Page 32 - Sumatra Geology, Resources and Tectonic Evolution

GRAVITY FIELD 19

precise where the basins are of only small lateral extent and Sumatra (onshore) from the Sunda (offshore) basin, and the
are often not apparent on regional maps. The examples of the dense basement rocks, which almost reach the surface along its
Ombilin intermontane basin and the Bengkulu forearc basin are crest, produce high gravity fields. However, the magnitudes of
discussed in more detail later in this chapter. the differences in gravity are smaller than those implied by the
changes in sediment thickness and suggest some degree of
crustal thinning beneath the basinal areas.
Toba-Tawar gravity low A number of southwards-convex curvilinear gravity trends are
superimposed on the local anomaly patterns in south and central
Low Bouguer gravity is to be expected in the mountainous regions Sumatra. These continue, and become even more prominent,
of northern Sumatra because isostatic balance requires mass offshore on the Sunda Shelf, where they are members of a set of
deficiencies at depth to support the topographic masses. Kadir curved anomalies that ring almost the whole of Borneo in an
et al. (1996) interpreted these low values as evidence for a apparent rotational swirl. The trend lines cut across a number of
structural model in which the crust is very thin and is underlain Late Tertiary boundaries between basins and structural highs,
by low density mantle. The alternative, and more conventional, including the Lampung High, and are therefore likely to be due
possibility is that the crust is in fact thicker in the vicinity of to sources within the basement rather than to basement relief.
the gravity low than elsewhere and is underlain by normal An origin in strain accompanying the rotation of Borneo is
mantle. Calculations based on a profile drawn across the strike possible, but the processes by which some of the observed
of the gravity low near Lake Toba indicate that this solution is gravity patterns could be generated by rotations are not clear.
perfectly feasible and that a satisfactory crustal model can be For example, the most prominent curved trend in the South
developed on this basis without undue difficulty. The mode of China Sea is the shelf-edge anomaly at the western margin of
compensation was discussed further by Masturyono et al. the central oceanic basin (Holt 1998), and it is hard to envisage
(2001), who drew attention to regions of low velocity (and a causal link between this and Borneo rotation. An alternative
hence, probably, of low density) in both the crust and uppermost explanation for the arcuate trendlines in Sumatra and on the
mantle in two areas beneath the Toba caldera. However, they Sunda Shelf is that these mark basement features associated
came to no firm conclusion as to the overall compensation with past subduction and accretion, implying that belts of former
mechanism. The Bouguer low covers an area vastly greater than arc basement have been 'wrapped around' the core of continental
the low velocity regions and the latter can therefore play only a SE Asia in Borneo and the Malay Peninsula. In eastern Sumatra
subsidiary role in its formation. It is, however, probable that there is some correlation between a curvilinear low sandwiched
some compensation does occur within the crust and that the between two positive curved features and the location of the
regional Bouguer low is due in part to the presence of a large Mutus assemblage that may mark the suture between the
granitic batholith that may still be in the process of formation. Malacca and Mergui microplates (Pulunggono & Cameron
The Toba-Tawar low is almost entirely onshore. There is a 1984). The rotation and basement suture hypotheses can be com-
weak possible extension out to sea to the north but this could bined by supposing that rotation of Borneo imposed curvature on
be fortuitous and merely a consequence of the presence of rela- sutures that were originally approximately straight.
tively deep water and light sediments on the Mergui Shelf.
A north-trending high that marks the western limit of the shelf
at about 96~ is associated in part with a low-amplitude bathy- Gravity effects of sedimentary basins
metric high known as the Mergui Ridge but is probably mainly
due to the transition from continental crust under the shelf to The regional map of Sumatra (Fig. 3.1) is sufficient to show the
oceanic crust in the Andaman Basin. Shelf-edge free-air highs broad gravity effects of most of the sedimentary basins but not
are the world-wide norm. They exist because the rapid shallowing the variations due to structures within them. Most of the
of the Moho beneath continental slopes affects gravity fields oil-company data that might define such details in the main
near the edges of shelves even though the crust immediately producing (back-arc) basins remain confidential, but there are pub-
beneath such locations is still thick and the sea is only a few lished studies of detailed work done by LEMIGAS in the Ombilin
hundred metres deep. intermontane basin (Situmorang et al. 1991) and the Bengkulu
The western limit of the Toba-Tawar low between about 96~ forearc basin (Yulihanto et al. 1995). The locations of these two
and 97~ is marked by a steep gradient defined by roughly surveys are indicated on Figure 3.1.
north-south contours, and the northwestern tip of Sumatra is The Ombilin Basin lies immediately to the east of the main
occupied by a gravity high with Bouguer values that in places Sumatran Fault (Fig. 3.1). It covers an area of some 1500 km 2
exceed ,1,100 reGal. The average gradient between the base and in places contains more than 3000 m of Eocene to Middle
stations at Banda Aceh airport and town (Bouguer values -t-39 Miocene sediments. It derives its economic importance from
and +53.5 reGal respectively: see Adkins et al. 1978) is about coal rather than oil or gas, and low density coals may well contrib-
one milligal per kilometre. The surface geology does not suggest ute to the gravity signature. The location suggests a genetic link
a terrane boundary in this region and the gravitational change to the Sumatran Fault, but Howells (1997b) interpreted the main
at the margin of the Toba-Tawar Low is probably largely a basin as a result of wrench modification of an earlier rift rather
lateral effect of high mantle beneath the forearc basin, coupled than as a simple strike-slip pull-apart. Only the much younger
with the effect of a change within the crust from young granitic Lake Singkarak rift (largely the area occupied by Lake Singkarak
rocks to an older and denser basement. in Fig. 3.3) is now interpreted as having formed as a recent pull-
apart within the Sumatran Fault (Sieh & Natawidjaja 2000).
There is good correlation between thin and thick sediments and
Eastern Sumatra gravity highs and lows (Fig. 3.3), both in relation to relatively
small structures (Situmorang et al. 1991) and also at a basin-
Away from the Barisan Mountains, gravity fields in the vast and wide scale. High Bouguer values define the main horst that
often swampy flatlands of eastern Sumatra are controlled by a separates the Ombilin Basin proper from Lake Singkarak. Low
number of competing factors. The most obvious of these is (<-20 mGal) Bouguer gravity characterizes the northern lobe
the subsurface presence in the region between the east coast of of the Palaeogene basin, but these values, some 50 mGal below
Sumatra and the eastern margin of the South Sumatra Basin of those on the horst block near Sulitair, are still higher than the
the roughly north-south oriented Lampung Structural High levels (of well below - 30 mGal) in the Singkarak rift. The differ-
(Pulunggono & Cameron 1984). The high separates the South ence could be due to differences in sediment thickness, to more