Fatma petroleum: TIMOR LESTE TEKTONIK

The Tectonic Evolution of East Timor and the Banda Arc

Gillian Hamson, #74189

Honours Literature Review submitted as part of the B.Sc.(Hons) degree in the School of Earth Sciences, University of Melbourne.

Submitted April 30th, 2004

I certify that this Literature Review contains less than 4,000 words.

Gillian Hamson

Table of Contents

1 Introduction...............................................................................................................1

2 Overview of regional geology...................................................................................3

3 Tectonic activity in the Banda Arc..........................................................................8

3.1 The Timor thrust: deformation in the Timor Trough..........................................8

3.2 The Wetar thrust: deformation in the back-arc thrust zone................................9

3.3 Earthquakes.........................................................................................................9

3.4 Volcanism.........................................................................................................11

4 Models of tectonic evolution..................................................................................12

4.1 Overthrust model..............................................................................................12

4.2 Imbricate model................................................................................................13

4.3 Autochthon model.............................................................................................14

5 Response to tectonism: uplift and exhumation of Timor....................................16

5.1 Thermochronological data................................................................................16

5.2 Raised reef terraces...........................................................................................17

5.3 Uplift mechanisms............................................................................................19

6 Discussion.................................................................................................................20

7 Thesis Plan...............................................................................................................22

8 References................................................................................................................23

Table of Figures

Figure 1.1 Map of the Banda Arc (adapted from Hinschberger et al., 2001)....................1

Figure 2.1 Simplified geology of East Timor (adapted from Charlton, 2002a).................4

Figure 2.2 Initial continental collision with subduction zone (from Charlton, 2000).......5

Figure 2.3 Pliocene collision at the proto-Timor region (from Charlton, 2000)...............6

Figure 2.4 Position of oceanic and continental crust (from Keep et al., 2003).................7

Figure 2.5 Schematic cross section of the Banda Arc at Timor.........................................7

Figure 3.1 Earthquake epicenters in the Banda Sea region (from Milsom, 2001)..........10

Figure 3.2 Regions of active volcanism in the inner Banda Arc (from Harris, 1991).....11

Figure 4.1 Overthrust model (from Richardson and Blundell, 1996)..............................12

Figure 4.2 Imbricate model (from Richardson and Blundell, 1996)................................13

Figure 4.3 Autochthon model (from Richardson and Blundell, 1996)............................14

Figure 5.1 Locality map of East Timor............................................................................18

Figure 5.2 Digital elevation model of East Timor (from SRTM data set, USGS)……... 18

The Tectonic Evolution of East Timor and the Banda Arc Gill Hamson #74189

1 Introduction

East Timor lies at the point at which the leading edge of the Australian continental margin impinged upon the Eurasian plate, giving a rare insight into the early stages of a major orogenic event. Subduction of buoyant Australian continental lithosphere effectively jammed northward subduction beneath the oceanic outer Banda Arc during the Neogene, resulting in arc-continent collision. Timor, the emergent core of the resulting Banda Orogen, comprises accretionary material of both Australian and Eurasian provenance (Bowin et al., 1980; Breen et al., 1989; Keep et al., 2003). The modern Banda Arc is today bound by an inner volcanic arc and an accretionary outer arc of which Timor is the largest island (Fig. 1.1).

Figure 1.1 The Banda Arc lies at the intersection of the Pacific, Eurasian and Indo-Australian plates. This

map shows the position of the inner and outer Banda Arcs, the plates and their direction of movement relative to the Eurasian plate (adapted from Hinschberger et al., 2001).

The Tectonic Evolution of East Timor and the Banda Arc Gill Hamson #74189

East Timor, on the southern arm of the outer Banda Arc, is located approximately 640 km northwest of Australia. It consists of the eastern part of Timor island, together with the small enclave of Oecussi on the north coast of Western Timor, and the islands of Atauro and Jaco (see Fig. 5.1, p.18). East and West Timor are divided along a 100 km north-south political boundary located roughly in the centre of the island, and East Timor extends 250 km to the east.

Geological knowledge of East Timor was acquired over three periods:

• Pre-1975: before Indonesian occupation, foreign access possible: reconnaissance mapping (e.g. Audley-Charles, 1968).

• 1975-1999: Indonesian occupation: political turmoil, limited foreign access: studies included seismic surveys and mapping of nearby islands (e.g. Hughes et al., 1996; Richardson & Blundell, 1996; Snyder & Barber, 1997)

• Post-1999: Independent East Timor; foreign access again possible: studies include computer modelling, thermochronology, stratigraphic and structural mapping (e.g. Charlton, 2002a, b; Harris et al., 2000)

An evaluation of the present-day tectonic activity in the Banda Arc region will be followed by a review of the three dominant tectonic evolution models for East Timor. Finally, the continuing uplift of East Timor in response to collision will be discussed. Landscape evolution as a consequence of this geodynamic change can be reconstructed from the young, variably uplifted rocks in East Timor. My study will involve constructing a tectonic geomorphological landscape history of East Timor based on the most recent phase of uplift since arc-continent collision was initiated in the Neogene.

The Tectonic Evolution of East Timor and the Banda Arc Gill Hamson #74189

2 Overview of regional geology

The geology of East Timor has long been recognised as highly complex, and many theories have been proposed for the island’s tectonic evolution (Audley-Charles, 1968; Chamalaun & Grady, 1978; Barber, 1979; Hamilton, 1979; Harris, 1991; Charlton, 2000). Despite the conflict of ideas, some broad geological observations allow a generalised model of the tectonic development of this region since the Neogene.

Gravity surveys have confirmed that the Australian continental crust extends as far as the north coast of Timor (Chamalaun et al., 1976; Hamilton, 1977). Overlying this gently deformed Australian basement are rocks derived from the distal Australian passive margin (para-autochthonous units), formed in response to the Middle-Late Jurassic breakup of eastern Gondwana and subsequent sea-floor spreading. Passive margin conditions prevailed until Neogene arc-continent collision, when rocks derived from the pre-collisional Banda forearc (allochthonous units) were incorporated into the collision complex. The rocks exposed in Timor (Fig. 2.1) include:

• Early-Permian to Early-Pliocene variably deformed and metamorphosed deep water sediments of the Australian passive margin (Gondwana and Kolbano Sequences)

• Late-Miocene-Early Pliocene Bobonaro Scaly Clay, an olistostrome thought to be emplaced as a gravity slide in response to the southward tilting of Timor during subduction (Johnston & Bowin, 1981).

• Banda Allochthon: pre-Cretaceous metamorphic rocks overlain by sedimentary deposits and ophiolites of upper Jurassic to Lower Pliocene age, all of which are derived from the pre-collisional Banda fore-arc

• Post-orogenic Upper-Miocene to Recent coral reefs, alluvial terraces and turbidites, unconformably overlying all other lithotectonic units.

The Tectonic Evolution of East Timor and the Banda Arc Gill Hamson #74189

Figure 2.1 Simplified map of geological units in East Timor (adapted from Charlton, 2002a).

The Tectonic Evolution of East Timor and the Banda Arc Gill Hamson #74189

Australia’s northern convergent margin encompasses New Guinea, Irian, Papua and Timor. This collision has been markedly diachronous, with collision beginning in the Oligocene at New Guinea (Fig. 2.2) (Charlton, 2000; Keep & Moss, 2000; Hall, 2002).

Figure 2.2 Initial collision of the Australian continental margin with the Eurasian-Pacific subduction zone (from Charlton, 2000).

The time at which subduction of continental lithosphere first began at proto-Timor (Fig.2.3) has been an issue of much controversy. Three different scenarios have been proposed:

• Mid-Pliocene (Carter et al., 1976; Hamilton, 1979; Bowen et al., 1980; Johnston & Bowin, 1981; Karig et al.,1987; Hall, 1996; Villeneuve et al., 1999).

• Late Miocene (Berry & Grady, 1981; Berry & McDougall, 1986; Charlton, 2000; Keep et al., 2003). This date coincides with the proposed incorporation of a microcontinent into the collision complex, inferred to have been the cause of the metamorphism in the Aileu Formation (Berry & McDougall, 1986).

The Tectonic Evolution of East Timor and the Banda Arc Gill Hamson #74189

• Early-Mid Miocene (Rutherford et al., 2001; M. Harrowfield, pers. comm., 2004) Rutherford et al. based their early age of collision on the related tectonic escape of Sumba into the forearc at this time (~16 Ma).

Diachronous collision of an oblique promontory on the Australian continental margin has been suggested, implying that timing of collision across Timor might have varied by as much as 5 Myr (Snyder et al., 1996a; Keep et al., 2003). This is further elaborated upon by Charlton (2002a, b), who claimed that earlier collision in East Timor relative to West Timor prompted greater uplift and denudation in the eastern half of the island.

Figure 2.3 Initial collision of the Australian continental margin with the subduction zone at the proto-Timor region (from Charlton, 2000).

Since 3 Ma, Timor progressively emerged from north to south, with southern Timor becoming fully emergent in the late Pleistocene (Veevers, 2000; Johnston & Bowin, 1981). Oceanic lithosphere to the west of Timor continues to subduct northward beneath the Eurasian plate (Fig. 2.4) (Audley-Charles, 1975; Chamalaun & Grady, 1978; McCaffrey & Nabelek, 1986; Lorenzo et al., 1998; Charlton, 2000). The island of Sumba, to the west of Timor is composed entirely of non-Australian-affinity rocks and marks the transition from subduction to oceanic lithosphere (Charlton, 2000).

The Tectonic Evolution of East Timor and the Banda Arc Gill Hamson #74189

Figure 2.4 Position of oceanic crust (blue) and continental crust (green) south of the subduction zone. Subduction no longer occurs south of Timor but continues at either end of the Timor Trough (from Keep et al., 2003).

It is clear that subduction and accretion are the driving mechanisms for tectonic activity in the Timor region. The regional structure is dominated by a divergent thrust style (Fig. 2.5). The surface expression of the south-directed Timor thrust is coincident with a bathymetric trough south of Timor (Johnston and Bowin, 1981), whereas the Wetar thrust is north-directed and outcrops north of the inner Banda Arc north of Timor (Richardson & Blundell, 1996; Harris et al., 2000). A dearth of deep earthquakes beneath East Timor suggests the presence of a seismic gap in the region, which coincides with the inactive segment of the inner Banda Arc. Elevated coral reefs of Quaternary age illustrate that uplift continues today, possibly as a response to thrust formation or isostatic rebound (Chappell and Veeh, 1978).

Figure 2.5 Schematic cross section of the Banda Arc at Timor.

The Tectonic Evolution of East Timor and the Banda Arc Gill Hamson #74189

3 Tectonic activity in the Banda Arc

3.1 The Timor thrust: deformation in the Timor Trough

Accretion of collision material to the Australian continent is thought by some to have resulted in formation of the Timor thrust, with its surface expression located south of Timor in the Timor Trough (Hamilton, 1979; von der Bosch, 1979; Karig et al., 1987; Masson et al., 1991). Seismic records show that the thrust dips northwards and stepped south during collision, suggesting that it may have been related to the former interface between the two plates ( Fig. 2.5) (Richardson & Blundell, 1996). However, this is inconsistent with evidence that Australian crust underlies the Timor Trough and Timor itself. The Timor thrust may instead represent a splay emanating from the main subduction-related thrust structure (M. Sandiford, pers. comm., 2004).

While some authors claim that plate convergence is accommodated on the Timor thrust, recent work using Global Positioning System geodetic measurements established that Timor and the inner Banda Arc are moving northward at the same rate as the Australian continent (Genrich et al., 1996). Thus, convergence may be transferred from the Timor thrust to the back arc region by cross-arc faulting zones (Masson et al., 1991), which account for the paucity of large thrust earthquakes concentrated at the Timor thrust (Johnston & Bowin, 1981; McCaffrey, 1988; McCaffrey, 1996). It is thus probable that movement on the Timor thrust ceased or at least slowed in the recent past. As such, the Timor Trough, “despite its developmental link with a subduction trench, may now be regarded as an intracontinental feature” (Johnston & Bowin, 1981).

The Tectonic Evolution of East Timor and the Banda Arc Gill Hamson #74189

3.2 The Wetar thrust: deformation in the back-arc thrust zone

North of Timor, the Wetar back-arc thrust zone evolved in order to accommodate convergence between Australia and the Banda Sea, through a combination of back-arc thrust faulting and strike-slip cross-arc faulting (Breen et al., 1989; McCaffrey & Abers, 1991; Snyder et al., 1996b; Snyder & Barber, 1997; Lorenzo et al., 1998; Veevers, 2000; Rutherford et al., 2001). Lateral shortening and crustal thickening have taken place along thrust structures dipping antithetic to subduction (Richardson & Blundell, 1996). Such north-directed thrusting is opposite to that occurring in the Java Trench and previously in the Timor Trough (Hamilton, 1979; McCaffrey, 1988; Rutherford et al., 2001). The small degree of shortening along the Wetar thrust suggests that it is a young feature, formed possibly as recently as 0.15 Ma (McCaffrey, 1996). The transference of convergence from the Timor thrust to the Wetar thrust is thought by some to represent the early stages of subduction polarity reversal (Silver et al., 1983; McCaffrey & Nabelek, 1986; McCaffrey, 1988; Breen et al., 1989; Snyder & Barber, 1997; Harris et al., 1998; Rutherford et al., 2001).

The degree of convergence located in the Wetar thrust zone is a matter of dispute. Masson et al. (1991) mapped shallow subsurface tectonic activity around the island of Timor, and found little evidence for major thrusting north of the Timor Trough. M. Norvick (pers. comm., 2004) believes that significant shortening is accommodated northward of the Wetar thrust, in the South Banda Sea. These contrary views suggest that more work is required to constrain the current convergent activity at the Wetar thrust and its effects.

3.3 Earthquakes

Shallow thrust and strike-slip earthquakes accommodate some of the collision-induced convergence, which is distributed throughout the forearc, island arc and backarc basin (McCaffrey, 1988). Convergence is transferred across the arc via zones of strike-slip faults such as the left-lateral transcurrent Wetar Fault. This particular fault is thought to offset the island arc north of Timor by ~50km (Masson et al., 1991). While earthquakes

The Tectonic Evolution of East Timor and the Banda Arc Gill Hamson #74189

show evidence of north-south convergence, normal faulting mechanisms also reveal that slow east-west extension is occurring in the forearc region (McCaffrey, 1988). This is supported by evidence of Plio-Pliestocene normal faulting in Timor (Audley-Charles, 1968).

Changes in seismicity are evident along the Banda Arc. Shallow earthquakes have occurred throughout the region, but deeper seismic activity is concentrated to the east and west of East Timor (Chamalaun & Grady, 1978). Shallow earthquakes in the vicinity of East Timor cannot corroborate the reversal of subduction polarity, despite evidence of northward thrusting in an opposite sense to that of the Timor thrust (McCaffrey, 1988).

Further studies of earthquakes at all depths in the region indicate that a notable seismic gap exists beneath East Timor (Fig. 3.1) (Milsom, 2001). The seismic gap may be a result of the detachment of the oceanic lithosphere at the former subduction zone beneath East Timor (Milsom, 2001). This zone coincides with the inactive section of the volcanic arc, as discussed below.

Figure 3.1 Earthquake epicenters in the Banda Sea region at depths of less than 100km and 100-125km (from Milsom, 2001).

The Tectonic Evolution of East Timor and the Banda Arc Gill Hamson #74189

3.4 Volcanism

The Alor/Wetar area of the inner arc directly north of East Timor has been volcanically inactive during the last 3 Myr (Abbott & Chamalaun, 1978). The severing of the connection between the magma chamber and the surface during the detachment of the subducting oceanic lithosphere in this region is a likely explanation for the absence of volcanism (Chamalaun & Grady, 1978; Johnston & Bowin, 1981). M. Norvick (pers. comm., 2004) suggested that the disparity in active volcanism along the inner arc is possibly due to a ‘locking’ of the collision zone at East Timor, a result of its proximity to the rigid Sahul Platform to the south (Fig.3.2). Locking of the collision zone has caused subduction to step northward into the Banda Sea, indicated by isolated active volcanism at Gunung Api, well north of Wetar (Fig. 3.2). According to Johnston and Bowin (1981), the Banda Sea thus remains an active subduction zone despite the lack of volcanism in the inner Banda Arc north of Timor,

Figure 3.2 Regions of active volcanism (shaded) in the inner Banda Arc (from Harris, 1991).

The Tectonic Evolution of East Timor and the Banda Arc Gill Hamson #74189

4 Models of tectonic evolution

Timor has been the focus of numerous geological studies since the early 20th Century. The seminal work on Timor is that of Audley-Charles (1968) who, despite describing the close geological ties between Timor and Australia, maintained a completely allochthonous origin for the material emplaced within the thrust sheets. Hamilton (1979) labeled the collision complex ‘tectonic chaos’ due to the complexity of the rock assemblage. Many workers have attempted to unravel the complex geology of Timor, and a variety of models concerning the island’s tectonic evolution have been proposed.

4.1 Overthrust model

North South

Figure 4.1 Schematic cross-section of the overthrust model for Timor. Allochthonous units are overthrust onto the folded Australian continental margin (from Richardson and Blundell, 1996).

The overthrust model was developed from early work on the surface geology where overthrust sheets of allochthonous material are well exposed. Its proponents suggested an almost completely allochthonous origin for the thrust sheets on Timor (Audley-Charles, 1968; Audley-Charles & Carter, 1972; Carter et al., 1976; Barber et al., 1977). They argued that allochthonous strata derived from the Eurasian plate to the north was thrust onto the Australian crust during the collision process. Large-scale folding and erosion of Australian continental margin sediments occurred before emplacement of the thrust sheets, which were not affected by folding.

The Tectonic Evolution of East Timor and the Banda Arc Gill Hamson #74189

The traditional justification for the overthrust model was the close juxtaposition of rocks of very different types and origins, often of the same age (Bowin et al., 1980). These rocks must have been widely separated at the time of deposition, then juxtaposed by compression and overthrusting at the time of collision (Barber et al., 1977). However, Grady and Berry (1977) amongst others, questioned the validity of the overthrust model due to the lack of field evidence for basal thrust planes: what should have been near-horizontal ‘thrust faults’ were in fact steeply dipping faults. Grady and Berry (1977) also stated that in some areas, allochthonous and autochthonous material were in normal stratigraphic relationship, and had experienced similar deformation.

4.2 Imbricate model

Figure 4.2 Schematic cross section of the imbricate model for Timor. Imbricated sheets of Australian and Eurasian affinity are thrust on top of one another during collision (from Richardson and Blundell, 1996).

Thrust sheets overlying the Australian basement are of both allochthonous and para-autochthonous origin and were pervasively imbricated during emplacement (Fitch & Hamilton, 1974; Hamilton, 1979; Charlton et al., 1991; Charlton, 2000). Rocks of distinctly different provenance were thrust together as a series of slices and are now juxtaposed in Timor, forming a chaotic complex of imbricated rocks and mélange.

Chamalaun and Grady (1978) disputed this model as their field observations did not support pervasive imbrication of units. Furthermore, there is little mixing of para-autochthonous and allochthonous material as would have been expected in the imbricate model. The incoherent chaotic mélange theory is too simplified a model for the tectonic

The Tectonic Evolution of East Timor and the Banda Arc Gill Hamson #74189

evolution of East Timor (Chamalaun, 1977). Bowin et al. (1980) also disputed the imbricate model, claiming that any Australian-affinity rocks on Timor were already present in the Outer Banda Arc prior to collision. However, they did not suggest how these rocks came to be in this present location: “the manner in which these continental blocks became detached from the Australian continent and incorporated into the frontal arc of the southern part of the Banda Arc is not well defined.”

A micro-continent of Eurasian affinity may form some of the crustal material within the collision complex (Carter et al., 1976; Karig et al., 1987; Whittam et al., 1996; Richardson & Blundell, 1996; Linthout et al., 1997; Hall, 2002). The proposed micro-continent lay to the north of the North West Shelf and was incorporated into the collision complex at around 8 Ma, coinciding with and causing the retrograde metamorphism of the Aileu Formation on the north coast of East Timor (Berry & Grady, 1981; Berry & McDougall, 1986). However, palaeomagnetic evidence suggests otherwise, showing Timor to be part of the Australian allochthon at least during the Upper Permian and Triassic (Chamalaun, 1977).

4.3 Autochthon model

Figure 4.3 Schematic cross section for the autochthon model. Timor represents the uplifted Australian continental margin. Uplift caused south-directed gravity sliding and decoupling of the oceanic slab (from Richardson and Blundell, 1996).

The autochthon model opposes both the overthrust and imbricate models. In this model, the accretionary wedge sediments were almost entirely derived from the cratonic sequence of the uplifted Australian plate (Grady, 1975; Grady & Berry, 1977; Chamalaun

The Tectonic Evolution of East Timor and the Banda Arc Gill Hamson #74189

& Grady, 1978). Material transfer across the plate boundary was limited to olistostrome mass transport, resulting in a single unit known as the Bobonaro Scaly Clay (Chamalaun & Grady, 1978; Harris et al., 1998). Proponents of the autochthon model cited a lack of field evidence for the overthrust and imbricate models. These authors suggested that the debate could be resolved if evidence for the basal thrust sheets was discovered through more detailed fieldwork on Timor.

Inherent in the autochthon model is the assumption that the oceanic and continental portions of the Indo-Australian crust became detached at the collision zone. The buoyancy of the continental slab caused it to rise rapidly, uplifting northern Timor and possibly causing reactivation of pre-existing faults, while the detached down-going slab was absorbed into the mantle (Milsom, 2001). Coupled with this uplift, the Bobonaro Scaly Clay became a gravity slide, moving southward across the continental margin. This model suggests a very steep contact between the oceanic crust of the island arc and continental crust to the south, which has been confirmed by an unusually steep positive northward gravity gradient on Timor’s north coast (Chamalaun et al., 1976).

The Tectonic Evolution of East Timor and the Banda Arc Gill Hamson #74189

5 Response to tectonism: uplift and exhumation of Timor

5.1 Thermochronological data

Rapid vertical uplift of Timor since the late Neogene has been well documented (De Smet et al., 1990; Harris et al., 2000; Veevers, 2000). Thermal histories of young orogenic belts can be reconstructed using apatite fission track analysis. This technique documents the post-orogenic cooling history below ~110ºC, and can provide estimates of the timing, magnitude and rates of tectonic uplift and denudation (Gleadow et al., 2002). Harris et al. (2000) used fission track data to analyse the amount of heating that occurred since the initial collision process. They determined that there was little or no heating within the collision complex during Neogene uplift and exhumation, most probably due to the lack of long-term burial suffered by the individual thrust units. Accreted Australian-margin material on Timor recorded peak palaeotemperatures very similar to unaccreted material in northwest Australia.

Harris et al. (2000) also concluded that the inversion of apatite fission track ages recorded (younger over older) was representative of rapid uplift and exhumation due to the emplacement of thrust sheets during collision. Stacking of thrust sheets of various origins within the collision complex created an inverted thermal profile with peak palaeotemperatures decreasing discontinuously downward. One example in a vertical section from East Timor yielded an abrupt change in fission track age from 280 Ma to 52 Ma over only 25 metres. Such a change is most likely due to the juxtaposition of rocks which have undergone different thermal histories, rather than due to steep thermal gradients.

Overall, measurements show a general but discontinuous (due to local disruption by faults) increase in palaeotemperatures northwards across the island, implying greater uplift and denudation in the north. This is corroborated by the observations of Price and Audley-Charles (1987), who observed the exposure of progressively deeper stratigraphic

The Tectonic Evolution of East Timor and the Banda Arc Gill Hamson #74189

intervals towards the north, suggesting that uplift is differential both temporally and spatially.

Analysis of apatite fission track data indicates rapid cooling and exhumation of the thrust sheets, which has preserved newly formed fission tracks. This is interpreted as implying the removal of between 1.2 - 4 km of overburden over the last 2 Myr, with a denudation rate of 0.6 – 2mm/yr (Harris et al., 2000). Similar denudation rates are derived from palaeobathymetry and chronostratigraphy of foraminifera (5-10 mm/yr) (de Smet et al., 1990) and 40Ar/39Ar dating of the Aileu Complex (~3mm/yr) (Berry & McDougall, 1986).

5.2 Raised reef terraces

Numerous authors have documented the raised reef limestone platforms which exist on Timor (Fig 5.1 and 5.2) and surrounding Banda Arc islands (Chappell & Veeh 1978; Hamilton, 1979; Vita-Finzi & Hidayat, 1991; Richardson & Blundell, 1996). Chappell and Veeh (1978) claimed that the uplift rate of these reef terraces averaged 0.5 mm/yr over the past 120,000 years, and continues today. The rates of uplift of Quaternary reef terraces are slower than those of the older rocks of the collision complex, suggesting that the earlier rapid uplift was a geomorphic response of the landscape to arc-continent collision. According to Carter et al. (1976), uplifted reef limestones on Timor become younger from north to south. This may be related to the possible gentle tilting of reefs observed by Hall and Wilson (2000), with northern Timor experiencing fast uplift than southern Timor. Extensional deformation may result in terraces of similar age being vertically offset. Further inquiry into the differential uplift rates and deformation history of coral reefs across Timor is necessary to correlate the ages of the reefs and establish their uplift history.

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Figure 5.1 Locality map of East Timor showing towns of Baucau and Lautem on north the coast.

Figure 5.2 Digital elevation model of East Timor showing locations of Baucau and Lautem. The horizontal nature of elevated coral reef terraces can be seen at the front and rear of the model (derived from SRTM data set, United States Geological Survey).

The Tectonic Evolution of East Timor and the Banda Arc Gill Hamson #74189

Age-height data collected from Quaternary uplifted coral reefs on the Huon Peninsula, Papua New Guinea could be used as an analogue for coral reefs on Timor (Ota & Chappell, 1999). The emergence of coral reefs on the Huon Peninsula during periods of Holocene sea level change and variable tectonic uplift was studied by Ota and Chappell (1999). They found that the age of reef emergence was dependent on the nature of uplift (stepwise vs. uniform) and the rate of reef growth. Such research has not yet been conducted on the reef terraces in East Timor.

5.3 Uplift mechanisms

The uplift of Timor may be due to a variety of mechanisms. The emplacement of thick thrust sheets on the Australian continental margin must have caused major isostatic disequilibrium, prompting rebound-related uplift on steep faults (Grady & Berry, 1977; Chamalaun & Grady, 1978; Norvick, 1979; Snyder et al., 1996b). Additionally, uplift may be a response of the landscape to the formation of thrust faults during the collision process (the Wetar and Timor thrusts). A significant thickening of the collision complex and crust underlying Timor and the Banda Arc has been indicated by seismic reflection profiles (Snyder et al., 1996b). This horizontal shortening would also account for the uplift of Timor and associated downwarping of the Timor Trough to the south (Johnston & Bowin, 1981).

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6 Discussion

Little is known about the details of early stage orogenesis and the role it plays in ensuing evolution of orogenic systems, as much of the evidence of these processes is destroyed by subsequent erosion or deformation. Such information can only be gleaned by studying young systems such as Timor. As a result of the end of the Indonesian occupation of East Timor in 1999, the country is once again accessible to foreign scientists. Studying East Timor and islands of the outer Banda Arc has important implications for understanding fundamentals of tectonic collision, including (e.g. Bowin et al., 1980; Karig et al., 1987; McCaffrey & Abers, 1991; Huang et al., 2000):

• early stage processes of collision

• ‘jamming’ of subduction zones

• terminal stages of arc evolution

• the role of changes in subduction polarity

Australia’s northern margin is its only convergent margin. The present tectonic activity at this margin bears the marks of vigorous tectonism, through its high relief (3 km in Timor), volcanicity, seismicity, rapid uplift and exhumation, as distinct from most other Australian margins which have long been inactive (McCaffrey & Nabelek, 1986; Hughes et al.,1996; Veevers, 2000).Many aspects of this young, evolving arc-continent collision remain poorly constrained: (e.g. Audley-Charles, 1968; Grady & Berry, 1977; Chamalaun, 1978; Barber, 1979; Hamilton, 1979; Harris, 1991; Charlton, 2000):

• the timing of initial collision in the proto-Timor region

• the origins of the rocks exposed on Timor

• the mechanisms by which ongoing convergence is accommodated

• the lack of volcanism north of East Timor and related paucity of deep seismic activity in the same region

• the evolution of the position of the plate boundary

• the role of thrusts, normal faults and strike-slip faults in the Banda Arc

• how the geomorphic expressions of this orogenic event manifested in the landscape today

The Tectonic Evolution of East Timor and the Banda Arc Gill Hamson #74189

Present-day tectonic activity in the Banda Arc includes deformation along divergent thrusts, earthquakes, volcanism and significant uplift. Three main models have been proposed for the tectonic evolution of East Timor: the overthrust, imbricate and autochthon models. These have been described on the basis of local structural, stratigraphic and geochemical evidence. The diversity of hypotheses raised suggests that a broader view of the tectonic process needs to be developed, possibly accommodating a variety of models of evolution at different stages of the collision process, in order to constrain models of Neogene collision.

The uplift of East Timor since onset of collision is clearly a response to tectonic processes including isostasy acting on thickened crust, deformation, erosion, and crustal underplating. What little is known suggests that uplift occurred in two major phases: rapid uplift related to emplacement of thrust sheets, followed by slower but continued uplift until the present day. To date, limited work has been carried out on this aspect of collision, however, the near-surface uplift history may hold the key to reconciling rival ideas and providing larger-scale constraints on collisional models. It is this most recent geodynamic manifestation of the landscape during the second phase of uplift which will form the basis of my project.

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7 Thesis Plan

The aim of my work is to constrain the more recent uplift history of East Timor. First, aspects of the geology of East Timor which provide evidence of uplift will be identified. Using both existing fission track data (Harris et al., 2000) and data to be obtained during fieldwork this year, ages and denudation rates will be measured, quantifying uplift in different regions over different timescales. With a lower temperature sensitivity than apatite fission track thermochronology, apatite U-Th/Helium thermochronology (e.g. Farley, 2002), will be used to obtain data from the rocks involved in initial uplift. U-series disequilibria (e.g. Edwards et al., 2003) will be used to date the elevated Quaternary coral reefs.

An attempt will be made to correlate the reef terraces present at various locations across northern East Timor (e.g. Fig. 5.2). These terraces, like those of the Huon Peninsula, Papua New Guinea, may show evidence of sea level change during the Holocene, further constraining the timing of reef formation (Ota & Chappell, 1999). Data acquired from both approaches will be combined to establish a first-order model of uplift over different timescales, providing an analysis of the geomorphic response of the landscape to plate collision. Uplift may also vary east-west and north-south across East Timor itself, indicated by the slight tilting of uplifted coral reefs. In the case of northern Timor being uplifted faster than southern Timor, the coral reefs ought to dip gently toward the south, which may be confirmed through fieldwork.

It is hoped that my work on constraining the recent uplift history of East Timor will add to the growing body of work, establishing a comprehensive knowledge of the recent geology of East Timor, and will help in the evaluation of natural hazards, an important asset in the rebuilding of infrastructure in this emerging nation. A copy of the report arising from my study will be lodged with the East Timor Department of Energy and Mineral Resources.

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8 References

Abbott, M.J., Chamalaun, F.H., 1978. New K/Ar age data for Banda Arc volcanics. Institute for Australasian Geodynamics, Flinders University of South Australia Publication 78.

Audley-Charles, M.G. 1968. The Geology of Portugese Timor. Memoirs of the Geological Society of London No. 4.

Audley-Charles, M.G., 1975. The Sumba Fracture: a major discontinuity between Eastern and Western Indonesia. Tectonophysics, 26, 213-228.

Audley-Charles, M.G., Carter, D. J., 1972. Palaeogeographical significance of some aspects of Palaeogene and early Neogene stratigraphy and tectonics of the Timor Sea region. Palaeogeography, Palaeoclimatology, Palaeoecology, 11, 247-264.

Barber, A.J., 1979. Structural interpretations of the island of Timor, eastern

Indonesia. Proceedings of the Southeast Asia Petroleum Exploration Society, 4, 9–21.

Barber, A.J., Audley-Charles, M.G., Carter, D.J., 1977. Thrust tectonics in Timor. Journal of the Geological Society of Australia, 24, 51–62.

Berry, R.F. 1981. Petrology of the Hili Manu lherzolite, East Timor. Journal of the Geological Society of Australia, 28, 453-469.

Berry, R.F., Grady, A.E., 1981. Deformation and metamorphism of the Aileu Formation, north coast, East Timor and its tectonic significance. Journal of Structural Geology, 3, 143–167.

Berry, R.F., McDougall, I., 1986. Interpretation of Ar40/Ar39 and K/Ar dating evidence from the Aileu Formation, East Timor, Indonesia. Chemical Geology, 59, 43–58.

Bowin, C., Purdy, G.M., Johnston, C., Shor, G., Lawver, L., Hartono, H.M.S., Jezek, P., 1980. Arc-continent collision in Banda Sea region. American Association of Petroleum Geologists Bulletin, 64, 868–915.

Breen, N. A., Silver, E.A., Roof, S. 1989. The Wetar Back Arc Thrust, Eastern Indonesia: The effect of accretion against an irregularly shaped arc. Tectonics, 8, 85-98.

The Tectonic Evolution of East Timor and the Banda Arc Gill Hamson #74189

Carter, D.J., Audley-Charles, M.G., Barber, A.J., 1976. Stratigraphical analysis of island arc–continental margin collision in eastern Indonesia. Journal of the Geological Society of London, 132, 179–198.

Chamalaun, F.H. 1977. Palaeomagnetic evidence for the relative positions of Timor and Australia in the Permian. Earth and Planetary Science Letters, 34, 107-112.

Chamalaun, F. H., Grady, A. E., 1978. The Tectonic Development of Timor: A New Model and its Implications for Petroleum Exploration. APEA Journal, 102-108.

Chamalaun, F.H., Lockwood, K., White, A., 1976. The Bouguer gravity field and crustal structure of eastern Timor. Tectonophysics, 30, 241–259.

Chappell, J., Veeh, H.H., 1978. Late Quaternary tectonic movements and sea level changes at Timor and Atauro Island. Geological Society of America Bulletin, 89, 356-368

Charlton, T.R. 2002a. The Petroleum Potential of East Timor. APPEA Journal, 42, 351-369.

Charlton, T.R. 2002b. The structural setting and tectonic significance of the Lolotoi, Laclubar and Aileu metamorphic massifs, East Timor. Journal of Asian Earth Sciences, 20, 851-865.

Charlton, T.R., 2000. Tertiary evolution of the Eastern Indonesian Collision Complex. Journal of Asian Earth Science, 18, 603-631.

Charlton, T.R., Barber, A.J., Barkham, S.T., 1991. The structural evolution of the Timor collision complex, eastern Indonesia. Journal of Structural

Geology, 13, 489–500.

De Smet, M.E.M., Fortuin, A.R., Troelstra, S.R., Van Marle, L.J., Karmini, M., Tjokrosapoetro, S., Hadiwasastra, S. 1990. Detection of collision-related vertical movements in the Outer Banda Arc (Timor, Indonesia), using micropalaeontological data. Journal of Southeast Asian Earth Sciences, 4, 337-356.

Edwards, R.L., Gallup, C.D., Cheng, H. 2003. Uranium-series dating of Marine and Lacustrine Carbonates. In B. Bourdon, G.M. Henderson, C.C. Lundstrom and S.P. Turner (Eds.) Uranium-Series Geochemistry. Reviews in Mineralogy and Geochemistry, 52, 353-406.

Farley, K. A. 2002. (U-Th)/He dating: Techniques, calibrations and Applications. In C. Porcelli, C. Ballentine and R. Wieler (Eds.) Noble Gases in Geochemistry and Cosmochemistry. Reviews in Mineralogy and Geochemistry, 47, 819–843.

The Tectonic Evolution of East Timor and the Banda Arc Gill Hamson #74189

Fitch, T.J. and Hamilton, W. 1974. Reply to a discussion by M.G. Audley-Charles & J. Milsom. Journal of Geophysical Research, 79, 4982-4985.

Genrich, J.F., Stevens, C.W., Subarya, C., Bock, Y., McCaffrey, R., Calais, E. 1996. Accretion of the southern Banda Arc to the Australian plate margin determined by global positioning system measurements. Tectonics, 15, 288-295.

Gleadow, A. J. W., Belton, D. X., Kohn, B.P., Brown, R. W., 2002. Fission Track Dating of Phosphate Minerals and the Thermochronology of Apatite. Reviews in Mineralogy and Geochemistry, 48, 579-630.

Grady, A.E. 1975 A reinvestigation of thrusting in Portugese Timor. Journal of the Geological Society of Australia, 22, 223-228.

Grady, A. E., Berry, R. F., 1977. Some Palaeozoic-Mesozoic stratigraphic-structural relationships in East Timor and their significance in the tectonics of Timor. Journal of the Geological Society of Australia, 24, 203-214.

Hall, R., 2002. Cenozoic geological and plate tectonic evolution of Southeast Asia and the southwest Pacific: computer-based reconstructions, models and animations. Journal of Asian Earth Sciences, 20, 353-431.

Hall, R., 1996. Reconstructing Cenozoic SE Asia. In R. Hall & D.J. Blundell (Eds.), Tectonic Evolution of Southeast Asia. Geological Society of America Special Publication 106, 153–184.

Hall, R., Wilson, M.E.J., 2000. Neogene sutures in Eastern Indonesia. Journal of Asian Earth Sciences, 18, 781-808

Hamilton, W. 1977. Subduction in the Indonesian Region. Island Arcs, Deep Sea Trenches and Back-Arc Basins. In M. Talwani, & W.C. Pitman, (Eds.), American Geophysical Union: Washington, D.C.

Hamilton, W., 1979. Tectonics of the Indonesian region. United States Geological Survey Professional Paper, 1078, 1–345.

Harris, R.A. 1991. Temporal distribution of strain in the active Banda orogen: a reconciliation of rival hypotheses. Journal of Southeast Asian Earth Sciences, 6, 373-386.

Harris, R.A., Kaiser, J., Hurford, A., Carter, A., 2000. Thermal history of Australian passive margin cover sequences accreted to Timor during Late Neogene arc-continent collision, Indonesia. Journal of Asian Earth

Sciences, 18, 47-69.

The Tectonic Evolution of East Timor and the Banda Arc Gill Hamson #74189

Harris, R.A., Sawyer, R.K., Audley-Charles, M.G., 1998. Collisional melange development: Geologic associations of active melange-forming processes with exhumed melange facies in the western Banda orogen, Indonesia. Tectonics, 17, 458–480.

Hinschberger, F., Malod, J.-A., Dyment, J., Honthaas, C., Rehault, J.-P., Burhanuddin, S. 2001. Magnetic lineations constraints for the back-arc opening of the Late Neogene South Banda Basin (eastern Indonesia). Tectonophysics, 333, 47-59.

Huang, C-Y., Yuan, P. B., Lin, C-W., Wang, T. K., Chang, C-P., 2000. Geodynamic processes of Taiwan arc–continent collision and comparison with analogs in Timor, Papua New Guinea, Urals and Corsica. Tectonophysics, 325, 1-21.

Hughes, B.D., Baxter, K., Clark, R.A., Snyder, D.B., 1996. Detailed processing of seismic reflection data from the frontal part of the Timor trough accretionary wedge, eastern Indonesia. In R. Hall & D.J. Blundell, (Eds.), Tectonic Evolution of Southeast Asia Geological Society of America Special Publication 106, 75-83.

Johnston, C.R., Bowin, C.O., 1981. Crustal reactions resulting from the mid-Pliocene to Recent continent-island arc collision in the Timor region. Journal of Australian Geology and Geophysics, 6, 223-243.

Karig, D.E., Barber, A.J., Charlton, T.R., Klemperer, S., Hussong, D.M., 1987. Nature and distribution of deformation across the Banda Arc–Australian collision zone at Timor. Geological Society of America Bulletin, 98, 18–32.

Keep, M., Moss, S. J., 2000. Basement reactivation and control of Neogene structures in the Outer Browse Basin, North West Shelf. Exploration Geophysics, 31, 424-432.

Keep, M., Longley, I., Jones, R. 2003. Sumba and its effect on Australia’s northwestern margin. In R.R. Hillis & R.D. Muller (Eds.), Evolution and Dynamics of the Australian Plate, Geological Society of Australia Special Publication 22 and Geological Society of America Special Paper 372, 309-318.

Linthout, K., Helmers, H., Sopaheluwaken, J., 1997. Late Miocene obduction and microplate migration around the southern Banda Sea and the closure of the Indonesian Seaway. Tectonophysics, 281, 17–30.

Lorenzo, J. M., O’Brien, G. W., Stewart, J., Tandon, K., 1998. Inelastic yielding and forebulge shape across a modern foreland basin: North West Shelf of Australia, Timor Sea. Geophysical Research Letters, 25, 1455-1458.

McCaffrey, R., 1988. Active tectonics of the eastern Sunda and Banda arcs. Journal of Geophysical Research, 93, 15163–15182.

The Tectonic Evolution of East Timor and the Banda Arc Gill Hamson #74189

McCaffrey, R., 1996. Slip partitioning at convergent plate boundaries of Southeast Asia. In R. Hall & D.J. Blundell, (Eds.), Tectonic Evolution of Southeast Asia Geological Society of America Special Publication 106, 3–18.

McCaffrey, R., Abers, G.A., 1991. Orogeny in arc–continent collision: the Banda Arc and western New Guinea. Geology, 19, 563–566.

McCaffrey, R., Nabelek, J. 1986. Seismological evidence for shallow thrusting north of the Timor Trough. Journal of Geophysical Research, 85, 365-381

Masson, D.G., Milsom, J., Barber, A.J., Sikumbang, N., Dwiyanto, B.,1991. Recent tectonics around the island of Timor, eastern Indonesia. Marine and Petroleum Geology, 8, 35-49.

Milsom, J. 2001. Subduction in eastern Indonesia: how many slabs? Tectonophysics, 338, 167-178.

Norvick, M. S., 1979. The tectonic history of the Banda Arcs, eastern Indonesia: a review. Journal of the Geological Society of London, 136, 519-527.

Ota, Y., Chappell, J. 1999. Holocene sea-level rise and coral reef growth on a tectonically rising coast, Huon Peninsula, Papua New Guinea. Quaternary International, 55, 51-59.

Price, N.J., Audley-Charles, M.G. 1987. Tectonic collision processes after plate rupture. Tectonophysics, 140, 121-129.

Richardson, A.N., Blundell, D.J. 1996 Continental collision in the Banda Arc. In: R. Hall and D.J. Blundell, (Eds.) Tectonic Evolution of Southeast Asia Geological Society of America Special Publication 106, 47-60.

Rutherford, E., Burke, K., Lytwyn, J., 2001. Tectonic history of Sumba island, Indonesia, since the Late Cretaceous and its rapid escape into the forearc in the Miocene. Journal of Asian Earth Sciences, 19, 453-479.

Silver, E.A., Reed, D., McCaffrey, R., Joyodirwiryo, Y., 1983. Back arc thrusting in the eastern Sunda arc, Indonesia: a consequence of arc-collision. Journal of Geophysical Research, 88, 7429–7448.

Snyder, D.B., Barber, A. J., 1997. Australia-Banda Arc collision as an analogue for early stages in Iapetus closure. Journal of the Geological Society of London, 154, 589-592.

The Tectonic Evolution of East Timor and the Banda Arc Gill Hamson #74189

Snyder D.B., Milsom, J., Prasetyo, H. 1996a. Geophysical evidence for local indentor tectonics in the Banda Arc east of Timor. In R.Hall and D.J. Blundell, (Eds.), Tectonic Evolution of Southeast Asia Geological Society of America Special Publication 106: 61-73.

Snyder, D.B., Prasetyo, H., Blundell, D.J., Pigram, C.J., Barber, A.J., Richarson, A., Tjokosaproetro, S., 1996b. A dual doubly vergent orogen in the Banda arc continent–arc collision zone as observed on deep seismic reflection profiles. Tectonics, 15, 34–53.

Veevers, J.J., 2000. Morphotectonics of the convergent northern margin. In J.J. Veevers (Ed.), Billion-year earth history of Australia and neighbours in Gondwanaland GEMOC Press: Sydney, 29-33.

Villeneuve, M., Harsolumakso, A.H., Cornee, J.J., Bellon, H. 1999. Structure of West Timor (East Indonesia) along a north-south cross section. Geologie Mediterraneenne, 26, 127-142.

Vita-Finzi, C., Hidayat, S. 1991. Holocene uplift in West Timor, Journal of Southeast Asian Earth Sciences, 6, 387-393.

Von der Borch, C.C., 1979. Continental-island arc collision in the Banda Arc. Tectonophysics, 54, 169–193.

Whittam, D.B., Norvick, M.S., McIntyre, C.L., 1996. Mesozoic and Cainozoic Tectonostratigraphy of Western ZOCA and adjacent areas. APPEA Journal, 36, 209-223.