Paper sarulla

Tectonic and stratigraphic evolution of the Sarulla graben geothermal
area, North Sumatra, Indonesia
R.G. Hickmana,*, P.F. Dobsonb
, M. van Gervenc
, B.D. Sagalad
, R.P. Gundersone
a
Structural Solutions, 1330 Sugar Creek Blvd., Sugar Land, TX 77478, USA
b
Earth Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA
c
Autodesk Inc., GIS Solutions Division, San Rafael, CA, USA
d
Unocal Geothermal Indonesia, Sentral Senayan-I Office Tower, Jln. Asia Afrika No. 8, Jakarta 10012, Indonesia
e
Unocal Geothermal Technology and Services, 1160 N. Dutton Ave., Santa Rosa, CA 95401, USA
Received 18 February 2003; revised 11 June 2003; accepted 16 June 2003
Abstract
The Sarulla graben is a composite Plio-Pleistocene basin developed along the northwest striking, dextral-slip Sumatra fault in a region
where the fault coincides with the Sumatra volcanic arc. Offset of the 0.27 ^ 0.03 Ma Tor Sibohi rhyodacite dome by an active strand of the
Sumatra fault, the Tor Sibohi fault (TSF), indicates a slip rate of about 9 mm/y. This value is lower than previous regional estimates of ,25–
30 mm/y for Holocene slip on the Sumatra fault determined from stream offsets in the Taratung region. This discrepancy may be due to (1) a
difference between Holocene and late Quaternary rates of slip and (2) additional slip on other faults in the Sarulla area. Since the magnitude
of undated stream offsets along the TSF in the Sarulla area is similar to those in the Taratung area, the discrepancy is likely to be due largely
to a change in slip rate over time.
Within the Sarulla area, major volcanic centers include the Sibualbuali stratavolcano (,0.7–0.3 Ma), the Hopong caldera (,1.5 Ma), and
the Namora-I-Langit dacitic dome field (0.8–0.1 Ma). These centers generated the majority of the ash-flow tuffs and tuffaceous sediments
filling the Sarulla graben, and appear to have been localized by structural features related to the Sumatra fault zone.
Four geothermal systems within the Sarulla area are closely linked to major faults and volcanic centers. In three of the systems, reservoir
permeability is clearly dominated by specific structures within the Sumatra fault system. In the fourth geothermal system, Namora-I-Langit
geothermal field, permeability may be locally influenced by faults, but highly permeable fractures are widely distributed.
q 2003 Elsevier Ltd. All rights reserved.
Keywords: Strike-slip faulting; Slip rate; Volcanism; Geothermal systems; Sumatra
1. Introduction
From mid-1993 through early 1998, Unocal Corporation,
under a Joint Operation Contract with Pertamina (the
Indonesian state-owned oil company) carried out an
exploration program for geothermal resources within the
15 by 63 km Sarulla contract area located in North Sumatra
(Fig. 1). This program included mapping of lithologic units,
hydrothermal alteration and structures, radiometric dating
of volcanic units, and locating, sampling, and analyzing
fluids from surface geothermal features within the contract
area (Gunderson et al., 1995). Structural mapping was
carried out by traverses, mainly along streams and roads
where the likelihood of outcrops was the highest. This was
supplemented by interpretation of 1:20,000 aerial photos.
These photos were acquired in 1974 at a time when the
forest cover was less extensive, thus facilitating aerial
mapping. The photos were especially useful in (1)
projecting faults identified in traverses, (2) identifying
other lineaments that may reflect fault or fracture trends, and
(3) in mapping the extent of alluvium, intensely cultivated
areas, and large areas affected by hydrothermal or supergene
alteration. An additional part of the program consisted of
conducting gravity, time-domain electromagnetic (TDEM)
and magnetotelluric (MT) surveys. Following these surveys,
13 exploration wells were drilled within the contract area
(Gunderson et al., 2000).
As a result of the geothermal prospects being
located within or near the Sarulla graben and adjacent to
1367-9120/03/$ - see front matter q 2003 Elsevier Ltd. All rights reserved.
doi:10.1016/S1367-9120(03)00155-X
Journal of Asian Earth Sciences 23 (2004) 435–448
www.elsevier.com/locate/jseaes
* Corresponding author. Tel.: þ1-218-240-1057; fax: þ1-281-240-8457.
E-mail address: rhickman@pdq.net (R.G. Hickman).

the Sumatra fault zone, these exploration efforts provide
new data regarding the geometry and displacement history
of the Sumatra fault zone and the interaction between the
fault system and the Quaternary-Recent volcanic arc, which
coincides with the fault zone in this region. Additionally,
this paper advances the understanding of the Quaternary
volcanic history of the region and the general development
of geothermal systems in the proximity of major strike-slip
fault zones.
2. Tectonic setting
Sumatra lies along the southern margin of the Eurasian
plate (Fig. 1). Late Paleozoic meta-sedimentary rocks
including limestones, argillites, and graywackes comprise
the oldest widely distributed rock unit in Sumatra. These are
part of the Sundaland craton, believed to have been accreted
to the Eurasian margin during Triassic time (Stauffer, 1983;
Cooper et al., 1989). These strata are overlain by Jurassic
and Cretaceous sediments, meta-sediments and mafic
volcanics, and are intruded by Late Cretaceous granitic
rocks (Page et al., 1979; Mitchell, 1993).
The backarc basins of southern, central, and northern
Sumatra developed as a result of initial extensional faulting
during the Eocene followed by subsequent sag-phase
deposition. This was followed by uplift and inversion of
both backarc and forearc basins beginning during the middle
Miocene (McCarthy, 1997). The Cretaceous through
Quaternary history of Sumatra has been interpreted as one
of continual convergence, during which periods of subduc-
tion were interrupted by obduction of arcs onto the western
margin of Sumatra during the late early Cretaceous and
during the middle Eocene (Mitchell, 1993, Fig. 2).
Currently, the Indo-Australian plate is moving northward
(azimuth of 003–0258) relative to Eurasia at about 60–
75 mm/y (Minster and Jordan, 1978; DeMets et al., 1990;
McCaffrey, 1992) and is being subducted beneath the Java-
Sunda trench. South of Java, the trend of the Java trench is
about 1008 and plate convergence is nearly orthogonal to the
trench (Bellier and Se´brier, 1994). In contrast, the Sunda
trench west of Sumatra has an average trend of about 1408
and plate convergence across that zone is oblique.
This oblique convergence is believed to result in strain
partitioning in which a sizeable portion of the margin-
parallel component of convergence is taken up by dextral
strike-slip motion on the Sumatra fault system (SFS, Fitch,
1972; Beck, 1983; Jarrard, 1986) and the Mentawai fault of
the Sumatran fore-arc basin (Diament et al., 1992, Fig. 1).
The SFS extends about 1650 km from the Sunda Strait
extensional fault zone to the Andaman Sea spreading center
where it acts as a transform fault (Page et al., 1979).
Displacement on the Mentawai fault appears to be
transferred to the northern SFS via the Batee fault (Bellier
and Se´brier, 1995).
Because the SFS is linked to the Andaman Sea
spreading center, the SFS likely has been active as a
dextral slip fault since at least the mid-Miocene, when
seafloor spreading started in the Andaman Sea (Curray
et al., 1979). The proposed range of total displacement of
the fault zone is large (McCarthy and Elders, 1997). Curray
et al. (1979) estimate of 460 km displacement based on the
amount of opening of the Andaman Sea since the mid-
Miocene represents an upper limit of possible displacement
Fig. 1. Map showing the general tectonic setting of Sumatra and the
location of the Sarulla area. SFS, Sumatra fault system; MF, Mentawai
fault; BF, Batee fault.
Fig. 2. Schematic diagram showing the successive closing of ocean basins
along the Asian continental margin during the cretaceous to present.
Modified after Mitchell (1993).
R.G. Hickman et al. / Journal of Asian Earth Sciences 23 (2004) 435–448436

on the fault. The largest proposed displacements based on
offset volcanic centers and rock units range from 90 to
150 km (Posevec et al., 1973; McCarthy and Elders, 1997).
Seih and Natawidjaja (2000) suggest that distension of
forearc structures and the trench near the Sunda Strait
implies about 100 km of arc-parallel stretching of the
forearc region since the early Pliocene. However, based on
offsets of major drainages, they argue that the total offset of
the Sumatra fault may be only about 20 km.
Recent slip rates based on stream offsets show an
increase from less than 10 mm/y where the SFS enters the
Sunda Strait to 23 ^ 2 mm/y adjacent to Lake Toba in
northern Sumatra (Bellier and Sebrier, 1995). Geodetic
measurements suggest that between 0.88 S and 2.78 N the
current slip rate is nearly uniform at about 25 mm/y
(Genrich et al., 2000).
From the Lake Toba area to the Sunda Strait, Quaternary
and Recent volcanoes of the Sumatra volcanic arc are in
very close proximity to the Sumatra fault zone (Fig. 1, Page
et al., 1979; McCarthy and Elders, 1997), and it seems
possible that the fault zone and the volcanic arc influence
each other.
3. Stratigraphy of the Sarulla area
The oldest rocks exposed within the study area are meta-
quartzites, phyllites, argillites, and limestones interpreted to
be of late Paleozoic age (Tapanuli Group and Kuantan
Formation of Aspden et al., 1982, Figs. 3 and 4). These
strata are exposed on both sides of the SFS in the Barisan
Mountains, along the margins of the Sarulla graben, and in
uplifted fault slivers. Mesozoic or early Tertiary granitic
intrusives are not exposed within the Sarulla area, but occur
within 15 km of the western margin of the map area
(Aspden et al., 1982). Five to ten kilometers to the east,
marine sandstones and limestones of Miocene age crop out
along the margin of the central Sumatra (backarc) basin
(Aspden et al., 1982).
Within the southern part of the study area, west of the
active Tor Sibohi strand of the SFS, lithic arenites, arkoses,
pebble conglomerates, and carbonaceous siltstone beds of
fluvial and lacustrine origin crop out in a small graben.
These beds are most likely of late Pliocene age, but could be
as old as late Miocene or as young as early Pleistocene
based on arboreal pollen (V.E. Williams, pers. com.). These
fluvio-lacustrine strata contain abundant detritus derived
from the Paleozoic quartzites, but are free of volcanic
material (Fig. 4). The strata are inferred to unconformably
overlie the Paleozoic rocks. Well-sorted quartz sandstones
that are silicified were encountered at a depth of about
1465 m in a well drilled east of the Tor Sibohi fault (TSF)
near the town of Sipirok; these rocks closely resemble
Tertiary (?) strata that are exposed along the eastern margin
of the Sipirok graben still further to the east. Other undated
pebble and cobble conglomerates, and sedimentary breccias
Fig. 3. Simplified geologic map of the Sarulla area. ASN Flt, Aek
Sitandiang Nemenek fault.
R.G. Hickman et al. / Journal of Asian Earth Sciences 23 (2004) 435–448 437

with clasts derived from the underlying Paleozoic strata
crop out in the northeastern part of the map area. All of these
units, which contain sediment derived from the Paleozoic
strata and lack significant volcanic material, may be the
same age.
Outcropping Pliocene (?) strata are overlain by similar,
but less indurated sandstones, siltstones, and conglomer-
ates that contain abundant volcanic detritus. The Silang-
kitang 1-1 well (Fig. 3) drilled within the Sarulla graben
encountered conglomerates with abundant volcanic clasts
that are overlain by more than 2 km of tuffs and
interbedded lacustrine siltstones. These are possibly
correlative with the outcropping volcanic-rich sediments
(Fig. 4). The possible Pliocene conglomerates of the
northern area that lie east of the TSF grade upward into
the conglomerates, pebbly mudstones, and tuffs containing
clasts derived from both the Paleozoic strata and
volcanics. These isolated units are thought to be
approximately the same age based on their general
stratigraphic and lithologic character, and are significant
in that they mark the start of recent volcanic activity
within the region. Radiometric dating of volcanic rocks
within the greater Sarulla area indicates that volcanic
activity in the area started about 1.8 million y ago
and suggests the approximate age of these strata
(Gunderson et al., 1995).
The greater Sarulla stratigraphic section younger than the
latter unit consists nearly entirely of volcanic and
volcaniclastic strata that complexly interfinger (Fig. 4).
These rocks consist of flows, lahars, and tuffs from volcanic
centers within the Sarulla area as well as tuffs derived from
more distant volcanic centers such as the Toba Caldera.
Exploration wells and gravity data demonstrate that the
thickness of this sequence exceeds 2000 m in the northern
Sarulla graben and is about 2500 m thick immediately east
of the Sibualbuali volcano. Near the volcanic centers, the
stratigraphic relationships of major flow and tuff units have
been partly determined by integration of mapping, radio-
metric dating and petrology. Wells drilled in the basinal
areas and margins of the volcanic centers have encountered
unwelded, but silicified tuffs and interbedded lacustrine
sediments that are probably partly age-equivalent to the
exposed section, but are difficult to correlate with it. The
geology and stratigraphy of the main volcanic centers are
described below from south to north.
3.1. Sibualbuali volcano
Sibualbuali volcano is a deeply dissected stratovolcano at
the southern end of the Sarulla contract area that is
predominately andesitic in composition. It consists of a
series of andesitic to dacitic lavas and breccias with some
Fig. 4. Diagram showing stratigraphic relationships across the Sarulla area.

interbedded tuffs that range in age from ,0.7 to 0.3 Ma
(Gunderson et al., 1995). Following construction of the
stratovolcano, more silicic dacite to rhyodacite lavas and
domes (0.12–0.30 Ma) were erupted along the western and
eastern flanks along the two major strands of the SFS. North
and east of the volcano, two distinctive sanidine-bearing
ash-flow-tuff units have been mapped. These are informally
referred to as the Old Sibualbuali tuff and the Young
Sibualbuali tuff and have 40
Ar/39
Ar sanidine ages of
0.57 ^ 0.003 and 0.38 ^ 0.002 Ma, respectively (all age
errors are 1s; Gunderson et al., 1995). The distribution and
elevation of the base of the Old Sibualbuali tuff suggests
that it may have traveled northward from Sibualbuali
volcano. The source of the Young Sibualbuali tuff is not
known, but pumice blocks up to 30 cm in length near the
town of Sipirok suggest a nearby source. The youngest
eruptive center in the area is the Lubukraya volcano, located
about 6 km southwest of Sibualbuali. Lubukraya consists
mainly of basaltic andesites; one flow yields a K–Ar age of
0.12 Ma (Gunderson et al., 1995).
3.2. Hopong caldera
Hopong caldera is a 9 km-diameter circular volcanic
collapse feature located on the eastern margin of the
Sarulla graben. Along the western margin of the caldera, a
sequence of interbedded andesitic flows and dacitic tuffs
are exposed that grade to the north and south into a more
tuff-rich sequence. The andesite and tuff sequence is
believed to be part of the volcanic center that preceded
development of Hopong caldera. Poorly welded rhyodacitic
to rhyolitic intracaldera tuffs are found within the most
deeply eroded parts of the caldera. A plagioclase separate
from a welded rhyodacite ash-flow tuff yielded a 40
Ar/39
Ar
plateau age of 1.46 ^ 0.12 Ma. Within the caldera,
laminated tuffaceous lacustrine sediments overlie these
tuffs. There are many deeply weathered and reworked
ash-flow tuffs found in the surrounding areas that are
thought to have been erupted from the caldera. A number
of dacitic to rhyolitic domes have been extruded along the
western margin of Hopong caldera. 40
Ar/39
Ar incremental
heating of a plagioclase separate from the largest of these
domes yielded a plateau age of 1.3 ^ 0.1 Ma. All of these
deposits are locally capped by the regional 73 Ka Young
Toba Tuff (Chesner et al., 1991).
3.3. Namora-I-Langit dome field
The Namora-I-Langit (NIL) volcanic center, in the
northwestern part of the study area and west of the SFS,
consists of a number of dacite and rhyolite domes and
andesitic flows (Fig. 3). K–Ar ages for this volcanic
center range from 0.75 ^ 0.06 Ma for a plagioclase
separate from an andesite lava to 0.16 ^ 0.08 Ma for a
biotite separate from an undissected andesite flow. The
center is located south of the Martimbang volcano, an
undated but geomorphologically even younger basaltic
andesite cone. Southeast of the NIL complex and just
west of the Tor Sibohi fault, a rhyolite dome near the
town of Sarulla yielded a K–Ar date on biotite of
0.12 ^ 0.08 Ma (Gunderson et al., 1995). The domes and
flows interfinger with the upper part of the sequence of
reworked tuffs and lacustrine deposits of the Sarulla
graben. Over much of the area, the Young Toba Tuff
overlies these deposits.
4. Structural geology
The Sarulla area is bisected by the SFS (Fig. 3), which
here consists of one through-going, active strand, the Tor
Sibohi fault (TSF), and several parallel, less active and
inactive faults. In the northern part of the area, the TSF is
closely paralleled to the southwest by the active Hutujulu
fault that merges with the TFS near the village of
Silangkitang. In the central part of the area, the TSF bounds
the eastern flank of a structural low, the Sarulla graben. In
the southern part of the area, the TFS is paralleled to the
southwest by the Aek Sitandiang Namenek (ASN) and Toru
Nabara faults. These latter faults and the TFS appear to form
a complex releasing step.
4.1. Small-scale structures of paleozoic rocks
Meta-quartzites, phyllites, argillites and limestones,
inferred to be of late Paleozoic age, are poorly exposed
as fault slivers along major faults and along the
northeastern margin of the study area. Because of the
poor quality of exposures, little can be said about
the regional structure of these strata. However, in
addition to having been subjected to low-grade meta-
morphism, all of these rocks have undergone strong pre-
Pliocene deformation. Bedding is generally steeply
dipping. Tight meter-scale upright folds are developed
in the argillites and phyllites. All of these rocks have
been subjected to a later brittle deformation that has
strongly fractured and locally brecciated them. Minor
hydrothermal veins composed of quartz or calcite and
pyrite typically fill these fractures. The proximity of
exposures to strands of the SFS and the occurrence of
hydrothermal mineralization likely related to the current
geothermal systems, suggest that this brittle deformation
is related to strain associated with the SFS.
4.2. Sumatra fault system—Tor Sibohi fault
The Sumatra fault system (SFS) forms a zone up to
10 km wide along the length of the study area. One active
strand of the fault, the Tor Sibohi fault (TSF), extends along
that entire distance (Fig. 3). Along much of this distance, the
fault zone occupies a linear valley or is bounded on one side
by steep slopes. Much of the valley is intensively cultivated,

and fault scarps have been removed or strongly modified by
extensive artificial terracing. However, the trace of the fault
is marked by springs, gas seeps, and narrow zones of steep
dips in tuffs and mudstones, and is locally identifiable on
aerial photos.
The fault has an overall strike of about N 358 W, but the
strike of individual segments of the fault ranges from about
N 558 W to N 208 W. Given the dextral strike-slip motion on
the fault, these strike changes create a slight constraining
bend near the village of Silangkitang, a releasing bend south
of Donatasik near the south end of the Sarulla graben, a
prominent constraining bend north of Sibualbuali volcano
and a releasing bend along the southeastern flank of
Sibualbuali (Fig. 3). At the point of this latter releasing
bend, a series of faults that are subparallel to the Tor Sibohi
fault are present. These have straight traces, are steeply
dipping, and are interpreted to be dextral strike-slip faults
that transfer some displacement from the Tor Sibohi via a
complex releasing step.
The fault is not well exposed, but near the northern limit
of the contract area (UTM Coordinates 500,340 m E;
217,590 m N), the fault is exposed in an excavated hillside.
There the fault strikes N 458 W and dips 678 to the
southwest. In the Silangkitang area, wells and surface
mapping indicate that the fault dips about 878 to the
southwest. Along the northeast flank of Sibualbuali (UTM
Coordinates 527,520 m E; 177,350 m N), the fault strikes N
468 W and dips about 858 to the southwest.
4.3. Tor Sibohi fault displacements
Numerous streams exhibit dextral jogs where they flow
across the Tor Sibohi fault. These bends are interpreted to
reflect dextral offset of the stream valleys by displacement
of the fault. In this situation, the greatest and oldest offsets
are typically shown by the largest, most deeply incised
streams (Wallace, 1968). In the study area, stream offsets
were estimated from aerial photos or topographic maps. The
recognized offsets range from about 130–1400 m (Table 1).
The maximum offsets here are similar to, but slightly
smaller than the maximum small stream offsets determined
in the Toba area from SPOT satellite images of
1660 ^ 100 m (Detourbet et al., 1993) and 1700–2100 m
offsets determined from aerial photographs and topographic
maps in the same area (Sieh and Natawidjaja, 2000).
Detourbet et al. (1993) and Sieh and Natawidjaja
(2000) assumed that these offsets are on drainages
developed after blanketing of the area by the Young
Toba Tuff. Based on an age of the Young Toba Tuff of
73,000 ^ 4000 y, the 1660 ^ 100 m offsets indicate an
offset rate of 23 ^ 3 mm/y (Detourbet et al., 1993); the
slightly larger offsets reported by Sieh and Rais (1991)
indicate an offset rate of about 28 mm/y. Unlike the Toba
area, in the Sarulla area the stream channels are incised
into rocks older than the Young Toba Tuff and are not as
useful for determining recent fault-slip rates as the stream
offsets to the north. However, the similar magnitude of
offsets suggests late Quaternary displacement rates similar
to the north assuming that the stream valleys were incised
at the same time. This may have occurred because the
Young Toba Tuff disrupted the existing drainages and
produced a new cycle of valley incision even though the
initially thinner blanket of tuff is now largely eroded from
the Sarulla area.
The Aek Welirang fault, one of the faults that make up
the releasing bend at the southern end of the map area,
shows abundant topographic evidence of recent movement
(Fig. 5). Three stream valleys along this fault each show
offsets of about 300 m (Table 1).
Southwest of the town of Sipirok, the late Quaternary Tor
Sibohi rhyodacite dome is dextrally offset by the Tor Sibohi
fault (Fig. 5). The dome is composed of biotite-hornblende
rhyodacite lava with flow banding defined by alternating
pink and gray layering. The distinctive lithology of the
rhyodacite and its original limited areal extent make it an
ideal marker of fault displacement.
The lateral separation of the northern contacts of the
rhyodacite across the fault is about 2.1 km; the lateral
separation between its southern contacts is about 2.9 km.
The mean of these two measurements is 2.5 km. The
maximum possible offset of the eroded southern contact is
3.3 km. 40
Ar/39
Ar incremental heating analysis yielded
isochron ages of 0.27 ^ 0.03 Ma for biotite and 0.26 ^ 0.1
Table 1
Offsets along faults within the Sarulla area
Offset feature UTM coordinates Offset (m)
Tor Sibohi
Fault
Aek Pargarutan 508,500 m E; 204,
650 m N
140
Stream 2 509,250; 203,350 230
Stream 3 509,400; 202,900 620
Stream 4 509,800; 202,200 130
Aek Sibarabara 510,700; 201,150 390
Stream 5 511,800; 199,900 630
Stream 6 511,950; 199,400 390
Stream 7 512,600; 198,700 700
Sarulla river 513,000; 198,100 540
Near Aek Sah 519,300; 187,100 300
Aek Simarjambu 520,300; 185,100 600–1200
Aek Sihoruhoru 523,950; 180,600 500–1400
Aek Sibue 528,500; 176,250 1000
Tor Sibohi Dome 528,700; 175,900 2500
Aek Mandurana 530,900; 172,600 1000
Aek Horsik 531,200; 172,100 400
Aek Situmba 532,100; 171,150 1000
Aek Weliran
fault
Aek Mandurana 529,800; 173,100 300
Aek Horsik 530,350; 172,250 300
Aek Weliran 531,050; 171,150 300
Hutujulu fault Stream A 501,800; 213,600 325
Aek Sitandiang
Namenek fault
Aek Nabara 523,400; 172,200 400
Aek Sinanap north 525,100; 169,400 300
Aek Sinanap south 524,800; 169,250 800

for plagioclase from the rhyodacite (Gunderson et al.,
1995). Combining an age of 0.27 Ma with the 2.5 km offset
gives a dextral slip rate of about 9 mm/y. The possible age
and offset extremes yield a range of possible offset rates
between 7 and 14 mm/y. This range of rates reflects
uncertainties due to (1) the effects of erosion, (2)
inaccuracies in locating the poorly exposed contacts of the
smaller western portion of the faulted dome, and (3) the
error in the age date. The facies change from largely
andesitic lavas to largely tuffs around the flanks of the
Sibualbuali volcano shows a similar amount of offset
(Fig. 3), but because of the transitional nature of this facies
change, it is not a good marker.
Assuming the stream offsets in the Taratung area and the
Tor Sibohi rhyodacite dome are accurately dated, the
discrepancy between the slip rate of 23 ^ 3 mm/y deter-
mined from stream offsets in the Taratung region (Detourbet
et al., 1993) and the 9 mm/y slip rate determined from the
offset dome may be due to (1) a difference between
Holocene and late Quaternary rates of slip and (2) additional
slip on other faults in the Sarulla area. However, because the
magnitude of undated stream offsets along the TSF in the
Sarulla area is similar to those in the Taratung area, the rate
difference is thought largely to be due a change in slip rate
over time.
4.4. Sipirok graben
The Sipirok graben intersects the Tor Sibohi fault in
the southern part of the study area near the town of Sipirok
(Fig. 3). Here, reconnaissance mapping indicates that the
basin is bounded on the east by a major steeply west-dipping
normal fault that strikes about N 58 E and gives the basin a
half-graben geometry. Satellite imagery suggests that the
extensional basin continues a considerable distance to the
northeast beyond the extent of field mapping. The exposure
of uplifted, hydrothermally altered Pliocene sandstones on
the footwall block of the Sipirok graben fault, as well as
topographic relief, indicate that the fault system is young
and perhaps currently active, and developed at least in part
contemporaneously with the Sumatra fault system.
Study of borehole breakouts in the Central Sumatra basin
75 km to the southeast of the Sarulla area indicate that the
orientation of the maximum horizontal stress, SHmax; varies
between north–northwest to northeast and that stresses at
borehole depth are compatible with a strike-slip regime
ðSHmax . SVertical . SHminÞ (Heidrick et al., 2000). North-
west–southeast extension across the Sipirok graben is
compatible with a northwest–southeast orientation of SHmin:
4.5. Aek Sitandiang Namenek/Toru Nabara fault system
Additional faults of the Sumatra fault system lie to the
west of the Tor Sibohi fault. Two major faults, the Aek
Sitandiang Namenek (ASN) and Toru Nabara faults form a
zone that cuts the southwestern flank of Sibualbuali
volcano. Within the ASN fault array, faults with horizontal
striations offset young terrace deposits and other faults may
offset stream channels up to a few hundred meters (Table 1).
These two faults appear to merge immediately south of
Sibualbuali. A remote-sensing interpretation shows that the
resultant fault zone extends many kilometers to the south-
east, paralleling the Tor Sibohi fault zone (Fig. 10).
The northern extent of these western faults is less clear.
This partly reflects poor exposures and incomplete mapping,
but also may reflect discontinuous faults. The Toru Nabara
fault does not appear to extend north of Sibualbuali and may
merge with the ASN fault along the northwestern flank of
the volcano. The ASN fault definitely extends a few
kilometers north of Sibualbuali volcano, where it exposes
Paleozoic rocks in the Batang Toru gorge (Aspden et al.,
1982) (Fig. 3). Photo lineaments suggest that it may extend
further northward to join faults that bound the southwestern
flank of the Sarulla graben.
However, part of the slip of the ASN fault system appears
to be transferred to the Tor Sibohi fault via a small pull-apart
basin on the north flank of Sibualbuali and a second
extensional zone further to the north comprised of easterly-
striking faults (Fig. 6). These latter faults all show oblique,
probably left-lateral, normal displacements. It is proposed
that this zone transfers dextral slip from the ASN and
Bulumario faults to the Tor Sibohi fault. Left lateral slip on
Fig. 5. Map showing the offset of the Tor Sibohi rhyodacite by the Tor
Sibohi strand of the Sumatra fault system. See Fig. 3 for location.

these faults would produce clockwise rotation of the
structural blocks between these faults and the major dextral
strike-slip faults. Such deformation would also produce
rotation of the east–northeast striking faults that originally
may have had a southwesterly strike. This style of
deformation is well known from other zones of wrench
tectonics (e.g. Dibble, 1977; Nicholson et al., 1986). The
Sibualbuali volcano is situated within the overall step
between the ASN and Tor Sibohi faults, suggesting that the
local extension produced by the releasing step served to
localize the volcano. If correct, earlier extension may be
hidden by the volcanic ediﬁce.
Fig. 6. Sketch map showing the structural setting of the Hopong caldera, southern Sarulla graben, and Tor Sibohi and ASN faults. See Fig. 3 for location.

4.6. The Sarulla graben
The Plio-Pleistocene Sarulla graben lies west of the Tor
Sibohi fault and extends from the Limestone Mountain area
north to the Namora-I-Langit area (Fig. 3). The graben is
bounded on the northeast by the Tor Sibohi fault and partly
bounded on the southwest by the Rebean and parallel faults
(Figs. 3 and 6). The northern extent of the Rebean fault is
not known because of incomplete mapping, but the fault
may join the Aek Parihanan fault further to the northwest.
The Aek Parihannan fault has a nearly vertical dip in
outcrops, but no indicators of the sense of displacement on
the fault are known.
Gravity data, wells, and limited seismic data show that
the Quaternary-Pliocene Sarulla graben is substantially
larger than the Recent alluvial depocenter in the Donatasik
area. Gravity models across the length of the basin and
seismic data in the Silangkitang area show that the basin
generally has an asymmetric graben to half-graben profile
that deepens toward the Tor Sibohi fault (Figs. 7 and 8). The
basin fill is cut by faults paralleling the TSF that show
normal separation.
The overall Sarulla graben is not a typical pull-apart
basin since it (1) is bounded along one entire margin by a
major strike-slip fault rather than occupying a releasing
step between two strike-slip faults, (2) generally has a half-
graben profile, and (3) is characterized by normal faults
that parallel the major strike-slip fault rather than oblique-
striking normal faults that form the sidewalls of a pull-
apart basin (Fig. 3). This basin geometry more closely
resembles the asymmetrical basins produced by fault-
normal extension along some strike-slip zones. The
formation of these asymmetrical basins has been ascribed
to situations where strike-slip fault zones are weaker than
the adjacent crust and the angle between the far-field
maximum principal stress (horizontal) and the strike of the
fault is less than 458 (Ben-Avraham and Zoback, 1992).
Under these conditions, stresses are reoriented near the
fault so that maximum horizontal stress, SHmax; is more
nearly parallel to the strike of the fault and the minimum
horizontal stress, SHmin is nearly perpendicular to the strike
of the fault. This situation promotes extension perpendicu-
lar to the strike of the wrench fault.
On a sub-basin scale, releasing and constraining bends
along the TSF do influence the geometry of the Sarulla
graben. In particular, the gentle releasing bend in the
Donatasik area increased subsidence of the southern part of
the basin (Figs. 3 and 6). The constraining bend further
south results in the termination of the graben, the uplift of
Paleozoic strata at Limestone Mountain, and relatively high
elevations east of Limestone Mountain.
4.7. Hopong caldera
The Hopong caldera lies east of the southern Sarulla
graben east of the Tor Sibohi fault. Satellite imagery and
topography indicate that the caldera margin has a slightly
elliptical shape. The caldera is about 9.6 km across in a
northeast–southwest direction and about 8.2 km across in a
northwest–southeast direction (Fig. 9). Gravity data show
that the thickest part of the caldera fill is in the northeast.
The southern margin of the caldera is formed by multiple
inwardly dipping normal faults. The elliptical map pattern
suggests that similar faults probably bound the eastern and
northern parts. In contrast, the southwestern part is bounded
by faults of the SFS and north-striking, right-lateral faults.
The association of rhyolite domes with these latter faults
(Fig. 3) suggests that the faults were active during caldera
formation and may have played a role in its formation.
4.8. Hutajulu fault
In the northern part of the map area, a second active
strike-slip fault, the Hutajulu fault, parallels and lies about
800–1600 m southwest of the Tor Sibohi fault (Fig. 3). To
the north, the strike of this fault becomes more westerly, and
the fault forms the southwestern margin of the Taratung
graben (Bellier and Se´brier, 1994). The southern extent of
the Hutajulu fault is not clear, but scattered exposures and
seismic lines suggest that it joins the Tor Sibohi fault near
the village of Silangkitang (Fig. 3). Thus, a very narrow
finger of the Taratung graben extends into the study area.
Fig. 7. Cross section across the southern Sarulla graben and Hopong caldera based on surface geology and gravity data. See Fig. 3 for location of cross section.

This narrow graben resembles the in-line grabens developed
near the ends of analog models of pull-apart basins formed
above releasing steps (Dooley and McClay, 1997).
A tributary of the Batang Toru shows a dextral offset of
about 325 m where it crosses the Hutujulu fault (Table 1).
The fault also juxtaposes distinctive types of young volcanic
rocks. Basaltic andesite flows and breccias, informally
referred to as the Sitonde basaltic andesite, are exposed west
of the fault from the northern limit of this map (Fig. 3)
northward to the flanks of the Martimbang volcano. Similar
basaltic lava flows, breccias and lahars on the east side of
the fault along the Batang Toru extend about 4.5 km south
of the southern contact of the western lavas, where they are
juxtaposed against dacitic lavas and tuffs on the west. This
pattern may reflect dextral offset of the lavas, but may also
result from flow of lavas and lahars southward along an
ancestral Batang Toru. The age of the lavas is not known.
They are overlain by the 73,000-year-old Young Toba Tuff
and overlie older tuffs, and are inferred to be of late
Quaternary age (Fig. 4).
5. Tectonic model for the Sarulla region
The Sumatra fault zone in northern Sumatra is charac-
terized by multiple fault strands that created a series of
elongate basins along the zone in the late Neogene (Fig. 10).
South of the Sarulla area, the elongate Purwodadi graben is
formed by an overstepping, releasing step between the Aek
Sitandiang Namenek/Toru Nabara fault zone and an
unnamed fault to the southwest (Fig. 10). The currently
active Tor Sibohi fault and Aek Sitandiang Namenek/Toru
Nabara fault zone define a present-day valley that parallels
the Purwodadi graben and may also be underlain by late
Neogene sediments. The northern end of this basin is a
complex releasing step that transfers displacement from
the ASN fault to the Tor Sibohi strand of the Sumatra fault
through a series of normal and sinistral oblique-slip faults in
the area north of Sibualbuali volcano. Thus, the Purwodadi
graben and the area between the Tor Sibohi and Aek
Sitandiang Namenek/Toru Nabara fault zone are a series of
pull-apart basins.
In contrast, the Sarulla graben is not a simple pull-apart
basin. The Tor Sibohi fault bounds the entire northeastern
side of the Sarulla graben and the northern part of the
‘graben’ has a half-graben profile and is internally cut by
normal-dextral slip faults that parallel the nearly linear trace
of the Tor Sibohi fault. Similar linear basins are described
along the Sumatra fault zone in central and southern
Sumatra (McCarthy and Elders, 1997). The Sarulla graben
appears to have been formed by extension nearly perpen-
dicular to the TSF. Within the Sarulla graben this overall
pattern of extension is locally modified by sub-basin scale
releasing and restraining bends along the TSF.
Further to the north, the Taratung graben is typical of a
pull-apart basin formed between two understepping strike-
slip faults (Bellier and Se´brier, 1994; Dooley and McClay,
1997). At the northern end of the Sarulla graben, the
Hutajulu fault branches off from the Tor Sibohi fault and
parallels the latter fault for several kilometers, forming a
narrow in-line graben before the two faults diverge at the
southern end of the main Taratung graben (Figs. 3 and 10).
The map pattern implies that at the latitude of Taratung
City, the bulk of strike-slip displacement occurs on
Fig. 9. Sketch structural map of Hopong caldera area. Contours are residual
Bouguer values in milligals.
Fig. 8. Gravity models across the northern Sarulla graben. Numbers refer to
densities used in the models. See Fig. 3 for location of transects.

the Hutujulu fault, and that the nearly north-striking
continuation of the Tor Sibohi fault has largely normal
displacement.
6. Relationship between the Sumatra fault zone
and volcanic centers
On a regional scale, the position of the Sumatra fault
zone and the volcanic arc are similar, although it has been
pointed out that the two features are not coincident, but
rather intertwine (Sieh and Natawidjaja, 2000). Because of
the similar orientation of the two features, it is possible that
the location and geometry of the Sumatra fault system are
controlled by the position of the volcanic arc (Hamilton,
1979; Bellier and Se´brier, 1994). The reason for this
presumably would be that higher heat ﬂow and local magma
accumulations along the arc produce a linear zone that is
weaker than the surrounding crust. Within the Sarulla area,
volcanic centers lie along or within a few kilometers of the
Sumatra fault system, and on a local scale, the fault system
may control the position of some of these volcanic features.
Sibualbuali volcano appears to have developed in a
releasing step between the Tor Sibohi and ANS faults. The
igneous center is probably localized because of the
extension produced by the releasing step. Lubukraya
volcano lies near the northern termination of the fault
that bounds the southwestern side of the Purwodad graben
(Fig. 10). Stress concentrations around the terminations of
faults are known to produce increased fracture permeability
(Curewitz and Karson, 1997), which may be responsible for
localization of the volcano.
At the Hopong caldera, minor right-lateral strike-slip
faults related to the Sumatra fault system appear to have
played a role in the collapse of the western margin of the
caldera, but it is not clear that the Sumatra fault played any
role in localizing the volcanic center. The Namora-I-Langit
volcanic center is bounded by the Hutajulu fault and is cut
by smaller, parallel faults. The volcanic center lies near, but
not at the inferred intersection of the Hutajulu and Tor
Sibohi faults. The Martimbang volcano, about 3 km north of
Namora-I-Langit, appears to lie on the projection of one of
these northwest-striking minor faults.
Thus, while there is not a one-to-one relationship in the
Sarulla area between volcanic features and faults, there is a
strong suggestion that volcanic features are localized at
steps between faults, fault intersections and near the tips of
faults. This supports previous studies that related stepovers
along the Sumatra fault system to volcanic centers (Bellier
and Se´brier, 1994).
7. Geothermal systems
Geothermal exploration in the Sarulla area was instigated
by the presence of numerous high-temperature surface
Fig. 10. Regional tectonic map showing the relationship of the Taratung,
Sarulla, and Purwodadi grabens.

features localized along structural features and near
volcanic centers. Exploration drilling in the Sarulla area
has resulted in the discovery and appraisal of three
geothermal systems and the recognition of a fourth,
unappraised system.
7.1. Sibualbuali geothermal system
At Sibualbuali volcano 19 areas of fumaroles, mud
pots, and other acid-sulfate thermal features are
distributed over an area of about 45 km2
, mainly along
faults of the Sumatra fault system. A regional gravity
survey found a large area of low gravity surrounding
the volcano, suggesting an underlying thick sediment or
tuff-filled basin. Drilling has shown this fill to be a
sequence of silicic tuffs more than 1 km thick. Resistivity
surveys found a central zone of high resistivity beneath
the central core of the volcano ringed by local, distinct
areas of low resistivity. These areas of low resistivity are
closely linked in most cases with acid-sulfate thermal
features and their associated alteration. They also correlate
with the faults of the Tor Sibohi, and ANS faults and
north-striking normal faults on the northwestern flank of
Sibualbuali (Gunderson et al., 2000).
Four wells (with measured depths of 1266–2439 m)
were drilled on the eastern flank of Sibualbuali. Three of the
wells were directionally drilled through faults of the Tor
Sibohi fault system into the thick sequence of rhyolitic
tuffs underlying the predominately andesitic rocks of
the Sibualbuali volcano. One well encountered a sub-
volcanic granitic intrusive. The wells were all productive,
finding a geothermal system whose temperature and
permeability structure is strongly controlled by the fault
system. Production temperatures for the wells are in the
range of 218–248 8C. Mineralogical and fluid inclusion
evidence for an earlier, hotter, and shallower phase of
hydrothermal activity was found above and within the
reservoir. The current system has strong vertical and lateral
temperature gradients, which are attributed to the channel-
ing of fluids along fault strands. Volumetric and reservoir
modeling evaluation of the drilled portion of the Eastern
Sibualbuali geothermal system suggests reserves of suffi-
cient energy to generate 40 MW of electricity for 30 y. It is
expected that further drilling on the northern, western and
southern flanks of Sibualbuali will lead to discovery of
significantly more reserves (Gunderson et al., 2000).
7.2. Donatasik geothermal system
In the Donatasik area, boiling chloride springs occur
along the SFS and Rebean fault on the east and west flanks
of the southern Sarulla graben (Fig. 6). Gas seeps and
fumaroles occur east of the valley and within the Hopong
caldera. The spring waters are generally similar to spring
waters in the Silangkitang area, but have higher magnesium
content and have equilibrated at lower temperatures. Based
on cation geothermometry, most of the Donatasik waters
equilibrated at 200–230 8C, but have since partially re-
equilibrated at lower temperatures (Gunderson et al., 1995).
7.3. Silangkitang geothermal system
A series of hot springs and fumaroles is located along the
margin of the Sarulla graben, near the village of Silangki-
tang in the central part of the Sarulla contract area. The
thermal features are concentrated in a 1 £ 3 km2
strip on and
west of the Tor Sibohi fault and about 1 km north of a
rhyolite dome with a K–Ar age of 0.12 ^ 0.08 Ma
(Gunderson et al., 1995). Seismic lines and gravity data
indicate that the thermal area lies above a local sub-graben
formed between the Tor Sibohi fault and the intersecting
Hutujulu fault.
Fault intersections are recognized areas of higher fracture
permeability (Curewitz and Karson, 1997), and it is likely
that the intersection of the Hutujulu fault with the Tor
Sibohi fault increases permeability in this region. Addition-
ally, it likely that the active Tor Sibohi fault is critically
stressed, and as a consequence, the fault zone has increased
permeability (Townend and Zoback, 2000).
Five wells (2031–2330 m) drilled at Silangkitang
encountered a geothermal system whose permeability is
strongly controlled by the Tor Sibohi fault. The wells all
drilled through a thin section of sediments beneath the
Sarulla graben valley floor, followed by more than 1500 m
of silicic tuffs from which the wells produce hot brines.
Two of the wells, located approximately 700 m and 1 km
from the Tor Sibohi fault, were drilled vertically. Per-
meability in each of these wells is relatively low, and the
dominant fracture type consists of microfaults with oblique
to horizontal slip, as indicated by slickensides. One of these
wells drilled the entire tuff sequence and penetrated
underlying conglomerates and sedimentary breccias con-
taining volcanic clasts. These strata resemble the late
Pliocene (?) strata exposed around the margins of the basin.
The conglomerates and breccias had low permeability and
were non-productive.
Three wells were deviated to the northeast toward the
main fault zone. One of these wells crossed the Tor Sibohi
fault into Paleozoic argillites, quartzites, and marbles. These
rocks contained thin, tight veinlets filled with quartz and
pyrite, and have very low matrix permeabilities. Two of
the Silangkitang wells that were targeted directionally into
the Sumatra fault zone found a very strong upflow in the
vicinity of the fault that is significantly overpressured with
respect to a normal hydrostatic gradient. Core recovered
from one of these wells was highly fractured and brecciated.
These fractures have been interpreted in terms of conjugate
Riedel shears and tension fractures associated with the
Sumatra fault zone (Moore et al., 2001). Fractures were
enlarged by dissolution. In this upflow zone, fluid
temperatures exceed 310 8C at a depth of around 2 km.
Following extensive testing of the wells, volumetric

evaluation and reservoir modeling of the geothermal system
have confirmed reserves that could generate 80 MW for
30 y at Silangkitang.
7.4. Namora-I-Langit geothermal system
The Namora-I-Langit volcanic complex consists of two
broad coalescent volcanoes made up of andesitic to rhyolitic
lavas and tuffs dated at 0.75–0.16 Ma. Associated with this
complex is an extensive array of surface thermal features
comprised primarily of fumaroles and acid sulfate springs,
but also including neutral chloride–sulfate–bicarbonate hot
springs, gas seeps, and numerous warm bicarbonate springs
covering an area of about 30 km2
.
The geothermal features largely lie west of the Hutajulu
fault, which may form the eastern boundary of the
geothermal system. Several smaller faults lie west of and
are aligned parallel to the Hutujulu fault. These faults are
discontinuous, and it is not clear whether they are minor
faults or more regional faults with only small displacements
in the young rocks exposed at the surface.
Four wells (1333–1722 m) have been drilled at Namora-
I-Langit. All of these wells drilled through the lavas into a
thick tuff section similar to that encountered at Silangkitang.
These wells found a large geothermal system whose
temperature and permeability distribution appear not to be
strongly controlled by faults. Instead, fracture permeability
is widely distributed, and vertical and lateral temperature
gradients within the reservoir are very low. The wells all
found high permeability and produced brines with tempera-
tures in excess of 260 8C. Based on the results of the wells
and their extensive flow testing, geothermal reserves
sufficient for generation of 210 MW have been reported to
Pertamina. Additional drilling throughout the remainder of
the geophysical target has the potential of increasing this
capacity significantly.
8. Conclusions
The Sarulla graben is a composite Plio-Pleistocene basin
developed along the currently active Tor Sibohi strand of
the Sumatra fault system. The geometry of the graben is
more complex than a simple pull-apart basin, but is clearly
controlled by overall dextral strike-slip deformation. The
Sumatra fault system in this area is up to 10 km wide and
consists of both active and inactive faults. For the last
0.27 Ma, slip on the Tor Sibohi fault has averaged about
9 mm/y.
Volcanic centers lie along the fault system, and several
appear to have been localized at fault steps, fault
intersections, and near fault tips. Significant geothermal
resources are developed in thick tuffs that fill the Sarulla
graben and underlie Sibualbuali volcano. At the Silangki-
tang, Donatasik, and Sibualbuali geothermal fields, fractur-
ing and faulting within the Tor Sibohi fault zone control
reservoir permeability. The Namora-I-Langit geothermal
field lies adjacent to the active Hutajulu fault, but
fracturing extends several kilometers from the fault and
may not be directly related to the faulting. Geothermal
activity in the four identified fields appears to be controlled
by the presence of volcanism and tectonism, resulting in
the development of high heat flow and enhanced
permeability.
Acknowledgements
We would like to thank the managements of Unocal and
Pertamina for permission to publish this paper, and the
people of North Sumatra for their hospitality and assistance
during our field surveys. We would also like to acknowledge
the contributions of our colleagues at Unocal, Unocal
Geothermal Indonesia and the assistance of Pertamina
geoscientists in this project. Warren Sharp carried out the
40
Ar/39
Ar analyses and offered suggestions to improve the
manuscript. We thank Ardyth Simmons and Dan Hawkes
for their careful reviews of the manuscript. Chris Elders and
Andrew Mitchell’s constructive reviews contributed greatly
to preparation of the final version of this paper.
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