Seismic Expression of Tectonic Features in the Lesser Sunda Islands,
The Sunda Arc is a chain of islands in the southern part of
cored by active volcanoes (Fig. 1). The western part of the Sunda arc is dominated
by the large islands of Sumatra and Java, and is commonly called ‘the Greater Sunda Islands’. The tectonics in this part is
dominated by the oceanic subduction below the Asian continental plate. Towards
the east the islands are much smaller and are called ‘the Lesser
Sunda Islands’. The transition from oceanic subduction to
continent-island arc collision developed in this area.
The Sunda Arc has long been considered as a classical accretionary margin system where the Indo-Australian oceanic plate is underthrust beneath the Asian continent, active since the Late Oligocene (
1979). At the eastern end of the Sunda Arc the convergent system changes from
oceanic subduction to continent-island arc collision of the Scott Plateau,
part of the Australian continent, colliding with the Banda island arc and
in between. Sumba Island
Figure 1. Map of Southeast Asia showing the different crustal type in the region and the location map of the
Lesser Sunda Islands are also called the inner-arc
islands. The formation of these islands are related to the subduction along the
Java Trench Bali Island is located in the west of the Lesser Sunda Islands and
Alor Island at the east end (Fig. 1). To the south of the inner-arc islands, an
accretionary wedge formed the outer-arc ridge. The ridge is subaerially exposed
in the east as Savu and . The northwest of
Island Lesser Sunda Islands are underlain by a
Late Cretaceous Accretionary Crust, which changes to an oceanic crust in the
northeast (Doust & Lijmbach, 1997). The Sumba Island
has a unique orientation and the origin of the island is still debated ( Rutherford et al., 2001).
The aim of this article is to provide a broad overview about the structures of the tectonic units based on some selected seismic lines. These lines also give a better geological understanding, including recent processes that developed in the area.
A number of surveys have been deployed to acquire seismic data in this area. Selected seismic data used for this article were acquired in the following expeditions:
- R. V. Vema cruise 28 and R. V. Robert Conrad cruise 11 (in
, 1979) Hamilton
- Rama 12 expedition (Prasetyo, 1992; Scripps Institute of Oceanography, http://www.ig.utexas.edu/sdc/)
- R. V. Baruna Jaya late 90’s (Krabbenhoeft, A., 2010) for bathymetric data acquisition.
- R. V. Sonne, cruise SO190 (Lüschen et al, 2011)
- CGG Veritas Spec. Survey (Rigg & Hall, 2012)
- ION-GXT JavaSPAN 2008 (Granath et al, 2011)
Apart from the surveys mentioned above, there are other surveys which contributed significant pieces of information to help geoscientists in understanding the geology of the region. During the Snellius-2 Expedition, for example, Van Weering et al., (1989) have also acquired a number of seismic sections in the area, but later surveys have gathered improved seismic images. Prasetyo (1992) reported seismic reflection and gravity data from this area as well.
The earlier surveys, such as R. V. Vema and R. V. Robert Conrad in
Hamilton (1979) provided limited data mainly
confined to information on bathymetry and shallow depth of image. The later
images, acquired by CGG Veritas are considered as a modern industry standard for
seismic, providing seismic images down to 8 seconds Two-Way-Time. Recent long
cable with improved technology by ION helped to acquire seismic more than 10 km
deep. These ION deeper sections help geoscientists to acquire a better
understanding about the basement.
Lesser Sunda Islands area consists of several tectonic
units (Fig. 2). Several regional seismic sections were shot across these features.
Some lines give a better geological understanding about the composition and the
Figure 2. Structural map of the
1. Outer-arc Ridge
The outer-arc ridge or also called the fore-arc ridge is an accretionary wedge formed by the subduction of the Indian plate. In the west of the Lesser Sunda Island region, the Outer Arc Ridge formed about 3000 m below sea level, parallel to the Inner Arc. To the east, the outer-arc ridge exposed sub aerially as the outer-arc islands of Roti and
(Fig. 2).These islands are mainly composed of raised shallow and deep marine
sediments. Mud diapers and mud volcanoes are common in the outer-arc islands
(Hamilton, 1979; Zaim, 2012). The outer arc is bounded by the Java Trench which
marked the subduction point in the south. The northern margin of the Outer Arc
Ridge is partly covered by the fore-arc basin sediment fill.
Figure 3 shows regional seismic sections acquired by the Sonne cruise in the region. Section - A, B, C and D in this figure (from Lüschen et al, 2011) show similar patterns of the outer-arc ridge. The subduction zone after the trench and the accretionary complex are well imaged. Lüschen (2011), also provide detail seismic images of Section B in Figure 4, showing the structures of the outer-arc ridge. The outer-arc ridge is a structurally complex unit with a series of thrust faults (Fig. 4A and further detail in Fig 4B). Some of these faults generated topographical relief on top of the outer-arc ridge and formed ‘piggy-back basins’, which are filled with recent sediments from surrounding structural highs. On seismic these sediments appear as brighter and relatively flat reflectors all the way up to the surface (Fig. 4C).
Section E and F in the east of the area show different patterns compared to the western 4 sections. The outer-arc ridge in Figure 3E has a gentle relief and the thrust faults are not as clear as the sections in the west. Figure 3F also shows a gentle relief but much wider (reformulate). The difference between the four seismic profiles in the west and the two in the east, reflect the transition from oceanic subduction to continent in the west to continent-island arc collision in the east (Kopp, 2011).
Depressions in the seabed between the inner volcanic arc and the outer-arc are known as fore-arc basins. The fore-arc basin in the west is called the
and the water depth of this basin is about 4000 to 5000 m deep (Fig. 2). Lombok Basin Savu Basin
is the fore-arc basin located in the east of the Lesser Sunda Islands,
separated from the Lombok basin by . In parts the
water depth of Sumba
Island is deeper than 2000
A series of north dipping thrust faults are clearly seen in the close up of these seismic sections as shown in Figure 4. A closer detail display of fault system in the trench is shown in Figure 4. A small sediment accommodation space developed in the trench. Recent sediments have filled this small depocentre, indicated by flat sea bottom on seismic sections.
Lüschen et al, 2011, also indicated a
developed in the centre of the
Outer Arc Ridge by the thrust fault system. These basins are generally small
and filled with recent sediments. Similar to the trench deposit, these basins
are characterized by semi parallel reflectors with flat surfaces (Fig.4). Piggy-Back Basin
The Savu fore-arc basin developed in the east of the
islands, where there is now a change from oceanic subduction to
arc-continent collision (Rigg and Hall, 2012). The Savu
Basin is bounded to the west by the and by a submarine ridge (the
Sumba Ridge) that crosses the fore-arc obliquely in an NW-SE direction. The
basin is narrowing to the east. To volcanic island arc bounded the north part
of the basin (Fig. 1). island of Sumba
Figure5A and 5B shows 2 regional seismic sections across the
acquired during Rama expedition in early 1980’s. The section on the west (Fig. 5A)
shows the narrow part of the basin, with the southern flank of the volcanic arc
( Savu Basin Flores Island)
in the north and the east continuation of the
high in the south. A detail section of the southern margin of the basin is
shown in Fig. 5C with seismo-stratigraphic interpretation (Fig 5 D) by Rigg and
Hall (2012). At the south end of this section Unit 1 is uplifted and thrust
northwards towards the basin and Units 2, 3 and 4 are largely missing and
interpreted to have been redeposited in the basin as Unit 4. Figure 5D shows a
significant southward thinning of Unit 3 and 4. Steep dipping of the base of
Unit2 are probably controlled by faults. Unit 3 contain a brighter reflective
package which wedges out to the north. A rather transparent seismic package
developed in the north part of the unit. The top of Unit 4 is relatively
undisturbed in the distal part. Sumba Island
3. Inner Arc – Volcanic
The Inner volcanic arc islands are some of the simplest geological structures within this complex region, and are certainly simpler than the outer-arc islands. The islands arc is basically a chain of young oceanic volcanic islands, often ringed by reef limestones or by other sedimentary material that has eroded from the main body of the island and built up between the tongues of lava and other extrusions. In general, the origins or basal materials of these islands become progressively younger from west to east, following the evolution of the Banda Arc eastward from the Sumba Fracture (Monk et al, 1997).
Figure 6 shows a seismic section acquired between Bali and Lombok island by Robert Conrad cruise 11 (
1979). The volcanic ridge is made irregular by volcanoes, fault blocks, and
folds which affect the sedimentary cover (Fig. 6). The southern flank of the
volcanic ridge is rich of volcanic deposits. A smaller sea bottom high in the
north is probably formed by volcanic intrusion (Fig. 7).
4. Continental shelf edge
The Australian Continental Shelf is located in the southeast of the
. The edge of this continent is interpreted
to be in the north side of Sumba and Lesser-Sunda Islands (Fig. 1, after
Harris et al, 2009). Unfortunately the seismic images acquire in these area are
either to shallow or too poor to see the edge of the edge of the Australian
Continent Shelf. Timor
The Sunda Shelf is located in the northwest of the studied area. A deep seismic section acquired by ION (Fig. 8) helped to understand the margin of the Sunda Shelf. Granath et al (2011) have interpreted the top basement based on this seismic image. The shallow basement in the WNW beneath NSA-1F well (Fig. 8A) and Kangean West-2 (Fig. 8B) is interpreted as the Sunda Shelf. The deeper basement in the ESE has been interpreted as Late Cretaceous accretionary crust (Doust & Lymbach, 1997).
identifies this area as Tertiary oceanic and arc crust.
Figure 9. Detail sections of the profiles shown in Figure 8. A) located near to NSA-1F and B) located near to Kangean West-2 well.
A west-east trend normal fault, which is dipping to the south, developed in the north of the Lesser Sunda Islands and formed
The map in figure 1 shows that the water depth in this basin reaches about more
than 4000 meters. A seismic section acquired by R. V. Robert Conrad (Fig. 10,
Hamilton, 1979) shows a deep trench developed by the fault. Recent sediment
accumulation is well imaged in this section at about 6.5 seconds. The Flores Basin
is poorly understood as it is deep and covered only by sparse data. Flores Basin
Prasetyo (1992) published a number of seismic lines which cover
and discussed the Flores Thrust Zone in great detail. The thrust zone is a
prominent E-W oriented structural feature extending from east to the west of
the Flores Basin . The fault zone separated south
dipping sedimentary sequences, including Paleocene rift and related sediments,
from the complex deformed material to the south (Prasetyo, 1992) Flores Basin
The position of the
is unique. It is not part of the
Sunda arc, which formed a lineation of volcanic islands in the north of Sumba Island Sumba. From the position it may be more related to Timor but it has different orientation (Fig. 1 and 2).
The origin of the island is still a debate amongst worker on this area.
is generally recognized as an exposed fore-arc basement which is located
between the Inner and Outer Arc. Several workers have considered Sumba Island
as a micro continent within a region of arc-continent collision
(Audley-Charles, 1975; Hamilton, 1979), and more recently as accreted terrane
(Nur and Ben-Avram, 1982; Howell et al., 1983). De Werff et al (1994) and
Harris et al (2009) conclude that the Sumba Island Sumba
Island is a continuation of Timor which is an arc-continent collision zone.
The Sunda Arc is known as an active convergence zone producing earthquakes, tsunamis and volcanic hazards. The Indo-Australian plate currently moves at 6.7 cm/a in a direction N11oE off western Java and thus almost normal to the trench (Tregoning et al. 1994). Convergence speed slightly increases from western Java towards the east at a very subtle rate. The movement is reaching 7 cm/a of
Bali (Simons et al, 2007) and has been
active since Eocene (Hall & Smyth, 2008). The overriding plate is
continental including Sumatra and western Java (Kopp et al, 2001) and the
basement below the forearc basin offshore Bali and Lombok is probably a rifted
crust of a continental character in transition to oceanic character at Sumbawa and further east (Banda Sea, Van der Weff, 1996).
The locations of the earthquake epicenters in the centre part of the Lesser Sunda Island reflect the subduction of the Australian Lithosphere under the Asian continenet (Fig. 11) . The Australian lithosphere, which is interpreted as Precambrian continental crust (
1979) moves northward. This subduction angle is also getting steeper
Figure 11. This plot shows the earthquake localizations on a South-North cross section for the lat -14°/-4° long 114°/124° quadrant corresponding to the
data source: USGS-NEIC; displayed in http://bigideasroots.wordpress.com/6-1/
Two major tectonic discontinuities separate the Banda Arc from the Sunda Arc in this area. The Pantar Fracture extends approximately north-south between the
island of Pantar
and Alor, and the Sumba Fracture separates Sumba and Flores islands from Sumbawa (Nishimura and Suparka, 1986). Unfortunately the
discontinuity of the arc, or the transition from Sunda to Banda arc is not
clearly seen on seismic section. Nishimura and Suparka (1986) use ‘fracture’ to
describe the separation, which indicates a small offset and therefore may not be imaged
well on seismic sections, especially by older sections
All seismic sections included in this are article were acquired sparsely offshore. Additional data around
may improve the
understanding of the origin of the island. This may bring the debate of the
origin of the island closer to conclusion. Sumba
Lesser Sunda Islands are a very active tectonic region,
formed by the subduction of the Indian Oceanic plate in the west and a
continent-island arc collision in the east.
This area is located between the Eurasian Continental Crust or also known as Sunda Shelf and the Australian Continental Crust or
The geology in the north of the Scott Plateau Lesser Sunda Islands
is poorly understood as it is poorly covered by seismic and lack of well
The west part of the Lesser Sunda Island is generally less complex compared to the east. The transition from Sunda arc to Banda arc, with Sumba and
in the east make the geology more complicated. Timor Island
Doust, H., & Lijmbach, G., 1997, Charge constraints on the hydrocarbon habitat and development of hydrocarbons systems in Southeast Asia Tertiary Basins, in Proceedings of the Petroleum Systems of SE Asia and Australasia Conference, Indonesian Petroleum Association.
Granath, J. W., Christ, J. M., Emmet, P. A., & Dinkelman, M. G., 2011, Pre-Cenozoic sedimentary section and structure as reflected in the JavaSPANTM crustal-scale PSDM seismic survey, and its implications regarding the basement terranes in the East Java Sea in: Hall, R., Cottam, M. A. &Wilson, M. E. J. (eds) The SE Asian Gateway: History and Tectonics of the Australia–Asia Collision. Geological Society,
London, Special Publications, 355, 53–74.
Hutchison, C., 1989, Geological Evolution of South-East Asia,
Krabbenhoeft, A., Weinrebe, R. W., Kopp, H., Flueh, E. R., Ladage, S., Papenberg, C., Planert, L., and Djajadihardja, Y., 2010, Bathymetry of the Indonesian Sunda margin-relating morphological features of the upper plate slopes to the location and extent of the seimogenic zone, Nat. Hazards Earth Syste. Sci., 10, p. 1899-1911.
Lüschen, E., Müller, C., Kopp, H., Engels, M., Lutz, R., Planert, L., Shulgin, A., Djajadihardja, Y. S., 2011, Structure, evolution and tectonic activity of the eastern Sunda forearc,Indonesia from marine seismic investigations, Tectonophysics, 508, p. 6-21
Monk, A., de Fretes, Y., Lilley, G. R., 1997, The ecology of Nusa Tenggara & Maluku, Periplus Edition.
Nishimura, S. and Suparka, S., 1986, Tectonic development of east
Journal of Southeast Asian Earth Sciences 1, 45-47.
Prasetyo, H., 1992, The Bali-Flores Basin: Geological transition from extensional to subsequent compressional deformation, Proceedings of Indonesian Petroleum Association, 21th Annual Convention
Rigg, J. W. D. & Hall, R., 2012, Neogene development of the Savu Forearc Basin, Indonesia, Marine and Petroleum Geology 32, p. 76-94
Simons, W. J. F., Socquet, A., et al., 2007, A decade of GPS in
Southeast Asia: resolving Sundaland
motion and boundaries, Journal of Geophysical Research, 112.
Tregoning, P., Brunner, F. K. Et al., 1994, First geodetic measurement of convergence across the Java Trench, Geophysical Research Letters, 21, p. 2135-2138.
Van Weering, T. C. E., Kusnida, D., Tjokrosapoetro, S., Lubis, S., Kridoharto, P. and Munadi, S. (1989) The seismic structure of the Lombok and Savu forearc basins, Indonesia Neth. J. Sea, Res. 24, 251-262