Crustal Structure of Continental Australia; Intra-Crustal Seismic Isostasy and Crustal Composition: a Review
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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