support for subduction now being active south
of the Banda Volcanic Arc (McCaffrey, 1996; Kreemer
et al., 2000). McCaffrey (1996) also emphasised that
earthquake seismology indicates that the movements
between the lower and upper plates of the subduction
system in the Timor region, that had been responsible
for the strong Neogene deformation including much
thrust stacking mapped in the Timor orogenic mountains (Barber, 1981; Harris, 1991; Harris et al., 2000,
Fig 4), have locked. Moreover, McCaffrey (1996),
using preliminary GPS results of Genrich et al. (1994),
added that the geophysical evidence indicated that
dTimor appears to be moving northward relative to the
Sunda Shelf at about the same rate as AustraliaT. He
added that the dnorthward motion of the extinct
volcanic islands north of Timor is revealed with GPS
to be similar to that of southern Timor (Genrich et al.,
1994); hence, little of the northward motion of
Australia is now accommodated by the internal
deformation of the Banda arc structure.T He also
observed that dThe island structure, bounded by the
Timor Trough and the Wetar backarc thrustT (also
known as the Wetar Thrust) d is rigidly thrusting over
the backarc basinT (also known as the South Banda
Sea). Kreemer et al. (2000) on the basis of GPS and
earthquake data concluded that active, southerly
directed thrusting on the Wetar and Flores backarc
thrusts suggest a jump in the locus of convergence
from the Java Trench to the backarc east of 1188E. The
entire region, comprising East Timor, Wetar and part of
the intervening Wetar Strait (Fig. 2), is without active
earthquakes at depths greater than 75 km (Engdahl et
al., 1998). This extraordinary aseismic feature may
accord with the suggestion that part of the Australian
lower plate in this region has ruptured (Price and
Audley-Charles, 1983; Tandon et al., 2000).
72
M.G. Audley-Charles / Tectonophysics 389 (2004) 65–79
4.2. Wetar Strait as part of the foundered forearc
Geological mapping in northern Timor has recognised that the deformation suture there includes the
roots of the thrust sheets belonging to the Banda
allochthon derived from the Banda forearc, and which
appear to continue offshore (Harris, 1991; Richardson
and Blundell, 1996; Harris et al., 2000). Moreover,
Masson et al. (1991) dredged samples from the Wetar
Strait of forearc volcanics analysed by Harris (1992).
These data indicate that the present Wetar Strait is a
subsided part of the Banda forearc, implying that the
former Banda Trench must now be south of the Strait
(Figs. 1 and 4).
5. Significance of the allochthonous Banda terrane
The metamorphic Lolotoi and Mutis Complexes
are rightly regarded by Harris (1991) as derived from
the basement of the Banda forearc on the evidence of
the rocks that rest on these metamorphic complexes.
The basement is unconformably overlain in different
places by volcanic and sedimentary rocks of the
Upper Cretaceous to Paleocene Palelo Group, neritic
Eocene limestones, and neritic Lower Miocene Cablac
Limestone. These are entirely allochthonous elements
of the Banda Terrane (Audley-Charles and Harris,
1990; Harris, 1992; Harris and Long, 2000) occupying a very high structural position. They are vastly
different from any age-equivalent deposits found in
the Australian passive margin sequences. They appear
to have been derived from the basement of the Banda
forearc, and transferred from the base of the upper
plate into the developing orogen (Fig. 4). This may
have occurred during the collision phase when the
northward thrusting Wetar Suture was active. The now
high level, relatively flat-lying thrusts of the Banda
Terrane together with the structurally underlying
folded and thrust para-autochthonous Permian to
Neogene strata are exposed widely throughout Timor.
Charlton (2002) suggested the metamorphic Lolotoi and Mutis Complexes were derived from the
Australian plate, but he arrived at this view by
ignoring their stratigraphic and structural relations
with the overlying volcanic and sedimentary Palelo
Group as well as with the Eocene Limestones, and his
interpretation depends on misreporting of work by
Carter et al. (1976) who had described the unconformable relationship of the Cablac Limestone to the
underlying Lolotoi Complex, an observation widely
reported from throughout the island of Timor (Tappenbeck, 1940; De Waard, 1957; Audley-Charles and
Carter, 1972).
6. Java Trench–Banda Trench evolution
6.1. Termination of Java Trench at 1208E: bathymetric, earthquake and lower plate characteristics
The abrupt termination of the 5–6 km deep and 50
km wide Java Trench at 1208E (Fig. 1) is reinforced
by the observations and conclusions by geophysicists
(Fitch, 1970; Chamalaun et al., 1982; McCaffrey,
1988; Engdahl et al., 1998; Kreemer et al., 2000) of
the dramatic change in Java Trench earthquake
seismicity east of 1188E, especially the abrupt
decrease eastwards of 0–75 km depth earthquakes in
the context of a lack of indications of subduction.
Moreover, east of what is now 1208E the Australian
continental lithosphere had begun before 5 Ma to
replace the Indian Ocean lithosphere as the lower
plate entered the subduction trench (Figs. 3 and 4).
Today, between 1208E and 1218E the Australian
continental slope shows no evidence of the passage
of the Java oceanic trench, and the oblique strike
difference between the existing, well observed Java
Trench at 1208E, and the western edge of the
Australian continental slope is notable, making an
angle of 708. Harris et al. (1998) attributed the small
re-entrant in the 4000 m bathymetric contour to it
representing the southern edge of the westernmost
part of the Timor Trough, it being the distal part of the
Banda forearc accretionary prism. It is notable that,
while the location of the Java Trench with respect to
the Sunda volcanic arc west of 1208E has been
relatively fixed since the early Cenozoic (Hall, 2002),
the nearby Australian continental slope, now abruptly
cutting across the strike of the Java Trench, has only
recently arrived at its present position. At about 10
Ma, the Australian continental margin, now immediately east of the Java Trench, was located about 600
km further south (Hall, 2002). Thus, details of the
bathymetry, such as the re-entrant revealed in the
4000 m contour (Fig. 1) on the present Australian
M.G. Audley-Charles / Tectonophysics 389 (2004) 65–79
slope next to the end of the Java Trench, must be a
relatively recent feature. It would have developed
after the slope occupied its present position with
respect to the Java Trench. Furthermore, the Australian continental lithosphere, cutting obliquely across
the end of the Java Trench, continues north of the
4000 m re-entrant to its tectonic junction with the
volcanic Sumba Ridge, a distance that varies from 100
to 150 km, marked by the contact indicated by the
Savu Thrust (Fig. 1). This accords with the suggestion
that this submarine morphology developed after the
Australian slope arrived at the Java Trench.
6.2. Plate reconstruction palaeogeographical
implications
Plate reconstructions (Hall, 2002) show the Java
Trench continuing east of Sumba into the west Pacific
from at least 45 Ma, supported by interpretation of
seismic tomography (Hall and Spakman, 2002). This
older part of the Java Trench was eliminated by
collision in eastern Indonesia during the early
Miocene. Between 15 and 10 Ma the Java Trench
developed eastward in a new position where it
functioned as the Banda Trench (not named as such
by Hall, 2002) in association with the beginning of the
Banda Volcanic Arc. This subduction trench was
located, from at least 10 Ma to at least 5 Ma, between
the Banda volcanic arc to the north and what was to
become the island of Timor to the south of this trench.
Hall (2002) reconstruction is used as the basis for the
palaeogeographical sketch map (Fig. 3). Timor is
shown as always on the northern edge of the
Australian continental slope to the south of the Banda
Trench. Because of later northeastward movement of
the Australian continental margin, at 12 Ma the
Timor–Savu region lay some hundreds of kilometres
to the south of the position it occupied at 5 Ma. The
Australian continental margin continued its convergence on the Banda Arc during the Neogene and
Quaternary.
6.3. Reconstruction of the Banda Trench at 5 Ma, and
dating cessation of subduction
The width of the Java Trench, where it is least
affected by incoming sediment from the ocean floor of
the India–Australia plate, is about 50 km. This
73
corresponds closely with the commonly observed
width at 5 km depth of ocean trenches that are less
than 7 km deep. Hence, 50 km is the postulated width
of the Banda Trench at about 5 Ma (Fig. 3). The Java
Trench has existed since at least the Eocene (Hall,
2002) whereas the Banda Trench cannot be much
older than the earliest volcanics in the Banda Arc
dated as 12 Ma (Abbot and Chamalaun, 1981; Barberi
et al., 1987). On the basis of biostratigraphy the
youngest deformed rocks in the Timor para-autochthon are N18 pelagic lutites in the Kolbano sequence,
and the oldest autochthonous rocks resting unconformably on the deformed para-autochthon belong to
the lowest part of the Batu Putih Limestone (Kenyon,
1974) determined as N18–19 which is between 5.0
and 3.5 Ma in age (Early to Mid-Pliocene). Timor
studies reveal a Late Pliocene–Quaternary regional
uplift that amounts to about 4 km; such a long-lived
and substantial regional uplift requires a fundamental
tectonic cause. Detailed studies have recognised both
local fault-related uplift and subsidence (Kenyon,
1974; De Smet et al., 1990). Uplift began with
Kenyon (1974) Fatu Laob uplift phase whose revised
date (Audley-Charles, 1986b) is Late Pliocene (N21)
ranging from 3.5 to 2.0 Ma. Beginning from between
~3.5 and 2.0 Ma, the regional uplift has continued,
with some local subsidence. The unconformable
relationships described above suggest that subduction
may have ceased in this part of the Banda Trench at
3.5 Ma. This implies that the active life of the trench
here was from ~12 to ~3.5 Ma, but if the onset of
uplift in this region was as late as 2.0 Ma and if this
uplift, which has continued throughout the Quaternary, is an isostatic response to cessation of subduction then Banda Trench subduction may have
stopped and collision began as late as 2 Ma.
6.4. Locating the present position of the defunct
Banda trench
The defunct Banda Trench has been located in this
paper (Figs. 1 and 4) by considering the consequences
of convergence between the northward-subducting
Australian plate and the Banda Volcanic Arc from 5
Ma (Fig. 3) to the date of onset of arc–continent
collision. Geological mapping indicates collision
began between about 3.5 and 2 Ma. As explained
above, collision is correlated here with the locking of
74
M.G. Audley-Charles / Tectonophysics 389 (2004) 65–79
the subduction trench. The important assumption is
that from 5 Ma to the time the trench was locked the
Banda forearc basement did not grow southwards, and
the distance from the volcanic arc to the north wall of
the Banda trench did not change significantly. Note
that in Fig. 3 the distance from the arc (at Wetar) to the
north wall of the Banda Trench was 100 km at 5 Ma. In
Figs. 1 and 4, representing present-day reconstructions, this distance has shortened by about 10%. The
consequences of the convergence of the Australian
margin with the Banda Volcanic Arc are revealed by
plotting the position of proto-Timor and the Australian
continental margin at 5 Ma in Fig. 3 in their relative
present-day positions as shown in Fig. 1. This indicates
that the present position of the Banda Trench (now
defunct) and its closely linked upper plate, comprising
the southern part of the Banda forearc basement, must
both be below Timor. The only important assumption
has been that the distance from the Banda Arc to the
north wall of the Banda Trench has not increased since
5 Ma (Figs. 1 and 4 imply a 10% shortening which
might well be expected to result from the tectonic
compression). The case for the location of the Banda
Trench at 5 Ma has already been set out above. It is
clear that there is little room for manoeuvring the
geographical positions of any feature once the width of
the Banda Trench at 5 km depth is taken to have the
value of 50 km, and that, following Hall (2002), the
position of the Java Trench in the region of Sumba has
not altered significantly during the last 5 Ma.
7. Distinguishing subduction from tectonic collision
When a passive continental margin converging on
a volcanic island arc system, whether by subduction
or by rollback, reaches the stage that all of the oceanic
lithosphere in any sector has passed below the inner
forearc wall of the trench so that the continental
lithosphere of the converging continental slope begins
to pass below this inner forearc trench wall the
condition of continental margin-arc collision may be
said to have commenced. Given that the shape of the
strike of the converging continental margins may be
irregular there maybe varying degrees of diachronism
in the tectonic collision.
Collision in the southern part of the Banda Arc was
initiated when the overriding forearc basement of the
upper plate reached the proximal part of the Australian slope whose basement was comprised of continental lithosphere, subduction probably began to
slow and stopped when the distal edge of the shelf
was at a distance of about 60 km to the south (Figs. 3
and 4). Where subduction or rollback of all the Indian
Ocean lithosphere below the Banda Arc was completed in the Sumba-Timor region the oceanic Banda
Trench began to form part of the orogenic northern
margin of the foreland basin evolving to become the
present Timor Trough.
The now missing, defunct Banda Trench can only
have been located, when it was actively subducting,
where the seismicity (Engdahl et al., 1998) implies its
former presence between the northwards subducting
Australian plate and the SE Asian Banda Volcanic
Arc. The now invisible, obliterated Banda Trench, that
probably ceased to subduct in the Timor region
between 3.5 and 2 Ma, is now located below the
islands of Timor and Roti (Figs. 1 and 4). Here it is
obscured by the thick pile of folded and thrust paraautochthonous cover rocks from the Australian continental rise and slope now thrust northwards by the
Wetar Suture over the overriding forearc.
These now para-autochthonous rocks first began to
form the Banda forearc accretionary prism from the
time the distal Australian continental rise sediments
began to arrive in the Banda Trench, when the Banda
Volcanic Arc began to appear at about 12 Ma. This
Banda forearc accretionary prism grew until the
subduction process was halted probably after slowing
as the Australian continental slope, with its crystalline
basement and great thickness of overlying Lower
Permian to Mid-Jurassic Gondwana sequences, and
post-rift Upper Jurassic to Pliocene Australian rifted
continental margin sequences, entered the subduction
trench between about 5 and 2 Ma.
The locking of the subduction process is associated
with the gradual onset of tectonic continental margin–
volcanic arc collision. It is interpreted here as having
begun between 3.5 and 2 Ma. Thickness of the lower
plate probably increased as the slope entered the
trench. A steepening of the subduction angle may
have impeded the downward motion of the lower
plate into the trench. Subduction may then have been
halted by the increasing gravitational resistance to the
upward movement of the forearc over the Australian
margin on the incline rising from the floor of the
M.G. Audley-Charles / Tectonophysics 389 (2004) 65–79
Banda Trench southwards up to arrive about 60 km
from the shelf break (Fig. 4), where the sea floor could
have been at less than 1000 m depth. Another braking
effect might have been the loss of pore-fluid pressure,
from below the overriding forearc basement moving
on a decollement of Jurassic shale and escaping into
the more permeable formations of the Australian
cover sequence. Continuing or reactivated crustal
shortening after the cessation of subduction constitutes tectonic collision. The collision in the southern
part of the Banda Arc resulted from compression of
the forearc accretionary wedge which consequently
became the Australian para-autochthon on both upper
and lower plates, with part of its forearc basement
becoming nappes of the allochthonous Banda Terrane.
This upper plate tectonic flake, in response to
continuing compressive stresses, produced northward-verging structures concentrated along the north
coast of Timor as mapped by Harris (1991) and
Prasetyadi and Harris (1996). This northward thrusting over the forearc helped close the forearc to only 30
km in the region of Atauro. Harris (1991) and
Prasetyadi and Harris (1996) also identified various
thrust slices of the Banda Terrane incorporated into
this northward-verging thrust wedge (Fig. 4) carried
on the Wetar Suture (Audley-Charles, 1986a).
Another characteristic of subduction and roll-back
related processes that distinguishes them from collision related processes is the structural level of the
principal decollement. In subduction and roll-back the
main decollement is the upper and lower plate
boundary, whereas in collision the principal decollements develop at a higher level even if they occur as
structurally low as to be within the cover rock
sequence on the autochthonous crystalline basement
of the lower plate, as in the Timor Trough (Hughes et
al., 1996). Both subduction and roll-back (Hamilton,
1988; 1995) appear to transfer much of the cover rock
sequence from the descending lower plate to the
leading edge of the upper plate as the lower plate
passes below the forearc basement that forms the
inner (arc) wall of the trench.
7.1. Main and later phases of collision
The recognition by Hughes et al. (1996) of
accretionary prism-type deformation in the northern
flank of the Timor Trough indicates they are much
75
simpler structures than those of the main collision
phase of structural deformation so evident in the
exposures on Timor (Fig. 4). Following the transfer of
the Australian cover rocks by subduction from the
lower plate into the forearc to form the accretionary
wedge of the upper plate, the main collision deformation involved at least three recognisable stages (i)
severe shortening of the forearc accretionary wedge to
form part of the Australian para-autochthon; (ii)
detachment of part of the underlying forearc basement
eventually to form the nappes of the allochthonous
Banda Terrane intimately associated with (iii) transport northwards by the Wetar Suture and associated
uplift of the para-autochthon and slices of the forearc
basement to become the Timor orogen. In contrast, the
Timor Trough accretionary prism structures appear to
be mainly the result of some probably reduced
convergent forces (McCaffrey, 1996) towards the
end of subduction and especially after that ceased in
the Timor region of the Banda Trench between 3.5
and 2.0 Ma. A major structural feature developed
towards the end of subduction and during collision
was the Late Pliocene Wetar Suture, now locked. The
crustal shortening resulted in the deformation that
characterises that part of the para-autochthon detached
from part of the unsubducted Australian lower plate
causing the detached para-autochthon to overthrust
the forearc tectonic flake (Fig. 4) on the Wetar Suture.
The compressional strain continued into the Pleistocene when the subduction polarity reversed and the
Wetar Thrust began to develop a southward-dipping
subducting plate margin (McCaffrey, 1996; Hall and
Wilson, 2000). The gentle folding of the Viqueque
Group basins (Kenyon, 1974) that affected some of
the reef terraces (Merritts et al., 1998) also indicates
that some crustal shortening continued into the late
Quaternary.
8. Sumba Ridge–Banda Trench relations
Fig. 1 shows the ENE-striking submarine Roti–
Savu Ridge has a southward-dipping thrust junction,
the Savu Thrust (Silver et al., 1983), with the southern
part of the south–east striking submarine Sumba
Ridge. Fortuin et al. (1994), Rutherford et al. (2001)
and Lytwyn et al. (2001) interpret the Sumba Ridge as
part of the Banda forearc. Comparison was made by
76
M.G. Audley-Charles / Tectonophysics 389 (2004) 65–79
Audley-Charles (1985) of the allochthonous Timor
Banda Terrane, thought to have been derived from the
Banda forearc, with some sections in West Sumba
described by Van Bemmelen (1949), Chamalaun et al.
(1982) and Von der Borch et al. (1983). For example,
both Timor (Tappenbeck, 1940) and West Sumba
sections have Lower Paleogene ignimbrites overlain
by neritic limestones with Middle and Upper Eocene
large foraminifera, and both have Lower Miocene
neritic limestones. Both have an angular unconformity
at the base of the Paleogene and an unconformity at
the base of the Miocene.
Rutherford et al. (2001) have interpreted Sumba as
having been part of the Sunda volcanic arc from Late
Cretaceous to Early Miocene. If the tentative correlation of the Palelo Group and associated sequences
unconformable on the allochthonous metamorphic
complexes in Timor with the Cretaceous–Lower
Miocene volcanics in Sumba is shown to be valid
then this would strengthen the correlation of the
Lolotoi and Mutis metamorphic complexes and their
unconformable Palelo cover (Banda Terrane) with the
Banda forearc basement and with the igneous basement of Sumba.
The Pliocene–Quaternary uplift in East Sumba
(Fortuin et al., 1994, 1997) suggests comparison with
the Late Pliocene–Quaternary uplift of 4 km in Timor.
The Timor uplift has been tentatively attributed (Price
and Audley-Charles, 1987) to isostatic rebound of the
blocked subduction zone of the defunct Banda Trench
no longer able to subduct the Australian continental
lower plate.
The suggestion by Rutherford et al. (2001) for the
initial collision (undefined and not distinguished from
subduction) between the Australian continental margin and Banda Arc having been as early as Early
Miocene, 16–18 Ma, conflicts with the results from
numerous field studies, including apatite FT ages for
dating of exhumation in Timor (Harris et al., 2000),
geological mapping across Timor and other islands in
the southern Banda Arc (Audley-Charles, 1986a,
1986b; Harris, 1991), offshore geophysics around
the Banda Arc (Bowin et al., 1980) and the review by
Hall (2002). Keep et al. (2003) suggest that the
collision (undefined and not distinguished from
subduction) of what is here described as the submarine Roti–Savu Ridge with the submarine Sumba
Ridge, that they agree represents a part of the Banda
volcanic forearc, occurred at about 8 Ma. These early
dates for Banda Arc collision in the Sumba–Timor
region cannot be reliably based on the radiometric
ages for metamorphism in the Permian Aileu Metamorphic Complex of Timor. Harris et al. (2000) on the
basis of vitrinite reflectance, conodont alteration and
illite crystallinity and structural mapping questioned
the interpretations of pre-Late Miocene tectonic burial
and pre-Pliocene exhumation of the metamorphism of
the Aileu Complex Australian continental margin in
Timor. They pointed out that dthe only possible
evidence of such deep burial is a thin Barrovian
metamorphic sequence along the north coast in the
Aileu ComplexT (Berry and Grady, 1981). They went
on to emphasise that dIt is also possible that the Banda
Volcanic Arc has thermally disturbed the Aileu
Complex, since it is as close as 30–40 km in placesT.
Furthermore, Hall (2002) and Hall and Wilson (2000)
have argued that such metamorphic dates in the
Permian Aileu Complex (that is part of the Gondwana
cratonic basin sequence and belongs structurally with
the Australian continental basement to the post-Late
Jurassic rift sequence) are more easily attributable to
an extension episode beginning about 12 Ma and
associated with Banda Arc volcanism. There is no
independent evidence for this early collision date, and
there is much in the Timor and Sumba islands to
contradict it, as indicated above.
The Banda forearc is made exceptionally wide by
the Sumba ridge which projects about 250 km south
of the existing narrow Banda forearc (Figs. 1 and 3),
so that a collision about 5 Ma earlier than the collision
in the Timor region between the Australian continental margin forming the Roti–Savu Ridge and the
Sumba Ridge of Banda forearc seems superficially
possible. However, their suggestion (Keep et al.,
2003, Fig. 10a) that the Roti–Savu Ridge at 8 Ma
formed a narrow peninsula that protruded orthogonally from the regional strike of the Australian
margin, the ridge having a rectilinear outline and
northwest strike, is difficult to reconcile with the
geological strike of the Australian margin in the
southern Banda Arc. Keep et al. (2003) appear to be
making a speculative exaggeration of the shape and
size of their projecting ridge (that they described as ba
fingerQ to indicate its length/width ratio) of the steprifted margin in the Sumba region. Their attribution of
the stepped morphology of the present rifted con-
M.G. Audley-Charles / Tectonophysics 389 (2004) 65–79
tinental margin to a pre-subduction age of pre-12 Ma
ignores the effects of subduction and collision-related
NNE transcurrent faulting that has affected the shape
of the present northern margin of the Australian
continent in the Banda orogen (Fig. 1). But even more
importantly it is impossible to reconcile the age of an
interpreted Late Miocene collision with the record of
uninterrupted pelagic deposition, perturbed only by
slumping in a deep water environment, at the
supposed site of the collision on Sumba itself (Fortuin
et al., 1992, 1994). The youngest pelagic sediments on
Sumba are late Early Pliocene (N19/20) in age (Roep
and Fortuin, 1996; Fortuin et al., 1997) suggest
emergence of Sumba did not occur before 3 Ma, an
age very close to that inferred from Timor for the
beginning of uplift. Thus, the evidence from
extremely well-exposed sections on Sumba support
the timing for the end of subduction and onset of
collision suggested in this paper.
Acknowledgements
In preparing this article, I have been most kindly
assisted by busy friends: Tony Barber, Graham Evans
and Paul Henderson provided timely practical help.
Robert Hall helped generously with art work for the
four figures, editing and in other practical ways. He
and Ron Harris both read and suggested improvements to earlier versions of this paper, and I benefited
too from their e-mailed tutorials for a superannuated
but formerly active toiler in the Banda Arc. I am also
indebted to the helpful suggestions from Myra Keep
and an anonymous referee for improvements to this
paper.
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