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Geological and Geophysical Studies in the Amadeus Basin, Central Australia

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Geological and Geophysical Studies in the Amadeus Basin, Central Australia DEPARTMENT OF PRIMARY INDUSTRIES AND ENERGY BUREAU OF MINERAL RESOURCES GEOLOGY AND GEOPHYSICS BULLETIN 236 Geologicaland geophysicalstudies in the AmadeusBasin, central Australia R.J. Korsch& J.M. Kennard Editors Onshore Sedimentary & Petroleum Geology Program AUSTRALIAN GOVERNMENT PUBLISHING SERVICE CANBERRA 409 Teleseismictravel-time anomalies and deep crustal structure of the northernand southernmargins of the AmadeusBasin K. Lambeckl Teleseismictravel-times recorded acrossthe central Australian basins and Musgrave and Arunta Blocks impose signifrcant constraints on crustal and upper mantle structure. Major discontinuities in lateral structure are required, particularly acrossthe Redbank-Ormiston Thrusts in the Arunta Block and the Woodroffe-Mann Thrusts in the Musgrave Block. The deep structure of these tectonic units exhibit considerablesimilarity, and in both instances the thrusts dip at about 45" through to the Moho. Major offsets in Moho depth are produced which have persisted since the time of the last movements on the faults, about 300 Ma ago in the case of the Redbank Thrust and much earlier in the case of the Woodroffe-Mann Thrusts. The teleseismic models are consistent with deep crustal seismic reflection observations across the Redbank Thrust Zone, and they confirm the conclusion drawn from gravity studies that the region as a whole is not in local isostatic equilibrium and that maximum stress- differenceswithin the crust and upper mantle are of the order of 100MPa. I ResearchSchool of Earth Sciences,Australian National University, PO Box 4,Canbena, A.C.'[.260I, Australia. lntroduction into which sedimentscan be deposited,rather than with the details of how this deposition occurs, although some form major feature Australia's Intracratonic basins a of of these models do specify the overall depositional pat- geology,yet the mechanisms leading to their formation terns (e.g.Beaumont & others, 1987)on the assumption poorly This is not remain understood. entirely a conse- that sediment accumulation follows the crustal warping. quence geophysicalprob- of inadequate exploration and The sedimentary record within the basins will often be ing of the basins and underlying crust, but also a much more complex than is allowed by these models consequenceof the complexity of basin formation mech- becausesedimentation will be partly controlled by local anisms for these intracratonic environments. Many of factors, including the nature of the sedimentsthemselves geological the basins have had very long histories of and localised brittle fracturing of the upper crust. Never- subsidence,rebound and deformations, during which no theless,the sedimentaryrecord, if generalitiesof deposi- single mechanism alone was responsiblefor the basin's tion can be established,provides one of the major initiation and evolution. In some cases, such as the constraints on these models of tectonic crustal evol- Amadeus Basin, these histories have spanned nearly ution. 1000million years. Any successful first-order model for the Amadeus Basin formation mechanisms can be characterisedas Basin evolution, as for any other basin's history must being driven by horizontal stresses (extensional and explain the subsidenceand sedimentation records con- compressive), thermal processes(including thermal tained within the basin sediments,as well as account for expansionand contraction, and changesin metamorphic the uplift histories of the adjacent exposed cratonic grade of the lower crust), or by passive gravitational blocks for the same time interval as sediments were loading. All mechanisms appear to contribute to a deposited within the basins or immediately prior to this basin's evolution at some time in its history and what deposition. Of particular importance will be the uplift distinguishes one basin from another is the relative histories of the southern part of the Arunta Block and importance of these complementarymechanisms at var- the MusgraveBlock. Also, the Amadeus Basin cannot be ious stagesof the basin's evolution, largely in response treated wholly in isolation from the other central Austra- to the tectonics that shaped the continent as a whole. lian basins, particularly the Ngalia Basin, the northern Furthermore, the responseof the crust to a given force Officer Basin and possibly the Georgina Basin, for it or load may be regionally variable becauseearlier tec- appearsthat all of these basins have had similar subsid- tonic events modifled the crustal properties, or because encehistories in responseto more regional forces (Shaw, of the boundary conditions imposed by the surrounding 1987;Shaw & others,in press;Lindsay & others, 1987). tectonic units. Early crustal and basin models for central Australia The Amadeus Basin is no exception to this and it is were almost entirely based on the large gravity anoma- improbable that a single mechanism can be found that lies observedover the basins and intervening blocks, and will explain all aspectsof the basin (Shaw, 1987, and did not attempt to answer the question as to how these this volume; Shaw & others, in press). Thus if a single structures had evolved to their present geometry. With mechanism model is proposed it can only lead to a peak to trough amplitudes in excessof 180 mgal in some frrst-order model, one which tries to explain the gross locations and with an approximate wavelength of features of the dominant crustal deformation and basin 200 km, these anomalies over the relatively flat terrain sediments.Second-order models must reflect a combina- of the Office4 Amadeus and Ngalia Basins and Musgrave tion of mechanical processeswhose relative importance and Arunta Blocks form the major features in the grav- has not remained unchanged through time in response ity fleld of the Australian continent; clearly the region is to the more regional forces that refined the shaping of out of isostatic equilibrium in any conventional sense, the Australian continent in Late Proterozoic and and the question is where do the subsurface density Palaeozoictimes. By their nature, frrst-order models are anomaliesoccur. Dynamic contributions associatedwith more concernedwith the deformation and tectonic evol- upper mantle small-scaleconvection must be ruled out ution of the crust, and with providing an environment because the last active tectonics occurred more than K. Lambeck t (years) 7 400 -f,.r o \!'ç9 g- ^s9 o L J +, (ü oL o. E |- ^ I t Çt.. 1000 Ë\b" Depth(km) Fig. 1. Effective mantle relaxation time as a function of crustal deformation can be tested. In particular, new temperature and stress-difference(for 100 MPa and seismic data, including teleseismictravel-time anomalies 400 MPa) for wet olivine accordingto Chopra & Paterson and deep crustal seismic reflection surveys, permit the (1981), and two geothermal profiles (1,2) according to models of present deep structure to be tested, although Sass & Lachenbruch (1979) spanning a range of values this does not directly test the process by which this that are consistent with surface geology,heat flow and structure was actually reached. The teleseismic data other geophysicalobservations for the central Australian consists of travel-time residuals observed at closely Shield. Qo is the surface heat flow (in heat flow units) spacedsites acrossthe basins and exposed blocks (Fig. and Ao is surface radioactivity (in heat generationunits). 2). Lambeck & Penney(1984) observedsignifrcant vari- The dotted line is the geotherm proposed by Cull & ations in the travel-times of teleseismic-wavesalong the Conþ (1983) for the central Australian region with a part twet' Central Australian line from the southern of the surface heat flow of 75 mWm'2. lThe flow law is Musgrave Block to north of the Ngalia Basin. Major (1981) found that adopted becauseChopra & Paterson azimuth-dependentstation anomalies were observed (0.010/oweight, havea evenvery snall amounts of water, near the Redbank Thrust Zone irt the southern Arunta weakening effect.) At a depth of 50 km the significant Block and near the Woodroffe and Mann Thrusts in the geotherm(1) predicts a temperatureof about 700'C and Musgrave Block. No evidence was found for major a 100 MPa stress-diflerence will relax with a time- crustal offsets or lateral boundaries beneath the constant of about 106 years. At 30 km the corresponding Amadeus Basin sedimentary succession.The result led time-constant exceeds the age of the Earth (from of two closer spaced, parallel, Lambeck, 1986). to the deployment north-south lines of instruments across the northern margin of the Amadeus Basin and across the southern 300 million years ago and, since then, the continent has Arunta Block (the Arunta and Redbank lines; Fig. 2), moved large distances relative to the underlying, sub- and these experiments confirmed the occurrenceof dif- lithospheric mantle. The source of the anomalous struc- travel-times of up to 1.5 seconds across the ture must therefore be in the lithosphere, and because ferential Zone (Lambeck & others, 1988). A the stress relaxation time constant also decreaseswith Redbank Thrust southern Amadeus depth (due to increasing temperature with depth), many fourth line of instruments acrossthe (the of these anomalies are likely to originate from within Basin and the Musgrave Block Musgraveline; Fig. the upper lithosphere (Fig. 1). Sucþ density anomalies 2) produced comparable large-amplitude travel-time must either be kept in place by the finite strength of this anomalies over relatively short distances. The second layer or be balanced by horizontal
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