Page 34 - Sumatra Geology, Resources and Tectonic Evolution

GRAVITY FIELD 21

A
S.I.O. RAMA 6 Line 58-59

Pleistocene - Recent

Banyak Is,

Interpretation simplified after Matson & Moore (1992)

ill

-b

X\x"X\.,
/.
.././/" I~
Fig. 3.5. Gravity variations in the central
forearc basin. In contrast to Figure 3.1,
///" Bouguer gravity is contoured in the offshore
as well as the onshore regions. Contour
interval 10 mGal. Offshore data are from
Scripps Institution of Oceanography (SIO)
shipborne readings along the tracks shown
by dashed lines. Areas of water depths in
excess of 500 m in the vicinity of the
Banyak and Singkel sedimentary basins
indicated by light stipple. The upper inset
shows SIO seismic line 58-59 across the
Banyak forearc basin east of Nias, from SIO
cruise Rama 6, with a simplified version of
Pini the interpretation provided by Matson &
///"
Moore (1992). Note the strong asymmetry
in the basin. The lower inset shows the
///" \ residual gravity anomaly over eastern Nias,
obtained by subtracting a linear regional
Equator
0 50kin ' ~ Batu Is. gradient parallel to the trends of Bouguer
~ - T 7 ~ s ~ ~2~l~I~E contours in the north of the island from the
98~ local values.

lies just north of the equator and straddles the forearc basin. The In part the low Bouguer and free-air values in the forearc basin
high gravity is evidently not due merely to the presence of the reflect the presence of the water column, which is up to 1500 m
bathymetric high, since a gravity low is associated with similar thick, but there is also a significant contribution from light
bathymetry in the Banyak group further north. A -80 mGal Neogene sediments The seismic stratigraphy of the area east
minimum was recorded on the most easterly of the Banyak of Nias was first described by Beaudry & Moore (1985), who
islands (Fig. 3.5). recognized three main sequences and assigned these tentatively
There is an obvious geographical correlation between the Pini to the Pleistocene (Unit 4), the Pliocene and uppermost Miocene
high and high free-air gravity associated with the Investigator (Unit 3) and to most of the remainder of the Miocene (Unit 2).
Fracture Zone on the Indian Ocean plate immediately to the Unit 2 was further subdivided into Units 2a and 2b, separated
south (IFZ; Fig. 3.1). A causal link between the two seems by a generally continuous, high-amplitude seismic event. Older
likely. Subduction of the fracture zone, which is a prominent stratified sediments (Unit 1) can be seen in places beneath a
bathymetric feature consisting of a deep linear trough flanked by strong regional unconformity at the base of Unit 2a, but elsewhere
two high standing ridges, has been suggested as a possible cause this region is devoid of reflectors and may comprise igneous or
for both the change in strike of the trench and forearc north of metamorphic basement or steeply dipping sediments.
Nias (the Nias 'elbow') and the enhanced volcanic activity in With one exception, Beaudry & Moore (1985) illustrated their
the Toba region (Fauzi et al. 1996). discussion with oil industry seismic sections which were of