2004-tectonic and stratigraphy evolution of the sarulla graben geothermal area, north...

7/26/2019 2004-Tectonic and Stratigraphy Evolution of the Sarulla Graben Geothermal Area, North Sumatra_Hickman Et Al

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Tectonic and stratigraphic evolution of the Sarulla graben geothermalarea, North Sumatra, Indonesia

R.G. Hickmana,*, P.F. Dobsonb, M. van Gervenc, B.D. Sagalad, R.P. Gundersone

aStructural Solutions, 1330 Sugar Creek Blvd., Sugar Land, TX 77478, USAbEarth Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA

c

Autodesk Inc., GIS Solutions Division, San Rafael, CA, USAdUnocal Geothermal Indonesia, Sentral Senayan-I Office Tower, Jln. Asia Afrika No. 8, Jakarta 10012, IndonesiaeUnocal 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 (,0.70.3 Ma), the Hopong caldera (,1.5 Ma), and

the Namora-I-Langit dacitic dome field (0.80.1 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) 435448www.elsevier.com/locate/jseaes

* Corresponding author. Tel.:1-218-240-1057; fax:1-281-240-8457.

E-mail address:[email protected] (R.G. Hickman).
http://www.elsevier.com/locate/jseaeshttp://www.elsevier.com/locate/jseaes


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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 comprisethe 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 0030258) 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 1008and plate convergence is nearly orthogonal to the

trench (Bellier and Sebrier, 1994). In contrast, the Sunda

trench west of Sumatra has an average trend of about 140 8

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 Straitextensional 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 Sebrier, 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 thelocation 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 afterMitchell (1993).

R.G. Hickman et al. / Journal of Asian Earth Sciences 23 (2004) 435448436


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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 KuantanFormation 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 offluvial 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 undatedpebble 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) 435448 437


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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 possiblycorrelative 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 aseries of andesitic to dacitic lavas and breccias with some

Fig. 4. Diagram showing stratigraphic relationships across the Sarulla area.



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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 (0.120.30 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 40Ar/39Ar 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 youngesteruptive 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 KAr 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 isbelieved 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 40Ar/39Ar

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. 40Ar/39Ar incremental

heating of a plagioclase separate from the largest of thesedomes 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. Thecenter 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 andinactive 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. Inthe 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 andpyrite 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 systemTor 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 sideby steep slopes. Much of the valley is intensively cultivated,



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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 358W, but the

strike of individual segments of the fault ranges from about

N 558W to N 208W. Given the dextral strike-slip motion onthe 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 steeplydipping, 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 1301400 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 17002100 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 asuseful 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. 40Ar/39Ar incremental heating analysis yieldedisochron 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 r iver 513,000; 198,100 540

Near Aek Sah 519,300; 187,100 300

Aek Simarjambu 520,300; 185,100 6001200

Aek Sihoruhoru 523,950; 180,600 5001400

Aek Sibue 528,500; 176,250 1000Tor 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



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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 rateover 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 58E 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 basin75 km to the southeast of the Sarulla area indicate that the

orientation of the maximum horizontal stress, SHmax;varies

between northnorthwest to northeast and that stresses at

borehole depth are compatible with a strike-slip regime

SHmax .SVertical .SHmin (Heidrick et al., 2000). North-

west southeast extension across the Sipirok graben is

compatible with a northwestsoutheast orientation ofSHmin:

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 azone 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 ofthe 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 southwesternflank 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 andBulumario 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. SeeFig. 3for location.



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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 ofdeformation 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 behidden 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. 3for location.



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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 inoutcrops, 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 Donatasikarea. 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 slightlyelliptical shape. The caldera is about 9.6 km across in a

northeastsouthwest direction and about 8.2 km across in a

northwestsoutheast 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

8001600 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 Sebrier, 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. SeeFig. 3for location of cross section.



10/14

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 faultthrough 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 grabenappears 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 Sebrier, 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 TaratungCity, 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. SeeFig. 3for location of transects.



11/14

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 Sebrier, 1994). The reason for thispresumably 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 offaults 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 ofthese 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 Sebrier, 1994).

7. Geothermal systems

Geothermal exploration in the Sarulla area was instigatedby the presence of numerous high-temperature surface

Fig. 10. Regional tectonic map showing the relationship of the Taratung,

Sarulla, and Purwodadi grabens.



12/14

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. Resistivitysurveys 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 ofthe 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 magnesiumcontent and have equilibrated at lower temperatures. Based

on cation geothermometry, most of the Donatasik waters

equilibrated at 200230 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 KAr 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 Sibohifault 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 3108C at a depth of around 2 km.Following extensive testing of the wells, volumetric



13/14

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 0.750.16 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 chloridesulfatebicarbonate 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 thegeothermal 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 (13331722 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 allfound 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 ofthe 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 acknowledgethe contributions of our colleagues at Unocal, Unocal

Geothermal Indonesia and the assistance of Pertamina

geoscientists in this project. Warren Sharp carried out the40Ar/39Ar 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 Mitchells constructive reviews contributed greatly

to preparation of the final version of this paper.

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