REGIONAL TECTONIC
Sumatra is formed by the
southwestern margin of Sundaland, the Southeast Asian promontory of the
Eurasian continental plate that stretches across the equator for 1760 km from
NW to SE, and is up to 400 km across. The island is product of a long and complex
geological history that spans a period of more than 250 Ma. Northeast
to Southwest, the island‟s geology is characterized by sedimentary basins in
the Northeast up to the Barisan mountains, which include the volcanic arc and
Sumatran fault, running along the length of the island near the Southwest
coast. The length is parallel to the offshore fore-arc basins, the fore-arc
highs (islands of the Simeulue-Enggano ridge), the deep trench, and the
subducting oceanic plate (McCaffrey, 2009).
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| Figure : Sumatran Physiography Map (McCaffrey, 2009) |
Hutchison (1994)
recognizes three terranes in the Malay Peninsula and Sumatera. Limestones
characterize the East Malaya Terrane with fusulinids in the east that linked to
Indochina and South China. To the west, Sinoburmalaya (cf. Sibumasu of
Metcalfe, 1996) is characterized by quartz sandstones that occupying the
western part of the Malay Peninsula and the Malacca Strait, and pebbly
mudstone-bearing formation (Bahorok fm) further to the west. The last terrane
is West Sumatra Block that characterized by lower to mid Permian with volcanic
arc and Carboniferous rocks. Hutchison (1994) concluded that the translation of
West Sumatra Block into its present position is occurred during Cenozoic,
however, the continuity of Middle to Upper Triassic sediments across the West
Sumatra Block, the Medial Sumatra Tectonic Zone, Sibumasu and East Malaya
indicates that these blocks had their present relationship before Mid-Triassic
times. Therefore, the translation of West Sumatra Block to its present position
must have occurred in very Late Permian or Early Triassic times (Barber et al.,
2005).
The active volcanoes of
Sumatra fall along the Barisan mountains and, like most arcs, are generally
parallel to the subduction zone and above the ~100 to 150 km depth contours of
the subducted plate (McCaffrey, 2009; Hatherton & Dickinson, 1967; Sieh
& Natawidjaja, 2000). This pattern of subduction-related volcanism has
persisted since at least Oligocene time (Hamilton 1979). The modern arc rocks
are andesite, dacite, and rhyodacite to rhyolite, and their strontium ratios
indicate variable amounts of crustal contribution. Thus the volcanic rocks
reveal a long history of subduction along the entire southwestern margin of
Sunda shelf.
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| Figure : Sumatran Tectonic Units (Hutchinson, 1994) |
REGIONAL STRUCTURE
The present geological
structure of Sumatra is dominated by the effect of the current subduction
system in which the Indian Plate is being subducted northeastwards beneath the
island. The structure of Sumatra was described by van Bemmelen (1949) and in
terms of plate tectonics by Hamilton (1979). The main structural elements of
Sumatra and its surrounded region are defined with respect to the Sumatran
subduction system. The oblique subduction of the Indian Ocean plate under
Sumatra gave rise to the major Sumatra Fault Zone, a NW-SE trending
right-lateral strike slip fault zone across the Barisan Mountains volcanic arc
(Barber et al., 2005).
Fitch (1972) explained the
presence of the Sumatran fault and other similar fault inboard subduction zone
by the process now known as slip partitioning, where the two plates do not
converge at a right angle to the strike of the trench, it requires smaller
overall shear force to share the shearing component of the relative motion
between two separate faults instead of on one fault. In the case of
partitioning, one fault is the subduction thrust, which takes up all of the
trench-normal slip and some fraction of the trench-parallel slip (strike slip
component). The subduction thrust and strike slip fault isolate a wedge of
forearc called the silver plate where bounded by Sumatran fault on the
northeast and on the southwest by the Sunda trench (McCaffrey, 2009). Sumatran
fault is purely strike slip fault and extend to the entire length of island and
coincides geographically with the volcanic arc through most of its length.
Bellier and Sebrier (1994) have claimed that numerous small and large volcanic
cones and calderas occur both current and ancient releasing step overs along
Sumatran fault.
Studies of the surface lineaments,
representing fault structures in the northern part of the North Sumatra Basin
using SAR (synthetic aperture radar) imagery showed that NW – SE (Sumatran) and
NE – SW (antithetic) trends are dominant throughout the basin (Sosromihardjo,
1988; Barber & Crow, 2005). In the NW Aceh Basin between Banda Aceh and
Lhokseumawe the fold trends are approximately east – west, parallel to the
north coast. This is surprising as the underlying basement structures trend north
– south. It has been suggested that the E – W orientation of folds is due to
the incipient development of southward-dipping subduction system in the
southern Andaman Sea, offshore northern Sumatra (Curray et al., 1979; Barber et
al. 2005).
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| Figure : Structural Map of the North Sumatran Basin (Barber and Crow, 2005) |
REGIONAL STRATIGRAPHY
The
basement of Sumatra consists of metamorphosed Permo-Carboniferous basalt and
andesites, as well as metasedimentary rocks. Pre-Tertiary rocks are exposed in
wide, yet isolated, areas mainly in the Barisan Mountains and in the Tin
Islands of Bintan, Singkep, Bangka and Belitung. The oldest rocks known are
Paleozoic (Carboniferous and older) mica schists with slate, phyllite,
quartzite, marble, and gneiss. The oldest rock identified by their fossils
content were also encountered in boreholes in eastern Sumatra. These rocks
contain palynomorphs from near the Devonian – Carboniferous boundary. The
better defined Pre-Tertiary rocks in Sumatra are grouped into the
Carboniferous-Permian Tapanuli Group, Permo-Triassic Peusangan Group and
Jurassic-Cretaceous Woyla Group. However, this terminology should be strictly
applied only to northern Sumatra where the units were firstly defined (Barber
& Crow, 2005).
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| Figure : Sumatran Pre-Tertiary Stratigraphic Units (Barber, 2005) |
Occurrences
of granites in the eastern part of Sumatra that extend into the Tin Islands
suggest that a large part of Sumatra is underlain by a highly differentiated
Pre-Carboniferous crystalline continental crust, the age of which extends back
into the Precambrian (Barber & Crow, 2005). These intrusions are unconformably
overlain by sedimentary and volcanic rock of Tertiary age or younger (Barber et
al., 2005). The Granites of Sumatera form two distinct groups. An older group
is widely distributed as isolated plutons and batholith over the whole island.
Some of these granites are tin-associated and have a narrow compositional range
of SiO2 values, generally above 70%. A younger group of Granites form the
plutonic component of a volcanic arc suite. They are confined to the Barisan
Range, where they form small batholiths and separated plutons with extended
compositional range from gabbro to monzogranite. Jurassic to Cretaceous
volcanic includes altered andesite and basalt lava, and volcanic breccia and
tuff. Unconformably overlying the Mesozoic rocks are Paleocene to Oligocene
volcanics and sedimentary rocks. The volcanic include altered andesitic to
basaltic breccia, lava, tuff, and welded tuff, with intercalations of sandy and
silty rocks, and in place, quartzitic sandstone or conglomerate. The main
sedimentary sequence consists of polymict breccia and conglomerate, sandstone,
mudstone and shale, with intercalations of welded tuff and in places quartzitic
sandstone. Paleocene to Eocene granodiorite and granite intrusive are also
known (Barber et al., 2005).
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| Figure : Sumatran Quarternary Center Main Volcanic and Tectonic Unit (Barber, 2005) |
These rocks are
unconformably overlain by an Oligocene-Miocene volcanic unit, and transgressive
sediments. The volcanics are altered and mineralised andesitic to basaltic
lava, breccia and tuff with intercalations of sandstone. The sediments are
coarse grained, locally quartzitic sandstone and conglomerate with
intercalations of mudstone, shale and coal in lower part. Fine-grained
sandstone, siltstone and mudstone with limestone intercalations occur in the
middle part, and mudstone and shale in the upper part. Volcanic rocks
associated with the active volcanics arc outcrop extensively in Sumatra, range
composition from rare basalts to abundant andesites and dacites (Gasparon, 2005
in Barber et al., 2005).
Reference
Barber,
A.J., Crow, M.J., & Milsom, J.S., 2005. Sumatra: Geology, Resources and
Tectonic Evolution. Geological Society, London.
Barber,
A.J., and Crow, M.J., 2005, Chapter 4: Pre-Tertiary Stratigraphy. In In Barber,
A.J., Crow, M.J., Milsom, J.S., (eds), Sumatra: Geology, Resources and Tectonic
Evolution, Geological Society, London, Memoirs, 31, pp. 24-53.
Curray, J.R.,
Moore, D.G., Lawver, L.A., Emmel, F.J., Raitt, R.W., Henry, M. &
Kieckheffer, R., 1979, Tectonic of the Andaman Sea and Burma, p. 189-198.
Fitch, T.J.,
1972, Plate convergence, transcurrent faults and internal deformation adjacent
to South-East Asisa and the western Pacific, Journal of Geophysical Research,
77, 4432-4460.
Hutchison, C.S.,
1994, Gondwana and Cathaysian blocks, Palaetethys Sutures and Cenozoic
Tectonics in South –East Asia, Geologische Rundshau, v. 82, 388-405.
McCaffrey, R.,
2009, The Tectonic Framework of the Sumatran Subduction Zone, Earth and
Environmental Sciences, Rensselaer Polytechnic Institute, Troy, New York,
37,p:3.1 - 3.22.
