Crustal Structure of Continental Australia - CiteSeerX

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crust in the outer parts of the Carnarvon, Browse and, to a lesser degree ... Key words: deep crustal structure, Australia, velocity models, petrology of the crust.
Crustal Structure of Continental Australia; Intra-Crustal Seismic Isostasy and Crustal Composition: a Review Alexey Goncharov Australian Geological Survey Organisation, Australia E-mail: [email protected]

SUMMARY The recent deep crustal studies in Australia are characterised by common use of refraction/wide-angle reflection seismic techniques, dense observations on refraction/wide-angle profiles and accurate seismic wave field analysis prior to seismic modelling. In constraining gravity models, more emphasis has been placed on accurate velocity information and less emphasis on seismic reflection data. This paper reviews the state of knowledge of Australia's deep crustal structure including its continental margins. It emphasises most recent results and advances in interpretation and processing methods used at the Australian Geological Survey Organisation (AGSO). The Moho depth variation in Australia shows little if any correlation with the boundaries of crustal megaelements. It is believed that Australian Proterozoic crust is thicker than Archaean due to underplating. It remains unclear why Archaean crust was not underplated. Selective, Proterozoic but not Archaean, underplating is either not consistent with plate tectonics, or for its explanation some isolation of crustal and lithospheric processes within a drifting plate is required. Analysis of seismic velocity and SiO2 distributions in full crustal column may distinguish between the regions where the vertical or horizontal mass transfer in the crust has prevailed. Regions with balanced SiO2 distribution in the crust are likely to have been affected by mostly vertical mass transfer. Prominent seismic reflectors and changes in reflectivity patterns in near-vertical reflection data do not necessarily correspond to significant bulk velocity discontinuities imaged by refraction/wide-angle data. This leads to re-assessment of sediment thicknesses, degree of crustal extension and role of underplating. Major crustal thinning has occurred beneath the North Western and Southern margins of the continent. The crust in the outer parts of the Carnarvon, Browse and, to a lesser degree, Canning basins at the North Western Australian Margin is not purely oceanic but rather transitional. Refraction seismic, sampling and geochemical results from the Kerguelen Plateau confirm that continental crust is present within the oceanic lithosphere in this large province. Key words: deep crustal structure, Australia, velocity models, petrology of the crust.

INTRODUCTION The Australian continent including its offshore continental margins contains geological provinces varying in age from Archaean to modern. It can be subdivided to several megaelements (Fig. 1) on the basis of different regional gravity and magnetic signatures. The western part of the continent (to the west of Tasman Fold Belt and North Queensland in Fig. 1) is underlain by Precambrian basement; while basement in the eastern part is Phanerozoic. The formation of Australia's continental margins within the plate tectonics concept is attributed to extensional processes which took place prior to separation of Australia, Antarctica and Greater India at 155-45 Ma. Since the mid-1990's scientists at AGSO have approached deep crustal studies with the following principles: • common use of refraction/wide-angle reflection seismic techniques; • dense observations on refraction/wide-angle profiles; • accurate seismic wave field analysis prior to seismic modelling; • in constraining gravity models, more emphasis has been placed on accurate velocity information and less emphasis on seismic reflection data; • downgrade of value of gravity modelling non-constrained by results of seismic interpretation; • recording of 3-component data with a purpose of studying S-waves of crustal origin, anisotropy and role of fluids in the crust. The main features of AGSO's new approach to seismic velocity characterisation of the crust are: • analysis of seismic velocity distribution in the interlinked fashion, when average velocity to any given depth becomes a new interpretational parameter; • use of petrophysical modelling technique to translate seismic velocity models into estimates of petrological composition of the crust. The two most advanced projects carried out with this new approach were the Mount Isa transect (Drummond et al. [1], Goleby et al. [2]) and the ocean-bottom seismograph (OBS) project at the North West Australian Margin (Goncharov et al. [3], [4]). This paper reviews the state of knowledge of Australia's deep crustal structure including its continental margins. It emphasises most recent results and advances in interpretation and processing methods used at the Australian Geological Survey Organisation (AGSO).

Structure and Composition of the Crust in Australia

CRUSTAL STRUCTURE ONSHORE Each of the mega-elements of the Australian continent (Fig. 1) represents a group of crustal elements with similar geophysical, geological and age characteristics (Shaw et al. [5]).

North Australia

Pinjarra Orogen

Western Australia

North Queensland

Central Australia South Australia

New England Tasman Fold Belt

Fig. 1. Mega-elements of the Australian continent, simplified from Shaw et al. [5].

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eastern part of the Tasman Fold Belt. For other parts of continental Australia 38-42 km Moho depths are typical which are close to global average values. Results of the deep seismic studies of the Australian Precambrian terranes support the concept of thickened Proterozoic crust compared to Archaean crust. Underplating by mafic melts near its base was suggested by Drummond and Collins [9] as the main process responsible for this thickening. It remains unclear why Archaean crust adjacent to the Proterozoic crust in the western part of the continent was not underplated during the same episodes of upper mantle melting. Essentially, a concept of such selective underplating is either not consistent with plate tectonics, or for its explanation some isolation of crustal and lithospheric processes within a drifting plate is required. Major crustal thinning has occurred beneath the North Western and Southern margins of the continent (Fig. 2). In cratonic areas, recent examples of good quality reflection profiling suggest crustal growth is dominated by thrusting and stacking in a compressional environment (Collins and Drummond [10]).

INTRA-CRUSTAL SEISMIC ISOSTASY Analysis of vertical seismic velocity distributions through the crust has led to a conclusion that anomalously high velocity rocks in Precambrian regions are underlain by anomalously low velocity rocks, and vice versa ('seismic isostasy' of Goncharov et al. [11]). The best characteristic to quantify this balancing effect is average velocity (ie. the ratio of any given depth to a vertical travel time to that depth). A good example of a region with 'balanced' seismic velocity distribution in the crust is the Proterozoic Mount Isa Inlier in Northern Australia where significant lateral variations in Pwave velocity at mid-crustal level (20-35km) are compensated well above the Moho. Average velocity isolines become almost horizontal by a depth of around 45 km. Above this depth, amplitude of average velocity isolines deviation from the horizontal position can be as high as 15 km (Fig. 3). Fig. 2. Location of seismic measurements and depth to the Moho from refraction seismic and receiver function studies, based on Collins [6] plus additional data from Drummond et al. [7].

The Moho depth variation (Fig. 2) shows little if any correlation with the mega-element boundaries (Drummond et al. [7]). In general, within the Archaean regions of Western Australia the Moho is relatively shallow. It is considerably deeper under the Proterozoic North Australian craton, under sedimentary basins of Central Australia and Phanerozoic

Sediments

0 10

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6.2-6.3 Depth, km

Most of these mega-elements have been studied by refraction and wide-angle reflection seismic profiles, although the density of such observations in Australia is less than on some other continents. Seismic velocity models of the Australian continent were summarised by Collins [8] and interpreted by Drummond and Collins [9] and Collins and Drummond [10]. A brief summary of these results follows.

Sediments

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6.3-6.4 Depth of ‘seismic’ isostatic compensation 6.5-6.6

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MOHO 6.8-6.9

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Fig. 3. Average seismic velocity distribution along the Mount Isa transect, from Goncharov et al. [11]. A similar observation can be made from the analysis of average velocity-depth functions from other Australian Precambrian terranes (Fig. 4).

Structure and Composition of the Crust in Australia

In order to gain an insight into crustal mass transfer, it is important to compute average velocity - depth functions in addition to the conventional interpretation of refraction/wideangle seismic data.

FROM SEISMIC VELOCITIES TO CRUSTAL COMPOSITION A commonly used method to translate seismic velocity models into models of crustal composition is to use velocity measurements in the laboratory under elevated PTconditions. The problem with this approach is that the number of rock types is too large (eg. 29 rock types in a recent compilation of Christensen and Mooney [12], Fig. 5) while their bulk geochemistry is not always clearly defined. 8.0-8.2 7.8-8.0

Depth, km

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ECL

DUN

PYX

HBL

GAB

ANO

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GGR

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DIO

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BZE

FGR

BBP

PGR

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QSC

SLT

PHY

BGN

QTZ

BAS

GGN

AND

55 SER

6.9-7.3 6.5-6.9

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7.7-8.1

Fig. 6. Velocity-depth-rock type diagram for the endmember rock types of Sobolev and Babeyko [13]. The Mount Isa region PT-conditions after Goncharov et al. [14]. Rock names define bulk geochemistry rather than specific mineralogical composition. By correlating seismic velocities observed in some region to a velocity-depth-rock type diagram constructed for appropriate PT-conditions, we define the proportion of various rock types at a number of depth ranges. As soon as the bulk geochemical composition within each type of rock is constant, we obtain SiO2 distribution in the crust as a by-product of this methodology (illustrated in Fig. 7 for the models of the Mt Isa refraction line).

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7.0-7.4

6.6-7.0

6.2-6.6

5.8-6.2

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MGW

8.1-8.3

Lherzolite 3

A limited data set from the region of the Kola superdeep bore hole (KSDB) in the Baltic Shield confirms that regions with high velocity and density anomalies in the crust seem to be isostatically compensated above the Moho in a conventional sense as well (Goncharov et al. [11]). However, the possibility of global translation of 'seismic isostasy' into conventional isostasy remains a subject for further studies.

0

Lherzolite 1

Unlike in the Precamrian terranes, seismic isostasy is not achieved above the Moho at the North West Australian Margin. For example, the Carnarvon transect (for its location see Fig. 8) shows a significant deviation of average velocity isolines from the horizontal position throughout the whole crust. Even at the deepest observed Moho the deviation is more than +3 km.

7.3-7.7

Fig. 4. Average seismic velocity-depth functions in different Precambrian regions, from Goncharov et al. [11]. Kola SDBH - vertical seismic profiling data from the Kola superdeep bore hole.

Gabbro 4

70

Trying to avoid these complications, we have developed a different approach relying on the petrophysical modelling technique of Sobolev and Babeyko [13]. We use this method to predict seismic velocities at depth for a range of assumed crustal compositions. The method considers igneous rocks only, and the possibility of meta-sedimentary rocks in the deep crust is ignored. An important feature of our approach is that we treat the crust as a mixture of a limited number of rock types ('granites', 'diorites', 'gabbros' and 'spinel lherzolites') represented by their end-members. The bulk geochemical composition within each type of rock is kept constant and the mineralogical compositions allowed to vary to account for equilibration at pressures and temperatures likely to have existed when the rock was formed (illustrated in Fig. 6 for the Mt Isa region in Australia).

Gabbro 1

Tennant Creek - Mount Isa Shallowest Moho

Diorite 4

Depth, km

Mount Isa region, east of Mount Isa Mount Isa - Tennant Creek Mount Isa region, near Mount Isa

Diorite 1

Kola SDBH

Granite 5

0

Christensen and Mooney [12]. SER - serpentinite, BAS basalt, GRA- granite-granodiorite, DIO - Diorite, GGR mafic garnet granulite, GAB - gabbro-norite-troctolite, PYX - pyroxenite, ECL - mafic eclogite, DUN - dunite, for other abbreviations of rock names and heat flow estimates refer to Christensen and Mooney [12].

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7.0

Depth, km

Average velocity, km/s

Granite 1

5.0

Goncharov

7.4-7.8

Fig. 5. Velocity-depth-rock type diagram constructed for the average heat flow conditions from the data of

In some regions models with balanced seismic velocity distribution along a vertical profile translate into petrological models with balanced distribution of felsic, intermediate and mafic rocks in the crust. The crust of the Mount Isa Inlier is a good example in that respect. Qualitatively this conclusion can be derived from the analysis of SiO2 variation with depth: there is a noticeable compensation for less felsic rock in the middle crust in the model of the anomalous middle part of the profile, by the more felsic rock underneath (Fig. 7).

Structure and Composition of the Crust in Australia

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SiO2, %

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Depth, km

Anomalous m iddle part of the profile

Whole profile average

Averaged in an expanding depth range window

MOHO Computed from the velocity models 70

Fig. 7. SiO2 distribution in the crust of the Mount Isa Inlier in Northern Australia, after Goncharov et al. [14]. One pair of the charts is the SiO2 distribution computed from velocity models (instantaneous value at any given depth), the other pair - SiO2 averaged in an expanding depth range window. Within each pair one model is whole line average, the other - representative model for the middle part of the line where a high-velocity body has been detected at mid-crustal level. Analysis of SiO2 contents averaged in an expanding depth range window (Fig. 7) clearly shows that whole-line average and anomalous middle-part curves merge to less than 1% difference at a depth of 45 km (about 10 km above the Moho). At this depth the model for the anomalous middle part becomes indistinguishable from the whole line average model. These results are more consistent with magmatic rather than tectonic emplacement of the mid-crustal high-velocity body because they imply some kind of crustal melting and fractionation during which lower crust underneath the highvelocity body might have been depleted of its mafic component.

Goncharov

As a result of continental extension, deep sedimentary basins have formed on the Australia's North Western and Southern margins. The geological configuration of Australia's southern margin has formed as a result of Late Jurassic to Early Cretaceous events. A number of sedimentary basins developed as part of this process which contain up to 15 km thick sedimentary section of ?Jurassic to Tertiary age (Totterdell et al. [16]). Deep structure of the crust in this region, particularly below the basement, is poorly known, and we are in the process of studying it on the basis of refraction seismic, gravity and deep seismic reflection data which have recently become available. Amongst the Australian continental margins, the North West Margin (NWAM), which has a large hydrocarbon potential, is the most studied. Its deep crustal structure has been studied using: • velocity information derived from AGSO’s unique refraction/wide-angle reflection data set recorded by ocean-bottom seismographs (OBS); and • interpretation of AGSO’s network of deep seismic reflection profiles. Integration of conventional reflection and refraction/wideangle reflection data from the NWAM has resulted in much more accurate seismic velocity estimates than those normally available from one data set only. We used the same approach to interpretation of refraction/wide-angle reflection data as discussed above. The OBS survey was carried out along 5 profiles (Fig. 8) of total length ~2800 km. Data in this experiment were recorded to maximum offsets of 300 km. All OBS transects coincided with previously recorded deep crustal reflection profiles.

If a thrust type tectonic emplacement of the mid-crustal highvelocity body was to be considered, than that thrust had to stop exactly in a position where the balance in the SiO2 distribution was achieved. Such a geochemical control over pure mechanical movement is hard to justify. Therefore, an alternative model involving large-scale tectonic movement with significant horizontal component suggested by MacCready et al. [15] is a less likely option for the evolution of the crust in the Mount Isa region. Generally, a degree of balance in the SiO2 distribution can be used as an additional criterion to distinguish between the regions where the vertical or horizontal mass transfer in the crust has prevailed. Regions with balanced SiO2 distribution in the crust are likely to have been affected by mostly vertical mass transfer. The situation at the NWAM where we do not observe a balance in seismic velocity distribution in the crust above the Moho is unclear in terms of the SiO2 distribution and its tectonic consequences. We have not yet developed a way to account for the effect of sediments on translation of seismic velocities into estimates of crustal composition.

CRUSTAL STRUCTURE OFFSHORE

Fig. 8. Locations of the profiles in the AGSO OBS experiment, and major structural features, North West Australian Margin. Dots show the locations of the OBS stations. The lines are numbered: 1-Carnarvon, 2Canning, 3-Browse, 4-Petrel, 5-Vulcan. The most noticeable observation from the co-interpretation of the OBS and conventional reflection data is that prominent seismic reflectors and changes in reflectivity patterns in conventional reflection data do not necessarily correspond to significant bulk velocity discontinuities imaged by refraction/wide-angle data.

Structure and Composition of the Crust in Australia

For example, a prominent 6.0 km/s refractor imaged along the Petrel line shows rather poor correlation with the reflectivity of the crust imaged by the coincident conventional reflection data (Pylypenko and Goncharov [17]). The depth to the crystalline basement defined on the basis of velocity information derived from the OBS studies along this line is considerably less than that defined from the conventional reflection data. This means less thick sedimentary cover with implications for modelling of crustal extension in the Petrel sub-basin. Similar examples from other coincident OBS/reflection lines lead to a conclusion that only a combination of both seismic techniques provides reliable information on the deep crustal structure. Crustal scale OBS-derived models of the NWAM show a very significant degree of variation, particularly in the lower crust. Velocity scans of these models on the basis of velocity values predicted by the petrophysical modelling technique discussed above do not reveal significant volumes of rocks with gabbrotype bulk geochemistry. Therefore, underplating, which is often associated with large-scale extension of the crust, does not look like a major crustal forming event in the region. It appears to have been restricted only to the offshore Canning compartment (labelled Roebuck in Fig. 8) and the outer, western part of the Carnarvon compartment where such material is present. Oceanic crust adjacent to the outer boundary of the NWAM (Fig. 8) is considerably thicker and has lower velocities than in global average model of oceanic crust (Fig. 9).

Velocity, km/s 1

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The Carnarvon and Browse transects are characterised by particularly low velocities in the oceanic crust. Only on the Canning (Roebuck) transect oceanic crust approaches the global average model of White et al [18]. But even after the water depth adjustment, this model still has almost 1 km/s lower velocities in the upper oceanic crust compared to the global average model. The anomalously low velocities in the oceanic crust may reflect the role of volcanism in the formation of this part of Australia's continental margin. Velocity-depth functions characterising oceanic crust on the Carnarvon and Browse transects are in between the global average oceanic and water depth-adjusted continental models. Velocities typical for rocks of pure gabbro-type bulk geochemistry are considerably higher than those observed on the Carnarvon and Browse oceanic segments (Fig. 9), which is not consistent with modern ideas on the formation of 'layer 3' in oceanic crust. An overall conclusion from these comparisons is that the crust studied by the outer parts of the Carnarvon, Browse and, to a lesser degree, Canning transects is not purely oceanic but rather transitional.

OCEANIC PLATEAUS AROUND AUSTRALIA An outstanding problem with plate tectonic reconstructions around Australia is the Kerguelen Plateau. It is one of the largest volcanic plateaus in the world and the largest in the Southern Ocean and is nearly three times the size of Japan or four times the size of the British Isles. It extends for more than 2200 km in a NW-SE direction and lies in relatively deep water (1000 to 4000 m). Geological sampling and drilling shows that it was emergent or under shallow water for up to 40 million years of its history (Borissova et al. [19]). Seismic refraction data (Charvis et al. [20], Borissova et al. [19]) indicate that crustal thickness reaches 18-22km under the central and southern parts of the plateau and only 7-8km under the Labuan Basin, which adjoins the plateau in the east. These results combined with some sampling data and geochemical results (Konnecke et al [21]) confirm that continental crust is present within the lithosphere of this large oceanic province.

5

Depth, km

Goncharov

10 15 20

CONCLUSIONS

25

1.

30 Carnarvon Canning Browse Global average oceanic crust Global average continental crust Gabbro-type velocity corridor

Figure 9. Seismic velocity models of oceanic crust studied by three transects (Carnarvon, Canning and Browse) at the NWAM compared to global average models of oceanic (White et al [18]) and continental (Christensen & Mooney [12]) crust. A 4.8 km-thick water layer added at the top of the global average model of continental crust.

2. 3. 4.

5. 6.

The Moho depth variation in Australia shows little if any correlation with the boundaries of crustal megaelements. It is believed that Australian Proterozoic crust is thicker than Archaean due to underplating. It remains unclear why Archaean crust was not underplated. Selective, Proterozoic but not Archaean, underplating is either not consistent with plate tectonics, or for its explanation some isolation of crustal and lithospheric processes within a drifting plate is required. Major crustal thinning has occurred beneath the North Western and Southern margins of the continent. Analysis of seismic velocity and SiO2 distributions in full crustal column may distinguish between the regions where the vertical or horizontal mass transfer in the

Structure and Composition of the Crust in Australia

7.

8.

9.

crust has prevailed. Regions with balanced SiO2 distribution in the crust are likely to have been affected by mostly vertical mass transfer. Prominent seismic reflectors and changes in reflectivity patterns in conventional reflection data do not necessarily correspond to significant bulk velocity discontinuities imaged by refraction/wide-angle data; this leads to re-assessment of sedimentary thicknesses, degree of extension of the crust and role of underplating in its formation. The crust studied by the outer parts of the Carnarvon, Browse and, to a lesser degree, Canning transects at the North West Australian Margin is not purely oceanic but rather transitional. Refraction seismic, sampling and geochemical results from the Kerguelen Plateau confirm that continental crust is present within the lithosphere of this large oceanic province.

Goncharov

[7]

[8]

[9]

[10]

ACKNOWLEDGEMENTS I thank AGSO and FORTUM PETROLEUM for providing financial support for my trip to Norway in May 2001 to present this paper at the International Workshop on Global Wrench Tectonics. Discussions with Clive Collins, Jim Colwell and Heike Struckmeyer helped me to formulate some of the ideas presented in this paper. Peter Petkovic and Jim Colwell provided valuable criticism of an earlier version of the manuscript. The author publishes with the permission of the Chief Executive Officer of the Australian Geological Survey Organisation. The AGSO Catalogue reference number for this paper is 36163.

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