The Lesser Sunda Islands

Seismic Expression of Tectonic Features in the Lesser Sunda Islands, Indonesia

Herman Darman

The Sunda Arc is a chain of islands in the southern part of Indonesia, 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 (Hamilton, 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 Sumba Island in between.
Figure 1. Map of Southeast Asia showing the different crustal type in the region and the location map of the Lesser Sunda Islands (after Doust & Lijmbach 1997). 

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 Timor Island. The northwest of the 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.

Seismic data
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 Hamilton, 1979)
- Rama 12 expedition (Prasetyo, 1992; Scripps Institute of Oceanography,
- 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.

Tectonic features
The 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 tectonic processes.
Figure 2.  Structural map of the Lesser Sunda Islands, showing the main tectonic units, main faults, bathymetry and location of seismic sections discussed in this paper.

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 Timor. (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. Six 15 km deep seismic sections acquired by BGR from west to east traversing oceanic crust, deep sea trench, accretionary prism, outer arc high and fore-arc basin, derived from Kirchoff prestack depth migration (PreSDM) with a frequency range of 4-60 Hz. Profile BGR06-313 shows exemplarily a velocity-depth model according to refraction/wide-angle seismic tomography on coincident profile P31 (modified after Lüschen et al, 2011).

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).

2. Fore-arc Basin.
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 Lombok Basin and the water depth of this basin is about 4000 to 5000 m deep (Fig. 2). Savu Basin is the fore-arc basin located in the east of the Lesser Sunda Islands, separated from the Lombok basin by Sumba Island. In parts the water depth of Savu Basin is deeper than 2000 m.
Figure. 4. Detail sections of BGR06-303. A) Outer-arc ridge with thrust faults which formed the accretionary complex. B) Detail section of A) showing the trench sediment fill and the thrust faults in the north of the section. C) Detail section of A) showing the sediment fills of the Piggy-Back Basin, with relatively undisturbed flat surface on the north. The active fault has disturbed the continuation of the sediments in the south of the section.

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 Piggy-Back Basin 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).

The Savu fore-arc basin developed in the east of the Lesser Sunda 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 island of Sumba 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).
Figure 5. North-south seismic sections across Savu Basin. A) Rama expedition seismic, shows the relationship of the outer-arc ridge, Sumba Island high, Savu basin and Flores Island in the north. B) Another Rama expedition seismic in the centre of Savu Basin. C) A CGG Veritas seismic lines parallel to section 6B with higher resolution image with the seismo-stratigraphic unit interpretation in D).

Figure5A and 5B shows 2 regional seismic sections across the Savu Basin, 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 (Flores Island) in the north and the east continuation of the Sumba Island 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.

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. Seismic-reflection profile across Bali-Lombok volcanic ridge, acquired by R. V. Robbert Conrad cruise 11 (Hamilton, 1979). The crest and north flank of the outer-arc ridge are mantled by pelagic sediments, whereas the south flank is not; this may record increasingly intensity of deformation within the mélange wedge southward toward the Java Trench. Strata within the outer-arc basin display basinal downfolding which decreases upward. The volcanic ridge is made irregular by volcanoes, fault blocks, and folds which affect the sedimentary cover.

Figure 6 shows a seismic section acquired between Bali and Lombok island by Robert Conrad cruise 11 (Hamilton, 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).
Figure 7. Block diagram of the southern part of Lombok Island. The surface is a gradients map of bathymetric data. Gradients are draped on perspective view of bathymetric relief. Trench, outer wedge, slope break and inner wedge are indicated. The sections is modeled based on sea bottom profile (after Krabbenhoeft et al, 2010).

4. Continental shelf edge
The Australian Continental Shelf is located in the southeast of the Lesser-Sunda Islands. The edge of this continent is interpreted to be in the north side of Sumba and Timor Island (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.

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). Hamilton, 1979, identifies this area as Tertiary oceanic and arc crust.
Figure 8. Two WNW-ESE seismic lines in the north of the Lesser Sunda Islands showing the potential margin of the Sunda Shelf or Eurasian Continental crust margin. These seismic sections were acquired by ION (Granath et al, 2011). A) Seismic line between NSA-1F and SG P-1 well with significant drop of basement (Horizon A) about 35 km ESE of NSA-1F. An isolated basement high raised about 30 km WNW of SG P-1 well. B) Seismic line between Kangean West-2 and ST Alpha-1. A significant horse-graben system developed in the east of Kangean West-2 which brought the basement (Horizon A) deeper towards the ESE.

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. 

5. Flores Basin
A west-east trend normal fault, which is dipping to the south, developed in the north of the Lesser Sunda Islands and formed Flores Basin. 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.
Figure 10. A N-S seismic section from Lamong Doherty Geological Observatory, acquired by R. V. Robert Conrad cruise 11 (Hamilton, 1979). This section shows little sediment on the narrow floor of the Flores Sea or Flores Basin, in contrast to the thick strata on the platform between that sea and the South Makassar Basin which probably consist of carbonate units.

Prasetyo (1992) published a number of seismic lines which cover Flores Basin 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)

6. Sumba Island
The position of the Sumba Island is unique. It is not part of the Sunda arc, which formed a lineation of volcanic islands in the north of 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.

The Sumba Island 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 is a continuation of Timor which is an arc-continent collision zone.

Tectonic activities
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 (Hamilton, 1979) moves northward. This subduction angle is also getting steeper northwards.
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 Lesser Sunda Islands region. The localizations are extracted from the USGS database and corresponds to magnitude greater than 4.5 in the 1973-2004 time period (shallow earthquakes with undetermined depth have been omitted.
data source: USGS-NEIC; displayed in

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 Sumba Island may improve the understanding of the origin of the island. This may bring the debate of the origin of the island closer to conclusion.

Closing Remarks
The 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 Scott Plateau. The geology in the north of the Lesser Sunda Islands is poorly understood as it is poorly covered by seismic and lack of well penetration.

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 Timor Island in the east make the geology more complicated.

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