Page 29 - Sumatra Geology, Resources and Tectonic Evolution

Chapter 3

The gravity field

JOHN MILSOM & ADRIAN WALKER

Data sources geodetic survey authority. A map showing the locations of the
BAKOSURTANAL stations and Bouguer gravity contours after
The gravity field of Sumatra and the surrounding marine areas is the application of a severe high-cut filter has been circulated
shown in Figure 3.1. Contours in the onshore area of Bouguer on a very limited basis, but these stations are not included in
gravity, but offshore are of free-air gravity. Terrain corrections Figure 3.2. An unfiltered but very small scale version of the
have not been applied. Although marine gravity measurements BAKOSURTANAL Bouguer map was published by Kadir et al.
have been made in the forearc basin and elsewhere on a number (1996), and the data may also have been used by GRDC in pre-
of research cruises (e.g. Kieckhefer et al. 1981), the data from paring the 1:10000000 Bouguer anomaly map of Indonesia
these generally widely spaced lines have not been used in pre- (Sobari et at. 1993). BAKOSURTANAL Bouguer values around
paring the maps because free-air gravity values obtained from the Toba caldera are generally 10-20mGal higher than those
inversion of satellite radar altimetry provide more systematic reported by the Japanese groups, a difference probably due to
coverage and can resolve anomalies with widths of as little as the lack of terrain corrections in the Japanese work.
7 km (Sandwell & Smith 1997). The onshore and satellite- The onshore contours in Figure 3.1 are based on actual point
derived offshore data were matched at coastlines without undue gravity data where available, supplemented where necessary by
difficulty, as should be the case because both free-air and values estimated at known BAKOSURTANAL station positions
Bouguer corrections are zero at sea level. However, gradients using the contours of Kadir et al. (1996). Accuracy is inevitably
tend to be steep at the coasts in the forearc region, partly low where this has been done, and even so some significant gaps
because of the change from free-air gravity, which is strongly remain. The problem of making full use of good regional coverage
correlated with local bathymetry, to Bouguer gravity, which is cor- where this exists and at the same time displaying in an acceptable
rected for local topography. way the results of interpolation across larger gaps has been
Figure 3.2 shows the locations of the onshore stations used in addressed by overlaying the map based on a relatively fine
preparing Figure 3.1, but not of the offshore estimates, distributed (0.1 ~ grid, which is blank in areas of inadequate coverage, on a
on a regular 2 minute grid. Onshore data were obtained from a map produced using a much coarser grid and a greater degree of
variety of sources, but unfortunately the results of the many interpolation. This is obviously unsatisfactory as a quantitative
detailed gravity surveys carried out by oil companies remain method, but Figure 3.1 is intended to be used only qualitatively
confidential. The largest single available data set was assembled and the general patterns can be considered sufficiently well estab-
as part of the collaboration between the British Geological lished to support regional interpretation. It is just possible on
Survey (BGS) and the Geological Research and Development Figure 3.1 to identify discontinuities in the colour patterns at the
Centre (GRDC) during the period 1988-1995. Almost all of edges of areas where the coarse grid has been used.
Sumatra south of the equator was covered at a reconnaissance Extending Figure 3.1 to include Billiton has brought western
level, although there are significant gaps in a few areas where Java within the boundaries of the map. The data used were
access would have been especially difficult. In addition to the obtained in 1970 by the BGS, working in conjunction with the
Sumatra mainland, measurements were made on Bangka and Geological Survey of Indonesia. The results of recent more
Billiton islands in the northeast and the Mentawai islands in the detailed work on Java by GRDC are not shown but are generally
west (Fig. 3.2). GRDC have published numerous Bouguer compatible with the BGS survey.
maps at 1:250 000 scale showing contours, generally at 2 mGal
intervals, and station locations. There are also two summary
maps at 1 000 000 scale (Padang and Palembang sheets), con- Regional gravity patterns
toured at 5 mGal intervals and without station positions. Terrain
corrections, of up to 12 mGal, were applied in preparing the The most prominent features in Figure 3.1 are offshore. Gravity
summary maps but were not used for any of the 1:250 000 detailed highs with north-south or NNE-SSW trends are associated
maps. The two versions of Bouguer gravity are therefore slightly with fracture zones and seamount chains on the Indian Ocean
different in the mountainous areas close to the Sumatran Fault Plate and these control the positions of individual culminations
but gradients in these areas are in any case steep, and overall on the broad flexural high at the outer margin of the Sumatra
patterns are very similar. Trench. Two deep NW-SE-trending free-air lows, associated
Coverage north of the equator, principally by GRDC and respectively with the trench and the forearc basin, intervene
LEMIGAS (the Indonesian Petroleum Research Institute), is less between this oceanic domain and the Sumatran mainland and
complete than in the south but is progressing rapidly. Moreover, are separated from each other by a high along the forearc ridge.
Japanese universities working between 1977 and 1979 obtained The low over the trench exists because the mass deficit of the
data along many of the more important roads in the Lake Toba water column is not in local isostatic equilibrium but is balanced
area (Fig. 3.2). In the northern forearc LEMIGAS collaborated elastically by the offset mass of the subducting slab.
with the University of London in surveys of all of the major Although the available gravity coverage is much less complete
islands (Milsom et al. 1991). Stations were mainly along the north of the equator than in the south, there can be no doubting
coasts, except on Nias. LEMIGAS/UofL stations on Siberut the existence of fundamental differences between SE and NW
were restricted to the southeastern corner, but the island was Sumatra. In the south the Barisan mountains are associated with
subsequently covered at a reconnaissance level by GRDC. a narrow, discontinuous and rather weak Bouguer low that,
In 1991 and 1992, stations were established along major roads where it exists, coincides quite precisely with the axis of the
throughout Sumatra by BAKOSURTANAL, the Indonesian mountain range, but in the north the low deepens and expands to