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Impact of lithospheric flexure on the evolution of shallow faults in the Timorforeland system
Laurent Langhi, Bozkurt Ciftci, Gilles D. Borel
PII: S0025-3227(11)00069-7DOI: doi: 10.1016/j.margeo.2011.03.007Reference: MARGO 4641
To appear in: Marine Geology
Received date: 25 May 2010Revised date: 25 February 2011Accepted date: 13 March 2011
Please cite this article as: Langhi, Laurent, Ciftci, Bozkurt, Borel, Gilles D., Impactof lithospheric flexure on the evolution of shallow faults in the Timor foreland system,Marine Geology (2011), doi: 10.1016/j.margeo.2011.03.007
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Impact of lithospheric flexure on the evolution of
shallow faults in the Timor foreland system
Laurent Langhi (corresponding author)
CSIRO Earth Sciences and Resource Engineering, ARRC, 26 Dick Perry Ave,
Kensington WA 6151, Australia. P.O.Box 1130, Bentley, WA 6102, Australia.
Email: [email protected]
Tel: +61 8 6436 8741, Fax: +61 8 6436 8550.
Bozkurt Ciftci
CSIRO Earth Sciences and Resource Engineering, ARRC, 26 Dick Perry Ave,
Kensington WA 6151, Australia. P.O.Box 1130, Bentley, WA 6102, Australia.
Email: [email protected]
Gilles D. Borel
Museum of Geology, Lausanne, UNIL, CH-1015 Lausanne, Switzerland.
Email: [email protected]
Abstract
Re-evaluation of structural evolution on the Laminaria High (Timor Sea, Australia),
shows that lithosphere flexure, associated with the Tertiary collision between the
Australian north-west margin and the Banda volcanic arc, is an important mechanism
for Neogene fault development and the reactivation of Jurassic structures.
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2D elastic models of the lithosphere flexure show that increasing flexural extension is
likely to affect the Laminaria High from the Late Miocene onwards.
The evaluation of the Neogene fault slip history, based on instantaneous throw
estimates, suggests an initiation of faulting during the Late Miocene when the
Laminaria High entered the flexed area (forebulge). Increasing fault growth is
recorded during the Pliocene and correlates with the Laminaria High reaching the
hinge of the forebulge structure. Maximum episodes of fault growth are recorded
between the Late Pliocene and Early Pleistocene when the Laminaria High was
located near the forebulge hinge or in the slope where curvature and flexural stress are
likely to be maximal.
NNW extension associated with plate flexure allowed the oblique extensional
reactivation of buried reservoir-bounding Jurassic structures. The orientation of the
Jurassic structures relative to the flexural extensional front appears as a key factor
controlling the type of segment linkage between the reactivated Jurassic faults and the
newly developed Neogene faults. Observations from the Laminaria High suggest that
with an angle >20° between σHmax and strike orientation of the Jurassic faults, soft-
linkage is likely to occur between Jurassic and Neogene segments. An angle <20° is
more likely to trigger hard-linkages between these segments. The development of
continuous fault zones results in a higher up-fault migration risk for hydrocarbon
leaking from fault-bounded reservoirs.
Keywords: plate flexure, fault reactivation, Timor Sea, Laminaria High
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1. Introduction
Active tectonism in foreland systems (area including a foredeep, forebulge and
backbulge) can affect depositional systems (Veevers 1978; Coakley and Watts, 1991,
Ford et al., 1999) and can impact structural evolution (Cloething et al., 1992) by
triggering the development of newly formed structures (Bradley and Kidd, 1991) and
by reactivating older inherited features (Lihou and Allen, 1996).
Bradley and Kidd (1991) showed that foredeeps formed by arc-passive margin
collision are zones of extension, which is induced by bending of the underthrusting
plate beyond its elastic limit. This flexural extension (Bradley and Kidd, 1991) leads
to normal faulting or normal reactivation of the pre-existing structures near the hinge
and along the slopes of the flexed plate where the bending stresses exceed rock
strength.
Flexural theory, implying that the lithosphere behaves as a rigid plate over an
asthenospheric fluid, associates the formation of a flexural deformation in a foreland
system with the lithospheric isostatic response to the tectonic load. It has been used to
successfully model and explain lithosphere deformation worldwide (Watts et al.,
1982; Cloething et al., 1992; Stewart and Watts, 1997; Ford et al., 1999, Chacin et al.,
2005).
Bending stress distribution and the magnitude of bending moment depend greatly on
the effective elastic thickness (EET), representing the theoretical thickness of a
perfectly elastic plate with similar elastic behaviour than the studied lithosphere, and
the curvature of the flexed lithosphere (Turcotte and Schubert, 1982). These elements
control the amount of induced stresses and the structural style of the foreland system
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and can be assessed by modelling methods (e.g. Cloething et al., 1992; Londono and
Lorenzo, 2004; Chacin et al., 2005).
The part of the Timor Sea (Australian North West Shelf) currently facing the Timor
Island is described an early foreland system (O’Brien et al., 1999) whose development
has been driven by the ongoing oblique collision between the Australian north-west
margin and Eurasian plates since the Miocene (Lorenzo et al., 1998; Tandon et al.,
2000). Thrusting on what is now the Timor Island and the formation of a proto-
foreland basin (i.e. Timor Trough) induced the flexure of the Australian margin in the
Bonaparte Basin (Fig. 1; Whittam et al., 1996; O’Brien et al., 1993, 1999).
By using simple bending elastic beam models Londono and Lorenzo (2004) proposed
that the reactivation of older Jurassic normal faults and development of new Neogene
structures in the Timor Sea could be the result of extensional stresses created during
bending of the north-western edge of the Australian plate under the load of the
overriding Eurasian plate. Bending strain here apparently exceeds the elastic limit and
is accommodated by inelastic yielding. Basin scale and field scale studies (e.g.
O’Brien et al., 1998; Shuster et al. 1998; O’Brien et al., 1999; Keep et al., 2007) also
suggest that elastic flexure of the lithosphere could trigger or impact Neogene
deformations in the Timor Sea. Harrowfield and Keep (2005) view Neogene faulting
as a thin-skinned response to subsidence and continuous amplification and flexure of
Permo-Carboniferous basement topography. The Neogene tectonics and the
development of extensional fault networks is considered to be the primary cause of
the high incidence of breached hydrocarbon traps in the Timor Sea (e.g. O’Brien et
al., 1999; Gartrell et al., 2006).
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In this paper we seek to compare the large scale tectonic history and the local fault
growth history over the Laminaria High (Bonaparte Basin, the Timor Sea). We
integrate flexural modelling, structural analysis from 3D seismic and regional
schematic plate reconstructions to illustrate and explain the lack of penetrative strike-
slip deformation in the northern Bonaparte Basin, despite the current oblique collision
between the Australian and Eurasian plates. We argue that the flexural deformation of
the lithosphere can explain the local Neogene structural style.
2. Regional setting
The Timor Sea covers the northern part of the Australian North West Shelf (NWS)
and includes the Bonaparte sedimentary basin (Fig. 1). This area is characterized by a
complex Phanerozoic tectonic history marked by three main Paleozoic-Mesozoic
extensional phases:
• A Late Devonian to Early Carboniferous NE-SW rifting phase responsible
for the NW–SE structural fabric present in the Petrel Sub-basin or the
Nancar Trough (Fig. 1).
• The Permo-Carboniferous development of the Neotethys rift system that
propagated from Australia to the eastern Mediterranean area (Borel and
Stampfli, 2002). On the NWS it generated extensive deformation (e.g.
Langhi and Borel, 2005) that defined the large-scale geometry of the
margin (Yeates et al., 1987). This phase is responsible for crustal thinning
on the Bonaparte Basin (Whittam et al., 1996).
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• A Late Jurassic to Early Cretaceous rifting associated with the opening of
the abyssal plains (AGSO, 1994; Baillie et al., 1994). This episode is
primarily responsible for the SW-NE structural trend commonly visible on
the NWS (Fig. 1). In the northern Bonaparte Basin, this trend can varies as
in the Nancar Trough and Laminaria High areas where structural fabric
changes to E-W (Fig. 1; de Ruig et al., 2000; Langhi and Borel, 2008).
The Tertiary geological evolution of the Timor Sea and Banda Arc has been
documented by several authors including Norvick et al. (1979); Bowin et al. (1980);
Price and Audley (1987); Daly et al. (1991); Lee and Lawver (1995); Hall (1996);
Charlton (2000); Hall (2002); Audley-Charles (2004) or Charlton (2004). Although
their interpretation may vary, a common feature is that the Tertiary Bonaparte Basin is
primarily influenced by the northward displacement of the Australian plate leading to
the complex oblique collision between the irregular Australian northern margin, the
Pacific plate and the Eurasian continent (Fig. 1). It results in the accretion and uplift
of the Timor accretionary prism (now Timor Island) and the development of an active
foredeep described as an underfilled proto-foreland basin (i.e. Timor Trough)
(O'Brien et al., 1993, 1999; Tandon et al., 2000).
The collision of the northern Australian margin initiated at the latest Oligocene or
Early Miocene (Fig. 2; ∼24 Ma, Keep et al., 2002) when the northern tip of the
Australian continent collided with the Philippine Sea plate Arc in the Papua New
Guinea region (Hall, 1996). This phase had little to no effect on the northern
Bonaparte Basin and the Laminaria High area. Audley-Charles (2004) places the
initiation of the Banda volcanic arc as known today to the Mid Miocene at ∼ 12 Ma
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(Fig. 2). Haig and McCartain (2007) report soft-sediment mixing on the Timor Island
(Bobonaro melange) that might be induced by tectonism and that suggests a phase of
deformation recorded between 9.8 Ma and 5.6 Ma. Woods (1994), Shuster et al.
(1998) and Charlton (2000) place the onset of the continent–arc collision in the Timor
Sea (i.e. northern tip of the Bonaparte Basin entering the subduction zone) to the Late
Miocene near 8 Ma (Fig. 2). This part of the margin is likely to include thinned
continental crust (Whittam et al., 1996) forming a plateau similar to the Exmouth
Plateau in the Carnarvon Basin (Fig. 1).
In the Timor Sea, the initial phase of extensional fault reactivation is usually dated
within the Miocene-Pliocene interval (O’Brien et al., 1999). Continuous deformation
has been reported during the Pliocene (∼ 4-3 Ma; Charlton et al., 1991) and is
probably coeval with the thrust and fold belt development and the emplacement of
terranes on the Timor Island (Haig and McCartain, 2007).
During the Late Pliocene, the northward subduction system jammed adjacent to the
Timor Island (Figs. 1 and 2; O’Brien et al., 1999; Longley et al., 2002; Audley-
Charles, 2004) but the northward movement of the Australian plate is maintained by
the development of a southward subduction zone along the Wetar and Flores thrust
system (Fig. 1; Genrich et al., 1996; Longley et al., 2002).
Fault activity decreased during the Pleistocene; however, deformation is ongoing with
some faults exhibiting offsets at the seafloor (Meyer et al., 2002).
2.1. Stratigraphy
The regional Mesozoic stratigraphy records the influence of the Jurassic-Cretaceous
rift and the subsequent passive margin stage (Fig. 2). The Early and Middle Jurassic
Plover Formation and the Callovian–Oxfordian Laminaria sandstones (deltaic and
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shelfal marine units, Labutis et al., 1998) represent the regional primary hydrocarbon
reservoir targets. The subsequent formations record the deepening of the depositional
environment with open marine shaly formations from the Oxfordian to the Barremian
(Frigate Fm, Flamingo Fm, Echuca Shoals Fm) (Whittam et al., 1996). It represents
the base of the regional hydrocarbon seal. The Aptian-Maastrichian Jamieson,
Woolaston, Gibson, Fenelon and Turnston Fm, are composed of northwesterly-
thinning progradational wedges mostly form of calcareous claystone and forming the
top of the regional seal.
The thick overlying Tertiary sequence is composed of prograding shelfal carbonates
recording the mature phase of the passive margin (Whittam et al., 1996). It comprises
calcarenites and calcilutites of the Johnson, Hibernia, Prion, Cartier, Oliver and
Barracouta Fm.
2.2. Laminaria High structure
The Laminaria High is located near the edge of the present-day Timor Trough (Fig.
1). It represents a small, east-west-orientated drowned platform-remnant (Fig. 3a;
Smith et al., 1996) with three structural levels (Fig. 3b; de Ruig et al., 2000):
• a N-S and NNE-SSW trending Permian block-faulted basement (hardly
resolvable in seismic data);
• a Jurassic-Cretaceous E-W trending horst and graben system (Fig. 3a);
• a series of WSW-ENE trending Neogene faults resulting in the formation
of normal faults nucleating above the reactivated Jurassic faults. Gartrell
et al. (2006) and Langhi et al. (2010) highlight that direct connection
between fault planes at Jurassic and Neogene levels is difficult to
ascertain because of the nature of: (1) intervening Cretaceous lithologies
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(shales, mudstones, and marls), (2) seismic signal degradation and (3)
subseismic faulting.
3. Lithospheric flexure in the Timor Sea
The development and reactivation of normal faults caused by plate bending are
expected to take place in areas of high curvature (hinge and slope) where greater
tensional stresses are located and produce brittle deformation (Bradley and Kidd,
1991).
In order to predict these stressed areas near the Laminaria High the early evolution of
the Timor Sea is examined using a series of simple 2D elastic half-beam models.
These models are based on flexural theory simulating the deflection of the elastic
lithosphere (seen as an elastic and homogeneous plate) under an end load. Detailed
theory is discussed by Schubert and Turcotte (1982).
The models are created perpendicular to the foredeep, in a NNW-SSE direction and
passing through the Laminaria High area (Fig. 1). The magnitude and wavelength of
the flexural response in the foreland basin are controlled by the flexural strength of
the lithosphere and the tectonic loading.
The analytical solution for the deflection of an elastic lithosphere affected by a
vertical load qa(x) and a horizontal force P is given by (Schubert and Turcotte, 1982):
qa(x) = D (d4w(x)/dx4) + P (d2w(x)/dx2) + (ρm - ρw) g w(x) (1)
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where w(x) is the vertical displacement of the lithosphere, g is the acceleration due to
gravity and ρm and ρw are the density of the mantle and the water respectively. D is
the flexural rigidity of the lithosphere given by:
D = E h3 / (12 [1-υ2]) (2)
where E is the Young’s modulus, υ is the Poisson’s ratio and h is the Effective Elastic
Thickness (EET). The EET is the thickness of a perfectly elastic plate which has the
same elastic flexural proprieties as lithosphere whose strength has been reduced by
brittle and plastic behaviour (Ford et al., 1999). The theoretical curves shown in the
models are produced primarily by variation in EET.
3.1. Effective Elastic Thickness
Continents can show a wide variation in EET with spatial variation in crustal strength
being previously reported in various foreland systems (Cardozo and Jordan 2001;
Tandon et al., 2000). This can be caused by (1) inherited rheologically heterogeneous
basement (Londono and Lorenzo, 2004) and thermal structure from previous rifting
(Bertotti et al., 1997; Stewart and Watts, 1997), (2) lateral variation in strain rate or
strain partitioning along the collision zone (Harris, 1991) and/or (3) inelastic yielding
(i.e. failure) in lithosphere (Tandon et al., 2000).
Londono and Lorenzo (2004) and Tandon et al. (2000) use elastic beam modelling
(infinite-beam and half-beam respectively) to infer EET values in the Timor Sea.
Because the flexure modelling in this study relies on half-beam model similar to the
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approach taken by Tandon et al. (2000) we will primarily rely on their EET values for
the Timor Sea and the Laminaria High area.
For simplicity and to focus primarily on the flexural extension of the Laminaria High
area we use constant EET estimates in our modelling (i.e. no variation of EET along a
NNW-SSE profile). The error in the estimation of EET using half-beam model may
be as high as 20% (Burov and Diament, 1995; 1996). This could be a significant error
if one attempts to match the seafloor bathymetric profile to the plate curvature.
Because our main aim is to define the relative variation and localisation of high
flexural stress due to lithosphere bending, we assume that the EET values based on
half-beam model represent a reasonable regional estimate.
Tandon et al. (2000) and Londono and Lorenzo (2004) suggest that the subdued
amplitude of the currently observed forebulge on northern Australian continental
lithosphere can be caused by inelastic/plastic failure on the continental slope. The
observation of an idealized forebulge can be also obliterated by reactivation of
inherited rift-related structures (Boehme, 1996; Lihou and Allen, 1996). Development
of inelastic failure and fault reactivation will decrease the capacity of the lithosphere
to transmit stresses and develop idealized forebulges (McAdoo et al., 1978). Thus,
conversely, the idealized forebulge hinge and slope location in the Timor Sea can be
taken as a proxy for high stress and inelastic yielding and reactivation locus.
Tandon et al. (2000) state that the change in EET (due to inelastic failure) occurred
near the Miocene-Pliocene boundary. Increasing bending on an initially rigid
lithosphere can reduce EET and Lorenzo et al. (1998) propose that once the yield
stress of the lithosphere is reached and bending stresses exceed rock strength,
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additional bending leads only to unrecoverable strain. Since the last tectonic event to
affect the NWS prior to bending was the Jurassic rifting, Londono and Lorenzo
(2004) suggest that the Australian continental lithosphere is expected to be initially
strong. Tandon et al. (2000) propose a constant EET value of around 75 km prior
continent-arc collision and reduction (Table 1).
3.2. Calculated lithospheric flexure
Equations (1) and (2) are used to carry out the half-beam flexural modelling (Fig. 4)
with the EET values listed in Table 1 and constant values for the mantle density (2.75
g/cm3), the water density (1.035 g/cm3), the Young’s modulus (5 1010 Pa), the
Poisson’s ratio (0.25).
The end load is also kept constant at ~2400 m below the isostatic level (Fig. 4). The
variation of this parameter impacts on the amplitude of the forebulge but has no effect
on its location. Uncertainties on this value will affect the bending stress magnitude but
not the stress distribution.
At the time of the initial collision we use an EET value of 75 km (Table 1; Tandon et
al., 2000). The resulting flexure shows a hinge located ~330 km from the foredeep
(Fig. 4a).
Lorenzo et al. (1998) propose that increasing bending on the lithosphere results in the
initiation of inelastic failure and Tandon et al. (2000) suggest that this process helped
to reduce EET. Therefore a reduced EET of 45-55 km is used for the Early Pliocene
(∼5 Ma). This range sits between pre-collision and present-day EET values defined by
Tandon et al. (2000). The resulting flexure shows a hinge located ~250 km from the
foredeep (Fig. 4a).
From the Pliocene onwards ongoing inelastic yielding helped to further reduce EET.
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An EET of 25 to 35 km was used for the part of the margin including the Laminaria
High area for the last 3 Ma as proposed by Tandon et al. (2000). The resulting flexure
shows a hinge located ~150 km from the foredeep (Fig. 4a).
When plotted over the present-day bathymetric profile running perpendicular to the
Timor Trough and through the Laminaria High (Fig. 4a), modelled flexures with EET
of 25 and 30 km match the present-day slope profile. The theoretical forebulge hinge
correlates with the shelf-slope break where most of the newly formed and reactivated
fractures (i.e. inelastic deformation) are located, highlighting area of high flexural
stress.
4. Neogene-Quaternary evolution
During the Neogene, the Laminaria High experienced a NNE (∼ 15°N) displacement
with respect to the Banda Arc and the Eurasian plate (Fig. 1; Charlton, 2000; Audley-
Charles, 2004). Consequently the Laminaria High also experienced a displacement
relative to the foredeep, slope and forebulge hinge. To define the impact of this
flexural deformation, the Neogene-Quaternary evolution of the NW Australian margin
around the Laminaria high was reconstructed with the following input constraints: (1)
the velocity of the Australian plate - this defines the distance of the Laminaria High to
the foredeep and consequently its location relative to the forebulge hinge and slope
and (2) the jamming of northward subduction (∼3 Ma, Longley et al. 2002; Audley-
Charles, 2004) - this defines when the “Australian plate-Timor prism” couple froze
and consequently when the Laminaria High was (quasi-) immobile relative to the
foredeep, forebulge hinge and slope.
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Lee and Lawver (1995) propose Australian plate velocities varying from 65 km/Ma to
83 km/Ma for the period from the Late Miocene to the Pliocene. An average NNE (∼
15°N) velocity of 75 km/Ma (±10%), corresponding to the present-day Australian
plate velocity (Fig. 1; Shuster et al., 1998; Charlton, 2000), was used in the
reconstructions for the period between the initiation of subduction in the Late
Miocene and the Early Pliocene. During the Late Pliocene, the initiation of the
jamming of subduction suggests a decrease of the relative velocity between the
Australian Plate and the prism. An average NNE velocity of 65 km/Ma (±10%) was
used in the reconstruction for the period between the Late Pliocene and the jamming
of northward subduction (∼3 Ma). The present-day distance (NNW) between the
Laminaria High and the Eurasian plate is ∼200 km (Fig. 5d) and these velocities imply
that ∼500 km of the Australian plate has been subducted in a NNW direction (Fig. 5a)
since the Late Miocene (∼ 8 Ma).
4.1. Location of the Laminaria High through time
During the Late Miocene (∼8 Ma), the Laminaria High was 490 km (±6%) away
from the subduction trench and entered the forebulge structure ∼150 km south of the
theoretical forebulge hinge (Fig. 5a). The flexure at that location is expected to be
minor (Figs. 4a and b).
Near the Early Pliocene the Laminaria High area had migrated 200 km (±10%) in a
NNE direction and was located 290 km (±15%) from the trench (Fig. 5b). Based on an
EET between 55 km and 45 km, the Laminaria High was located in the vicinity of the
forebulge hinge and had entered the zone of flexure. This correlates with the initiation
of inelastic failure and fault reactivation in the area (O’Brien et al., 1999).
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During the Late Pliocene (Fig. 5c), the Laminaria High had further migrated 100 km
(±10%) in a NNW direction. At that time the thrusting and deformation of Australian
para-autochthonous rocks had already led to the development of the Timor prism
(Whittam et al., 1996; Haig and McCartain, 2007). The load point on the lithosphere
at that time was located southward of the prism. Although uncertainties remain on the
precise evolution of the Timor prism during the Pliocene we can assume that its
NNW-SSE width was slightly smaller than the present-day width of the prism (∼100
km) as this marginally pre-dated the jamming of northward subduction. With an EET
between 25 km and 35 km and a load point that migrated 70 km to the SSE, the
Laminaria High was located 110 km (±50 km) from the foredeep in the vicinity of the
forebulge hinge and was in a zone of high flexural stress (Fig. 5c).
The theoretical forebulge hinge for the present-day, based on EET=30 km (Fig. 4a),
correlates with the shelf-slope break.
5. Discussion
The impact of flexural stress on the development of Neogene faults and the
reactivation of Mesozoic features on the Laminaria High is discussed here. The timing
of fault activity, the location and development of Neogene faults and the characteristic
fault networks pattern are discussed on the basis of forward deformation modelling of
the Laminaria High during the Early Pliocene and the structural interpretation of two
specific faults (Figs. 3 and 6):
• the north-dipping fault bounding the Corallina oil field and
• the north-dipping fault bounding the Vidalia structure.
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The structural interpretation relies on the analysis of the Laminaria 3D seismic survey
provided by Woodside Energy Ltd (Perth). The final filtered migration volume used
in this study presents an SEG negative polarity and a high signal-to-noise ratio for the
Cainozoic section.
5.1. Neogene fault timing
Age data from Vidalia-1 (Geoscience Australia, 2010) was used to estimate the
variation in sedimentation rate during the Neogene and the age of the mapped seismic
markers (Fig. 7). Before decompaction, the 8 samples dated between the base
Miocene and the seabed suggest a sedimentation rate (based on median ages,
Geoscience Australia, 2010) between 45 m/Ma and 115 m/Ma for the middle and
upper Miocene, between 45 and 90 m/Ma for the early Pliocene and of 90 m/Ma for
the upper Pliocene to seabed.
In order to assess the timing of fault growth, the difference between the hanging-wall
and the foot-wall thicknesses (compacted and decompacted) has been calculated for
the Corallina and the Vidalia faults (Fig. 8). With a sedimentation rate exceeding the
fault displacement rate this value represents the instantaneous throw.
On the Laminaria High, this assumption is supported by two elements.
1) The present-day maximum (compacted) throws on the Corallina and Vidalia
faults (i.e. cumulative fault movement from the Late Miocene-Early Pliocene) are
90 m and 160 m respectively. This is in the range of the overall sedimentation
rate from the middle Miocene to the seabed (Fig. 7).
2) The seismic reflectors on the hanging-wall compartments are largely flat, parallel
and concordant. A fault displacement rate exceeding the sedimentation rate would
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create a depocentre on the hanging-wall compartments where onlap or discordant
patterns are likely to occur during subsequent deposition cycles.
If the fault displacement rate periodically exceeds the sedimentation rate the
difference between the hanging-wall and the foot-wall thickness values represent an
underestimate of the instantaneous throw.
Neogene fault timing is shown in Fig. 8. The instantaneous throw distribution
suggests that faulting initiated during the late Miocene with instantaneous throws up
to 15 m (20 m decompacted) on the Corallina fault from ∼10 Ma to ∼7 Ma (markers
Mio2 and Mio5 respectively). The Vidalia fault (Figs. 6 and 8b) only shows restricted
activity during that period (<5 m offset). The timing of this initial phase correlates
with the arrival of the Laminaria High in the forebulge structure (Fig. 5a). This is
based on a theoretical flexure calculated with EET = 75 km in agreement with a pre-
collision EET value proposed by Tandon et al. (2000).
A significant increase in fault growth occurred during the latest Miocene-Early
Pliocene (Fig. 8) with uncompacted offsets >25 m and >40 m recorded on the
Corallina and Vidalia faults respectively. This phase appears to be responsible for the
regional development of the major Neogene structures in the Laminaria High area.
Based on the regional reconstruction and the theoretical flexure calculated with EET =
45-55 km, this episode correlates with the arrival of the Laminaria High in the vicinity
of the forebulge hinge (Figs. 4a, 4c, 5b) where flexural stress is expected to increase.
Both the Corallina and Vidalia faults recorded a peak of fault activity near the
Pliocene-Pleistocene boundary with maximum instantaneous offsets between 37 m
and 47 m (decompacted, Fig. 8d). Biostratigraphic data from the shallow sections of
Ludmilla–1, Mandorah–1 and Alaria–1 in the Laminaria High area also indicate fault
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activity occurring in Late Pliocene time (de Ruig et al., 2000). Despite uncertainties
on the location of the Laminaria High relative to the forebulge structure due to the
developing Timor prism, the paleogeographic reconstruction for the Pliocene-
Pleistocene boundary (Fig. 5c) suggests that the Laminaria High was located either at
the forebulge hinge or north of it in the area of increased flexural stress.
During the Pleistocene, the Corallina and Vidalia faults recorded decreasing
instantaneous offsets suggesting a reduction in tectonic activity. During that period,
the Laminaria High area is still located in the close vicinity of the forebulge hinge.
However, as inelastic deformation has already been affecting the lithosphere for at
least 2 Ma (since the first main tectonic phase at ~ 5 Ma), the recorded fault activity
might be locally attenuated due to previous stress release.
5.2. Neogene fault locations
In the Timor Sea, the Neogene faults often cluster above buried Late Jurassic
structures, indicating that their development is influenced to some extent by the
reactivation of these underlying rift faults (Gartrell et al., 2006).
However, data from the Laminaria High area (Ciftci and Langhi, 2010) suggest that
the Neogene deformation (i.e. reactivation) stages are not primarily accommodated by
upward reactivation and propagation of pre-existing Late Jurassic parent faults as
proposed for the nearby Vulcan Subbasin (Woods, 1992) or elsewhere (e.g.
Holdsworth et al., 1997; Nicol et al., 2005; Baudon and Cartwright, 2008).
Structural mapping of the north Corallina fault system shows that direct connection
(hard linkage) between Jurassic and Neogene fault traces is most probably not fully
realised, with the Cretaceous formations (shales, mudstones, and marls) potentially
acting as ductile horizons (Gartrell et al., 2006; Fig. 3b). Seismic signal degradation
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below the Tertiary limestone also complicates the interpretation of fault connection,
although the drastic decrease of offset in the vicinity of the Cretaceous formations
further suggests a detachment of the upper and lower fault traces (Fig. 3b). The offset
distribution for the Corallina fault shows a steep downward decrease from the base
Miocene (Fig. 9) indicating that the Neogene fault has grown downward (Ciftci and
Langhi, 2010).
Even when direct connection does probably occur such as in the Vidalia-Claudea
structure (Fig. 3b), the fault offset gradient (Fig. 9) still shows a steep decrease
indicating that the Neogene and reactivated Jurassic fault planes evolve separately, in
opposite directions and connect by dip linkage (Baudon and Cartwright, 2008; Ciftci
and Langhi, 2010).
In both cases (i.e. hard-linked or soft-linked), Neogene and Jurassic fault systems
grow individually, with the first primarily developing near the Miocene-Pliocene
boundary (i.e. maximum displacement level, Fig. 9) and propagating downwards and
the latter propagating upward but probably accommodating less strain.
This growing process is consistent with flexural extension as proposed by Bradley and
Kidd (1991) and has already been reported in the Timor Sea (Meyers et al., 2002;
Harrowfield et al., 2005).
When the flexure estimated for the Early Pliocene (based on an EET=45 km and the
Laminaria High located ∼250 km from load point) is applied to a profile that
schematically captures the Laminaria High stratigraphic and structural elements (Fig.
10a), the highest extensional stress is located on the upper part of the Tertiary
limestone (Fig. 10b). This correlates with the distribution of the maximum Coulomb
shear stress that will develop on optimally oriented conjugate shear fractures (Jaeger
and Cook, 1979), which can then be used as a proxy for fault location and density
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(Fig. 10c; Maerten et al., 2002; 2006 and Maerten and Maerten, 2006). This further
suggests that fractures are expected to develop within the shallow Neogene limestone
in response to bending and stress concentration. Deformation modelling also records
reactivation of the buried Jurassic fault planes with areas of more intense regional
reactivation correlating with anomalies of extensional stress and maximum Coulomb
shear stress (Fig. 10c).
5.3. Neogene faults pattern and stress regime
Structural mapping at the level of the maximum Tertiary displacement (Late Miocene,
Fig. 6) shows fault strike-lengths ranging from 150 m to ∼ 20 km with major fault
planes generally longer than 5 km and cumulative offset > 100 m. The general trend is
ENE following the orientation of the underlying Jurassic structures.
5.3.1. North Corallina fault system
At the top Miocene level, the north Corallina fault system (1 in Fig. 6) is composed of
a series of main right-stepping en-echelon fault planes (6 km to 12 km long) that
overlap and create relay ramps. The Neogene deformation was primarily
accommodated by the main north dipping faults; the remaining part of the strain was
accommodated by secondary synthetic and antithetic faults.
Previous studies proposed that the obliquity of the continent-arc collision primarily
controled the Neogene structural style in the Timor Sea (Shuster et al., 1998; Keep et
al., 2000), imposing a strike-slip reactivation of the underlying Late Jurassic fault
systems. However observations from the Laminaria High area report only Neogene
net normal fault offsets (de Ruig et al., 2000), suggesting an extensional to possibly
slightly transtensional regime during the main Neogene deformation phase leading to
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a component of left-lateral wrenching (de Ruig et al., 2000). This stress regime is
consistent with the Laminaria High area being affected by a WSW-ENE flexural
extensional front (i.e. parallel to the forebulge hinge; Fig. 5 and 11), with σ1 vertical
and σ2 and σ3 horizontal and oriented to the WSW and NNW respectively from the
Late Miocene onward (Fig. 11a and b). It is also supported by paleostress estimations
from Gartrell and Lisk (2005) who propose a NNW-SSE extensional regime during
the Late Miocene period form the Timor Sea.
At the Late Jurassic level, the north Corallina fault system is formed by a series of E-
W fault segments connected by WSW-ENE fault tip (Fig. 3a and 11b). A NNW σhmin
(i.e. σ3), during the Neogene, implies then an oblique extension (oblique reactivation)
of the E-W Late Jurassic structures (McCoss, 1988; Morley et al., 2004). A
characteristic feature of such oblique normal reactivation is the development of
geometries similar to those that form during strike-slip deformation. En-echelon
overlapping fault patterns as observed on the Laminaria High can develop and local
transtension can lead to the breaching of relay ramps (Fig. 11c and d).
5.3.2. Vidalia-Claudea fault system
The Vidalia-Claudea structure is formed by a long ENE-oriented Neogene graben that
nucleates above a narrow (<2km wide) Late Jurassic horst (Fig. 3b, 6 and 11b).
The main Neogene faults are fairly continuous (10 km to 20 km long, Fig. 6) with a
few jogs but not major relay or breach relay. These structures are connected (i.e. hard-
linked) to the continuous Late Jurassic fault segments that delimit the Vidalia-Claudea
horst (Fig. 3a).
We propose that (1) the continuity of the Late Jurassic fault planes but above all (2)
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their orientation sub-parallel to the flexural extensional front (i.e. forebulge hinge,
Fig. 11b) provide a better framework to accommodate the Neogene strain and that
these elements control the Neogene structural style. This leads to a high probability of
connectivity between upper (Miocene-Pliocene) and lower (Jurassic) structural levels
and a favourable growth of continuous Neogene features by preventing the
development of overlapping fault segments.
5.4. Impact of flexural extension on hydrocarbon traps
integrity
Seal breach caused by fault reactivation has been recognized as a critical risk for
hydrocarbon preservation in the Australian context (Jones and Hillis, 2003).
The mechanism(s) by which fault reactivation results in the leakage of hydrocarbons
are yet not well proven; however, active faulting in the brittle upper crust is associated
with the generation of dilatant strains and consequently increased permeability.
Whether leakage occurs due to critical stressing of faults or fault zone fractures
(Barton et al., 1995), slip-induced dilation (Wilkins and Naruk, 2007) or is linked to
the build-up and release of geopressured fluids (e.g. Sibson, 1992, 1996) remain
uncertain. However, documentation of fluid flow from deep to shallow levels or to
Earth’s surface in the seismically active Timor Sea region (Cowley and O’Brien,
2000; O’Brien et al., 2000; Rollet et al. 2006) and others (Anderson et al., 1994; Losh,
1998; Losh et al., 1999; Hovland et al., 2002; Haney et al., 2005) demonstrate a clear
link between active faulting and vertical fluid flow.
As a consequence it is proposed that the Neogene tectonic phases affecting the Timor
Sea lead to the creation of fracture permeability and up-fault fluid discharge and are
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commonly considered to be the cause of the high incidence of fault-bounded breached
traps (Shuster et al., 1998; O'Brien et al., 1999; Jones and Hillis, 2003; Gartrell and
Lisk 2005; Mildren et al., 2005; Langhi et al., 2010).
All wells drilled to date in the Laminaria High area are underfilled with evidence of
paleo-oil columns (de Ruig et al., 2000; George et al., 2004; Gartrell et al., 2006).
Gartrell et al. (2006) and Langhi et al. (2010) show that the magnitude of the
reactivational strain accommodated by trap-bounding faults controls the trap integrity
and that this strain distribution is in turn controlled by fault length, fault height and
the distribution of jogs and fault tips overlaps.
The linkage style between the Neogene and Jurassic fault planes represents an
additional key parameter as it controls the ability of the hydrocarbon to migrate away
from the reservoirs given the presence of a free surface expulsion site or a shallow net
pay horizon.
The orientation of the Jurassic structures relative to the flexural extensional front (i.e.
σ2 or σHmax during the Neogene) appears to be critical in differentiating between hard-
and soft-linkage. With an angle of ~15° between the Jurassic Vidalia-Claudea
structure and the inferred σHmax (Fig. 11b) the connection between upper and lower
fault planes was fully realised and both closures drilled at Vidalia-1 and Claudea-1 are
totally dry (de Ruig et al., 2000). The Jurassic north Corallina fault system is more
discontinuous and is oriented at ~25° from the inferred σHmax during the Neogene
(Fig. 11b). We propose that this leaded to a more segmented Neogene fault network
with overlapping segments and no or ambiguous direct connection between upper and
lower fault planes. This setting still allowed some localised up-fault migration of
hydrocarbon (Langhi et al., 2010); however the north Corallina fault delimits the
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Corallina Field hosting a 77 m oil column at the time of discovery (de Ruig et al.,
2000).
6. Summary and conclusions
1. Flexural models and reconstructions suggest that the Laminaria High area was
affected by plate flexure during the Late Miocene when it entered the
forebulge structure; by the Early Pliocene it reached the vicinity of the
forebulge hinge where the flexure increases. At the Pliocene-Pleistocene
boundary, the Laminaria High area was most likely at the forebulge hinge or
north of it, in zones of high flexural stress.
2. The timing of tectonic episodes recorded by instantaneous throws on the
Vidalia and Corallina faults consistently correlates with the development and
evolution of lithosphere flexure and flexural stress. Fault activity initiates
during the Late Miocene, maximum fault activity occurs during the Pliocene
and Early Pleistocene, with decreasing fault activity from the Late Pleistocene
onward.
3. Local deformation modelling, incorporating the schematic lithological
succession and structural architecture, associates lithosphere flexure with
extensional stress near the top of the Tertiary limestone. This supports the
development of shear failure near the maximum flexure and the association
with reactivation of buried Jurassic structures. These observations are
consistent with (a) the development of normal faults at the base Pliocene and
their downward propagation as suggested by structural analysis from 3D
seismic and (b) the clustering of the main Neogene faults above Jurassic
structures.
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4. The modelled flexure for the Pliocene-Pleistocene period implies an
extensional stress regime with σ1 vertical and σ2 and σ3 horizontal and
oriented to the WSW and NNW respectively. This attests to a rotation of the
stress field since the Late Jurassic extensional event (σ1=σ vertical > σ2=σHmax
>σ3=σhmin and σ3 azimuth = 3°N; Langhi and Borel, 2008). This setting
therefore implies an oblique extensional reactivation of the Late Jurassic
structures. This type of reactivation is able to produce en-echelon overlapping
faults similar to those forming the north Corallina fault system.
5. The geometry and the orientation of the Jurassic structures relative to the
flexural front impact on connectivity between Neogene and Jurassic fault
segments. On the Laminaria High, a transition between full connection and
poor connection or disconnection seems to occur when the angle between the
Jurassic structure and the Neogene σHmax reaches ~20°. This element is a
factor controlling hydrocarbon trap integrity, along with Jurassic fault length,
height, geometry and tip distributions.
Acknowledgments
We wish to thank IGEOSS for providing Dynel2D software. Schlumberger is
acknowledged for supplying Petrel seismic software.
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Figures captions
Figure 1. Geological elements of the North West Shelf and the Timor Sea, modified
after AGSO (1994) and Harrowfield et al. (2003). COB=continent ocean boundary,
L.H.=Laminaria High, N.T.=Nancar Trough, F.H.=Flamingo High.
Figure 2. Schematic stratigraphic column for the Laminaria High and Nancar Trough
area and tectonic episodes, modified from de Ruig et al. (2000).
Figure 3. Structural elements of the Laminaria High. A) Base Oxfordian sandstone
time-structure map and distribution of the wells and oil fields. Datum UTM 51S.
Location on Fig. 1. B) Cross-section across the Laminaria High. Location on Fig. 3a.
1 and 2 are respectively is the north-dipping faults bounding the Corallina Field and
the Vidalia-Claudea structure.
Figure 4. Theoretical flexure of the Timor Sea foreland on a NW-SE section through
the Laminaria High area. Location on Fig. 1. A) Theoretical flexure using 2D elastic
half-beam model for Effective Elastic thickness (EET) varying from 75 km to 25 km.
The error on the location is given by the grey area. The upper part shows the
Laminaria High location based on reconstruction in Fig. 5 and using EET values
discussed in the text and shown in Table1. Note that at ~ 3 Ma the location of the
Laminaria High is affected by the development of the Timor prism, given the width of
the accreted material the location of the Laminaria High relative to the load point
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might vary as shown by the grey area. B) C) and D) Detail of the theoretical flexure
and the Laminaria High position respectively at 8 Ma, 5 Ma and 3 Ma.
Figure 5. Reconstruction of the Timor foreland basin for the Late Miocene onward
and distribution of the flexural front. The size of the star represents the uncertainty on
the location of the Laminaria High A) Late Miocene initiation of the continent-arc
collision based on Woods (1994), Shuster et al. (1998) and Charlton (2000). The
Laminaria High is located north of the forebulge structure calculated with
EET=75km. The arrow (2) represents the 500 km of subducted Australian margin
since 8 Ma based on plate velocities detailed in the text. B) Early Pliocene
reconstruction. The Laminaria High is located on the theoretical forebulge hinge (with
EET=55 km to 45 km). The Timor accretionary prism is initiating. The error on the
Laminaria High location due to velocity estimate is ± 25 km and represents the size of
the star. C) Late Pliocene reconstruction. The Laminaria High is located on the slope,
north of the theoretical forebulge hinge (with EET=25 km to 35km). The error on the
Laminaria High location due to velocity estimate is ± 20km and is smaller than the
size of the star. D) Present-day configuration and theoretical forebulge structure
calculated with EET=30 km. The forebulge hinge correlates with the shelf break. A
southward subduction initiated north of the Timor prism. The arrow (1) represents the
distance (200 km) from the Laminaria High to the inferred limit of the Eurasian plate.
Figure 6. Base Pliocene time-structure map. Datum UTM 51S. Location on Fig. 1. 1
and 2 are the north-dipping faults bounding the Corallina Field and the Vidalia-
Claudea structure respectively.
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Figure 7. Burial curve for Vidalia-1 well modified from Geoscience Australia (2010).
The maximum, minimum and median ages of samples are plotted against the
compacted depth (present-day depth below rotary table). The uncomapcted depths are
calculated following Watts (1992) and depth porosity trends from Sawyer et al.
(1982).
Figure 8. Neogene fault evolution on the Laminaria High with instantaneous throw
calculated as the difference between footwall and hanging-wall thicknesses. A)
Corallina Field north-dipping fault (1). Location on Fig. 6. B) Vidalia-Claudea
structure north-dipping fault (2). The maximum displacement level (a) is located near
the Miocene-Pliocene boundary. Both faults growth downward from that point and
display a growth section overlying a displacement section where no instantaneous
throw is recorded. Location on Fig. 6. C) Instantaneous throw (compacted
thicknesses) for the Corallina and Vidalia faults. D) Instantaneous throw
(decompacted thicknesses) for the Corallina and Vidalia faults.
Figure 9. Offset versus horizon age plot for the Corallina Field north-dipping fault (1)
and the Vidalia-Claudea structure north-dipping fault (2). The greyed area indicates
the linkage zones. The general C shape of both curves indicates a downward and
upward propagation respectively for the Tertiary and the Jurassic faults.
Figure 10. Model configuration and results of the forward deformation model of the
Laminaria High (mechanical properties in Table 2). A) Original six-layer model and
target horizon (i.e. flexure with EET=45 km for a location ∼250 km from load point).
B) Computed σxx or normal stress along the x axis (extension is negative,
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compression positive) and reactivation offset on the lower Jurassic faults. C)
Computed maximum Coulomb shear stress (MCSS) used as a proxy for fault location
and density.
Figure 11. Stress regime and fault pattern on the Laminaria High. A) Schematic block
diagram showing the Timor foreland basin near the Miocene-Pliocene boundary and
the local stress regime for the Laminaria High. Not to scale. B) Structural map on the
Laminaria High showing the Late Jurassic (grey) and base Pliocene (black) fault
traces for the north-dipping faults bounding the Corallina Field (1) and the Vidalia-
Claudea structure (2). The local stress field is inferred from the orientation of the
forebulge hinge. C) Base Pliocene upper breach ramp observed on the Laminaria High
and attributed to oblique extensional reactivation. D) Example from Morley et al.
(2004) of a similar upper breach ramp due to oblique extension of buried structure.
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Age Foreland stage EET value
8 Ma Pre-collision and collision initiation 75 km
5 Ma Collision stage, Timor prism development and inelastic yielding in the foreland basin
45-55 km
3 Ma onward Collision continue and inelastic yielding in the foreland basin
35-25 km
Table 1. Effective elastic thicknesses used for the half-beam flexural modelling. Layer Poisson’s ratio Young’s
modulus (Pa) friction angle
(°) Density (kg m-3)
porosity
Carbonate 0.2 4 1010 30 2500 0.3 Shale 0.35 2 1010 22 2400 0.15 Sandstone 0.25 2 1010 30 2450 0.18 Table 2. Mechanical properties used for the deformation models.
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Fig. 1
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Fig. 2
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Fig. 3
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Fig. 4
Fig. 5
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Fig. 6
Fig. 7
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Fig. 8
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Fig. 9
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Fig. 10
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Fig. 11
