M ORPHO-STRUCTURAL UNITS IN THE CENTRAL INDONESIAN REGION
4.1 INTRODUCTION
The Indonesian region is constructed o f two major blocks o f continental lithosphere (Fig. 3.1), these being Sundaland, which is part o f the southeast Eurasian Plate, and the Australian platform which extends northwards into the Eastern Indonesian region. Between these two blocks lies a collection o f landmasses or small continental fragments and a number o f deep sea basins bounded by island arcs. The smaller continental
fragments are interpreted as having been rifted either from the South China continental shelf or the northern Australian continental margin. They display a variety o f tectonic styles and activity and sediment thicknesses, dictated largely by their proximity to arcs or continental margins (Prasetyo 1992).
4.2 GEOLOGICAL PROVINCES
On the basis o f the magnetic contour patterns and values (Fig. 3.10) and the Bouguer anomaly map (Fig. 3.16), in addition to the structural map o f the region, the Central Indonesia Region (CIR) and nearby areas can be divided into five major provinces (Fig. 4.1), these being;
1. Bone Bay Province 2. Makassar Strait Province 3. Central Java Sea Province 4. East Java Sea Province 5. Flores Sea Province
The geological and geophysical data available in each province will be examined in order to understand the tectonic styles and evolution. Subsequently, the evolution o f each province will be related to the others in order to obtain a complete history o f the
evolution o f subduction at the eastern margin o f Sundaland since the Late Cretaceous.
SULAWESI
KALIMANTAN
one Bay Provinc
entrai Java Sea Province akassar Strait Previn East Java S ea Province JAVA lores S e a Provinc FLORE SlIM H A W A 0 k m 2 0 0 k m 106® 1 08° Legends 110° 112° 114° 116° 118° 120°
□
Bone Bay Province Makassar Strait Province Central Java Sea Province
East Java Sea Province Flores Sea Province
Bouguer anomaly contour Gravity high
Gravity low Province boundary Lineation
122° 12 4 °
4.3 SUBDUCTION ROLL-BACK
The Central Indonesian Region (CIR) is recently located in a back-arc region. Geology, gravity and seismic interpretation indicate that the region is occupied by deep basins which experienced crustal extension. Therefore, the term back-arc basin or marginal basin can be used in the CIR. However, the development and evolution o f marginal basins are still poorly understood. Several hypotheses have been suggested (cf. Uyeda and Kanamori 1979) as follows;
1. Entrapment o f a marginal part o f a pre-existing ocean by the formation o f an island arc
2. Back-arc spreading caused by or related to subduction 3. Opening related to a leaky transform fault
4. Opening related to the subduction o f a ridge
5. Subsidence caused by oceanization o f continental crust
Karig (1971) suggested that many marginal seas in the Southwest Pacific region have developed by crustal extension during subduction o f oceanic lithosphere, usually called back-arc spreading.
The CIR is situated between Cretaceous-Early Tertiary basement complexes (see Fig. 3.3) and it is believed that basin formation has been associated with migration o f subduction. The mechanism can be explained through the relationship between the age o f the subducting slab and the gravitational instability o f oceanic lithosphere (Molnar and Atwater 1978). Young lithosphere resists subduction, possibly causing compression behind the trench, while an old subducting slab may experience active roll-back o f the trench axis and thereby cause back-arc spreading (Jarrad 1986). Dewey (1980) defined trench roll-back as the gradual seaward migration o f a trench caused by the gravitational pull o f the descending slab. This motion is greater for old, cold ocean plates than
younger, warmer ocean plates. Several authors (e.g. Chase 1978; Molnar and Atwater 1978) have suggested that a back-arc basin occurs when the resultant of the velocities o f the overriding plate and the trench roll-back motion has a component directed away from the trench, when viewed in a hot-spot frame o f reference. Daly et a/. (1991) described the mechanism o f back-arc extension in relation to the relationship between the velocity o f roll-back (Vt) and the velocity o f the overriding plate (Vu) (Fig.4.2). If Yu > Vt, the
oo U)
X
Vu Vt > Vu \ / / extensional \ / / regime / \ Vu compressional regime Vt < Vu VuFast Strain Rate
larg e p
Slow Strain Rate
sm all f l
Figure 4.3: The two models showing the different styles of extensional deformation expected with fast and slow rates of lithosphere extension (Park 1988)
Figure 4.2: Sketch showing the relationship between the relative velocity of subduction zone rollback (Vt) and the velocity of the upper plate (Vu) in relation to extensional and compressional stress behind the arc
overriding plate advances over the trench line resulting in a compressive arc. If Vu < Vt, an extensional arc will be generated and may result in back-arc basin formation. The roll-back velocity (Vt) depends on the age o f the slab since the older the slab the more dense it is and the faster it sinks. Therefore, subduction o f an old slab is more likely to result in subduction roll-back.
Park (1988) pointed out two opposing effects resulted from the process o f lithosphere extension; (i) steepening o f the geotherm, brought about by bringing the hotter
asthenosphere nearer to the surface, which weakens the lithosphere, and (ii) thinning o f the crust, which will act to strengthen the lithosphere. W hich o f these two effects predominates depends on whether extension is slow or fast. Park (1988) presented two models showing the different styles o f extensional deformation expected with fast and slow rates o f lithosphere extension (Fig.4.3). The figure showed that fast extension rates are only possible for hot, thermally young lithosphere in which there will be locally intense extensional deformation, with strain softening, leading to large (3 values and ultimately, if the force persists, to the complete rifting o f the continental crust and the formation o f an ocean. The second model (slower extension rate) produced strain
hardening and generated a p value o f around 1.5. As each section o f lithosphere hardens, intense deformation is localised along listric faults which would be expected to spread laterally to involve a much wider region o f extensional deformation. This critical p value o f 1.5 is in remarkable agreement with the estimated p values from a wide range o f intra continental extensional basins which show an average o f p value o f 1.4 - 1.5 (Park 1988). [ Note: The p value is the lithosphere stretching factor o f McKenzie (1978), equal to the ratio o f the lateral extent o f the stretched and unstretched crust. Thus a value o f P = 2 corresponds to a doubling o f the original width and a halving o f the original thickness o f the lithosphere (Park 1988)].