High-resolution aeromagnetic data over central Australia assist Grenville-era ( Ma) Rodinia reconstructions

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1 GEOPHYSICAL RESEARCH LETTERS, VOL. 35, L01306, doi: /2007gl031563, 2008 High-resolution aeromagnetic data over central Australia assist Grenville-era ( Ma) Rodinia reconstructions Alan R. A. Aitken 1 and Peter G. Betts 1 Received 15 August 2007; revised 1 November 2007; accepted 4 December 2007; published 12 January [1] High resolution aeromagnetic data covering the polydeformed and poorly exposed Musgrave Province of Australia reveals the Grenville-aged crustal architecture. A combination of upward continuation and the potential field tilt filter out magnetic anomalies relating to a late Neoproterozoic orogenic event, and emphasize the more subtle magnetic structural grain formed during the ca Ma Musgravian Orogeny. The resulting images indicate that crustal architecture is defined by the distribution of ca Ma magnetic granitoids within less magnetic basement and is dominantly northeast trending, continuous beneath the Amadeus and Officer basins, and defines an orogenic belt that connects Mesoproterozoic provinces. The orogenic belt is truncated within Australia and precludes a direct east trending connection between the Musgrave Province and contemporaneous orogens in Laurentia. Clockwise rotation of the South Australian Craton and subsequent collision with the North and West Australian cratons is our preferred model to explain the Grenville-aged architecture observed in Australia. Citation: Aitken, A. R. A., and P. G. Betts (2008), High-resolution aeromagnetic data over central Australia assist Grenville-era ( Ma) Rodinia reconstructions, Geophys. Res. Lett., 35, L01306, doi: /2007gl Introduction [2] Paleocontinental reconstructions typically rely on a combination of paleomagnetic data [e.g., Meert and Torsvik, 2003; Pisarevsky et al., 2003], and correlation of continental stitching points such as orogenic belts [Karlstrom et al., 2001], and crustal blocks [Burrett and Berry, 2000]. These methods fail to unambiguously assess connectivity between stitching points, whose relationships are often underpinned by non-unique and indirect geological inferences. As a result, significant conjecture remains concerning the configuration of the proposed supercontinent Rodinia. The recent development of continental scale merged magnetic data sets and increasing availability of high-resolution regional aeromagnetic data provides a new way to constrain the architecture and connectivity of Rodinia fragments. [3] In many Rodinia reconstructions such as AUSWUS [Karlstrom et al., 2001] and SWEAT [Moores, 1991] a continuous Grenville-aged orogenic belt connecting Laurentia, Australia and Antarctica is a key constraint. In support of these hypotheses, intercontinental stitching points are invoked between the Albany Fraser Province 1 Australian Crustal Research Centre, School of Geosciences, Monash University, Melbourne, Victoria, Australia. Copyright 2008 by the American Geophysical Union /08/2007GL and the Bunger Hills of Antarctica, and the Musgrave Province and western Laurentia. [4] Whilst connectivity between the Albany-Fraser Province and the Bunger Hills in Antarctica is well established [Duebendorfer, 2002], links between the Musgrave Province and western Laurentia are more subjective. SWEAT (Figure 1a) aligns the Musgrave Province with the Wopmay Province of northern Canada [Moores, 1991], and AUSWUS (Figure 1b) proposes that the Oaxaca terrane of Mexico connects the Musgrave Province to the Grenville Belt [Karlstrom et al., 2001]. These proposed stitching points have significantly different metamorphic and magmatic evolutions to the Musgrave Province [c.f. Laughton et al., 2005; Solari et al., 2003; White et al., 1999]. [5] Paleomagnetic studies indicate that neither AUSWUS nor SWEAT are viable, and that ca Ma North America and Australia were moving independently, and may have been separated by an ocean (Figures 1d, 1e, and 1f) [Pisarevsky et al., 2003]. By ca Ma an AUSMEX configuration [Wingate et al., 2002] that connects Australia with the Grenville Belt through present day Mexico (Figure 1c) is paleomagnetically viable. [6] These reconstructions have assumed, in connecting the Musgrave Province to western Laurentia, that the presently observed east trending architecture of the Musgrave Province was valid at Grenvillian times, despite intense overprinting during the ca 550 Ma Petermann Orogeny, which is interpreted as the principal influence on present crustal architecture [Camacho and McDougall, 2000]. [7] This paper presents an analysis of aeromagnetic data to see through the Petermann Orogenic overprint and determine the structural architecture of the Musgrave Province at Grenvillian times. The separation of magnetic anomalies related to Grenvillian-era architecture from those representing late Neoproterozoic architecture was possible because regional aeromagnetic data over the Musgrave Province (Figure 2) are of sufficient resolution (200 m line spacing) to define magnetic signatures of rock packages and correlate these with exposed rocks. 2. Imaging Inherited Crustal Architecture Using Magnetic Signatures [8] Polydeformation and the discontinuous surface expression of Proterozoic terranes are major obstacles in understanding Proterozoic tectonics. Regional-scale, high resolution aeromagnetic grids can be used to map the subsurface expression of terranes and constrain their crustal architecture at depth [e.g., Finn and Sims, 2005], allowing the validity of geological connections between these terranes to be tested. L of6

2 Province and the Albany-Fraser and Warumpi Provinces are covered by the mid-neoproterozoic to Devonian Officer and Amadeus Basins [Lindsay, 2002]. [12] The Musgrave Province basement comprises highgrade gneiss, metamorphosed during the two stage Musgravian Orogeny, with stage I at Ma, and stage II at Ma [White et al., 1999]. Musgravian gneissic rocks are preserved in two subdomains that are separated by the late Neoproterozoic Woodroffe Thrust (Figure 2), south of which amphibolite to granulite facies metamorphism is observed and north of which greenschist to amphibolite facies metamorphism is observed [Camacho and Fanning, 1995]. Basement gneiss is intruded by syn-tolate Musgravian Pitjantjatjara Supersuite granitoids ca Ma [Camacho and Fanning, 1995], and Giles Complex mafic/ultramafic intrusions ca Ma [Sun et al., 1996]. [13] The late Neoproterozoic Petermann Orogeny formed an east-trending network of crustal-scale dextral transpressional shear zones that accommodated rapid burial and exhumation of the Musgrave Province [Camacho and McDougall, 2000]. Deformation was focused on discrete crustal boundaries causing reorientation of the Grenville structural grain within km of these shear zones (Figure 2b). Figure 1. Selected Rodinia configurations for Grenvillian Australia and Laurentia. (a) SWEAT, after Moores [1991]. (b) AUSWUS, modified from Karlstrom et al. [2001] and Burrett and Berry [2000]. Symbols indicate correlations of pre-grenvillian terranes. (c) AUSMEX, after Wingate et al. [2002]. (d, e, f) Paleomagnetically viable reconstructions ca Ma, after Pisarevsky et al. [2003]. NAC, SAC, WAC, North, South, and West Australian cratons; MP, Musgrave Province; AFP, Albany Fraser Province; CB, Coompana Block; BH, Bunger Hills; WP, Wopmay Province. [9] Definition of preserved architecture can be achieved by relating magnetic signatures to rock types or alteration textures of known age. Magnetic signature is described in terms of field intensity, anomaly amplitude and magnetic texture, which is described by its wavelength, orientation and amplitude. [10] Image processing techniques can be applied to selectively amplify or attenuate particular magnetic signatures, thus enhancing or reducing the signal from specific rock units. If the magnetic signatures of younger events are sufficiently different from those of an older event, an image of preserved early architecture can be produced. Significant deformation of the inherited architecture may have occurred during the younger event(s) and must be considered in any interpretation. 3. Musgrave Province [11] In Australia, igneous and metamorphic rocks of Grenville-age are preserved in the Albany-Fraser Province, the Musgrave Province and the Warumpi Province. Assessing the connectivity of these terranes has been hampered by poor exposure because the regions between the Musgrave 4. Identifying Grenvillian Architecture Using Magnetic Signatures [14] Musgravian gneissic and granitic rock packages cause broad, low amplitude magnetic anomalies compared to the relatively short wavelength, high amplitude anomalies associated with Petermann Orogeny shear zones. Five categories of magnetic anomalies were defined and by correlation with outcrop, were interpreted in terms of lithology. [15] Category 1 (C1) anomalies are broad and moderately negative, often with a low amplitude, short wavelength (1 5 km) and northeast oriented sub-linear fabric (Figure 2b). Where present, this fabric is cross cut by Category 3, 4 and 5 anomalies. C1 anomalies are interpreted to represent basement gneiss, with a Musgravian Orogeny fabric. Category 2 (C2) anomalies are moderately positive to weakly negative, and linear to sub-circular (Figure 2c) with similar texture to C1 anomalies. C2 anomalies are interpreted as an early phase of Pitjantjatjara Supersuite with a Musgravian Orogeny fabric. Category 3 (C3) anomalies are strongly positive, relatively broad (10 30 km) and sub-circular to sub-rectangular. The texture of many C3 anomalies is a secondary feature, but primary texture can be stippled or smooth. C3 anomalies are interpreted as a late phase of Pitjantjatjara Supersuite. Category 4 (C4) anomalies are high amplitude, typically strongly negative, but occasionally strongly positive or dipolar, and sub-linear to sub-circular with smooth texture. C4 anomalies correlate with Giles Complex intrusions. Category 5 (C5) anomalies are high amplitude, generally strongly negative, linear and narrow (<10 km) with smooth texture. Most are east to south east trending and form a network that transects the entire Musgrave Province. C5 anomalies cross-cut C1 C4 anomalies, and correlate with outcropping mylonite zones that have been dated ca. 550 Ma [Camacho and Fanning, 1995]. 2of6

Figure 2. (a) Pseudocolor image of the Musgrave Province aeromagnetic dataset, showing major province boundaries.

(c) An example of the distribution and orientation of C2 and C3 anomalies (Pitjantjatjara Supersuite) within C1 anomalies (basement gneiss).

These anomalies are therefore interpreted to represent Petermann Orogeny shear zones.

The reduced to pole magnetic field was upward continued by 5 km (Figure 3a) and 15 km (Figure 3b).

3 Figure 2. (a) Pseudocolor image of the Musgrave Province aeromagnetic dataset, showing major province boundaries. (b) Detailed view of C1 and C3 anomalies, and their local reorientation by dextral motion on the Petermann Orogeny shear zone (POSZ). (c) An example of the distribution and orientation of C2 and C3 anomalies (Pitjantjatjara Supersuite) within C1 anomalies (basement gneiss). (d) Part of the western Musgrave Province showing extensive Giles Complex intrusions (C4), and a dense network of Petermann Orogeny shear zones (C5). These anomalies are therefore interpreted to represent Petermann Orogeny shear zones. [16] With the aim of producing an image of Musgravian architecture, upward continuation was applied to remove the short wavelength C5 anomalies, and enhance the longer wavelength C1 C3 anomalies. The reduced to pole magnetic field was upward continued by 5 km (Figure 3a) and 15 km (Figure 3b). C5 anomalies have been greatly reduced in amplitude, and the anomalies defining the overall distribution of the Pitjantjatjara Supersuite (C2 and C3) and basement gneiss (C1) are relatively enhanced, although the short wavelength texture of these anomalies has been lost. However, the juxtaposition of granulite facies and amphibolite facies subdomains within the Musgrave Province results in a large amplitude, long wavelength east trending anomaly within which C1 C4 anomalies are present, but are poorly defined (Figure 3a). The potential field tilt [Miller and Singh, 1994] is relatively independent of the input signal amplitude and therefore allows small amplitude anomalies to be resolved better within a large amplitude regional field. [17] The tilt of 5 km upward continued data (Figure 3c) shows a pervasive northeast trending magnetic grain at moderate wavelengths (10 km) that is well developed in the central and eastern Musgrave Province, and correlates with the distribution and orientation of the Pitjantjatjara Supersuite (positive tilt phase) within gneissic basement (negative tilt phase). This trend is only weakly observed in the western Musgrave Province due to extensive Giles Complex intrusions, and the dense network of Petermann Orogeny shear zones (Figure 2d). The tilt of 15 km upward continued data (Figure 3d) shows that, at the continental scale, arcuate northeast trending magnetic highs are observed extending beneath the sedimentary rocks of the Amadeus and Officer basins. 5. New Constraints on Rodinia Reconstructions [18] The northeast trending anomalies in Figure 3d suggest that chains of magnetic granitoids, similar to the Pitjantjatjara Supersuite, extend beneath the Amadeus Basin and connect with the contemporaneous Teapot granites [Black and Shaw, 1995] of the Warumpi Province. In a similar way, magnetic highs above the Officer Basin (Figure 4a) suggest chains of magnetic granitoids that link the Pitjantjatjara Supersuite to the Grenville-aged Nornalup Complex granitoid suites [Clark et al., 2000] of the Albany Fraser Province. [19] The magmatic origins of the compositionally diverse Pitjantjatjara Supersuite, Nornalup Complex granites, and Teapot granites are enigmatic due to a lack of systematic geochemical analysis. A compilation of geochemical data suggests that they are predominantly I-type to transitional A-type granitoids [Budd et al., 2001]. Most granitoids likely 3of6

Figure 3. Pseudocolor images showing RTP data upward continued by (a) 5 km and (b) 15 km. The tilt of the upward continued data (c) 5 km and (d) 15 km.

In addition to the orogenic magmatic suites fragments of arc magmatism may have been incorporated during accretion at the onset of orogenisis.

Mesoproterozoic Albany-Fraser, Musgrave and Warumpi Provinces (Figure 4a).

Importantly, the Grenville-era orogenic belt is abruptly and obliquely terminated against the Palaeoproterozoic North Australian Craton, (Figure 4a).

4 Figure 3. Pseudocolor images showing RTP data upward continued by (a) 5 km and (b) 15 km. The tilt of the upward continued data (c) 5 km and (d) 15 km. reflect syn-to-late orogenic magmatism occurring within the thickened crust of a collisional orogen. In addition to the orogenic magmatic suites fragments of arc magmatism may have been incorporated during accretion at the onset of orogenisis. [20] These chains of granitoids suggest a continuous Grenville-aged orogenic belt trending northeast from the continental margin to the southern margin of the Arunta Inlier, connecting the Mesoproterozoic Albany-Fraser, Musgrave and Warumpi Provinces (Figure 4a). This orogenic belt is surrounded on three sides by older terranes and does not extend eastwards to the Tasman Line (Figure 4a). Importantly, the Grenville-era orogenic belt is abruptly and obliquely terminated against the Palaeoproterozoic North Australian Craton, (Figure 4a). The North Australian Craton is thrust over the Warumpi Province via the Redbank Thrust Zone [Goleby et al., 1989; Selway et al., 2006] which was last active during the ca Ma Alice Springs Orogeny but is interpreted to record activity dating back to ca Ma [Biermeier et al., 2003]. [21] The architecture imaged within this orogenic belt imposes new constraints that must be satisfied in reconstruction models of Australia. Any such model must be able to explain dominant north east trending architecture, intracontinental termination of the orogenic belt and voluminous granitoid magmatism. [22] Tectonic reconstructions proposing an east-trending link between the Musgrave Province and Laurentia [Karlstrom et al., 2001; Moores, 1991; Wingate et al., 2002] cannot explain either the observed architecture within the orogenic belt or its termination against the North Australian Craton. Giles et al. [2004] proposed a model in which the Musgravian and Albany Fraser Orogenies resulted from collision between the South Australian Craton and the joined West Australian and North Australian cratons following 52 of clockwise rotation about a pole located at 136 E and 25 S (Figure 4). This rotation was interpreted to be caused by asymmetric rollback of an initially northeast dipping subduction zone outboard of the South Australian Craton, commencing ca Ma and complete by ca Ma. [23] The Giles et al. [2004] model implies that the Musgravian and Albany Fraser orogenies would have occurred simultaneously as part of a northeast trending orogenic belt that is terminated in the continental interior. The geodynamic model of Giles et al. [2004] (Figure 4b) is a generalist model that did not consider the internal architecture of this orogenic belt, and assumed that the Mus- 4of6

5 Figure 4. (a) Our interpretation of a continuous Grenville-aged orogenic belt within Australia, showing intracontinental termination on 3 sides, the locations of major granitoid regions that define the internal architecture of the belt, and the pole and direction of rotation proposed by Giles et al. [2004]. NAC, SAC, WAC, North, South, and West Australian cratons; RTZ, Redbank Thrust Zone. (b) Schematic after Giles et al. [2004] showing the starting configuration ca Ma (i), rotation (ii), and final configuration ca Ma (iii) of their model. Also shown is our redefined extent of the orogen. gravian Orogeny did not extend beyond the present day limits of the Musgrave Province. Our imaging defines the internal geometry of the Musgravian and Albany Fraser Orogenic belt, and illustrates that the orogen extends beyond the limits of the Musgrave Province, extending to the Warumpi Province. 6. Conclusion [24] Image-processing of high resolution aeromagnetic data has been effective in enhancing anomalies from Grenville-aged gneiss and granitoids and reducing those of a strongly overprinting younger event. A continuous northeast trending orogenic belt with dominant northeast trending internal architecture is defined that links Mesoproterozoic Provinces of Australia and is abruptly terminated against the Paleoproterozoic North Australian Craton. This orogenic belt requires any model of Grenville-era orogenisis in Australia to explain northeast trending architecture and intracontinental termination. Models proposing eastwards continuation of the Musgrave Province to connect with broadly contemporaneous Laurentian orogens are therefore not viable. A model proposing rotation of the South Australian Craton and collision with the North Australian and West Australian cratons [Giles et al., 2004] is consistent with our results, and is our preferred model to explain Grenville-era orogenisis in Australia. [25] Acknowledgments. This work was supported by Primary Industry and Resources South Australia (PIRSA) and Australian Research Council Linkage Grant LP The Musgrave Province aeromagnetic dataset was supplied by PIRSA. Continental Magnetic anomaly grid (# Commonwealth of Australia, 2002) obtained under licence from Geoscience Australia. Fausto Ferraccioli is thanked for his constructive review. References Biermeier, C., et al. (2003), Aspects of the structural and late thermal evolution of the Redbank Thrust system, central Australia: Constraints from the Speares Metamorphics, Aust. J. Earth Sci., 50, Black, L. P., and R. D. Shaw (1995), An assessment, based on U- Pb zircon data, of Rb-Sr dating in the Arunta Inlier, central Australia, Precambrian Res., 71, Budd, A. R., et al. (2001), The metallogenic potential of Australian Proterozoic granites, Rec. 2001/12, Geosci. Aust., Canberra, A. C. T. Burrett, C., and R. Berry (2000), Proterozoic Australia-Western United States (AUSWUS) fit between Laurentia and Australia, Geology, 28, Camacho, A., and C. M. Fanning (1995), Some isotopic constraints on the evolution of the granulite and upper amphibolite facies terranes in the eastern Musgrave Block, central Australia, Precambrian Res., 71, Camacho, A., and I. McDougall (2000), Intracratonic, strike-slip partitioned transpression and the formation of eclogite facies rocks: An example from the Musgrave Block, central Australia, Tectonics, 19, of6

6 Clark, D. J., et al. (2000), Geochronological constraints for a two-stage history of the Albany-Fraser Orogen, Western Australia, Precambrian Res., 102, Duebendorfer, E. M. (2002), Regional correlation of Mesoproterozoic structures and deformation events in the Albany-Fraser Orogen, Western Australia, Precambrian Res., 116, Finn, C. A., and P. K. Sims (2005), Signs from the Precambrian: The geologic framework of the Rocky Mountain region derived from aeromagnetic data, in The Rocky Mountain Region An Evolving Lithosphere: Tectonics, Geochemistry, and Geophysics, Geophys. Monogr. Ser., vol. 154, edited by K. E. Karlstrom and G. R. Keller, pp , AGU, Washington, D. C. Giles, D., et al. (2004), Ga links between the north and south Australian cratons and the early-middle Proterozoic configuration of Australia, Tectonophysics, 380, Goleby, B. R., et al. (1989), Geophysical evidence for thick-skinned crustal deformation in central Australia, Nature, 337, Karlstrom, K., et al. (2001), Long-lived ( Ga) convergent orogen in southern Laurentia, its extensions to Australia and Baltica, and implications for refining Rodinia, Precambrian Res., 111, Laughton, J. R., et al. (2005), Early Proterozoic orogeny and exhumation of Wernecke Supergroup revealed by vent facies of Wernecke Breccia, Yukon, Canada, Can. J. Earth Sci., 42, Lindsay, J. (2002), Supersequences, superbasins, supercontinents; evidence from the Neoproterozoic-early Palaeozoic basins of central Australia, Basin Res., 14, Meert, J. G., and T. H. Torsvik (2003), The making and unmaking of a supercontinent; Rodinia revisited, Tectonophysics, 375, Miller, H. G., and V. Singh (1994), Potential field tilt: A new concept for location of potential field sources, J. Appl. Geophys., 32, Moores, E. M. (1991), Southwest US-East Antarctic (SWEAT) connection: A hypothesis, Geology, 19, Pisarevsky, S. A., et al. (2003), Late Mesoproterozoic (ca 1.2 Ga) palaeomagnetism of the Albany-Fraser orogen: No pre-rodinia Australia- Laurentia connection, Geophys. J. Int., 155, F6 F11. Selway, K., G. Heinson, and M. Hand (2006), Electrical evidence of continental accretion: Steeply-dipping crustal-scale conductivity contrast, Geophys. Res. Lett., 33, L06305, doi: /2005gl Solari, L. A., et al. (2003), 990 and 1100 Ma Grenvillian tectonothermal events in the northern Oaxacan Complex, southern Mexico; roots of an orogen, Tectonophysics, 365, Sun, S. S., et al. (1996), A major magmatic event during Ma in central Australia, and an emplacement age for the Giles Complex, AGSO Res. Newsl., 24, White, R. W., et al. (1999), SHRIMP U-Pb zircon dating of Grenville-age events in the western part of the Musgrave Block, central Australia, J. Metamorph. Geol., 17, Wingate, M. T. D., et al. (2002), Rodinia connections between Australia and Laurentia: No SWEAT, no AUSWUS?, Terra Nova, 14, A. R. A. Aitken and P. G. Betts, Australian Crustal Research Centre, School of Geosciences, Monash University, Melbourne, VIC 3800, Australia. (alan.aitken@sci.monash.edu.au) 6of6

2 Britain s oldest rocks: remnants of

Britain s oldest rocks: remnants of Archaean crust 15 2 Britain s oldest rocks: remnants of Archaean crust 2.1 Introduction Owing to the complex nature of extremely old deformed rocks, the standard methods