Geophysical evidence for a deep crustal root beneath the Yilgarn Craton and the Albany-Fraser Orogen
Hugh Tassell* Alexey Goncharov Geoscience Australia, Geoscience Australia, Australia Australia [email protected] [email protected]
Line 19 of survey GA280, extending across the offshore Bremer Sub-basin, the Albany-Fraser Orogen and into the SUMMARY Yilgarn Craton. The resulting crustal model provides constraint to the crustal structure beneath the Albany-Fraser Using a marine seismic reflection survey as a source for Orogen and the southern margin of the Yilgarn Craton. onshore and offshore seismic refraction recorders, high quality refraction data were recorded to a maximum offset of 400 km. A two-dimensional velocity model for Line 19 of Geoscience Australia survey 280 in the Bremer Sub-basin and its onshore extension was derived by forward modelling using iterative ray tracing. Our velocity model indicates the presence of a deep crustal root located under the southern margin of the Yilgarn Craton and the Albany-Fraser Orogen. Comparison of the gravity response derived from this model with the gravity profile extracted from regional grids shows reasonable correlation. However, fine tuning of the seismic model is required to better define the complex geometry of the crustal root. Deep crustal roots are common under Proterozoic mobile belts that surround Archaean cratons. However, these results also indicate the presence of a crustal root under the southern margin of the Yilgarn Craton. This is consistent with the view that Proterozoic taphrogenesis penetrated tens of kilometres into the Yilgarn Craton.
Key words: SW Australia, Bremer Sub-basin, Yilgarn Craton, Albany-Fraser Orogen, seismic refraction, gravity. Figure 1. Location of survey GA280 onshore land seismic stations (black dots) and seismic reflection lines (red). Background image is a high-pass filtered Bouguer gravity INTRODUCTION anomaly.
As part of the Australian Government’s New Oil initiative, GEOLOGICAL SETTING Geoscience Australia undertook a geophysical survey (Southwest Frontiers, GA280) of the south-western Australian Exposed to the north of the Bremer Sub-basin are rocks of the continental margin in late 2004. The survey acquired 2700 km Palaeoproterozoic Albany-Fraser Orogen (Fig. 1), which of industry-standard, 106 nominal fold seismic data recorded extends along the southern margin of the Archaean Yilgarn to 12 seconds two-way time using a 6–8 km digital streamer Craton (Myers, 1990). Rocks of the Albany-Fraser Orogen and 4900 cui gun array. Marine seismic reflection acquisition comprise granitoid intrusions, orthogneisses, metagabbros, was supplemented by recording of refracted data from the mafic dykes and metasediments (Fitzsimons et al., 2003, same source by sonobuoys at sea and land recording stations Abeysinghe, et al., 2002 & Myers, 1995). These are basement (Fig. 1). The key scientific objectives of the refraction to the onshore Eocene sediments of the Bremer Sub-basin, component of this study were: which are extensively eroded, and fill the low-relief basement topography in discontinuous pockets close to the coast 1. Provide estimates of crustal thickness to better constrain (Hocking, 1990). The area has been subject to prolonged interpretation of tectonic evolution of the region, erosion and the subdued topography is extensively covered by 2. Provide accurate seismic velocity information to define the Cainozoic regolith (Abeysinghe, et al., 2002; Myers, 1990). nature of the basement and crust, and 3. Provide accurate seismic velocity information to improve The Albany-Fraser Orogen is thrust against and truncates depth conversion of reflection seismic data. granite-greenstones of the Yilgarn Craton to the northwest and extends eastwards under the Eucla Basin to the Coompana This paper discusses the crustal velocity structure derived Block and Gawler Craton (Fitzsimons, 2003 & Gee, 1979). from modelling and interpretation of the north-south oriented Rocks of the Albany-Fraser Orogen which formed between
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2200 and 1200 Ma were emplaced during the collision of the starting model offshore was the measured water depth and an Yilgarn Craton margin and east Antarctica between 1300 and estimate of basement depth from interpretation of depth 1100 Ma (Myers, 1993, 1995). The Recherche Granite converted reflection data. Forward modelling of the sediment dominates the eastern region of Albany-Fraser Orogen and upper basement velocity profile was based on sonobuoy (Fitzsimons & Buchan, 2005). Early stage deformation of the data (Goncharov et al., 2005). Modelling was an iterative granite was followed by deposition of the Mount Ragged process involving revised identification of refracted and metasedimentry group comprising quartzite with minor pelite, reflected phases and aiming to minimise the difference which are assumed to unconformably overly the Recherche between computed and observed travel times of these phases. Granite (Fitzsimons & Buchan, 2005). Low grade metasedimentary rocks of Stirling Range and Mount Barren Travel times of main phases were modelled for all nine land units outcrop in the western part of the Albany-Fraser Orogen refraction stations. The general pattern of seismic phases next to its northern border (Fitzsimons & Buchan, 2005). identified in the experimental data is well reproduced by the final model, as illustrated in Fig.2, where modelled phase PREVIOUS INVESTIGATIONS arrivals closely match phase arrivals observable in the field data. Travel time mismatches for individual phases in the final Crustal velocity profiles from previous land seismic refraction model (simplified version presented in Fig. 3) are generally surveys reported in Mathur (1974) ,marked as ‘PMA’ in Fig. within + 0.2 s although at some anomalous locations 1, Dentith et al. (2000), marked as ‘PDN’ in Fig. 1, and mismatches of up to 0.5 s were observed. Given the Drummond (1988) are consistent with the granitic and complexity of the final model, this represents a close fit gneissic granite composition of rocks outcropping onshore in between observed and calculated arrivals. the region. Both in the southern margin of the Yilgarn Craton, and the Albany-Fraser Orogen, velocities in the range 5.9-6.2 BASEMENT AND CRUSTAL STRUCTURE km/s dominate the upper crust and extend as deep as 15 km in onshore measurements. Velocities in the range 5.2-5.7 km/s On a crustal scale, our velocity model correlates well with dominate in the basement imaged by the offshore sonobuoy others in the region. Mathur’s (1974) crustal thickness of 34 refraction study in Talwani et al. (1979) (PTA measurements km near Albany at PMA05 location (see Fig. 1) is consistent in Fig. 1). with our results as is a sub-crustal velocity of 8.1 km/s. However, our results do not support a three layer crust with REFRACTION DATA AND VELOCITY MODEL velocities 6.1, 6.6 and 7.3 km/s proposed by Mathur (1974). The two layer model of Dentith et al. (2000) with an upper
crustal velocity gradient from 5.95-6.31 km/s and a lower The density of observations in the onshore component of this crustal from 6.90-7.05 km/s is also consistent with our model investigation is a significant improvement over previous in terms of crustal velocity stratification, but our mid-crustal experiments. Land recorder stations were deployed at 25-50 boundary is up to 12 km deeper than the 18 km proposed in km intervals and shots fired every 37.5 metres in contrast to Dentith et al. (2000). This mismatch suggests that there may Dentith et al. (2000) with three shots for each of two ~400 km be a sharp SW-NE deepening of the mid-crustal refractor long orthogonal transects with 27 stations recording half of between the Dentith et al. (2000) PDN02-PDN05 line (Fig. 1) each transect simultaneously. However, our observation and our land refraction line. scheme is limited in that data were recorded in only one direction, and with no recording at near offsets (for the most A key feature of the velocity model is the presence of a deep remote inland station the nearest shot was fired at 295 km crustal root located ~100 km inland from the coast (Fig. 3). offset (see Fig. 1)). As a result, the velocity model of the The presence of this root is well constrained by first arrivals upper crust for inland stations is not well constrained. and some subsequent phases recorded at several stations. This However, there is good agreement between the main phases is illustrated in Fig. 4 which displays a common offset gather recorded by the station at near offsets and those from more for travel times recorded at three land refraction stations. distant stations. This suggests that no significant inland lateral variation in the upper crust has been missed due to these limitations.
The quality of the data in terms of continuity of arrivals, clarity of phase changes in the first arrivals, and signal-to- noise ratio is very high (Fig. 2). Useful signal was recorded up to a maximum offset of 400 km for the station located 295 km inland from the coast. Three major phases were identified in the first arrivals: refractions in the basement and upper crust, refractions in the lower crust, and upper mantle refractions propagating at ~8 km/s (Fig. 2). This pattern is consistent from station to station. In addition, at some stations deep reflections were identified and were originally interpreted as Figure 4. Common offset gather of travel times recorded reflections from the Moho (PMP). However, subsequent by three of the nine land refraction stations indicating the modelling has shown that they most likely originate beneath need for a crustal root (centred around 200km in Fig.3) the Moho (Fig. 2). underlying the southern margin of the Yilgarn Craton and the Albany-Fraser Orogen. A two-dimensional velocity model for Line GA280-19 (lower panel in Fig. 3) was derived by iterative forward modelling Upper mantle arrivals recorded at stations 150 and 175 km using the ray-tracing algorithm of Zelt and Smith (1992). The from the coast (AB and EF respectively) are significantly
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delayed at small offsets compared to those recorded at the minimal impact upon the presence and geometry of the crustal station 100 km from the coast (as indicated by the black root. Any increase or decrease in the velocity of basement in arrows). A possible explanation is that energy arriving at the Bremer Sub-basin would simply cause a bulk static time these stations has spent relatively more time travelling through shift of the first arrivals recorded by the land refraction thicker crust than energy arriving at the 100 km station. stations, as all energy from the marine source passes through However, at larger offsets the delay in first arrivals from the the Bremer Sub-basin basement. This will result only in upper mantle phases for stations at 150 km and 175 km (CD increasing or decreasing all phase arrival times by the same and GH) has decreased and arrival times are close to those amount and would not affect the relative offset between from station at 100 km. This suggests that as offset increases mantle refraction phases that define the crustal root. and ray paths become progressively deeper, energy spends less time travelling through the thicker crust within the root. GRAVITY MODELLING Such configuration of first arrivals requires that a crustal root or crustal thickening of some form is situated approximately Gravity modelling was used as an additional validation to test 200 km along the profile (from its inland origin) to explain the the crustal structure and velocity distribution predicted by our observed upper mantle arrival delay at this offset. model. The density model in the middle panel of Fig. 3 was derived from the velocity model (simplified version presented A limitation of the model presented in Fig. 3 in effectively in Fig. 3. We took the average velocity from each polygonal imaging the crustal root is that it produces mantle refractions block of the input velocity model and used the empirical at the 150 km station preceding observed first arrivals. relationship from Dooley (1976) between seismic velocity and However, the neighbouring station (175 km) imaging the same density to calculate an average density for that block. Dooley feature does not produce mantle arrivals from the model (1976) used a modified version of the Nafe-Drake curve although they are recorded in the experimental data. We (which is approximated by Equation 1 (N. Direen., speculate that amplitude effects due to the complex shadow pers.comm.)) with additional constraint for higher velocities zones forming from the crustal root may explain these and densities provided from empirical functions presented in mismatches and will require further research. Woollard (1959,1968).
The presence of a crustal root in this region has been 2 ρapp = −0.6997 + 2.23×Vp − 0.598×Vp suggested in earlier refraction studies. Drummond (1988) (1) + 0.07036×V 3 − 0.0028311×V 4 reported crustal thickening beneath the Albany-Fraser Orogen p p based on continuing weak reflections arriving later than the expected Moho travel time as recorded over the Yilgarn where ρapp is the apparent density and Vp is the p-wave Craton. Land refraction interpretations of crustal thickness by refraction or stacking velocity in km/s of the interval for Dentith et al. (2000) and Mathur (1974), when combined, also which a density is sought (after Ludwig et al., 1971). suggest crustal thickening in this area, but not to the extent that our model suggests. Our interpretation of a crustal root in The gravity response of the density model was then calculated this location is also supported by Wellman (1978) who, based using the method of Talwani et al. (1959) for infinitely long on gravity considerations, predicted that the crust beneath polygonal prisms along axis perpendicular to the model mobile belts such as the Albany-Fraser Orogen is thicker and profile. The gravity response was then compared with the denser than in the adjacent older cratonic areas. Our model anomaly profile extracted from available onshore free air also suggests that the crustal root extends under the southern anomaly measurements and from the grid of Murray and margin of the Yilgarn Craton. This is consistent with Petkovic (2001) offshore. This yielded a computed vs. Wellman’s (1988) assessment that Proterozoic taphrogenesis observed RMS error of ~8 mGal, a reasonable match given the penetrated tens of kilometres into the craton. complexity of the velocity model and the averaging assumptions of the gravity modelling process. However, to An unexpected feature of the velocity model derived from GA achieve this match, the density of the crustal root at a depth of 280 is that velocities in the basement underlying Mesozoic 30-60km had to be increased by 0.26 g/cm3, beyond the sediments of Bremer Sub-basin are generally in the 5.0-5.7 density of 2.99 g/cm3 predicted by Dooley (1976) for km/s range, consistent with old offshore velocity velocities in the range 7.0-7.1 km/s. measurements (Goncharov et al., 2005). Possible basement rock types recovered by several sea floor dredges in the High pressure laboratory measurements of seismic velocity Bremer area included granite, gneiss and schist, (Blevin, for crustal rocks in Christensen and Mooney (1995) indicate 2005; Exon et al. 2005). Sonic velocity measurements were that petrologies including garnet bearing mafic granulite and not carried out on these samples, but seismic velocity range hornblendite have a sufficiently low velocity to density ratio well above the 5.0-5.7 km/s in our model would be generally to explain the velocity and density distribution in the crustal expected to correspond to these lithologies. However, seismic root identified in our model. Thus, it appears that the presence velocity models derived from refraction data characterize a far of a crustal root is consistent with and cannot be dismissed on greater volume of the basement than dredge samples. the basis of velocity and density ratios in the lower crust. Therefore, granites, gneisses and schists, dredged from the seafloor, appear to represent only a small fraction of the CONCLUSION heterogeneous basement in the area. These lithologies may be restricted to basement highs where higher velocities have been The velocity model developed from combined iterative ray detected by refraction work (Goncharov et al., 2005). tracing of first arrivals from land refraction and sonobuoy data indicates the presence of a deep crustal root beneath the The nature of the basement of the Bremer Sub-basin, whether southern margin of the Yilgarn Craton and the Albany-Fraser it is comprised of granite, gneiss and schist or a different Orogen. The presence of this root is well constrained by first lower velocity lithology, as our model suggests, will have
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arrivals and some subsequent phases recorded at several and Breakup. Geological Society, London, Special Publication stations. Comparison of the gravity response derived from this 206, 93-130. model with the observed gravity shows reasonable correlation. Fitzsimons, I.C.W., Buchan, C., 2005, Geology of the western However, fine tuning of the seismic model and further Albany-Fraser Orogen, Western Australia – a field guide, research is required to explore possible causes for mismatch Western Australia Geological Survey, Record 2005/11. between observed and calculated mantle arrivals in the area of the crustal root. Our results are consistent with the view that Gee, R.D., 1979, Tectonics of the Western Australian shield, the crust beneath mobile belts is thicker and denser and that Tectonophysics, 58, 327-369. Proterozoic taphrogenesis penetrated tens of kilometres into the Yilgarn Craton. Goncharov, A., Petkovic, P., Leitchenkov, G., Tassell, H., 2005, Basement and crustal results from the Bremer Sub- ACKNOWLEDGMENTS basin, SW Australia and its Antarctic counterpart drive Australia-Russia cooperation, Preview, 119, December 2005, Fred Kroh, Damien Ryan, the onboard acquisition team on 15-22. M/V “Pacific Sword”, Tanya Fomin, Chris Heath, Alan Hocking, R.M., 1990, Bremer Basin, in, Geology and mineral Crawford and Jack Pittar participated in the offshore and resources of Western Australia, Western Australia Geological onshore components of the refraction seismic survey. Survey, Memoir 3, 561-563. Equipment and support was provided by the Australian National Seismic Imaging Resource thanks to Brian Kennett, Ludwig, W.J., Nafe, J.E. & Drake, C.L., 1971. Seismic Tim Barton and Armando Arcidiaco. Alan Whitaker, Richard Refraction in the Sea, Vol 4. in Maxwell A.E. (ed), Willey- Blewett, Bruce Goleby and Barry Drummond reviewed earlier Interscience, New York, 53-84. versions of the manuscript and provided valuable remarks. Mathur S.P. 1974. Crustal structure in southwestern Australia Peter Petkovic provided software development and sonobuoy from seismic and gravity data. Tectonophysics, 24, 151-182. data modelling. The authors publish with permission of the Chief Executive Officer, Geoscience Australia. Murray, A.S. & Petkovic, P., 2001. Digital gravity grid of the Australian region, CD-ROM, Geoscience Australia, Canberra. REFERENCES Myers, J.S., 1990. Albany-Fraser Orogen, in, Geology and Abeysinghe, P.B., Flint, D.J., Luckett, J., McGuinness, S.A., mineral resources of Western Australia, Western Australia Pagel, J., Townsend, D.B. & Vanderhor, F., 2002. Geology Geological Survey, Memoir 3, 255-265. and mineral resources of the Southern Cross-Esperance region Myers, J. S., 1993, Precambrian history of the West Australian of Western Australia: Western Australia Geological Survey, Craton and adjacent orogens, Annual Review of Earth and Record 2002/3. Planetary Science, 21, 453-485. Blevin, J.E. (editor), 2005, Geological framework of the Myers, J. S., 1995, Geology of the Esperance 1:l 000000 Bremer and Denmark sub-basins, southwest Australia, R/V sheet: Western Australia Geological Survey, 1:100 000 Southern Surveyor Survey SS03/2004, Geoscience Australia Geological Series Explanatory Notes. Survey 265, post-survey report and GIS, Geoscience Australia, Record 2005/05. Talwani, M., Worzel, J.L., & Landisman, M., 1959. Rapid Gravity Computations for Two-Dimensional Bodies with Christensen, N.I., and Mooney, W.D., 1995, Seismic velocity Application to the Mendocino Submarine Fracture Zone, structure and composition of the continental crust: a global Journal of Geophysical Research, 64, 49-61. review. Journal of Geophysical Research, 100, 9,761–9,788. Talwani, M., Mutter, J.C., Houtz, R.E. & Konig, M., 1979. Dentith, M.C., Dent, V.F. & Drummond, B.J., 2000. Deep The Crustal Structure and Evolution of the Area Underlying crustal structure in the southwestern Yilgarn Craton, Western the Magnetic Quiet Zone on the Margin South of Australia, in Australia. Tectonophysics, 325, 227-255. Watkins, J.S., Montadert, L. and Dickerson, P.W., Eds., Drummond, B.J., 1988. A review of crust/upper mantle Geological and Geophysical Investigations of Continental structure in the Precambrian areas of Australia and Margins, Am. Assoc. Petrol. Geol., Memoir 29, p. 151-176. implications for Precambrian crustal evolution. Precambrian Wellman, P., 1978. Gravity evidence for abrupt cnages in Research, 40/41, 101-116. mean crustal density at the junction of Australian crustal Dooley, J.C., 1976, Variation of crustal mass over the blocks. BMR Journal. Australian Geology and Geophysics, 3, Australian Region, BMR Journal of Australian Geology & 153-162. Geophysics, 291-296. Wellman, P., 1988. Development of the Australian Exon, N., Blevin, J. and Hocking, R., 2005. Regional Proterozoic crust as inferred from gravity and magnetic geological setting. In: Blevin, J.E. (ed.), Geological anomalies, Precambrian Research, 40/41, 89-100. framework of the Bremer and Denmark sub-basins, southwest Woollard, G.P., 1959. Crustal structure from gravity and Australia, R/V Southern Surveyor Survey SS03/2004, seismic measurements, Journal of Geophysical Research, 64, Geoscience Australia Survey 265, post-survey report and GIS. 1521-1544. Geoscience Australia, Record 2005/05. Woollard, G.P., 1968. The interrelationship of the crust, the Fitzsimons I.C.W. 2003. Proterozoic basement provinces of upper mantle, and isostatic gravity anomalies in the United southern and south-western Australia, and their correlation States in Knopoff, L., Drake, C.L., & Hart, P.J. (ed) The crust with Antarctica. 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Zelt, C.A. & Smith, R.B., 1992. Seismic travel time Journal International, 108, 16–34. inversion for 2-D crustal velocity structure. Geophysical
Figure 2. Example of data from station 48 deployed on the land extensions of line GA280-19 of the Southwest Frontiers Survey GA280 at 48 km location shown in Fig. 1. An 8 km/s travel time reduction, 1:2:1 weighted average running mix and digital coherency enhancement have been applied to the refraction data. Calculated travel-times from the model in Fig. 3 are superimposed on top of the record section as colour coded lines: red – refraction from the basement and upper crust, dark blue – reflection from the top of the lower crust, yellow – refraction in the lower crust, light blue – Moho reflection, green – mantle refraction, magenta – sub-Moho reflections.
Figure 3. Line GA280-19 and its onshore extension. Simplified velocity model (C) derived from combined interpretation of sonobuoy and land refraction data. Note the presence of a thick crustal root centred around 200 km and low (5.2-5.6 km/s) velocities in the Bremer Sub-basin basement. Simplified density model (B) derived from the velocity model using empirical relationships between seismic velocity and density. Note that the density of the crustal root in the depth range of 30-60km
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was increased to 3.25g/cm3 to provide an acceptable match between modelled and observed gravity anomalies (A). Velocity values in km/s and densities in g/cm3.
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