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 INTRODUCTION The Australian continent including its offshore continental The recent deep crustal studies in Australia are margins contains geological provinces varying in age from characterised by common use of refraction/wide-angle Archaean to modern. It can be subdivided to several mega- reflection seismic techniques, dense observations on elements (Fig. 1) on the basis of different regional gravity refraction/wide-angle profiles and accurate seismic wave and magnetic signatures. The western part of the continent field analysis prior to seismic modelling. In constraining (to the west of Tasman Fold Belt and North Queensland in gravity models, more emphasis has been placed on Fig. 1) is underlain by Precambrian basement; while accurate velocity information and less emphasis on basement in the eastern part is Phanerozoic. The formation of seismic reflection data. This paper reviews the state of Australia's continental margins within the plate tectonics knowledge of Australia's deep crustal structure including concept is attributed to extensional processes which took its continental margins. It emphasises most recent results place prior to separation of Australia, Antarctica and Greater and advances in interpretation and processing methods India at 155-45 Ma. used at the Australian Geological Survey Organisation (AGSO). Since the mid-1990's scientists at AGSO have approached deep crustal studies with the following principles: The Moho depth variation in Australia shows little if any

correlation with the boundaries of crustal mega- · common use of refraction/wide-angle reflection seismic elements. It is believed that Australian Proterozoic crust techniques; is thicker than Archaean due to underplating. It remains · dense observations on refraction/wide-angle profiles; unclear why Archaean crust was not underplated. · accurate seismic wave field analysis prior to seismic Selective, Proterozoic but not Archaean, underplating is modelling; either not consistent with plate tectonics, or for its · in constraining gravity models, more emphasis has been explanation some isolation of crustal and lithospheric placed on accurate velocity information and less processes within a drifting plate is required. emphasis on seismic reflection data; · downgrade of value of gravity modelling non-constrained

Analysis of seismic velocity and SiO2 distributions in by results of seismic interpretation; full crustal column may distinguish between the regions · recording of 3-component data with a purpose of where the vertical or horizontal mass transfer in the studying S-waves of crustal origin, anisotropy and role of crust has prevailed. Regions with balanced SiO2 fluids in the crust. distribution in the crust are likely to have been affected by mostly vertical mass transfer. The main features of AGSO's new approach to seismic velocity characterisation of the crust are: Prominent seismic reflectors and changes in reflectivity · analysis of seismic velocity distribution in the inter- patterns in near-vertical reflection data do not linked fashion, when average velocity to any given depth necessarily correspond to significant bulk velocity becomes a new interpretational parameter; discontinuities imaged by refraction/wide-angle data. · use of petrophysical modelling technique to translate This leads to re-assessment of sediment thicknesses, seismic velocity models into estimates of petrological degree of crustal extension and role of underplating. composition of the crust.

Major crustal thinning has occurred beneath the North The two most advanced projects carried out with this new Western and Southern margins of the continent. The approach were the Mount Isa transect (Drummond et al. [1], crust in the outer parts of the Carnarvon, Browse and, to Goleby et al. [2]) and the ocean-bottom seismograph (OBS) a lesser degree, Canning basins at the North Western project at the North West Australian Margin (Goncharov et Australian Margin is not purely oceanic but rather al. [3], [4]). transitional. Refraction seismic, sampling and geochemical results from the Kerguelen Plateau confirm This paper reviews the state of knowledge of Australia's deep that continental crust is present within the oceanic crustal structure including its continental margins. It lithosphere in this large province. emphasises most recent results and advances in interpretation and processing methods used at the Australian Geological Key words: deep crustal structure, Australia, velocity Survey Organisation (AGSO). models, petrology of the crust. Structure and Composition of the Crust in Australia Goncharov

CRUSTAL STRUCTURE ONSHORE eastern part of the Tasman Fold Belt. For other parts of continental Australia 38-42 km Moho depths are typical Each of the mega-elements of the Australian continent (Fig. which are close to global average values. 1) represents a group of crustal elements with similar geophysical, geological and age characteristics (Shaw et al. Results of the deep seismic studies of the Australian [5]). 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 North North not underplated during the same episodes of upper mantle Australia Queensland melting. Essentially, a concept of such selective underplating is either not consistent with plate tectonics, or for its Central Australia explanation some isolation of crustal and lithospheric Western New processes within a drifting plate is required. Pinjarra Australia England Orogen South Australia Tasman Fold Belt 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 Fig. 1. Mega-elements of the Australian continent, Drummond [10]). simplified from Shaw et al. [5]. 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 P- wave 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 Sediments Sediments Moho from refraction seismic and receiver function 0 studies, based on Collins [6] plus additional data from 6.0-6.1 6.1-6.2 10 Drummond et al. [7]. 6.2-6.3 20

Most of these mega-elements have been studied by refraction 6.3-6.4 30 and wide-angle reflection seismic profiles, although the Depth of ‘seismic’ isostatic compensation 6.4-6.5 40 density of such observations in Australia is less than on some Depth, km 6.5-6.6 6.6-6.7 50 6.7-6.8 other continents. Seismic velocity models of the Australian MOHO 6.8-6.9 60 continent were summarised by Collins [8] and interpreted by 6.9-7.0 Drummond and Collins [9] and Collins and Drummond [10]. 0 100 200 300 400 500 A brief summary of these results follows.

The Moho depth variation (Fig. 2) shows little if any Fig. 3. Average seismic velocity distribution along the correlation with the mega-element boundaries (Drummond et Mount Isa transect, from Goncharov et al. [11]. al. [7]). In general, within the Archaean regions of Western Australia the Moho is relatively shallow. It is considerably A similar observation can be made from the analysis of deeper under the Proterozoic North Australian craton, under average velocity-depth functions from other Australian sedimentary basins of Central Australia and Phanerozoic Precambrian terranes (Fig. 4). Structure and Composition of the Crust in Australia Goncharov

Christensen and Mooney [12]. SER - serpentinite, BAS - Average velocity, km/s 5.0 7.0 basalt, GRA- granite-granodiorite, DIO - Diorite, GGR - 0 mafic garnet granulite, GAB - gabbro-norite-troctolite, Kola SDBH PYX - pyroxenite, ECL - mafic eclogite, DUN - dunite, for Mount Isa region, east of Mount Isa other abbreviations of rock names and heat flow estimates Mount Isa - Tennant Creek refer to Christensen and Mooney [12]. Mount Isa region, near Mount Isa

Depth, km Tennant Creek - Mount Isa Trying to avoid these complications, we have developed a Shallowest Moho different approach relying on the petrophysical modelling 70 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 Fig. 4. Average seismic velocity-depth functions in only, and the possibility of meta-sedimentary rocks in the different Precambrian regions, from Goncharov et al. deep crust is ignored. An important feature of our approach is [11]. Kola SDBH - vertical seismic profiling data from the that we treat the crust as a mixture of a limited number of Kola superdeep bore hole. rock types ('granites', 'diorites', 'gabbros' and 'spinel lherzolites') represented by their end-members. The bulk Unlike in the Precamrian terranes, seismic isostasy is not geochemical composition within each type of rock is kept achieved above the Moho at the North West Australian constant and the mineralogical compositions allowed to vary Margin. For example, the Carnarvon transect (for its location to account for equilibration at pressures and temperatures see Fig. 8) shows a significant deviation of average velocity likely to have existed when the rock was formed (illustrated isolines from the horizontal position throughout the whole in Fig. 6 for the Mt Isa region in Australia). crust. Even at the deepest observed Moho the deviation is 0 more than +3 km. 8.1-8.3

A limited data set from the region of the Kola superdeep bore hole (KSDB) in the Baltic Shield confirms that regions with km 6.9-7.3 7.7-8.1 high velocity and density anomalies in the crust seem to be 6.1-6.5 7.3-7.7 isostatically compensated above the Moho in a conventional Depth, 6.5-6.9 sense as well (Goncharov et al. [11]). However, the possibility of global translation of 'seismic isostasy' into 70 conventional isostasy remains a subject for further studies. 1 3 Diorite 1 Diorite 4

In order to gain an insight into crustal mass transfer, it is Granite 1 Granite 5 Gabbro 1 Gabbro 4 important to compute average velocity - depth functions in Lherzolite Lherzolite addition to the conventional interpretation of refraction/wide- angle seismic data. Fig. 6. Velocity-depth-rock type diagram for the end- member rock types of Sobolev and Babeyko [13]. The FROM SEISMIC VELOCITIES TO CRUSTAL Mount Isa region PT-conditions after Goncharov et al. COMPOSITION [14]. Rock names define bulk geochemistry rather than specific mineralogical composition. A commonly used method to translate seismic velocity models into models of crustal composition is to use velocity By correlating seismic velocities observed in some region to a measurements in the laboratory under elevated PT- velocity-depth-rock type diagram constructed for appropriate conditions. The problem with this approach is that the PT-conditions, we define the proportion of various rock types number of rock types is too large (eg. 29 rock types in a at a number of depth ranges. recent compilation of Christensen and Mooney [12], Fig. 5) while their bulk geochemistry is not always clearly defined. As soon as the bulk geochemical composition within each type of rock is constant, we obtain SiO2 distribution in the 8.0-8.2 crust as a by-product of this methodology (illustrated in Fig. 7 7.8-8.0 5 for the models of the Mt Isa refraction line). less than 15 5.8 5.8-6.2 6.2-6.6 6.6-7.0 7.0-7.4 In some regions models with balanced seismic velocity 25 distribution along a vertical profile translate into petrological 35 models with balanced distribution of felsic, intermediate and

Depth, km 45 mafic rocks in the crust. The crust of the Mount Isa Inlier is a

55 good example in that respect. Qualitatively this conclusion can be derived from the analysis of SiO2 variation with depth: DIA DIO SLT QTZ BZE PYX ECL SER PHY HBL BAS BBP FGR AND BGN QSC GRA PGR BGR AMP AGR MBL ANO GAB DUN GGN GGR MGR MGW there is a noticeable compensation for less felsic rock in the 7.4-7.8 middle crust in the model of the anomalous middle part of the Fig. 5. Velocity-depth-rock type diagram constructed for profile, by the more felsic rock underneath (Fig. 7). the average heat flow conditions from the data of Structure and Composition of the Crust in Australia Goncharov

40 SiO2, % 80 0 As a result of continental extension, deep sedimentary basins have formed on the Australia's North Western and Southern Anomalous margins. middle part of Whole the profile profile average

km The geological configuration of Australia's southern margin has formed as a result of Late Jurassic to Early Cretaceous

Depth, Averaged in an events. A number of sedimentary basins developed as part of expanding depth range window this process which contain up to 15 km thick sedimentary section of ?Jurassic to Tertiary age (Totterdell et al. [16]). MOHO Deep structure of the crust in this region, particularly below Computed from the velocity models the basement, is poorly known, and we are in the process of 70 studying it on the basis of refraction seismic, gravity and deep seismic reflection data which have recently become

Fig. 7. SiO2 distribution in the crust of the Mount Isa available. Inlier in Northern Australia, after Goncharov et al. [14]. One pair of the charts is the SiO2 distribution computed Amongst the Australian continental margins, the North West from velocity models (instantaneous value at any given Margin (NWAM), which has a large hydrocarbon potential, is depth), the other pair - SiO2 averaged in an expanding the most studied. Its deep crustal structure has been studied depth range window. Within each pair one model is whole using: line average, the other - representative model for the · velocity information derived from AGSO’s unique middle part of the line where a high-velocity body has refraction/wide-angle reflection data set recorded by been detected at mid-crustal level. ocean-bottom seismographs (OBS); and · interpretation of AGSO’s network of deep seismic Analysis of SiO2 contents averaged in an expanding depth reflection profiles. range window (Fig. 7) clearly shows that whole-line average and anomalous middle-part curves merge to less than 1% Integration of conventional reflection and refraction/wide- difference at a depth of 45 km (about 10 km above the angle reflection data from the NWAM has resulted in much Moho). At this depth the model for the anomalous middle more accurate seismic velocity estimates than those normally part becomes indistinguishable from the whole line average available from one data set only. We used the same approach model. to interpretation of refraction/wide-angle reflection data as discussed above. These results are more consistent with magmatic rather than tectonic emplacement of the mid-crustal high-velocity body The OBS survey was carried out along 5 profiles (Fig. 8) of because they imply some kind of crustal melting and total length ~2800 km. Data in this experiment were recorded fractionation during which lower crust underneath the high- to maximum offsets of 300 km. All OBS transects coincided velocity body might have been depleted of its mafic with previously recorded deep crustal reflection profiles. component.

If a thrust type tectonic emplacement of the mid-crustal high- velocity 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 Fig. 8. Locations of the profiles in the AGSO OBS in the crust are likely to have been affected by mostly vertical experiment, and major structural features, North West mass transfer. Australian Margin. Dots show the locations of the OBS stations. The lines are numbered: 1-Carnarvon, 2- The situation at the NWAM where we do not observe a Canning, 3-Browse, 4-Petrel, 5-Vulcan. balance in seismic velocity distribution in the crust above the Moho is unclear in terms of the SiO2 distribution and its The most noticeable observation from the co-interpretation of tectonic consequences. We have not yet developed a way to the OBS and conventional reflection data is that prominent account for the effect of sediments on translation of seismic seismic reflectors and changes in reflectivity patterns in velocities into estimates of crustal composition. conventional reflection data do not necessarily correspond to significant bulk velocity discontinuities imaged by CRUSTAL STRUCTURE OFFSHORE refraction/wide-angle data. Structure and Composition of the Crust in Australia Goncharov

For example, a prominent 6.0 km/s refractor imaged along The Carnarvon and Browse transects are characterised by the Petrel line shows rather poor correlation with the particularly low velocities in the oceanic crust. Only on the reflectivity of the crust imaged by the coincident conventional Canning (Roebuck) transect oceanic crust approaches the reflection data (Pylypenko and Goncharov [17]). The depth to global average model of White et al [18]. But even after the the crystalline basement defined on the basis of velocity water depth adjustment, this model still has almost 1 km/s information derived from the OBS studies along this line is lower velocities in the upper oceanic crust compared to the considerably less than that defined from the conventional global average model. The anomalously low velocities in the reflection data. This means less thick sedimentary cover with oceanic crust may reflect the role of volcanism in the implications for modelling of crustal extension in the Petrel formation of this part of Australia's continental margin. sub-basin. Velocity-depth functions characterising oceanic crust on the Similar examples from other coincident OBS/reflection lines Carnarvon and Browse transects are in between the global lead to a conclusion that only a combination of both seismic average oceanic and water depth-adjusted continental models. techniques provides reliable information on the deep crustal Velocities typical for rocks of pure gabbro-type bulk structure. geochemistry are considerably higher than those observed on the Carnarvon and Browse oceanic segments (Fig. 9), which Crustal scale OBS-derived models of the NWAM show a very is not consistent with modern ideas on the formation of 'layer significant degree of variation, particularly in the lower crust. 3' in oceanic crust. Velocity scans of these models on the basis of velocity values predicted by the petrophysical modelling technique discussed An overall conclusion from these comparisons is that the above do not reveal significant volumes of rocks with gabbro- crust studied by the outer parts of the Carnarvon, Browse type bulk geochemistry. Therefore, underplating, which is and, to a lesser degree, Canning transects is not purely often associated with large-scale extension of the crust, does oceanic but rather transitional. not look like a major crustal forming event in the region. It appears to have been restricted only to the offshore Canning OCEANIC PLATEAUS AROUND AUSTRALIA compartment (labelled Roebuck in Fig. 8) and the outer, western part of the Carnarvon compartment where such An outstanding problem with plate tectonic reconstructions material is present. around Australia is the Kerguelen Plateau. It is one of the largest volcanic plateaus in the world and the largest in the Oceanic crust adjacent to the outer boundary of the NWAM Southern Ocean and is nearly three times the size of Japan or (Fig. 8) is considerably thicker and has lower velocities than four times the size of the British Isles. It extends for more in global average model of oceanic crust (Fig. 9). than 2200 km in a NW-SE direction and lies in relatively deep water (1000 to 4000 m). Geological sampling and Velocity, km/s drilling shows that it was emergent or under shallow water for up to 40 million years of its history (Borissova et al. [19]). 1 2 3 4 5 6 7 8 9 0 Seismic refraction data (Charvis et al. [20], Borissova et al. [19]) indicate that crustal thickness reaches 18-22km under 5 the central and southern parts of the plateau and only 7-8km under the Labuan Basin, which adjoins the plateau in the 10 east. These results combined with some sampling data and geochemical results (Konnecke et al [21]) confirm that 15 continental crust is present within the lithosphere of this large oceanic province. Depth, km 20 CONCLUSIONS

25 1. The Moho depth variation in Australia shows little if any correlation with the boundaries of crustal mega- 30 elements. Carnarvon 2. It is believed that Australian Proterozoic crust is thicker Canning than Archaean due to underplating. Browse 3. It remains unclear why Archaean crust was not Global average oceanic crust underplated. Global average continental crust 4. Selective, Proterozoic but not Archaean, underplating is Gabbro-type velocity corridor either not consistent with plate tectonics, or for its explanation some isolation of crustal and lithospheric Figure 9. Seismic velocity models of oceanic crust studied processes within a drifting plate is required. by three transects (Carnarvon, Canning and Browse) at 5. Major crustal thinning has occurred beneath the North the NWAM compared to global average models of oceanic Western and Southern margins of the continent. (White et al [18]) and continental (Christensen & Mooney 6. Analysis of seismic velocity and SiO2 distributions in [12]) crust. A 4.8 km-thick water layer added at the top of full crustal column may distinguish between the regions the global average model of continental crust. where the vertical or horizontal mass transfer in the Structure and Composition of the Crust in Australia Goncharov

crust has prevailed. Regions with balanced SiO2 Drummond B.J. (Ed.), The Australian Lithosphere, distribution in the crust are likely to have been affected Geological Society of Australia Special Publication 17 by mostly vertical mass transfer. (1991) 67-80. 7. Prominent seismic reflectors and changes in reflectivity [7] Drummond, B.J., Goncharov, A.G. and Collins, patterns in conventional reflection data do not C.D.N., Crustal structure in the Archaean and necessarily correspond to significant bulk velocity Proterozoic provinces in Australia, in:. Rutland, discontinuities imaged by refraction/wide-angle data; R.W.R and Drummond, B.J. (Eds.), Paleoproterozoic this leads to re-assessment of sedimentary thicknesses, tectonics and metallogenesis: comparative analysis of degree of extension of the crust and role of underplating parts of the Australian and Fennoscandian shields, in its formation. AGSO Record 1997/44, 1997, pp. 37-41. 8. The crust studied by the outer parts of the Carnarvon, [8] Collins, C.D.N., Seismic velocities in the crust and Browse and, to a lesser degree, Canning transects at the upper mantle of Australia, in: Bureau of Mineral North West Australian Margin is not purely oceanic but Resources, Geology and Geophysics Report 277, 1988, rather transitional. 160 pp. 9. Refraction seismic, sampling and geochemical results [9] Drummond, B.J., and Collins, C.D.N., Seismic from the Kerguelen Plateau confirm that continental evidence for underplating of the lower continental crust is present within the lithosphere of this large crust of Australia, Earth and Planetary Science Letters oceanic province. 79 (1986) 361-372. [10] Collins, C.D.N. and Drummond, B.J., Crustal ACKNOWLEDGEMENTS thickness patterns in the Australian continent, in: Skilbeck, C.G. and Hubble, T.C.T. (Eds.), I thank AGSO and FORTUM PETROLEUM for providing Understanding Planet Earth: Searching for a financial support for my trip to Norway in May 2001 to Sustainable Future, 15th Australian Geological present this paper at the International Workshop on Global Convention, 59, 2000, p. 91. Wrench Tectonics. Discussions with Clive Collins, Jim [11] Goncharov, A.G., Lizinsky, M.D., Collins, C.D.N., Colwell and Heike Struckmeyer helped me to formulate some Kalnin, K.A., Fomin, T.N., Drummond, B.J., Goleby, of the ideas presented in this paper. Peter Petkovic and Jim B.R.G., and Platonenkova, L.N., Intra-crustal "seismic Colwell provided valuable criticism of an earlier version of isostasy" in the Baltic Shield and Australian the manuscript. Precambrian cratons from deep seismic profiles and the Kola superdeep bore hole data, in: Braun, J., et al. 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