The anatomy of a deep intracontinental orogen

Transcription

1 TECTONICS, VOL. 29,, doi: /2009tc002504, 2010 The anatomy of a deep intracontinental orogen Tom Raimondo, 1 Alan S. Collins, 1 Martin Hand, 1 Althea Walker Hallam, 1,2 R. Hugh Smithies, 3 Paul M. Evins, 3,4 and Heather M. Howard 3 Received 25 March 2009; revised 8 April 2010; accepted 27 April 2010; published 12 August [1] The crustal architecture of central Australia has been profoundly affected by protracted periods of intracontinental deformation. In the northwestern Musgrave Block, the Ediacaran Cambrian ( Ma) Petermann Orogeny resulted in pervasive mylonitic reworking of Mesoproterozoic granites and granitic gneisses at deep crustal levels (P =10 14 kbar and T = C). SHRIMP and LA ICPMS dating of zircon indicate that peak metamorphic conditions were attained at circa 570 Ma, followed by slow cooling to C at circa 540 Ma driven by exhumation along the Woodroffe Thrust. Strong links between regional kinematic partitioning, pervasive high shear strains and partial melting in the orogenic core, and an anomalous lobate thrust trace geometry suggest that north vergent shortening was accompanied by the gravitational collapse and lateral escape of a broad thrust sheet. Like the present day Himalayan Tibetan system, the macroscopic structural, metamorphic, and kinematic architecture of the Petermann Orogen appears to be dominantly shaped by large scale ductile flow of lower crustal material. We thus argue that the anatomy of this deep intracontinental orogen is comparable to collisional orogens, suggesting that the deformational response of continental crust is remarkably similar in different tectonic settings. Citation: Raimondo, T., A. S. Collins, M. Hand, A. Walker Hallam, R. H. Smithies, P. M. Evins, and H. M. Howard (2010), The anatomy of a deep intracontinental orogen, Tectonics, 29,, doi: /2009tc Introduction [2] Intracontinental orogens are enigmatic deformational zones produced at a considerable distance (>1000 km) from active plate boundaries. Consequently, any account of their evolution must be framed outside conventional plate tectonic models, which can explain only the spatially restricted effects of convergent plate margin interactions [e.g., Dewey 1 Centre for Tectonics, Resources and Exploration, School of Earth and Environmental Sciences, University of Adelaide, Adelaide, South Australia, Australia. 2 Now at Heathgate Resources, Adelaide, South Australia, Australia. 3 Geological Survey of Western Australia, East Perth, Western Australia, Australia. 4 Now at Swedish Museum of Natural History, Stockholm, Sweden. Copyright 2010 by the American Geophysical Union /10/2009TC and Bird, 1970; England and Jackson, 1989]. Several authors have identified intracontinental orogens in the present Earth, the most prominent being the Tien Shan Orogen in central Asia [Molnar and Tapponnier, 1975, 1977; Hendrix and Davis, 2001; Cunningham, 2005; De Grave et al., 2007; Omuralieva et al., 2009]. In the ancient Earth, arguably the best examples of these unusual orogens are found in central Australia, where there is a remarkable record of intracontinental deformation spanning the late Proterozoic and Phanerozoic [Duff and Langworthy, 1974; Hand and Sandiford, 1999; Sandiford et al., 2001]. The Ediacaran Cambrian ( Ma) [Wade et al., 2005] Petermann Orogeny and the Ordovician Carboniferous ( Ma) [Haines et al., 2001] Alice Springs Orogeny are both major intraplate events affecting the crustal architecture of this region [Shaw and Black, 1991; Shaw et al., 1991; Lambeck and Burgess, 1992; Flöttmann and Hand, 1999; Scrimgeour and Close, 1999; Flöttmann et al., 2004; Aitken et al., 2009a, 2009b]. [3] Despite the good exposure of these orogenic systems, the precise details of their evolution remain poorly understood. This is most apparent with regard to the Petermann Orogeny, which has only recently become the subject of more focused studies [e.g., Camacho et al., 1997, 2001, 2009; White and Clarke, 1997; Scrimgeour and Close, 1999; Camacho and McDougall, 2000; Aitken and Betts, 2009a, 2009b; Aitken et al., 2009a, 2009b; Gregory et al., 2009; Raimondo et al., 2009]. By integrating its macroscopic structural, metamorphic, geochronologic and kinematic patterns, Raimondo et al. [2009] argued that the Petermann Orogen may preserve evidence for lower crustal channel flow, similar to what has been attributed to the present day Himalayan Tibetan system [e.g., Bird, 1991; Nelson et al., 1996; Beaumont et al., 2001, 2004]. If this interpretation is viable, it has profound implications for the behavior of intracontinental crust during episodes of reworking, in particular the spatial and temporal scales of deformation, the rate and kinematics of exhumation, and the overall exhumation and cooling history [Holdsworth et al., 2001]. Perhaps most significantly, the apparent correspondence between the lower crustal behaviors of both the Himalayan and Petermann Orogens suggests that there may not be fundamental differences between the thermal and rheological structures, and hence deformational responses, of collisional and intracontinental orogens. [4] In order to illuminate better the characteristic anatomy of an intracontinental orogen, this paper presents in more detail the structural, metamorphic and geochronologic record of deep intracratonic reworking associated with the Petermann Orogeny. It specifically addresses an area of shear zones on the northwestern margin of the orogen (the Bates 1of31

2 region; Figure 1), which form part of a lower crustal orogenic core characterized by high grade metamorphism and pervasive mylonitic deformation [Raimondo et al., 2009]. Structural mapping, thermobarometry, Sensitive High Resolution Ion Microprobe (SHRIMP) and Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA ICPMS) dating of zircon, and trace element (Zr and Ti) thermometry are used to document the style of orogenesis, allowing an integrated and multifaceted approach to understanding the dynamics of intracratonic reworking. Ultimately, this also enables the development of critical tests for the applicability of the channel flow hypothesis to the Petermann Orogen, and its distinction from alternative orogenic models such as critical taper [Kohn, 2008], wedge and general shear extrusion [Burchfiel and Royden, 1985; Vannay and Grasemann, 2001; Vannay et al., 2004], tectonic wedging [Webb et al., 2007] and extensional collapse [Coney and Harms, 1984; Dewey, 1988; Platt and Vissers, 1989; Fossen, 2000]. 2. Geologic Setting and Previous Work [5] The Musgrave Block (Figure 1) is a Mesoproterozoic mobile belt that forms a broad E W trending gravity high straddling the South Australian, Western Australian and Northern Territory borders [Wade et al., 2008]. It is bounded to the north by the Amadeus Basin and to the south by the Officer Basin, which contain correlatable sequences with depocenters several hundred kilometers from any Neoproterozoic plate margin [Shaw et al., 1991; Lindsay and Leven, 1996; Sandiford and Hand, 1998; Collins and Pisarevsky, 2005]. The stratigraphic continuity between these basins strongly suggests that they comprised part of an extensive intracratonic sedimentary basin initiated after the amalgamation of the Australian craton during the assembly of Rodinia [Lindsay et al., 1987; Walter et al., 1995; Wade et al., 2008]. This depression probably blanketed much of central Australia and has been termed the Centralian Superbasin. It remained intact until fragmentation occurred during late Neoproterozoic, when the Musgrave Block was exhumed from beneath it and a series of smaller sub basins resulted. [6] Current exposures throughout the Musgrave Block are dominated by Ma ortho and paragneisses, widespread Ma granites, and Ma mafic ultramafic intrusives and volcanics [Edgoose et al., 2004; Wade et al., 2008; Smithies et al., 2009]. These outcrops preserve evidence of a complex history of polyphase metamorphism, magmatism and deformation, beginning with felsic magmatism and granulite facies metamorphism in the western Musgraves at circa 1300 Ma [White et al., 1999]. This was followed by regionally extensive granulite facies metamorphism during the Ma Musgravian Orogeny, which reached conditions of T = C and P =5 6 kbar [Clarke et al., 1995; White et al., 2002; Kelly et al., 2006; Wade et al., 2008]. Accompanying this event was voluminous granitic magmatism between Ma, resulting in the emplacement of Pitjantjatjara Supersuite lithologies that dominate outcrops in the Northern Territory and parts of Western Australia [Camacho and Fanning, 1995; Edgoose et al., 2004; Smithies et al., 2009]. The layered mafic ultramafic sills and dikes of the Giles Complex were subsequently emplaced at circa 1080 Ma, followed by syntectonic gabbros and granites at circa 1075 Ma [Clarke et al., 1995; Glikson et al., 1995, 1996; Smithies et al., 2009]. Related bimodal magmatism continued until circa 1026 Ma [Smithies et al., 2009; Evins et al., 2010]. From this period until the onset of the Petermann Orogeny, activity within the Musgrave Block was punctuated only by the emplacement of two mafic dike suites at circa 1000 and circa 800 Ma [Sun and Sheraton, 1992; Zhao et al., 1994; Sun et al., 1996]. [7] An influx of basement derived sediments into the Officer Basin at circa 600 Ma is interpreted to mark the exhumation of the Musgrave Block from beneath the Centralian Superbasin, and thus the beginning of the Petermann Orogeny [Wade et al., 2005]. The main locus of deformation was subsequently focused on the northern margin of the Musgrave Block, producing a series of major E W trending fault structures that dissect the deep crust. These include the Hinckley and Mann Faults and the Woodroffe Thrust (Figure 1). The latter is a shallowly south dipping mylonite and pseudotachylyte zone up to 3 km thick that offsets the Moho by 20 km [Bell, 1978; Lambeck and Burgess, 1992; Camacho et al., 1995; Aitken et al., 2009a, 2009b]. In the northwestern Musgraves, its position is inferred from aeromagnetic and gravity data due to lack of exposure [Edgoose et al., 2004]. Figure 1. (a) Regional geologic map of the Musgrave Block, showing its position relative to the intracratonic Amadeus and Officer basins. Also shown are the locations of key E W trending fault structures of the Petermann Orogen and the boundaries of the field areas discussed in this study. Modified from Edgoose et al. [2004] and Aitken et al. [2009a, 2009b]. (b) Schematic cross section (Y Y ) across the central Musgrave Block, showing the structural arrangement of major faults and lithologies. Note the overall crustal scale dextral transpressive shear system, involving significant Moho displacement and deep exhumation along the Woodroffe Thrust/Mann Fault. Modified from Hand and Sandiford [1999] and Aitken et al. [2009b]. (c) Comparison of structural measurements from three key areas of the orogenic core outlined in Figure 1a. Stereonets summarize foliation and lineation data from mylonitic shear zones using equal angle, lower hemisphere projections. Note the coincidence between the best fit pole to regional fold corrugations and the maximum density of lineation data. Abbreviations: AB, Amadeus Basin; BBZ, Bloods Back Thrust Zone; CL, Caroline Lineament; HF, Hinckley Fault; LL, Lindsay Lineament; MAF, Mount Aloysius Fault; MF, Mann Fault; OB, Officer Basin; PDZ, Piltardi Detachment Zone; WDZ, Wankari Detachment Zone; WHL, Wintiginna Hinckley Lineament; WL, Wintiginna Lineament; WT, Woodroffe Thrust. 2of31

3 Figure 1 3of31

4 Figure 2 4of31

5 [8] The Woodroffe Thrust accommodated significant displacement during N S shortening, assisted by rapid initial unroofing of the Musgrave Block and sustained sediment production throughout the orogenic cycle [Lindsay and Leven, 1996; Calver and Lindsay, 1998; Wade et al., 2005]. This facilitated the exhumation of granulite facies gneisses from depths of km and their structural juxtaposition against amphibolite facies gneisses to the north [Maboko et al., 1992; Scrimgeour and Close, 1999]. Middle to upper crustal rocks are exposed in a large scale imbricate thrust stack (the Petermann Nappe Complex) in its footwall, and contain interleaved sedimentary sequences of the Amadeus Basin. The northern margin of the Musgrave Block also includes a preserved foreland conglomerate (the Mount Currie conglomerate) [Camacho and McDougall, 2000; Edgoose et al., 2004; Flöttmann et al., 2004]. The Petermann Orogen thus features a near complete range of crustal exposure. The youngest record of deformation is preserved by synkinematic biotite and muscovite growth from the Woodroffe Thrust in the eastern Musgrave Block at circa 530 Ma [Maboko et al., 1992; Camacho and Fanning, 1995]. [9] The dimensions of the Petermann Orogen and its extension through northwestern Australia are comparable to typical plate margin orogens, spanning 1500 km E W and up to 300 km N S. Sequential cross section restorations suggest that north vergent shortening during orogenesis exceeded 100 km and was accommodated by substantial crustal thickening [Flöttmann et al., 2004]. This is supported by thermobarometric constraints from the high grade orogenic core exposed between the Woodroffe Thrust and The Mann Fault, which indicate pressures of kbar and temperatures of C [Clarke et al., 1995; Camacho et al., 1997; White and Clarke, 1997; Scrimgeour and Close, 1999; Gregory et al., 2009]. Metamorphic grade decreases toward the foreland to the north and parallel to major structures toward the east [Edgoose et al., 2004]. [10] The Petermann Orogen is usually regarded as a crustal scale dextral transpressive shear system [Camacho and McDougall, 2000; Edgoose et al., 2004; Flöttmann et al., 2004]. North vergent transport was principally concentrated along the Woodroffe Thrust, while south directed overthrusting was accommodated along the southern margin of the Musgrave Block. Dextral strike slip offset is primarily recorded along deeply penetrating shear zones in the axial zone of the orogen, including the Mann Fault and the Wintiginna Lineament (Figure 1) [Aitken et al., 2009a, 2009b]. It is likely that intracontinental orogenesis was driven by N S compression of the Australian plate during the amalgamation of Gondwana in the Neoproterozoic [Collins and Pisarevsky, 2005; Hand and Sandiford, 1999; Aitken and Betts, 2009a]. The stationary position of northwestern Australia relative to anti clockwise rotation of the southern plate margin may be responsible for the generation of large scale dextral transpression within the continental interior. 3. Structural Relationships [11] The Bates region (Figure 1) is situated at the junction of the South Australian, Western Australian and Northern Territory borders, on the northwestern margin of the Musgrave Block [Howard et al., 2006]. It forms part of a deep crustal wedge exhumed between the Mann Fault and the Woodroffe Thrust, and is characterized by high grade metamorphism and pervasive mylonitic deformation [Raimondo et al., 2009]. Mesoproterozoic Pitjantjatjara Supersuite granites and various granitic gneisses are extensively reworked to form structures ranging from discrete 1 5 cm recrystallized shear bands to m wide mylonitic shear zones. Some of the best preserved and most extensive outcrops occur at Spaghetti Hill in the central Bates region (Figure 2). This area has been studied in detail to document the interactions between mylonitisation, metamorphism and migmatization accompanying Petermann Orogeny reworking. The outcrop scale structural relationships observed in this location are then compared to the broad patterns evident across the Bates region, and further east in the Mann Ranges (Figure 1), to build up an overview of the structural arrangement of the orogenic core Outcrop Structure (Spaghetti Hill) [12] At the outcrop scale, the structural organization of mylonitic shear zones is quite complex. Spaghetti Hill comprises a megacrystic rapakivi granite that is strongly recrystallized to form felsic mylonites with relict K feldspar augen and prominent elongation lineations defined by aligned aggregates of quartz, biotite and feldspar. The mylonites are arranged in an anastomosing pattern that dissects the country rock into multiple discrete blocks, each one completely enveloped by the mylonitic fabric (Figure 2a). The strike of this fabric mimics the outline of the undeformed blocks, creating complex convergence zones where Figure 2. Summary of structural relationships at Spaghetti Hill, central Bates region. (a) Outcrop map showing the complex anastomosing trend lines of mylonitic shear zones, which wrap around discrete blocks of undeformed granitic country rock. Orientation data from mylonitic fabrics (M.F.) and elongation lineations (E.L.) are summarized using equal angle, lower hemisphere projections. Also shown are the sample and photograph locations mentioned in the text. (b) Detailed structural map of the central Spaghetti Hill section, showing complex convergence zones where anastomosing mylonite strands with very oblique strikes are juxtaposed without truncation. Also note strain variations within the shear zones, particularly around granitic inliers. See text for explanation. (c) Smooth strain gradient from protomylonite to ultramylonite at the margins of an undeformed granitic block, entirely enveloped by the mylonitic fabric. Scale bar is 10 cm. (d) Rotated garnet porphyroblast showing top to the SW (normal sense) kinematics (delta clast). Field of view is 5 cm. (e) Deformed leucosome from the eastern Spaghetti Hill section, containing coarse (centimeter scale) garnet porphyroblasts. 5of31

6 blocks are in close proximity or shear zones with highly oblique strikes are juxtaposed, as illustrated in detail for the central section of Spaghetti Hill (Figure 2b). This results in widely dispersed foliation data, with considerable variation in the stereonet distributions of the central and NW sections (containing several convergence zones) compared to the SW and eastern sections (containing fairly uniform strike). However, when the outcrop is considered as a whole, moderately to steeply NE to NW dipping orientations are most common (probably representative of a larger scale bifurcation of the mylonitic fabric at the southern tip of Spaghetti Hill), and define a steeply dipping fold profile plane whose best fit pole (25 036) closely corresponds to the predominant NE plunging elongation lineations of all sections (maximum density at ). [13] Strain is considerably varied across individual shear zones at Spaghetti Hill, particularly at the margins of undeformed granitic pods, which often show smooth strain gradients from protomylonite to ultramylonite (Figure 2c). High strain packages feature asymmetric sigma and deltaclasts, along with S C and C fabrics, that invariably record top to the SW tectonic transport (Figure 2d) [Howard et al., 2006]. Tight to isoclinal intrafolial folds are also common and typically have hinges orthogonal to the lineation orientation. In contrast, low strain packages do not record rotational kinematics, but instead contain symmetrical porphyroclasts and meter scale tight to open folds with N/S plunging elongation lineations (slightly oblique to the predominant NE/SW plunging orientation) parallel to fold hinges. [14] Strain patterns are particularly complex at the convergence of oblique shear zones, where L S tectonites progressively lose their planar fabric and are converted into L tectonites that plunge parallel to the strike of the adjacent units (Figure 2b). However, detailed observation of the contact relationships between multiple anastomosing mylonite strands in the central section of Spaghetti Hill showed a consistent conformable transition between shear zones of different strike; that is, no evidence was found for truncation of earlier mylonitic fabrics Regional Structure [15] At the regional scale, the structural organization of mylonitic shear zones throughout Bates (Figure 3) appears to reflect the broad patterns evident at Spaghetti Hill. Mylonitic fabrics are considerably dispersed, but again define a steeply dipping profile plane whose best fit pole (10 234) closely corresponds to the predominant shallowly NE/SW plunging elongation lineations of the region (maximum density at ). In addition, top to the SW kinematic indicators (red double barbed arrows in Figure 3) are consistently observed from the immediate hanging wall of the Woodroffe Thrust in the north to the Mann Fault in the south. There is some scatter in the lineation data, with a smaller cluster of N/S plunging lineations again evident. However, as documented at Spaghetti Hill, they are contained within the same shear zones that feature the more abundant NE/SW plunging lineations, without any evidence of truncation or overprinting. On the contrary, at Heather s Hill in southeast Bates (see Figure 3), lineations are observed to rotate progressively from SW to south plunging orientations between high and low strain domains in a single shear zone of approximately 60 m width. Similarly to Spaghetti Hill, south plunging lineations are restricted to protomylonites with symmetrical flattening fabrics, while SW plunging lineations are contained in mylonite packages associated with pervasive asymmetric augens showing topto the SW kinematics. Based on structural criteria, therefore, the oblique lineation arrays throughout Bates are temporally indistinguishable from one another, and are invariably associated with a single kinematic vector (top to the SW). [16] The structural arrangement of the Bates region is strongly reflected in the western Mann Ranges, further east of this location (Figure 1c). Here, elongation lineations are predominantly west to SW and east to NE plunging (maximum density at ), and again closely match the best fit pole to a profile plane defined by poles to the mylonitic foliation (5 260). Kinematic indicators are top to the WSW in this area [Raimondo et al., 2009]. A distinct group of south plunging lineations is also apparent, but rotational strain and crosscutting relationships are again absent from their host fabrics [Scrimgeour and Close, 1999; Scrimgeour et al., 1999]. The adjacent eastern Mann Ranges, in contrast, are very structurally distinct. They feature invariably south dipping mylonitic fabrics with pervasive south plunging elongation lineations. Kinematic indicators are uniformly top to the N in this region [Scrimgeour and Close, 1999; Scrimgeour et al., 1999; Edgoose et al., 2004]. [17] A lateral shift from east to west through the orogenic core, therefore, appears to be marked by gradual change from uniformly planar mylonitic fabrics to broad regional fold corrugations. The profile plane of the corrugations shows increased strike rotation toward the west between the western Mann Ranges and the Bates region, as indicated by the shift of its best fit pole from shallowly WSW to SW plunging (Figure 1c). This transition is accompanied by the change from WSW to SW directed normal sense kinematics, which strongly disagrees with the north vergent reverse kinematics observed in the eastern Mann Ranges. It is also marked by the development of a pronounced lobate geometry along the Woodroffe Thrust that is convex toward the foreland, in the direction of net thrust displacement, Figure 3. Regional geologic map of the Bates region, showing the distribution of mylonitic shear zones, the orientations and transport directions of shear sense indicators (red double barbed arrows), and key field locations mentioned in the text. The locations of all samples and their associated average P T, age and Zr/Ti thermometry estimates are displayed with reference to the data key (top right). Inset shows equal angle, lower hemisphere stereographic projections of orientation data for mylonitic fabrics (as poles) and elongation lineations. Map is modified from Howard et al. [2006] and includes a superimposed pseudocolor aeromagnetics image (total magnetic intensity) sourced from Geological Survey of Western Australia [2006]. Coordinates derived from the Map Grid Australia Zone 52 (MGA94). 6of31

7 Figure 3 7of31

8 Figure 4. Photomicrographs of petrological relationships. All images except Figure 4c are in plane polarized light and have a mylonitic foliation parallel to their base. Mineral abbreviations are after Kretz [1983]. (a) Recrystallized leucosome containing coarse garnet porphyroblasts wrapped by a mylonitic fabric composed of finely disseminated biotite, hornblende and ilmenite. Sample (b) Relict porphyroclastic garnet in contact with metamorphic biotite and hornblende needles and separated from fragmented primary hornblende grains by a plagioclase moat. Sample (c) Linear inclusion trails in relict hornblende surrounded by coarse titanite porphyroblasts associated with ilmenite and a recrystallized matrix of biotite, hornblende, plagioclase, K feldspar and quartz. Sample (d) Diamond shaped titanite porphyroblasts parallel to mylonitic foliation defined by elongate biotite and hornblende needles. Sample and parallel to the spreading direction of the regional fold hinges. 4. Metamorphic Petrology and Thermobarometry 4.1. Field Relationships and Sample Descriptions [18] Mineral assemblages associated with mylonitic shear zones have limited variability throughout the Bates region, and largely reflect compositional variations in their granitic and gneissic precursors. The majority formed from quartzofeldspathic bulk compositions and consist of fine grained recrystallized biotite, hornblende, garnet, clinopyroxene, quartz, K feldspar and plagioclase, with titanite, clinozoisite, ilmenite, magnetite, apatite and zircon present as accessories (Figure 4). Coarse fragmented porphyroclasts of clinopyroxene, hornblende, garnet, K feldspar and plagioclase are variably preserved, except in migmatized domains where primary igneous minerals are absent and leucosomes contain coarse garnet and hornblende porphyroblasts. Metamorphic mineral populations are distinguished on the basis of reduced grain size and the absence of abundant 8of31

9 Table 1. Summary of THERMOCALC Average Pressure and Temperature Estimates From Mylonitic Shear Zones in the Bates Region Sample Average P ±1s (kbar) Average T ±1s ( C) Average P T ±1s (kbar; C) Average Geothermal Gradient ( C km 1 ) ± ± ± 72; 11.3 ± ± ± ± 76; 10.1 ± ± ± ± 35; 10.7 ± ± ± ± 37; 11.8 ± ± ± ± 40; 11.8 ± B ± ± ± 66; 10.7 ± ± ± ± 47; 12.0 ± ± ± ± 47; 10.7 ± ± ± ± 38; 10.7 ± ± ± ± 34; 9.6 ± ± ± ± 70; 10.8 ± A ± ± ± 35; 11.8 ± Recalculated P T Estimates Using the Mineral Compositions of White and Clarke [1997] M010 (Core) ± ± ± 28; 11.0 ± (Rim) 8.98 ± ± ± 33; 9.1 ± M213 (Core) ± ± ± 46; 13.0 ± (Rim) ± ± ± 36; 11.4 ± M214 (Core) ± ± ± 37; 13.1 ± (Rim) ± ± ± 40; 11.6 ± inclusions, consistent with their having undergone extensive recrystallization. In addition, metamorphic aggregates of hornblende and biotite are dominantly acicular and exhibit a strong preferred orientation that defines the mylonitic fabric, allowing them to be differentiated from their porphyroclastic igneous equivalents. [19] In the northern Bates region, immediately south of the Woodroffe Thrust, mylonitic mineral assemblages are sparsely garnet bearing, typically annealed, and feature extensive pseudotachylyte veining. The proportion of garnet and hornblende increases moving southward toward the Mann Fault, concomitant with an increase in the regional distribution of migmatitic shear zones. Localized migmatization in the form of melt veins along thin shears is evident at Spaghetti Hill and Mount Gosse (see Figure 3), while more extensive partial melting occurs further south of these locations. Migmatitic domains are structurally confined to discrete shear zones, and are characterized by fine grained recrystallized leucosomes, irregular layering, and coarse (centimeter scale) porphyroblastic minerals (Figures 2e and 4a) THERMOCALC Average P T Methodology [20] Mylonitic lithologies from the Bates region are conducive to thermobarometry because they contain a large number of minerals in textural equilibrium. All samples selected for thermobarometry are garnet bearing and contain metamorphic minerals that either define the mylonitic fabric or exhibit strong grain size reduction relative to primary igneous phases. Metamorphic mineral compositions were obtained using a Cameca SX51 Electron Microprobe with four wavelength dispersive spectrometers, located at the University of Adelaide. Quantitative analyses were done using an accelerating voltage of 15 kv and a beam current of 20 na. The mineral chemistry of all samples is summarized in Text S1 in the auxiliary material. 1 Pressure and temperature (P T) calculations were performed using the compositional analyses presented in Data Set S1, with activity and composition relationships for mineral endmembers determined using the software A X [Powell et al., 1998]. Additional P T calculations were also completed using the published mineral analyses of White and Clarke [1997] from samples M010, M213 and M214. These samples are sourced from the same outcrop as sample , 12 km west of Mount Gosse (see Figure 3). Pressure and temperature estimates were obtained using the average P, average T and average P T approaches of Powell and Holland [1988, 1994]. These multiple equilibria techniques deploy the software THERMOCALC (version 3.26) and the updated internally consistent data set of Holland and Powell [1998] to calculate the optimal metamorphic conditions from the thermodynamics of an independent set of mineral reactions Average P T Estimates [21] A summary of average P T estimates from mylonitic shear zones is presented in Table 1. Sample locations and their associated P T conditions are also shown in Figure 3. In general, the results show a gradual trend of increasing temperatures toward southeast Bates (Figure 5). Conditions vary from 620 C in the immediate hanging wall of the Woodroffe Thrust to 780 C approximately 30 km further south, adjacent to the Mann Fault. Estimates are consistently C in the central domain that comprises Spaghetti Hill, Mount Gosse and additional outcrops to the west, coinciding with evidence of localized migmatization in these areas. In contrast, temperatures range between 650 and 700 C 1 Auxiliary material data sets are available at ftp://ftp.agu.org/apend/tc/ 2009tc Other auxiliary material files are in the HTML. 9of31

10 Figure 5. Comparison of THERMOCALC average pressure and temperature estimates from mylonitic lithologies with Zr/Ti thermometry values, showing the metamorphic field gradient of the Bates region and its correspondence to an increase in the regional distribution of migmatitic shear zones between the Woodroffe Thrust and the Mann Fault. in the regions surrounding Mount Daisy Bates, where no migmatization is observed. [22] The general trend toward elevated temperatures in southeast Bates is accompanied by a steady fluctuation in pressure, with peak values in the central zone and lower values immediately adjacent to both the Woodroffe Thrust and the Mann Fault (Figure 5). Conditions vary from 9.5 kbar in the north to 11 kbar in the south. Estimates are higher in the regions west of Mount Gosse, reaching 13 kbar using the recalculated core assemblages of White and Clarke [1997]. Several variations are also evident throughout the domain south of Spaghetti Hill, but generally these occur over a range of ±0.5 kbar, which remains within error of the typical THERMOCALC pressure uncertainties. 5. U Th Pb Geochronology [23] SHRIMP U Th Pb analysis was conducted on zircons from sample , while LA ICPMS U Th Pb analysis was performed on zircons from samples , , and All samples are moderately to highly strained felsic mylonites. Additional geochronology of titanites from the Bates region is discussed by Raimondo et al. [2009]. Both SHRIMP and LA ICPMS operating procedures and data reduction methods are outlined in Text S Zircon Descriptions [24] Samples , and show convolute internal zoning when viewed under cathodoluminescence (CL) (Figures 6a 6c). In some cases, oscillatory zoned domains appear to be overprinted by patchy segments with variable luminescence. In others, convolutely zoned cores are mantled by thin concentric banding. Overall, however, most grains feature reasonably homogeneous or patchily zoned cores with low CL responses. They are enveloped by moderately luminescent, irregular rims, which in some cases extend deep into the core. As an exception, sample displays much darker rims with highly luminescent cores and several grains with relatively featureless CL domains. [25] Samples and display much greater regularity of internal features when viewed under CL (Figures 6d and 6e). Grain interiors are characterized by patchy or convolute zoning with moderate luminescence, usually mantled by distinct oscillatory zoned domains with overall low luminescence. Featureless dark cores are also occasionally present in some grains. However, all grains display highly luminescent rims of variable thickness, which are observed to either truncate concentric banding or irregularly contact homogeneous cores. Sample contains a number of thick rims with ghost textures, regions of subtle banding that appear to mimic the primary oscillatory zoning. Several large fractures through individual grains also appear to have completely re healed, and appear as highly luminescent bands that intersect rim areas Age Estimates [26] Age data for all samples are presented in Figures 7 and 8 and summarized in Table 2, and analytical data are 10 of 31

11 Figure 6. Cathodoluminescence images of representative zircon grains from all geochronology samples, showing the range of textural features. Note in particular the ghost zoning exhibited by highly luminescent rim areas of sample , regions of subtle banding that appear to mimic the primary oscillatory pattern. Displayed spot ages <1000 Ma and >1000 Ma are 206Pb/238U and 207Pb/206Pb ages, respectively. Spot size shown is 30 mm. 11 of 31

12 Figure 7 12 of 31

13 Figure 8. Concordia plots of zircon data from samples , , and Age uncertainties are quoted at the 95% confidence level unless indicated otherwise. Inset probability density plots use 206 Pb ages for data included in the regression lines with >90% concordance. (a) All analyses from sample , showing the calculated regression line. Black ellipses indicate data included in Figure 8b. Gray ellipses indicate data included in the regression but excluded from the concordia calculation for reasons mentioned in the text. (b) Concordant data from sample , showing the weighted average error ellipse of the concordant age calculation (shaded gray). (c and d) All analyses from samples and , respectively. Black ellipses indicate data included in the regression lines. Gray ellipses indicate data excluded from the regressions for reasons mentioned in the text. Figure 7. Concordia plots of zircon data from samples and Age uncertainties are quoted at the 95% confidence level unless indicated otherwise. Inset probability density plots contain analyses included in the regression lines and use 206 Pb/ 238 U and 206 Pb ages for data <1000 Ma and >1000 Ma, respectively. (a) All analyses from sample Black ellipses indicate data included in Figures 7b and 7c. Gray ellipses indicate data excluded from age calculations for reasons mentioned in the text. (b) Rim analyses, showing the weighted average error ellipse of the concordant age calculation (shaded gray). (c) Core analyses, showing the regression line and age calculations. (d) All analyses from sample Black ellipses indicate data included in Figures 7e and 7f. Gray ellipses indicate data excluded from age calculations for reasons mentioned in the text. (e) Tera Wasserburg plot of rim analyses, showing the regression line and age calculations. Common lead composition used as an anchor value for the regression (0.814 ± 0.023) is based on SHRIMP analysis of titanites from the same sample [see Raimondo et al., 2009]. (f) Core analyses, showing the regression line and age calculations. 13 of 31

14 Table 2. Summary of Zircon Characteristics Sample Size (mm) Number of Grains/ Number of Spots Average Th/U Average U (ppm) Average f 204 (%) Age (Ma) /14 Rims: 0.15 Cores: ± ± / ± / ± /60 Rims: 0.11 Cores: ± ± / ± 13 provided in Tables 3 and 4. All errors in the data tables are quoted at the 1 sigma (1s) level, while intercept ages and weighted averages are at the 95% confidence level. All age calculations were performed using Isoplot/Ex version 3.57 [Ludwig, 2003]. The spatial distribution of all samples is shown in Figure 3, along with their associated age estimates. See also Figure 2a for Spaghetti Hill sample locations Sample (Highly Strained Felsic Mylonite) [27] Fourteen SHRIMP analyses of eleven zircon grains were obtained from sample , targeting the weakly luminescent cores and highly luminescent rims. Two age maxima are apparent on a probability density plot at circa 570 Ma (rims) and circa 1140 Ma (cores). Rim analyses are concordant to reversely discordant, while core analyses define a linear Pb loss trend with concordant to moderately discordant components. Two outliers (shown as gray ellipses in Figure 7a) are excluded from subsequent age calculations because they were positioned on narrow rims and probably represent mixed ages. [28] A weighted average of 206 Pb/ 238 U ages from rim analyses yields an estimate of 568 ± 12 Ma, with the mean square of weighted deviates (MSWD) equal to This estimate is comparable to the concordant age calculation (568 ± 6 Ma; MSWD = 0.94), which takes into account the equivalence and concordance of 206 Pb/ 238 U, 235 U and 206 Pb ages simultaneously [Ludwig, 1998], with analytical and decay constant errors included (Figure 7b). A weighted average of 206 Pb ages from core analyses yields an estimate of 1118 ± 31 Ma (MSWD = 1.50). This is within error of a model 1 solution for the discordia chord (Figure 7c), which yields an upper intercept age of 1158 ± 61 Ma and a lower intercept age of 422 ± 340 Ma (MSWD = 0.82). The upper concordia intercept age is used as the best estimate of the true zircon core age, as it takes into account a small number of concordant grains that have undergone minimal Pb loss Sample (Moderately Strained Felsic Mylonite) [29] Sixty LA ICPMS analyses of forty zircon grains were obtained from sample , targeting the weakly luminescent cores and highly luminescent rims. Two age maxima are apparent on a probability density plot at circa 575 Ma (rims) and circa 1160 Ma (cores). Rim analyses are strongly discordant, and plot on a linear array that approximates a common Pb trend. In contrast, core analyses define a linear Pb loss trend with concordant to moderately discordant components. A number of outliers (shown as gray ellipses in Figure 7d) are excluded from subsequent age calculations for the following reasons. First, several analyses showed clear mixing between core and rim zones, but with no resolvable parts of the spectrometry signal that could be isolated. This is largely the result of both the fine width of the rims (occasionally <20 mm) and the high intensity of the laser. Second, a small number of analyses (particularly Table 3. SHRIMP Zircon U Th Pb Age Data for Sample Spot Name U (ppm) Th (ppm) Th/U f 204 (%) 206 Pb ±1s (%) 206 Pb/ 238 U Isotopic Ratios a ±1s (%) 235 U ±1s (%) 208 Pb/ 232 Th ±1s (%) Rho b 206 Pb/ 238 U ±1s Age Estimates a 206 Pb ±1s a Displayed ratios and ages are corrected for common Pb following the method of Stacey and Kramers [1975]. b Error correlation; defined as [(err. 206 Pb/ 238 U)/(measured 206 Pb/ 238 U)]/[(err. 235 U)/(measured 235 U)]. c Effective age; for ages <1000 Ma and >1000 Ma, this corresponds to calculated 206 Pb/ 238 U and 206 Pb ages, respectively. Eff age c 14 of 31

15 Table 4. LA ICPMS Zircon U Th Pb Age Data Spot Name Th/U 206 Pb ±1s Isotopic Ratios a Rho b Age Estimates a Conc c 206 Pb/ 238 U ±1s 235 U ±1s 208 Pb/ 232 Th ±1s 206 Pb ±1s 206 Pb/ 238 U ±1s 235 U ±1s 208 Pb/ 232 Th ±1s (%) Eff Age d Sample Spot Spot Spot Spot Spot Spot Spot Spot Spot Spot Spot Spot Spot Spot Spot Spot Spot Spot Spot Spot Spot Spot Spot Spot Spot Spot Spot Spot Sample Spot Spot Spot Spot Spot Spot Spot Spot Spot Spot Spot Spot Spot Spot Spot of 31

16 Table 4. (continued) Spot Name Th/U 206 Pb ±1s Isotopic Ratios a Rho b Age Estimates a Conc c 206 Pb/ 238 U ±1s 235 U ±1s 208 Pb/ 232 Th ±1s 206 Pb ±1s 206 Pb/ 238 U ±1s 235 U ±1s 208 Pb/ 232 Th ±1s Sample Spot Spot Spot Spot Spot Spot Spot Spot Spot Spot Spot Spot Spot Spot Spot Spot Spot Spot Spot Spot Spot Spot Spot Spot Spot Spot Spot Spot Spot Spot Spot Spot Spot Spot Spot Spot Spot Spot Spot Spot Spot Spot Spot Spot Spot Spot Spot Spot (%) Eff Age d 16 of 31