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Late Jurassic rifting along the Australian NorthWest Shelf: margin geometry and spreading ridgeconfigurationC Heine & RD Müllera University of Sydney Institute of Marine Science and School of Geosciences, Universityof Sydney , NSW, 2006, AustraliaE-mail:Published online: 02 Feb 2007.

To cite this article: C Heine & RD Müller (2005) Late Jurassic rifting along the Australian North West Shelf: margingeometry and spreading ridge configuration, Australian Journal of Earth Sciences: An International Geoscience Journal ofthe Geological Society of Australia, 52:1, 27-39, DOI: 10.1080/08120090500100077

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Late Jurassic rifting along the Australian North WestShelf: margin geometry and spreading ridgeconfiguration

C. HEINE* AND R. D. MULLER

University of Sydney Institute of Marine Science and School of Geosciences, University of Sydney, NSW 2006,Australia.

The Argo and Gascoyne Abyssal plains in the easternmost Indian Ocean document the last stages ofeastern Tethys evolution before the breakup of eastern Gondwana. Thus they provide crucialinformation not only for modelling the evolution of the eastern Tethys and Proto-Indian Ocean, but alsoto understand the complex geodynamic history of the North West Shelf. We have revisited the marinemagnetic anomaly record of the Argo and Gascoyne Abyssal Plains in combination with othergeological and geophysical data from the North West Shelf and southeast Asia. Based on thecombined data, we have created a revised plate-tectonic model and a set of palaeogeographicreconstructions for the evolution of the North West Shelf for the early stages after the breakup. The maindifference between this model and previously published models is that we have interpreted acomplete section of anomalies, M25A–M22A, in the Gascoyne Abyssal Plain, northwest of the ExmouthPlateau. The magnetic anomalies have the same trend as in the Argo Abyssal Plain. Our new plate-tectonic reconstructions show that continental breakup in the Argo and northern Gascoyne AbyssalPlains, east and northwest of the Exmouth Plateau, respectively, started simultaneously in the Oxfordianwith M25A identified as the oldest anomaly. In the Gascoyne Abyssal Plain, the oldest anomalysequence, M25A–M22A (154.5 – 150.4 Ma) indicates that the ‘Argo’ spreading ridge continued aroundthe northern margin of Greater India, and was probably linked with the Somali Basin. Sea-floorspreading continued until M14, separating the West Burma Block and possibly other smaller continentalfragments like the Sikuleh Terrane of Western Sumatra from the northern Australian margin. Asouthward-directed ridge jump at M13 (134 Ma) transferred segments of Australian Plate oceanic crustto the West Burma Plate. Contemporaneously, an anticlockwise change in spreading direction fixedthe West Burma Block relative to Greater India until its collision with the southern Eurasian margin.

KEY WORDS: Argo Abyssal Plain; Gascoyne Abyssal Plain; North West Shelf; plate tectonics; platemargin.

INTRODUCTION AND PREVIOUS WORK

The Australian North West Shelf is the oldest passive

margin of the continent and extends over 2400 km from

the Arafura Sea between northern Australian and Irian

Jaya in the east, up to the Exmouth Plateau off the

Northwest Cape in the west (Figure 1). It formed as a

result of multiple rifting episodes with subsequent

removal of continental slivers during Palaeozoic –

Mesozoic times (Stagg et al. 1999; AGSO Northwest

Shelf Study Group 1994). The Argo Abyssal Plain is

located adjacent on the North West Shelf in the

easternmost corner of the Indian Ocean, and represents

one of the few remaining patches of Jurassic ocean

floor. Bound by the Java Trench to the north,

submerged continental crust of the Scott Plateau,

Rowley Terrace and Exmouth/Wombat Plateaus limits

this ocean basin to the east, south and southwest,

respectively (Figure 1). The volcanic Joey and Roo

Rises north of the Platypus Spur separate the Argo

Abyssal Plain from the Gascoyne Abyssal Plain to

west. Occupying an intermediate position between the

Argo Abyssal Plain in the east and the Wharton Basin/

Christmas Island area in the west, the Gascoyne

Abyssal Plain is bound by the western margin of the

Exmouth Plateau to the south and the Java Trench to

the north (Figure 1).

Early surveys resulted in recognition of the oceanic

character of both the Argo and Gascoyne Abyssal

plains (Veevers et al. 1991; Fullerton et al. 1989; Powell

& Luyendyk 1982; Cook et al. 1978; Heirtzler et al.

1978). This was later confirmed by ODP (Ocean

Drilling Program) drilling partly penetrating oceanic

basement (Ludden 1992; Mihut & Muller 1998a; Sager

et al. 1992). Magnetic anomalies in both abyssal plains

document the last rifting episode and final breakup of

eastern Gondwana between Australia and India in

*Corresponding author: christian@geosci.usyd.edu.au

Australian Journal of Earth Sciences (2005) 52, (27 – 39)

ISSN 0812-0099 print/ISSN 1400-0952 online ª Geological Society of Australia

DOI: 10.1080/08120090500100077

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Late Jurassic/Early Cretaceous (Stagg et al. 1999;

Mihut & Muller 1998b; Sager et al. 1992; Veevers et

al. 1991; Fullerton et al. 1989). Whereas the opening of

the Argo Abyssal Plain has been attributed to the

rifting of a smaller continental sliver off the passive

margin of northeastern Gondwana in the Late Jur-

assic (presumably the West Burma Block of Metcalfe

1999), oceanic crust along the western Australian

margin, including the Gascoyne Abyssal Plain, has

been related to the India –Australia breakup and

subsequent sea-floor spreading in the Early Cretaceous

(Stagg et al. 1999; Mihut & Muller 1998b; Fullerton et

al. 1989).

Most of the published models for the sea-floor

spreading history of the Argo Abyssal Plain show a

similar pattern of isochrons. A northeast – southwest

trend has been observed throughout the abyssal plain,

starting with the Mesozoic M25 [154.1 Ma, according to

the Gradstein et al. (1994) timescale] or M26 (155.0 Ma)

anomaly in the southernmost corner, with a general

younging towards the northwest (Powell & Luyendyk

1982; Fullerton et al. 1989; Sager et al. 1992; Mihut 1997).

The most problematic areas for correlating magnetic

anomalies are the north and west of the Argo Abyssal

Plain because of intraplate volcanic activity partly

disturbing the magnetic record and preventing unequi-

vocal correlation of magnetic anomalies.

Powell and Luyendyk (1982) proposed a complete

M25 –M14 (154.1 – 135.8 Ma) anomaly sequence in eastern

parts of the basin and a M25 –M5 (154.1 – 126.7 Ma)

sequence with a southward ridge jump around M14

(135.8 Ma) in the western spreading segment, separated

by a transform fault. They related this ridge jump to the

formation of a triple junction, because of the north-

northeast – south-southwest-trending M6–M0 (128.2 –

120.4 Ma) sequence in the Gascoyne Abyssal Plain that

intersects obliquely with the older Argo magnetic

lineations. However, after the acquisition of new

magnetic data, a revised set of isochrons and a simpler

model for the Argo and the Gascoyne Abyssal Plains

was published by Fullerton et al. (1989), in which a

continuous N608E trending M26 –M16 (155.0 – 137.9 Ma)

sequence was interpreted. It intersects with the N308E-trending isochrons of the Gascoyne Abyssal Plain, along

transform faults north of the Wombat Plateau. This

model was reviewed by Sager et al. (1992) on the basis of

results of ODP drilling in the southern Argo Abyssal

Plain near the continent – ocean boundary (COB). In

order to resolve large discrepancies between initial

sediment ages (Berriasian –Valanginian) and radio-

metric/magnetic basement dating (Oxfordian, around

156 Ma: Ludden 1992) a M11 –M0 sequence was proposed.

Although this sequence matches the observed data, it

does not explain correlatable anomalies northwest of

anomaly M0, in what should be the Cretaceous Normal

Superchron, so at that time the first model was

considered to fit best (Sager et al. 1992). Later revised

ages of the sediments of Site 765 and DSDP-Site 261 as

Tithonian and Kimmeridgian –Early Tithonian, respec-

tively, decreased the gap between basement age and first

sediments to 10 and 3 – 8 million years (J. Mutterlose

pers. comm. 2000), explained by extremely low deposi-

tional rates (Sager et al. 1992).

In the Gascoyne Abyssal Plain and southerly adja-

cent Cuvier Abyssal Plain, Fullerton et al. (1989)

interpreted anomalies M10 –M0 (130.2 – 120.4 Ma) strik-

ing N308E on the basis of new magnetic data. Anomaly

M10 is observed along western Exmouth Plateau,

suggesting a simultaneous onset of sea-floor spreading

between Australia and India at around 130 Ma, with a

westward ridge jump around M5, that transfers pieces of

the Indian Plate onto the Australian Plate (Fullerton et

al. 1989). Mihut (1997), Mihut and Muller (1998b) and

Muller et al. (1998) have analysed the magnetic anomaly

data together with gravity data from satellite altimetry

and constructed a revised isochron map for the western

Australian margin. They identified M11 (132.0 Ma) as the

oldest anomaly preserved and three major fracture

zones, as well as two northward ridge propagation

events, which are also found in the Cuvier Abyssal

Plain. In the central part of the basin the M11 –M0

sequence is observed with the conjugate Indian M11 –

M7 (132.0 – 128.4 Ma) anomalies that were transferred to

the Australian Plate during a westward ridge jump and

northward propagation (Muller et al. 1998). The M11 –

M5 anomalies show a N458E trend and the anomalies

younger than M5 trend N358E (Muller et al. 1998).

A problem, which has not been addressed fully by

previous models, is the geodynamic implications of the

spatial and temporal proximity of the different rifting

events and the resulting configuration of the active

spreading ridges north and northwest of Australia at

that time. Most models assume the northward propagat-

ing India –Australia ridge to have cut off the magnetic

lineations of the western Argo Abyssal Plain (Mihut

Figure 1 Combined GTOPO-ETOPO morphology of north-

western Australia and the adjacent oceanic areas in 5 km

resolution. Depth in 1000 m contour intervals (thin grey

lines). Numbers indicate DSDP/ODP well sites, white

colours indicate that oceanic basement was reached. AAP,

Argo Abyssal Plain; GAP, Gascoyne Abyssal Plain; CAP,

Cuvier Abyssal Plain; RR, Roo Rise; JR, Joey Rise; RT,

Rowley Terrace; CT, Carnarvon Terrace; ScP, Scott Plateau;

ExP, Exmouth Plateau; WP, Wombat Plateau; PS, Platypus

Spur; CRFz, Cape Range fracture zone; NWC, Northwest

Cape; CI, Christmas Island. Inset shows the location of the

Wharton Basin area (WB) and the Arafura Sea (AfS) with

respect to main map.

28 C. Heine and R. D. Muller28 C. Heine and R. D. Muller

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1997; Sager et al. 1992; Fullerton et al. 1989) or implicitly

assume a triple junction or abandoned spreading centre

in the eastern Tethys (Metcalfe 1999, 1996; Veevers et al.

1991). Also, the extended continental crust promontory

of the Exmouth Plateau, representing the northwestern-

most extent of Australian continental crust, has been

regarded as the major geological boundary between the

two different spreading corridors and probably biased

the magnetic anomaly interpretation in the Gascoyne

Abyssal Plain. As oceanic crust north of the Argo and

Gascoyne Abyssal Plains has already been subducted

along the southern Sundaland margin, only an inte-

grated approach can help to unravel the plate-tectonic

history of the northwestern Australian margin for the

Late Jurassic/Early Cretaceous. For our revised model

we attempted to bridge this gap by combining our

revised marine magnetic anomaly interpretation with

available geological and geophysical data from south-

east Asia and the North West Shelf, embedded in a

regional, self-consistent set of finite rotations. This also

allows us to reconstruct subducted oceanic lithosphere

of the eastern Tethys, modelling the convergence

history of the Sundaland margin since the Late Jurassic

(Heine et al. 2004).

REVISED MAGNETIC ANOMALY INTERPRETATION

Magnetic anomalies in the Argo Abyssal Plain and

eastern Gascoyne Abyssal Plain have been correlated by

visual comparison of a computed synthetic magnetic

sequence, using the parameters listed in Table 1. It was

assumed that the average sea-floor spreading rate for

the M25 –M10 interval was constant around 40 mm/year

(half-spreading rate). Magnetic data were obtained from

the GEODAS archive and interpreted jointly with

satellite-derived gravity (Sandwell & Smith 1997) in

order to identify structural trends like fracture zones

and continental margin offsets. Figure 2 shows the ship

tracks that have been used for the interpretation. For

the correlation of magnetic anomalies the Gradstein et

al. (1994) geomagnetic time-scale was used.

Argo Abyssal Plain

We correlated an M25A–M10N (154.5 – 130.8 Ma) se-

quence in both the Argo Abyssal Plain and Gascoyne

Abyssal Plain (Figures 3, 4). The best correlation of

synthetic and observed anomalies in the Argo Abyssal

Plain were found on the a9314 and um63 tracks (Figure

2), extending from ODP Site 765 (Figure 1) in the south,

across the central part of the basin to the northwest

(Figure 3). Ages determined at ODP Site 765 acted as a

tiepoint for the interpretation. By following track a9314

to the northwest (Figure 3), the synthetic profile was

matched, and afterwards the interpretation was ex-

tended to the adjacent wiggles, covering the complete

Argo Abyssal Plain and the northwest of the Exmouth

Plateau in the eastern Gascoyne Abyssal Plain (Figure

5). M26 (155.0 Ma) was the oldest anomaly identified in

the southern Argo Abyssal Plain south of ODP Site 765,

limited to the a9314 track. This confirms the dating and

interpretation by Sager et al. (1992). The anomalies

M26 –M24A (155.0 – 153.1 Ma) are limited to the east and

to the southwest by the COB of the Exmouth Plateau –

Rowley Terrace – Scott Plateau margin. In the northeast

the M24 –M22A (152.1 – 150.4 Ma) anomalies are bound

by the Scott Plateau/Java Trench. The magnetic linea-

tions at DSDP Site 261 are M24 –M23 (152.1 – 150.7 Ma)

result in a basement age of late Kimmeridgian [accord-

ing to the Gradstein et al. (1994) time-scale], matching

the ages of the oldest dated sedimentary rocks well. A

continuous sequence is identified until M22A (150.4 Ma)

where a high-amplitude negative anomaly of about

7 500 nT is observed in the recorded profiles (Figure

3). This negative anomaly is likely related to an

interpreted southward ridge jump at M14 (135.8 Ma), as

the correlation with the normal sequence (M22 and

younger) north of this anomaly is lost. Instead, the

M13 –M10N (135.5 – 130.8 Ma) sequence shows a good fit

to the recorded profiles (Figure 3) with a slight

successive anticlockwise rotation of the spreading

direction from *N608E in the southern parts of the

Argo Abyssal Plain to N458E in the northern part

Table 1 Parameters used to generate synthetic magnetic

anomalies for the Argo and Gascoyne Abyssal plains.

Parameter Value

Present field

Location 148S, 1168EInclination 7468Declination 18Strike of ridge N608EPhase shift 322.588Top of layer 6 km

Base of layer 6.5 km

Remanent field

Inclination 1238Declination 1348Strike of ridge N308EHalf-spreading rate 40 mm/y

Palaeopole position 708S, 1408E

Figure 2 Ship-track database used for this study. Bold black

lines indicate ship-tracks used for correlating synthetic

magnetic sequence. White lines represent bathymetric

contours in 1000 m intervals.

Jurassic North West Shelf rifting 29

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(Figures 5, 6). On the gravity map, an east-northeast –

west-southwest-trending, concave signature matches

with the magnetic data representing the remnant

signature of the ridge jump. Six northeast-trending

fracture zones segment the magnetic lineations into

smaller compartments (Figure 5) and correlate well with

the offsets of the magnetic lineations (Figure 6). North of

the pseudofault no unequivocal evidence for fracture

zones was found either in the magnetic or in gravity

data.

Higher amplitude magnetic anomalies in the north-

ern part could be related to the decrease in water

depth due to the regional southward tilt of the basin

and the outer swell of the subduction zone in the Java

Trench. The frequency of polarity reversals signifi-

cantly changes in the M26 –M10 sequence from higher

rates (M25 –M22A) to lower (M22 –M16) and back to

higher rates for the M15 –M10 anomalies. The mag-

netic record of the Argo Abyssal Plain instead reflects

a more or less constant rate of reversals with a slight

decrease towards the centre of the basin (Figure 3). If

the spreading had continued normally from M22

onwards, one would expect the low-reversal frequency

sequence M22 –M15 in the central and northern part of

the abyssal plain. Instead, the spacing between the

anomalies is largest in the centre of the basin with a

sudden increase in the reversal frequency north of the

interpreted pseudofault, supporting the interpretation

of a southward ridge jump with the anomalies M22/

M21 to M14 being transferred onto the conjugate plate

rifting away from Australia (Figure 7). Volcanic

activity in the Joey Rise and Roo Rise areas biases

the magnetic signal (Figure 5). In the north the

identification of magnetic anomalies younger than

M10N (130.8 Ma) is limited by subduction at the Java

Trench.

A second model has been tested for a younger M-

sequence in order to decrease the discrepancy between

oldest sedimentary rocks and basement ages in the

DSDP/ODP wells in the Argo Abyssal Plain. However,

in this case correlatable magnetic anomalies in the

northern part of the Argo Abyssal Plain would be

located within oceanic crust generated during the

Cretaceous Normal Superchron. Thus this model was

discarded.

Gascoyne Abyssal Plain

In the eastern Gascoyne Abyssal Plain the magnetic

anomaly interpretation was more problematic due to

the azimuth of the recorded profiles and the sparse data

coverage (Figure 2). Magnetic lineations have been

interpreted previously to be related to the India –

Australia breakup along the western Australian margin

in Valanginian –Hauterivian times and to have the

same orientation as in the Cuvier and Perth Abyssal

Plains (Muller et al. 1998; Mihut 1997; Sager et al. 1992;

Fullerton et al. 1989; Powell & Luyendyk 1982). However,

Figure 3 Selected stacked magnetic profiles and synthetic

magnetic model for the Argo Abyssal Plain. Spreading

velocity for synthetic profile is 80 mm/year full spreading

rate. Ship-tracks are indicated in Figure 2.

Figure 4 Selected stacked magnetic profiles and synthetic

magnetic model for the Gascoyne Abyssal Plain. Spreading

velocity for synthetic profile is 80 mm/year full spreading

rate. Ship-tracks are indicated in Figure 2.

30 C. Heine and R. D. Muller30 C. Heine and R. D. Muller

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our revised interpretation has identified the anomalies

M25A–M22A (153.5 – 150.4 Ma) parallel to the lineations

in the Argo Abyssal Plain.

The identified magnetic anomalies trend obliquely

to all available ship tracks in the area (Figure 5). A

good correlation was found for the M24B–M22A

magnetic anomalies over the io050 – io055 tracks (Fig-

ure 4). North of the northern tip of the Cape Range

Fracture Zone anomaly M25A was traced over four

profiles, limited to the east by the extended continental

crust promontory of the Wombat/Exmouth Plateau

(Platypus Spur). A continuous sequence of magnetic

anomalies is identified until M22A, with the same

azimuth as anomalies in the Argo Abyssal Plain. The

lineations are segmented by two fracture zones. North

of the Platypus Spur anomaly M24 is the oldest

identified, probably linking the two spreading compart-

ments of the Gascoyne and Argo Abyssal Plains. A

north – south-striking pseudofault, caused by a north-

ward propagating ridge event (Muller et al. 1998; Mihut

& Muller 1998b), cuts off the anomalies to the west and

is expressed by a combination of a smaller negative

and major positive magnetic anomaly (Figure 4, 5). It is

also preserved as a minor positive gravity anomaly

(Figure 6). Anomalies M4–M0 (126.7 – 120.4 Ma) con-

tinue west of the pseudofault, related to the spreading

between India and Australia.

CONTINENT –OCEAN TRANSITION AROUND THENORTH WEST SHELF

The COB along the western and northern Exmouth

Plateau, eastern Wombat Plateau, northern Rowley

Terrace and western Scott Plateau has been revised

based on satellite gravity data and seismic refraction

profiles across the boundary (Goncharov 2004; Fomin et

al. 2000).

The Cape Range Fracture Zone separates the con-

tinental crust of the southwestern Exmouth Plateau

from the oceanic crust of the Cuvier Abyssal Plain. At

the northern tip of the Cape Range Fracture Zone, the

COB bends to the northeast following in a staircase-

shaped manner the diffuse and broad negative gravity

anomaly on the southern margin of the Gascoyne

Abyssal Plain. This trend is subparallel to the identified

magnetic lineations. A longer transform fault at the

western margin of the Platypus Spur offsets the COB

around the northwesternmost tip of Australian Plate

continental crust. All around the Argo Abyssal Plain a

major negative gravity anomaly coincides with the COB

(Veevers et al. 1985). The boundary, slightly curved and

cut by transform fault offsets, extends to the Rowley

Terrace where again a staircase pattern is observed.

Along the northwestern Scott Plateau the gravity

signature gets diffuse, with the COB following a smaller

negative anomaly to the north. South of Sumba island

the COB follows Java and Timor Trench to the east. The

transition from continental to oceanic crust is also

constrained by three seismic refraction profiles across

the outer Exmouth Plateau, the Rowley and Terrace and

the Scott Plateau.

The age of the COB had to be extrapolated for the

plate-tectonic reconstructions as it represents the onset

of sea-floor spreading. We have assumed a constant

spreading rate within the COB?M25 (154.1 Ma)?M24

(152.11 Ma) sequence and derived rotations for the

M25?COB stage (7 2.528). With a half-spreading rate

of 48.4 mm/year and a stage rotation angle of 7 2.788 forthe M24 –M25 stage (Dt = 1.99 million years), the age of

the initialisation of the continental margin around the

Argo Abyssal Plain and northwest Exmouth/Wombat

Plateau area was calculated to be 155.9 Ma.

EXTERNAL TIME CONSTRAINTS FOR THESEA-FLOOR SPREADING IN THE ARGO ANDGASCOYNE ABYSSAL PLAINS

Backstripping of exploration wells and stratigraphic

data from the North West Shelf have been used to

constrain the plate-tectonic reconstructions and as a

control for the interpreted magnetic anomaly data.

Stratigraphic data was compiled from wells in the

Bonaparte and Browse Basins and has been outlined

and discussed in detail by Baldwin et al. (2003) and

Muller et al. (1998, 2000).

Stratigraphy

Sequence-stratigraphic analysis shows the existence of

several so-called megasequences that document the

last rifting episode on the Australian North West

Shelf. The syn- and post-rift sequences in particular

contain vital age constraints for dating the breakup

along the North West Shelf margin (Jablonski 1997;

Labutis 1994).

The syn-rift megasequence is subdivided into two

stages separated by the main Oxfordian unconformity

(Jablonski 1997). A Callovian transgressive surface

marks the onset of the first syn-rift megasequence that

culminated in a major regional uplift in the northern

Carnarvon Basin (including Exmouth and Wombat

Plateaus) by the Oxfordian. Large areas were exposed

to subaerial conditions, resulting in erosion of the

complete Jurassic sequence and the J-WSS15(JO) trans-

gressive surface of the main unconformity (Jablonski

1997; von Rad et al. 1992). At the base of the second syn-

rift stage, a major sea-level lowstand (156.7 Ma) corre-

lates with the continental breakup event implied by our

model and with the onset of sea-floor spreading in the

Argo Abyssal Plain (M26, 155.0 Ma). This is followed by a

rapid increase in rate of deposition and the development

of submarine fans (Jablonski 1997; von Rad et al. 1992).

The second syn-rift phase is limited to the top by the

major Tithonian transgressive surface that marks the

transition to the late syn-rift megasequence (Jablonski

1997). Increasing volcanic activity on the outer Wombat

Plateau (von Rad et al. 1992) and the reactivation of the

older Indo-Australian rift zone (Veevers & Tewari 1995;

Jablonski 1997) indicate the beginning separation of the

Greater India Block and Australia. Uplift of up to 120 m

(Jablonski 1997) in the south and west of the Exmouth

Plateau during Valanginian times terminated the Bar-

row Delta progradation and was accompanied by

Jurassic North West Shelf rifting 31

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volcanic activity (Jablonski 1997). The Valanginian

transgressive surface represents the beginning of the

post-rift sequence and subsequent sea-floor spreading

(M14, 135.8Ma) in the Cuvier Abyssal Plain. A north-

ward ridge propagation event in the Gascoyne Abyssal

Plain during the mid-Valanginian (Mihut & Muller

1998b) and subsequent sea-floor spreading was followed

by thermal subsidence of the western, northwestern and

northern Australian margins.

The stratigraphic data supports the separation of the

West Burma continental block and the start of sea-floor

spreading in the Oxfordian, at around 155 Ma.

Backstripping

The backstripping technique allows modelling of the

tectonic basement subsidence and uplift as a function of

time, if palaeo-water depths are sufficiently well con-

strained. Phases of lithospheric extension reveal

periods of rifting. In this study, stratigraphic data from

Hadrian 1, Longleat 1, Taltarni 1 and Yampi 1 wells were

selected for backstripping to further constrain the

breakup age and sea-floor spreading in the Argo Abyssal

Plain (Figure 8). All wells are located on the outer North

West Shelf in the Bonaparte and Browse Basins south of

the Timor Trough.

Figure 8 shows the tectonic subsidence curves of the

four wells. Hadrian 1 shows a rifting event between 150

and 130 Ma followed by thermal subsidence until 70 Ma.

The Yampi 1 well indicates a rifting event at around

150 – 130 Ma, also followed by thermal subsidence. Both,

Longleat 1 and Taltarni 1 show a phase of rifting

between 170 and 155 Ma followed by thermal subsidence.

All data show the onset of significant extension and

accelerated subsidence at a time that correlates well

with a Late Jurassic breakup and onset of sea-floor

spreading in the Argo Abyssal Plain. Whereas the

duration of fast syn-rift subsidence at the Longleat and

Taltarni sites matches well with the onset of sea-floor

spreading determined from magnetic anomaly data, the

transition from rifting to thermal subsidence implied by

the data from Yampi 1 and Hadrian 1 is slightly younger.

Figure 5 Magnetic wiggle map for the northwest Australian shelf and adjacent abyssal plains. Wiggle azimuth is 408. Solidlines represent isochrons: M25, orange; M24, blue; M22A, magenta; M13, light blue; M11A, grey; M10N, lilac; M6, apricot; M4,

yellow; M2, green; M0, black. Fracture zones are shown as solid red lines, interpreted transition from continental to oceanic

crust as dashed black line, pseudofaults as dashed dark-green lines, and extinct ridges as short-dashed red lines. DSDP/ODP

sites annotated with their respective site number. Abbreviations as in Figure 1. Map extent is shown in Figure 6.

32 C. Heine and R. D. Muller32 C. Heine and R. D. Muller

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Given the errors in dating, these differences are not

significant.

Generally, an Oxfordian –Kimmeridgian age of sea-

floor spreading in the Argo Abyssal Plain and along the

northern Australian margin, followed by thermal sub-

sidence is supported by the well data, as opposed to a

later onset of sea-floor spreading (e.g. in Tithonian –

Berriasian times) suggested by previous interpreta-

tions.

SEA-FLOOR SPREADING AROUND THE NORTHERNAND NORTHWESTERN AUSTRALIAN MARGIN

The last major period of rifting on the southern Tethyan

margin of northeastern Gondwana started in the

Triassic and ended with the final breakup between

Australia and India in the Valanginian.

Margin geometry

Post-Rhaetian tectonic activity along the western

margin of Australian Gondwana (von Rad et al. 1992;

Veevers & Tewari 1995) had probably created a zone of

weakness extending from the Carnarvon Terrace area

in the north, down to the Perth Basin in the south,

which acted as a preferred major tectonic boundary

between Indian and Australian Gondwana from the

Late Triassic to the Cretaceous (Veevers & Tewari

1995). Along the northern Australian Gondwana mar-

gin, the present-day Argo embayment represents a

globally unique feature in terms of its geometry. A

triangular-shaped block of continental crust was rifted

out of the coherent northern Australian margin,

between the Exmouth Plateau in the west and the

Scott Plateau/North West Shelf proper in the east.

Additionally, a set of basins (the Carnarvon Basin/

Rankin Trend and the Browse/Bonaparte Basins),

converge towards the southern tip of the Argo Abyssal

Plain, indicating that breakup in the southern Argo

Abyssal Plain was probably triggered by a localised

lithospheric weakness.

Recent extensional modelling of lithospheric rheol-

ogy of the Northern Carnarvon Basin assumes a

symmetric wide rift (Westralian Superbasin) caused by

a Permo-Carboniferous extension. This extension was

followed by a Late Permian –Late Triassic sag phase and

asymmetric breakup with deformation localised in

marginal narrow rift basins in the Late Jurassic

(Gartrell 2000). It is likely that the breakup of the

Sibumasu continental sliver off the northeastern Gond-

wana margin in the early Late Permian (Metcalfe 1999)

Figure 6 Interpreted isochrons superimposed on satellite-derived gravity map. Solid lines represent isochrons: M25, orange;

M24, blue; M22A, magenta; M13, light blue; M11A, grey; M10N, lilac; M6, apricot; M4, yellow; M2, green; M0, black. Fracture

zones as solid red lines, interpreted transition from continental to oceanic crust as dashed black line, pseudofaults as long-

dashed red lines and extinct ridges as short-dashed red lines. DSDP/ODP sites annotated with their respective site number.

Abbreviations as in Figure 1; T, Trough. White rectangle shows extent of Figure 5.

Jurassic North West Shelf rifting 33

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caused the development of a uniformly extended/

thinned wide continental margin, representing the wide

side of a rift basin. The Late Jurassic Argo rifting

episode affecting the northern margin has then resulted

in another localised deformation, reactivating older

extensional structures and pre-existing lithospheric

weaknesses. On the North West Shelf this scenario is

supported by the present-day COB and a southern line of

abandoned rift basins (Figures 9, 10c, d – the Carnarvon,

Browse and Bonaparte Basins. As Gartrell (2000)

pointed out, one side of the rift system is likely to be

weaker than the other, leading to asymmetric breakup.

One possible explanation of this localised weakness

causing the unusual margin geometry around the Argo

Abyssal Plain is a proposed Permo-Triassic extraterres-

trial event at the Bedout High Structure (Becker et al.

2004).

Plate-tectonic evolution

In Late Jurassic –Early Cretaceous times, the northern

margin of Australian Gondwana was likely a wide

continental shelf underlain by extended continental

crust. Following previous work by Metcalfe (1994, 1996,

1999) and including further evidence from geological

data compiled from southeast Asia, we regard the

West Burma Block as a conjugate part to the Argo

Abyssal Plain embayment prior to the rifting (Figure

10a). Lithological and facies architectural affinities

between Triassic sequences containing Halobia bi-

valves found in both Timor and the Indo-Burman

Ranges represent constraints for a pre-rift position of

the West Burma Block adjacent to the North West

Shelf (UNESCAP 2002; Socquet et al. 2002; Gramann

1974). In the vicinity of the future line of breakup,

uplift was dominant (von Rad et al. 1992), causing

erosion of the Jurassic sedimentary section on con-

Figure 7 Sketch illustrating the southward ridge jump in the

northern Argo Abyssal Plain. (a) Normal spreading between

Australian Plate (grey) and the West Burma microplate

(white). (b) Initiation of the Argo Plate that is later attached

to the West Burma Plate. (c) Southward ridge jump at M15–

M14 (136.7 – 135.8 Ma) abandoning the northern spreading

centre and continued spreading in the south between M22A

and M22/21 of the Australian Plate. This detaches the

conjugate M21 –M15 anomalies of the Australian Plate and

attaches them to the West Burma Plate. Only isochrons used

in the reconstruction are shown. AUS, Australia; ARGO,

Argo Plate; BUR, West Burma Plate; ASR, abandoned

spreading ridge; SR, spreading ridge.

Figure 8 Calculated tectonic subsidence curves of four wells

located on the outer North West Shelf in the Browse and

Bonaparte Basins. All wells show the end of significant

accelerated tectonic subsidence in an interval between 155

and 145 Ma, indicating that most of the younger extension

was taken up by the sea-floor spreading ridge. The Yampi 1

well is located close to the line of abandoned Late Jurassic

rift basins on the inner North West Shelf, where extension

was likely to have lasted longer. Vertical error bars indicate

uncertainties in water depth.

34 C. Heine and R. D. Muller34 C. Heine and R. D. Muller

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jugate rift shoulders. This erosional unconformity is

also found in the Indo-Burman Ranges of the West

Burma Block (Bender 1983; Mitchell 1993), the Wombat

Plateau and on the outer North West Shelf (Gradstein

1992; von Rad et al. 1992). In reconstructing the pre-

collision outline and size of the West Burma Block, a

rough estimate of 150% of the present-day length and

width was used. It is assumed that the southern COB

of the present-day West Burma Block was conjugate to

the North West Shelf (Figure 10a) and that the part

rifted from the Gascoyne Abyssal Plain margin was

subducted beneath the eastern Himalayas when the

Indian Plate collided with the southern Eurasian

margin.

The Sikuleh and the Meratus Blocks, now located in

western Sumatra and southeastern Kalimantan, respec-

tively, are potential candidates for a northern Gondwana

origin (Gorur & Sengor 1992; Metcalfe 1996), separated

together with the West Burma Block by the Argo

spreading ridge. Although Barber (2000) doubted the

allochthonous character of the Woyla Blocks in South-

west Sumatra, a pre-rift position of the Sikuleh fragment

east of the West Burma Block (approximately between

Timor and the present-day Bird’s Head region) is

supported by the drift path in our reconstructions (Heine

et al. 2004). In our palaeogeographic reconstruction of the

pre-rift configuration of the northern Australianmargin,

the Sikuleh andMeratus Blocks occupy a position east of

the West Burma Block (Figure 10a).

Regional uplift and extensive volcanic activity on the

Wombat/Exmouth Plateau, Rowley Terrace and Scott

Plateau during the Callovian –Oxfordian (von Rad et al.

1992; Crawford & von Rad 1994) initiated the separation

of the West Burma Block from the northwest Australian

Tethys margin. Interpreted magnetic anomaly data from

the eastern Gascoyne Abyssal Plain also indicates

rifting and subsequent sea-floor spreading west of the

Exmouth Plateau (Figure 10b). Tilting and uplift in the

Wombat Plateau area may indicate that the ridge tried

unsuccessfully to propagate a rift to the continental

promontory of the Platypus Spur/Wombat Plateau,

starting from both sides—from the Argo Abyssal Plain

to the west, and from the Gascoyne Abyssal Plain to the

east, respectively.

Because the Oxfordian transgression is documented

in the stratigraphic record from the North West Shelf

to Irian Jaya (Pigram & Symonds 1991; M. Norvick

pers. comm. 2000) sea-floor spreading is considered to

have continued all along the northern margin. It is

likely that the rift graben and later sea-floor spreading

developed in either highly extended continental crust

or small compartments, which followed pre-existing

structural weaknesses. The present-day Weber Deep in

which the subduction zone of the eastern Banda Arc

migrated, could have also represented such an embay-

ment, similar to the Argo Abyssal Plain. Therefore,

isochrons have been constructed from the Gascoyne

and Argo Abyssal Plains up to the Bird’s Head area of

Irian Jaya. Recent analysis of basalts from the

Mesozoic Central Ophiolite Belt of Papua New Guinea

show a geochemical signature that reveal a backarc

origin in a subduction regime (Monnier et al. 2000). A

subduction zone extending along the eastern margin of

Australia up to Papua New Guinea is also supported by

the existence of Tasmanide basement in Irian Jaya (M.

Norvick pers. comm. 2000). This does not allow a

continuation of the Argo-trend sea-floor spreading all

around Irian Jaya into the Coral Sea, although base-

ment subsidence curves from the Papuan Basin show

accelerated subsidence synchronous to the breakup in

the Argo Abyssal Plain (Pigram & Symonds 1991).

Large uncertainties in the reconstruction of the

Tethyan –Pacific transitional zone have been another

argument to limit the isochrons to the east in the

Bird’s Head region.

Formation of oceanic crust on the northwest

margin started in the Oxfordian, documented by the

anomalies M26 –M24A (154.8 – 153.1Ma) in the Gas-

coyne and Argo Abyssal Plains (Figure 10b). They

are bound by the continental crust of the Platypus

Spur, northern margin of the Exmouth/Wombat

Plateau and Scott Plateau area, implying sea-floor

spreading in isolated compartments.

Mapped Jurassic sedimentary basins on the North

West Shelf south of Timor are parallel to the Argo

spreading direction and fracture zone trend. They may

indicate that extension was probably focused in a

northern and southern zone of rifting, supporting the

model of Gartrell (2000) for large parts of the northern

Australian margin at that time. The northern rift

succeeded whereas the southern rift zone was aban-

doned.

In our reconstructions, the M24 (152.1Ma) isochron

corresponds to the oldest continuous magnetic anom-

aly along the northern margin, seaward of the

Figure 9 Geometry and possible rift width of the northern

Australian Gondwana margin at 130 Ma after the West

Burma Block breakup. Australia fixed in present-day

coordinates. ExP, Exmouth Plateau; ScP, Scott Plateau;

CT, Carnarvon Terrace; BE, Banda Embayment; BH, Bird’s

Head; CRFz, Cape Range Fracture Zone; RR, Roo Rise; JR,

Joey Rise.

Jurassic North West Shelf rifting 35

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Figure 10 Palaeogeographic reconstructions of the North West Shelf at (a) 156 Ma, (b) 150.0 Ma, (c) 136.0 Ma and (d) 130.0 Ma.

Australia fixed in present-day coordinates. ExP, Exmouth Plateau; ScP, Scott Plateau; CT, Carnarvon Terrace; BE, Banda

Embayment; BH, Bird’s Head; Sik, Sikuleh allochthonous terrane; Mer, Meratus Block; MBT, Main Boundary Thrust; SB,

Shan Boundary fault; SDRS, seaward-dipping reflectors; CRFz, Cape Range Fracture Zone; WZFz, Wallaby–Zenith Fracture

Zone; JR, Joey Rise; PF, Pseudofault; ASC, abandoned spreading centre.

Table 2 Finite rotation poles calculated for motions of the West Burma Plate relative to the Australian Plate (except where otherwise

indicated).

Chron Time (Ma) Latitude (8N) Longitude (8E) Angle (8 + =clockwise)

Fit reconstruction 155.9 10.36 115.73 125.28

M25 154.1 11.10 115.23 123.31

M24 152.1 11.93 114.66 121.16

M22A 150.4 12.58 114.21 119.54

M21 146.7 14.11 113.15 115.81

M16 137.9 18.04 110.32 107.06

Ridge jump 135.8 19.01 109.61 105.11

M13 135.3 19.09 109.40 104.39

M11A 133.3 19.65 107.67 102.45

M10N 130.9 20.40 106.86 99.67

M10N relative to India 130.9 43.77 68.70 97.41

36 C. Heine and R. D. Muller36 C. Heine and R. D. Muller

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undulating COB of the Late Jurassic –Early Cretaceous

margin up to the Banda embayment and half-way

around the Bird’s Head, joining the different compart-

ments. North of the Platypus Spur M24 is the oldest

anomaly identified, joining the spreading centres of the

Gascoyne and Argo Abyssal Plains. In the northeastern

corner of the Argo Abyssal Plain, M24 links up directly

with the Java Trench and is the oldest anomaly off the

COB.

The role of the Banda embayment is not yet

resolved and remains problematic, Charlton (2001)

proposed that exhumed basement and the West Burma

block originated here, later rifted by sea-floor spread-

ing, although this is difficult to reconcile with the

required spreading geometries. However, according to

our reconstructions, it may represent an embayment

like the Argo Abyssal Plain, formed by isolated sea-

floor spreading earlier in the Kimmeridgian/Oxfor-

dian with M24 as the first continuous isochron around

the northern margin. Alternatively, highly extended

continental crust could also make up basement of

the Banda embayment. As this still remains

speculative, isochrons for this area were not recon-

structed.

The M24 isochron for the Argo Abyssal Plain was

extended along the northern margin up to the Bird’s

Head, using the derived stage and finite rotation poles

(Figure 10b; Table 2). A major transform fault is likely to

have set off the isochron around the Bird’s Head

promontory. Because of the southward ridge jump at

135.8Ma (M14), incorporating the anomalies M22/M21

(148.1/146.7 Ma) –M15 (136.6 Ma) onto the Burma micro-

plate, a transitional ‘Argo Plate’ has been invoked for

the model. All isochrons located on this transitional

plate, now completely lost by subduction, were recon-

structed on the base of symmetrical spreading and with

the same stage rotation poles until M15, prior to the

ridge jump (Figure 10c).

Following the southward ridge jump in the Argo

Abyssal Plain, a successive 158 anticlockwise change in

spreading direction occurred (Figure 10d). The rotation

in the spreading direction is documented by fan-shaped

magnetic lineations north of the pseudofault in the Argo

Abyssal Plain (Figures 6, 10d). The volcanic Joey Rise

area (von Rad et al. 1992) is probably related to this

event. Regional uplift and termination of the Barrow

Delta on the Exmouth Plateau indicates the onset of the

India –Australia breakup. By the time of chron M10N

(130.8 Ma) the former Argo spreading direction was

parallel to the opening between India and Australia,

most likely with a transform offset along the Cape Range

Fracture Zone and continued production of oceanic

crust in the Cuvier Abyssal Plain.

A northward propagating ridge event captured

Indian ocean crust in the Cuvier Abyssal Plain (Mihut

1997; Mihut & Muller, 1998b). North of the Cape Range

Fracture Zone a small eastward ridge jump trans-

ferred pieces of recently produced Australian Plate

ocean crust onto the Indian Plate, breaking down the

long offset of the Cape Range Fracture Zone into

smaller, energetically economic, staircase-shaped seg-

ments and producing a north – south-trending

pseudofault in the Gascoyne Abyssal Plain. Sedimen-

tation in the Jurassic basins stopped synchronously

when the whole western and northern margin ther-

mally subsided in the mid-Valanginian. The West

Burma Block reached the southern Sundaland margin

in the vicinity of present-day western Thailand at ca

80 Ma (Heine et al. 2004).

DISCUSSION

Our revised interpretation for the magnetic anomaly

record clearly indicates spreading in the Argo and

Gascoyne ocean basins started in Oxfordian times with

the separation of the West Burma Block from the Argo

embayment. The initiation of the boundary between

continental and oceanic crust can be dated to 155.9 Ma,

based on interpolation of the oldest isochrons and

calculated spreading velocities. Spreading started east

and west of the Exmouth Plateau with M25 (154.1 Ma)

as the first correlatable magnetic anomaly, and M26

(155.0 Ma) as the oldest identifiable anomaly in the

southern Argo Abyssal Plain. By M24 (152.1 Ma) the

two different spreading compartments were joined in a

single spreading ridge which extended from the Bird’s

Head region of Irian Jaya along the northeastern

margin of Gondwana.

This model for sea-floor spreading at the north-

western margin of the Australian Plate solves the

problematic intersection of Argo- and India –Australia-

related M anomalies north of the Platypus Spur.

According to our interpretation the Argo spreading

ridge continued around northern Greater India and was

likely connected to the Somali Basin spreading ridge.

After an anticlockwise rotation of the spreading ridge

axis between the West Burma Block and Australia the

spreading ridge north of Greater India was abandoned,

fixing the Burma microplate to the plate motions of the

Greater India Plate until the collision of the West Burma

Block with the southern Eurasian margin. The model

further confirms the identification of the West Burma

Block as the continental terrane rifting off the Austra-

lian North West Shelf in Late Jurassic as proposed by

the work of Metcalfe (1991, 1994, 1996, 1999). Other

allochthonous terrane fragments along the Sundaland

margin of Eurasia, like the Sikuleh Block of Western

Sumatra could have been part of the West Burma sliver.

We relate the unusual margin geometry around the

Argo Abyssal Plain to a localised zone of lithospheric

weakness, which may have been caused by a possible

Permo-Triassic impact at the Bedout Structure. The

model simplifies the geological evolution of this area,

and integrates most of the available structural, se-

quence stratigraphical and geophysical data. It has

important implications for the evolution of the eastern

Tethys, the convergence history of the southeast Asian

continental margin and the initial opening of the Indian

Ocean.

ACKNOWLEDGEMENTS

We would like to thank Myra Keep and Robert Iasky for

their constructive reviews which helped to vastly

Jurassic North West Shelf rifting 37

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improve the initial manuscript of this paper. Christian

Heine holds a PhD scholarship of the German Academic

Exchange Service (DAAD). We are grateful to Martin

Norvick for sharing his knowledge of the northwestern

Australian margin and southeast Asia. Tara Deen is

acknowledged for supplying the backstripping data.

Maps have been created using free GMT software

(Wessel & Smith 1991).

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