Alan R.A. Aitken 1,*, Peter G. Betts 1, and Laurent Ailleres 1 1. inlier (e.g., Collins and Shaw, 1995; Sandiford,

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1 The architecture, kinematics, and lithospheric processes of a compressional intraplate orogen occurring under Gondwana assembly: The Petermann orogeny, central Australia Alan R.A. Aitken 1,*, Peter G. Betts 1, and Laurent Ailleres 1 1 SCHOOL OF GEOSCIENCES, MONASH UNIVERSITY, WEINGTON ROAD, CLAYTON, VICTORIA 38, AUSTRALIA ABSTRACT We ally aeromagnetic interpretation with constrained three-dimensional (3D) gravity inversion over the Musgrave Province in central Australia to produce a 3D architectural and kinematic model of the ca. 55 Ma compressional intraplate Petermann orogeny. Our model is consistent with structural, metamorphic, and geochronological constraints and crustal-scale seismic models. Aeromagnetic interpretation indicates that divergent thrusts at the margins of the province are cut by transpressional shear zones that run along the axis of the orogen. Gravity inversion indicates that the marginal thrusts are crustal-scale and shallow-dipping, but that the transpressional shear zones of the axial zone are more steeply dipping, and penetrate the crust-mantle boundary, accommodating offsets of 1 km. This thick wedge of mantle within the lower crust has been in isostatic disequilibrium for more than 5 Ma, and we suggest that this load may be supported by local lithospheric strengthening resulting from the emplacement of relatively strong lithospheric mantle within the relatively weak lower crust. Other orogenic processes inferred from the model include: probable inversion of relict extensional architecture; crustal-scale strain partitioning leading to strain accommodation by the vertical and lateral extrusion of relatively undeformed crustal blocks; and escape tectonics directed toward the relatively free eastern margin of the orogen. These processes are consistent with the concept that mechanical and thermal heterogeneities in the lithosphere, and the resulting feedbacks with deformation, are the dominant controls on intraplate orogenesis. This model also demonstrates that the architecture and kinematics of the Petermann orogeny require modification of leading models of Gondwana assembly. LITHOSPHERE; v. 1; no. 6; p doi: 1.113/L39.1 INTRODUCTION The architecture and kinematics of orogenic belts have been the topic of many studies, mostly focused on actively deforming or recently deformed plate margins, such as the Himalaya (e.g., Molnar, 1988; Yin, 26) or the European Alps (e.g., Bruckl et al., 27; Ebbing et al., 21; Luschen et al., 24). In contrast, compressional intraplate orogens have been the subject of comparatively few studies. To date, studies of the structure and kinematics of intraplate compression have concentrated on the inversion of extensional basins in both backarc-hinterland and forearc-foreland settings (e.g., Dickerson, 23; Sandiford, 1999; Turner and Williams, 24; Ziegler et al., 1995), with others studying the dynamic evolution of the currently active intraplate compressional orogens of the Tien Shan (Burov et al., 199; Tapponnier and Molnar, 1979; Zhao *Corresponding author alan.aitken@sci. monash.edu.au. et al., 23) and Altai (Cunningham, 25) regions of central Asia. These studies highlight the diverse range of settings for intraplate compressional deformation, and also the variety of lithospheric processes that can occur. However, one finding that is common to all studies is the importance of thermal and mechanical heterogeneities in the continental lithosphere as a control on crustal architecture (e.g., Cunningham, 25; Dickerson, 23; Hand and Sandiford, 1999; Sandiford and Hand, 1998; Ziegler et al., 1998). Late Neoproterozoic to Devonian tectonic reworking of central Australia is interpreted to reflect intraplate compressional orogenesis (e.g., Betts et al., 22; Camacho and Fanning, 1995; Camacho et al., 22; Hand and Sandiford, 1999; Sandiford and Hand, 1998). Two discrete orogens are recognized: the ca. 6 5 Ma Petermann orogeny that reworked the Mesoproterozoic Musgrave Province (e.g., Major and Conor, 1993; Wade et al., 28) and the ca Ma Alice Springs orogeny that reworked the Paleoproterozoic Arunta inlier (e.g., Collins and Shaw, 1995; Sandiford, 22). The initiation of these orogens has been the topic of much study, and several quite different models may explain the initiation and some of the main features of these orogens (cf. Braun and Shaw, 21; Camacho et al., 22; Neil and Houseman, 1999; Neves et al., 28; Sandiford et al., 21). A detailed regional-scale model of the 3D architecture and kinematics of these orogens is lacking. This is important because it may indicate the orientation and intensity of the forces driving the system and characterize the feedback processes that control the interaction between crustal architecture and the dynamics of the orogen. In addition, these orogens provide a record of intraplate continental lithospheric deformation under the influence of one of Earth s most dramatic periods of tectonism associated with the assembly of Gondwana, and their architecture and kinematics may help to recognize the most (and least) credible of many competing tectonic models (e.g., Boger and Miller, 24; For permission to copy, contact editing@geosociety.org 29 Geological Society of America 343

2 AITKEN et al. Cawood, 25; Collins and Pisarevsky, 25; Jacobs and Thomas, 24; Meert and Van Der Voo, 1997; Meert, 23; Rino et al., 28; Veevers, 23). A combination of aeromagnetic data and gravity data can be used to image architecture from the near surface to crust-mantle boundary geometry (e.g., Williams and Betts, 27), and therefore these data provide the ideal tool to unify the concepts of previous studies of orogenic architecture at multiple scales. In this paper, we combine interpretation of highresolution aeromagnetic data with 3D gravity inversions to produce a crustal-scale model of the architecture and kinematics of the intraplate Petermann orogeny in the eastern Musgrave Province. This model is constrained by geological observations at a number of scales, including pressure-temperature-time (P-T-t) data, structural interpretations, petrophysical sampling, and macro-scale geological observations, and constraint is also derived from crustal-scale seismic reflection lines and passive seismic models. From this architectural and kinematic model, we infer the most influential lithospheric processes that have shaped, and been controlled by, the architecture and kinematics of the Petermann orogeny. THE GEOLOGIC SETTING OF THE PETERMANN OROGENY The Musgrave Province preserves a variety of Mesoproterozoic gneissic rocks of dominantly felsic lithology with precursors dated at ca. 16 Ma (Gray, 1978; Wade et al., 26) that were metamorphosed at amphibolite to granulite facies during the ca. 12 Ma Musgravian orogeny (Camacho and Fanning, 1995; Gray, 1978; Maboko et al., 1991; Sun and Sheraton, 1992; White et al., 1999). Although, in outcrop, the structural trend of this event is variable throughout the province (cf. Aitken et al., 28; Aitken and Betts, 29b; Clarke et al., 1995; Edgoose et al., 24; Major and Conor, 1993), linking these observations to a coincident structural grain in aeromagnetic data defines this structural trend at the regional scale and shows that it is dominantly northeast trending (Aitken et al., 28; Aitken and Betts, 29b). The emplacement of the granitic plutons of the Pitjantjatjara Supersuite occurred during and shortly after this orogeny (Major and Conor, 1993), and their emplacement pattern dominantly reflects the northeast-trending structural grain of the Musgravian orogeny (Aitken et al., 28; Aitken and Betts, 29a, 29b; Edgoose et al., 24; Major and Conor, 1993). These chains of magnetic granitoids are interpreted to be continuous beneath the Amadeus and Officer basins, defining the extent of the ca. 12 Ma Musgravian-Albany-Fraser orogeny (Aitken and Betts, 28). Subsequent to the Musgravian orogeny, the voluminous mafic intrusions of the Giles Complex and coeval mafic dikes and granitoids were emplaced within the Musgrave Province during the extensional Giles event at ca. 18 Ma (Glikson et al., 1995; Sun et al., 1996), along with surficial volcanic rocks now exposed at the margins of the Musgrave Province (Glikson et al., 1995). Although not well defined, the extent and orientation of this event may be defined by east-to-east-southeast trending shear zones that predate or are synchronous with the dike emplacement events (Aitken et al., 28; Aitken and Betts, 29b; Clarke et al., 1995); the alignment of Giles Complex mafic intrusions along the axis of the Musgrave Province with no geophysical evidence for buried Giles Complex plutons outside of this zone (Glikson et al., 1995; Glikson et al., 1996); and the orientation and extent of the Warakurna large igneous province (LIP), of which the Giles Complex is a key constituent, which extends from northern Western Australia to the Musgrave Province (Wingate et al., 24). After a hiatus of ca. 2 million years, mafic dikes were emplaced at ca. 8 Ma along eastsoutheast and southeast-oriented structures (Zhao et al., 1994). The inception of the Officer and Amadeus basins is broadly contemporaneous with these dikes, and probably formed as part of the once contiguous Centralian Superbasin (Walter et al., 1995). This ca. 8 Ma extensional event may represent a northwest-trending aulacogen, related to a mantle plume centered beneath the Adelaide Rift Complex to the southeast (Betts et al., 22; Zhao et al., 1994). A second hiatus of ca. 2 million years followed this event, before the Musgrave Province was intensely reworked during the ca. 55 Ma Petermann orogeny. Although locally derived vertical driving forces may have played a significant role (Neil and Houseman, 1999), the Petermann orogeny is typically interpreted to represent the intraplate response to far-field stresses related to Gondwana assembly. Due to the uncertainties regarding the assembly of Gondwana, defining a specific tectonic driver for this event is not straightforward, and several major orogens may have contributed to the stress field. The closest active plate-margin orogen may be found in the ca Ma collision of India with Australia s western margin, termed the Kuunga orogeny (Collins and Pisarevsky, 25; Meert et al., 1995; Meert and Van Der Voo, 1997; e.g., Meert, 23). This orogeny was originally interpreted to represent the ca continent-continent collision of Gondwana, resulting in the suturing of Australia, east Antarctica, and the Kalahari craton, onto the remainder of Gondwana, which was previously assembled during the ca Ma east African orogen (Meert, 23). However, a lack of accreted arc fragments or continental blocks and the limited extent of its component terranes led Squire et al. (26) to suggest that the Kuunga orogeny is an intracratonic response to the East African Antarctic orogen, which is interpreted by several authors to record the major event in the amalgamation of Gondwana (Jacobs and Thomas, 24; Stern, 1994). A third hypothesis for Gondwana assembly recognizes the dominance of transpressional orogenic belts, and proposes that oblique subduction along the Pacific margin of Gondwana from ca. 56 Ma onwards led to continental blocks becoming a counter-rotating cog in Gondwana (Veevers, 23). The localization of strain in the Musgrave Province during the Petermann orogeny has been the subject of some discussion. An early model suggested thermal blanketing of an upper crust high in heat-producing elements by the thick sediments of the Centralian Superbasin as a mechanism to create anomalously weak lithosphere beneath the deepest part of the basin, which was interpreted to overlie the Musgrave Province (Hand and Sandiford, 1999; Sandiford and Hand, 1998). This model has since been disputed on the grounds of an emergent Musgrave Province as the source for detrital zircons in ca. Ma to ca. 5 Ma Amadeus Basin sedimentary rocks (Camacho et al., 22), and the possibility of high heat production in the lithospheric mantle as a mechanism for strain localization has also been raised (Neves et al., 28). Other alternatives have been suggested to drive this orogenesis, including Rayleigh-Taylor instability of the lithospheric mantle (Neil and Houseman, 1999) and strain localization at the interface between regions of contrasting mechanical strength, often related to the weakening effects of previous deformation events (Braun and Shaw, 21; Camacho et al., 22). In outcrop, Petermann orogeny deformation is characterized by the development of mylonite, ultramylonite, and pseudotachylite zones, varying from a few meters in width to several kilometers (Clarke et al., 1995; Edgoose et al., 24). The primary structure in outcrop, the Woodroffe thrust, is a shallowly south-dipping mylonite zone with an apparent thickness of up to 3 km, and a strike length greater than 5 km (Fig. 1). The Woodroffe thrust forms the boundary between the Fregon subdomain to the south, which is dominated by granulite-facies metamorphic rocks, and the Mulga Park subdomain to the north, which contains Volume 1 Number 6 LITHOSPHERE

3 28 S 27 S 26 S S MF HF 1 PDZ ^ 2 HF WHL Amadeus Basin MF LG 3 Officer Basin EL PL FF ^ ^ ^ ^^ ^ MYF PL ^ MG EL Teleseismic station Deep seismic reflection line ^ Petrophysical sample site 1 P-T-t study location Major shear zone Major thrust Musgravian Gneiss units Granulite facies Transitional amphibolitegranulite facies Wataru Gneiss Amphibolite facies Greenschist facies Other lithological units Pitjantjatjara Supersuite Giles Complex Volcanic/sedimentary sequences Syn-Petermann Orogeny grabens Kilometers E 13 E 131 E 132 E 133 E 134 E Figure 1. Map showing the locations of teleseismic stations (Lambeck and Burgess, 1992), deep seismic reflection surveys (Korsch, et al., 1998; Lindsay and Leven, 1996), petrophysical sampling sites, and pressure-temperature-time (P-T-t) studies: 1 Tomkinson Ranges (Clarke et al., 1995), 2 Mann Ranges (Scrimgeour and Close, 1999), and 3 Musgrave Ranges (Maboko et al., 1992). Solid geology including metamorphic grade transitions is reinterpreted from Glikson et al. (1995), and major shear zones are delineated from magnetic data. Shear zone nomenclature follows Major and Conor (1993), where possible: Woodroffe thrust, MF Mann fault, HF Hinckley fault, FF Ferdinand fault, MYF Marryat fault, EL Echo lineament, PL Paroora lineament, WHL Wintiginna-Hinckley lineament (new name), Wintiginna lineament, Lindsay lineament, LG Levenger graben, MG Moorilyanna graben. amphibolite-facies metamorphic rocks (Camacho and Fanning, 1995; Maboko et al., 1992). Within the Fregon subdomain, several other major shear zones are recognized in outcrop, including the Mann fault, Marryat fault, Ferdinand fault, and Hinckley fault; however, many more that are not observed at the surface due to extensive cover successions are evident in the aeromagnetic data (Fig. 1). As well as defining major metamorphic grade transitions, Petermann orogeny shear zones also delineate the margins of the Levenger and Moorilyanna grabens, interpreted as syntectonic transtensional grabens that formed during the Petermann orogeny (Gravestock et al., 1993; Major and Conor, 1993). The Musgrave Province records little tectonic activity subsequent to the Petermann orogeny, with deformation being restricted to infrequently observed low metamorphic grade shear zones, thought to be related to the Alice Springs orogeny (Edgoose et al., 24; Major and Conor, 1993). In contrast, the Officer and Amadeus basins record extensive deformation and subsidence during the Ma Alice Springs orogeny (e.g., Haddad et al., 21; Hoskins and Lemon, 1995; Lindsay, 22), including a major thrust complex at the southern margin of the Musgrave Province that has deformed the Ordovician to Devonian strata of the Officer Basin (Lindsay and Leven, 1996). Geochronological and Metamorphic Studies of the Petermann Orogeny Geochronological and metamorphic studies relevant to the Petermann orogeny have focused on defining the evolution of Petermann orogeny shear zones and the contrast in crustal blocks across them. These studies have been undertaken in three regions: the Musgrave Ranges, the Tomkinson Ranges, and the Mann Ranges (Fig. 1). In the Musgrave Ranges (Fig. 1), geochronological studies indicate that the Woodroffe thrust was active during the late Neoproterozoic to Early Cambrian (56 5 Ma) (Camacho and Fanning, 1995; Maboko et al., 1992). The similar geochronological evolution either side of the Woodroffe thrust is interpreted to indicate that the metamorphic grade difference across this shear zone reflects differing crustal levels of the same terrane (Camacho and Fanning, 1995). Geochronologically constrained P-T data defined five metamorphic events for this region. The first four reached up to granulite facies and may represent the ca. 12 Ma Musgravian orogeny (Maboko et al., 1991). The fifth metamorphic event is characterized by muscovite development in mylonite zones, and occurred under greenschist-facies conditions (~4 kbar, <4 ºC) at 54 ± 1 Ma (Maboko et al., 1991). In contrast to this greenschist-facies metamorphic event, P-T estimates from the Davenport shear zone, located ~1 km south of the Woodroffe thrust, contain evidence for a subeclogite-facies event (~12 kbar, ~65 ºC) dated at 547 ± 3 Ma (Camacho et al., 1997; Ellis and Maboko, 1992). A geodynamic model based on these P-T data proposes crustal thickening in the early stages of the Petermann orogeny, before exhumation begins, progressing to a crustal-scale flower structure (Camacho and McDougall, 2). A similar evolution is observed in the Tomkinson Ranges in the western Musgrave LITHOSPHERE Volume 1 Number

4 AITKEN et al. Province (Fig. 1), where geochronologically constrained P-T data indicate several late Mesoproterozoic granulite-facies events followed by the development of ultramylonite and pseudotachylite zones, and a metamorphic overprint at up to eclogite facies (14 ± 1 kbar, 75 ºC). These rocks were subsequently overprinted by mica-rich retrograde shear zones (Clarke et al., 1992, 1995). Although not radiometrically dated in this locality, these shear zones connect into the regional network of major Petermann orogeny shear zones, and are interpreted to be Petermann orogeny aged (Clarke et al., 1995). Analysis of Petermann orogeny overprints in the Mann Ranges (Scrimgeour and Close, 1999) showed that metamorphosed granites north of the Woodroffe thrust in the Mulga Park subdomain underwent amphibolite-facies metamorphism (6 7 kbar, 65 ºC) during the Petermann orogeny, whereas in mylonites immediately across the Woodroffe thrust granulite-facies conditions were observed (9 1 kbar, ºC) and, ~4 km farther south, subeclogite-facies conditions were observed (12 13 kbar, 75 ºC). These subeclogite-facies mylonites are cut by high metamorphic grade migmatitic shear zones that have been U-Pb sensitive high-resolution ion microprobe (SHRIMP) dated at 56 ± 11 Ma (Scrimgeour et al., 1999). These were then cut by mylonites at amphibolite facies (7 ± 2 kbar, 66 ± 5 ºC). These overprinting relationships are interpreted to reflect the exhumation of the province from subeclogite facies to amphibolite facies during the Petermann orogeny (Scrimgeour and Close, 1999). These sharp transitions in crustal level across shear zones indicate the juxtaposition of crustal blocks, within which P-T estimates can be fairly consistent (Scrimgeour and Close, 1999). This is echoed in the mineralogy of igneous rocks from the Giles event, which shallow southwards across sharp, shear zone related transitions. Ultramafic plutons in the northern Fregon subdomain were emplaced at ~6 kbar (Clarke et al., 1995). Moving south, coeval rocks show a transition through gabbro-pyroxenite, troctolite, and ultimately surface volcanics at the margins of the province (Glikson et al., 1995). This indicates that since ca. 18 Ma, the northern Fregon subdomain has been uplifted by ~2 km relative to the margins of the province. The Architecture of the Petermann Orogeny Shear zones are important in defining the kinematics and architecture of the Petermann orogeny; however, very little work has been done to define the architecture and kinematics of these shear zones, either in the near surface or at depth. Structural studies (Clarke et al., 1995; Edgoose et al., 24; Flottmann et al., 25) have defined the kinematics of some of these shear zones in localized areas, although the lack of a regional framework makes these results difficult to integrate with the lithospheric-scale architecture. Models of the lithospheric-scale architecture of the Musgrave Province based on passive seismic data are characterized by steep lithospheric-scale shear zones, correlated with the Mann fault, Wintiginna lineament, and Lindsay lineament, that define an upwards Moho offset beneath the central Musgrave Province (Lambeck and Burgess, 1992). Foreland basins are sensitive indicators of the isostatic and geodynamic processes of orogenic belts, and as a result, provide an important record of orogenic evolution (e.g., Berge and Veal, 25; Burbank, 1992; Karner and Watts, 1983; Lambeck, 1983). The Officer and Amadeus basins that flank the Musgrave Province should therefore record the evolution of the Petermann orogeny. Provenance studies of both the Amadeus and Officer basins have detected a large influx of sediments between ca. 6 Ma and 5 Ma, with Grenville-aged detrital zircon populations consistent with the erosion of the Musgrave Province during the Petermann orogeny being observed in each basin (Maidment et al., 27; Wade et al., 25; Camacho et al., 22; Zhao et al., 1992). In the Amadeus Basin, the Musgrave Province is considered a source of zircon throughout the evolution of the Amadeus Basin, providing a small to moderate contribution prior to 56 Ma, the dominant contribution during the period 54 5 Ma and progressively less influence in later samples (Maidment et al., 27). In the eastern Officer Basin, provenance studies have detected a large influx of Musgrave Province derived sediments ca. 6 Ma, indicating the onset of the Petermann orogeny, but a lack of Musgrave Province derived sediments during the Ma period (Wade et al., 25). Eastward transport of sediments along east-trending structures was proposed as the most likely reason for this lack of sediment input from the Musgrave Province (Wade et al., 25). Subsidence analysis in the Officer Basin indicates a period of subsidence during the Petermann orogeny, followed by a brief period of nonsubsidence, and then further subsidence until ca. 5 Ma, attributed to the Delamerian orogeny (Haddad et al., 21). AN AEROMAGNETIC INTERPRETATION OF PETERMANN OROGENY STRUCTURES The high-resolution (2 4 m line spacing) regional aeromagnetic data covering the Musgrave Province permits the definition of plutons, basins, and shear zones in the near surface by their magnetic character, and also the definition of the principal structural trends and their overprinting relationships (Aitken et al., 28; Aitken and Betts, 29a, 29b). Major Petermann orogeny shear zones are defined by narrow, high-amplitude magnetic lows, and were mapped throughout the eastern Musgrave Province (Fig. 2A). In addition to defining the locations of these shear zones, kinematic information for these shear zones was interpreted from the aeromagnetic data using similar methods to those used in structural geology (Betts et al., 27). The aeromagnetic data show a variable but broadly northeast-trending magnetic grain, which is interpreted to be Musgravian orogeny aged (Aitken et al., 28; Aitken and Betts, 29a, 29b). This magnetic grain, along with ca. 12 Ma Pitjantjatjara Supersuite granitoids and ca. 18 Ma Giles Complex plutons, act as magnetic marker horizons that have been deformed by Petermann-aged shear zones, permitting a kinematic interpretation of the main Petermann orogeny shear zones. The shallow south-dipping Woodroffe thrust (Fig. 2A) is defined by an abrupt change in magnetic texture from more magnetic rocks with high-amplitude magnetic fabrics to the south of the thrust, to less magnetic rocks with lower amplitude magnetic fabrics to the north of the thrust, reflecting ca. 55 Ma juxtaposition of granulite-facies and amphibolite-facies rocks (Maboko et al., 1992). The aeromagnetic data do not reveal any kinematic information for the Woodroffe thrust, but kinematic indicators within the shear zone consistently indicate south-overnorth movement (Edgoose et al., 24). The Mann fault is defined in the aeromagnetic data by a broad (2 3 km) and intense aeromagnetic low, and can be traced from the western edge of the interpretation area, through the Levenger graben, to its connection with the Echo lineament. Deflection of the Musgravian structural trend proximal to the Mann fault indicates right-lateral shear sense, consistent with folding of the Levenger Formation within the Levenger graben (Major and Conor, 1993). The Ferdinand fault extends northeast from the Levenger graben, and the deflection of the Musgravian structural trend indicates that this shear zone is left-lateral, consistent with surface mapping (Major and Conor, 1993). In the aeromagnetic data, the Ferdinand fault is cut by the southeast-trending Marryat fault, which has caused large apparent right-lateral offsets to magnetic granitoids and also the Woodroffe thrust. These two major shear zones may form a conjugate set, indicating N-S compression Volume 1 Number 6 LITHOSPHERE

5 695N N 5N 71N 715N A MF FF LG EL WHL PL MYF MG 5E 1E 15E 2E E 3E 35E 4E 695N N 5N 71N 715N B 5E 1E 15E 2E E 3E 35E 4E Kilometers RTP magnetic intensity (nt) Bouguer Gravity (mgal) 5 75 GDA94 zone Figure 2. (A) Reduced to pole aeromagnetic data, showing the interpretation of Petermann orogeny shear zones and their kinematics. Abbreviations are as in Figure 1. Dip-slip shear sense is interpreted from the inversion model result (Fig. 6B). (B) The gravity data distribution (gray dots), the resulting grid, and its relation to major shear zones. The northeast-trending heavy dashed line indicates a broad gravity low of unknown but probably lower-crustal origin. LITHOSPHERE Volume 1 Number

6 AITKEN et al. The Mann fault, Ferdinand fault, and Marryat fault define the northern limit of the axial zone of the orogen, which extends south to the Wintiginna lineament, a shear zone that extends across the whole province (Fig. 1). This axial zone is characterized by an anastomosing network of shear zones, many showing apparent right-lateral offset of magnetic marker horizons (Fig. 2A). Major shear zones within this zone include the Wintiginna-Hinckley lineament and Paroora lineament, neither of which show any strike-slip kinematic indicators in the aeromagnetic data, and the Echo lineament, which shows prominent deflection of the Musgravian structural trend indicating right-lateral shear sense. In the aeromagnetic data, the Wintiginna lineament shows apparent right-lateral offset of several magnetic marker horizons (Fig. 2A). South of the Wintiginna lineament, a lack of strike-slip kinematic indicators indicates that dip-slip movement is dominant in this region. In particular, the major shear zone in this region, the Lindsay lineament, shows no evidence of strike-slip motion. Although they were active during the Alice Springs orogeny in places, the margins of the Musgrave Province are also interpreted to have been active during the Petermann orogeny (Edgoose et al., 24; Flottmann et al., 25; Scrimgeour et al., 1999). The prominent crustal-scale nappe complexes in the vicinity of the Petermann Ranges (Flottmann et al., 25; Scrimgeour et al., 1999) are characteristic of basement-cored nappes throughout the Mulga Park subdomain (Edgoose et al., 24) that are interpreted to represent pervasive Petermann orogeny deformation. The southern margin of the Musgrave Province may also have been active during the Petermann orogeny but was extensively reactivated during the Alice Springs orogeny (Aitken and Betts, 29a; Lindsay and Leven, 1996; Hoskins and Lemon, 1995). Well-defined crosscutting relationships between these shear zones (Fig. 2A) indicate that the Petermann orogeny had at least two stages of deformation: the first phase, in which granulite-facies crust was emplaced above amphibolite-facies crust, is characterized by north- and south-directed movement on divergent shallowdipping, crustal-scale thrust faults, principally the Woodroffe thrust, the Lindsay lineament, the Piltardi detachment zone, and possibly also the margins of the province. The second phase is characterized by dextral transpressional movement on more steeply dipping crustal to lithospheric-scale shear zones in the axial zone of the orogen, principally the Mann-Ferdinand-Marryat fault system and the Wintiginna lineament. Although the relative timing of these deformation events is clear, geochronological estimates for the Petermann orogeny are not sufficiently precise to detect this multiphase evolution, and the absolute age difference between these events is therefore unconstrained. A 3D DENSITY MODEL OF THE EASTERN MUSGRAVE PROVINCE The Gravity of the Eastern Musgrave Province The kinematics indicated in the aeromagnetic interpretation gives an estimate of twodimensional motion in the near surface, but constraining the depth penetration and vertical component of motion on these shear zones is more difficult. If these shear zones are associated with crust mantle-boundary offsets and the juxtaposition of crustal blocks from different levels, then gravity modeling should indicate their deeper geometry. The granulite-facies Fregon subdomain is associated with a very high amplitude regional gravity anomaly (~15 mgal) and steep regional gravity gradients (~3 eotvos). This intense high, indicating a large subsurface load, is situated within a broad, subcircular gravity low in central Australia, 1 km in diameter, that corresponds to a region of thick crust (Clitheroe et al., 2) and may represent a long-wavelength flexural depression. Gravity data coverage in the eastern Musgrave Province is relatively good (Fig. 2B), with regional grids at km spacing supplemented by more recent high-resolution profiles at ~1 km spacing (Gray and Flintoft, 26; Gray and Aitken, 27; Gray et al., 27). The main shear zones interpreted in the aeromagnetic data are broadly correlative with the major steep gradients in the gravity field (Fig. 2B), although there are significant differences: The principal surface boundary, the Woodroffe thrust, is not associated with the principal gravity gradient, which is located farther south, whereas in other areas steep gravity gradients occur with no evidence of major Petermann orogeny shear zones in the near surface. A broad, northeast-trending low crossing the regional high (Fig. 2B) is not associated with any structure defined in the magnetic field, and may relate to a deep crustal or lithospheric boundary. The Gravity Inversion Method Petrophysically constrained gravity modeling has been shown to be an effective method for defining the architecture of the near surface (Farquharson et al., 28; Fullagar et al., 28; McLean and Betts, 23). These methods can be extended to model the whole crust because constraint on the upper crustal density distribution greatly reduces the uncertainty in modeling the geometry of the lower crust and upper mantle (e.g., Ebbing et al., 21). To produce density models of the eastern Musgrave Province, three-dimensional inversion was conducted using VPmg software (Fullagar et al., 28), which by iteratively modifying an input geological model containing lithological units and density information, seeks to optimize the fit to gravity data. With a uniform target misfit, the fit to the gravity data is defined by the root mean square of the residual anomaly (the RMS misfit). With VPmg, inversion terminates when the RMS misfit is less than the target misfit (convergence), or when the algorithm fails to reduce this parameter on successive iterations (a stalled inversion). Lithological units in the model can be either homogenous (i.e., density is the same throughout) or heterogeneous (i.e., density can be varied within the unit). The software has two main gravity modeling modes heterogeneous density optimization and geometry optimization. For heterogeneous density optimization, the subsurface is discretized into cells of regular x, y, and z extent, each represented by a single density value, and the inversion algorithm seeks to replicate the gravity data by modifying the density distribution represented by these cells. In this inversion mode, the density of homogenous units and the boundaries between units cannot change. For geometry optimization, the subsurface is discretized into vertical prisms, within which the depths to lithological boundary intersections are recorded, and the inversion algorithm seeks to replicate the gravity data by varying the depths to these lithological boundaries. In this inversion mode, the density within each prism cannot change, although any preexisting heterogeneity is maintained. The much-documented inverse problem (e.g., Parker, 1994; Zhdanov, 22) means that changes in density during inversion must be controlled to avoid an unrealistic density structure. During heterogeneous density optimization the user can impose upper and lower bounds on the range of densities permissible for each lithology and control the maximum change in density permitted per iteration. During geometry optimization, a user-defined parameter controlling the maximum relative change in interface depth per iteration is applied. This constraint means that it is mathematically easier to change the geometry of units at depth, and acts as depth weighting to counteract the loss of sensitivity with depth. More rigid constraints can also be imposed on the model geometry by defining regions in 3D space within which the boundaries of a lithological unit or units cannot change Volume 1 Number 6 LITHOSPHERE

7 The Initial Model Parameters, Constraints, and Boundary Conditions The spatial limits of the gravity model are broadly defined by the limits of high-resolution gravity data (Fig. 2B). The base of the model was set at 9 km depth, and precise topography from the gravity data was maintained in modeling as the upper surface bound. Prior to inversion, the free-air gravity data were minimum curvature gridded with 5 km cell size, upward continued by m to remove short wavelength content irresolvable with 5 km cell size, and detrended along a planar surface, to remove the need for density sources outside the model. This planar trend slopes from the south to the north over a total range of 11 mgal. To provide constraint on the upper crustal density distribution, 146 measurements of the density of major lithologies were made on samples and core from throughout the Fregon subdomain (Fig. 1). These measurements showed that the density distribution is heterogeneous at small scales (tens of meters), and this means that individual density measurements do not reflect the bulk density of large modeling cells, and are not therefore used to directly constrain the densities of measurement localities. However, the statistical distribution of these measurements (Fig. 3) is important in constraining the probable density distribution in the near surface. Deep seismic reflection studies (Korsch et al., 1998; Lindsay and Leven, 1996) and passive seismic models (Clitheroe et al., 2) constrain the Moho depth to ~5 km beneath central Australia. To correspond with this constraint, and Density (g/cm 3 ) the crustal layering observed in the seismic reflection studies (Korsch et al., 1998; Lindsay and Leven, 1996), a four-layer model was constructed with the mantle (3.3 g/cm 3 ), eclogitic crust (3.1 g/cm 3 ), lower crust (2.85 g/cm 3 ), and upper crust separated by boundaries at 5 km, 35 km, and km, respectively. The upper crust was subdivided in accordance with the major geological boundaries (Fig. 1), with units representing the Amadeus and Officer basins (2.55 g/cm 3 ), amphibolitefacies crust (2.67 g/cm 3 ), granulite or transitional granulite-facies crust (2.77 g/cm 3 ), and also the transitional granulite-amphibolite-facies Wataru gneiss (2.75 g/cm 3 ) in the southwest of the area (Gray, 1978) and the Ammaroodinna inlier (2.85 g/cm 3 ) in the southeast (Krieg, 1993). These density values are constrained by both petrophysical data (Fig. 3) and the density contrasts required to satisfy the short-wavelength gravity gradients revealed in high-resolution data across the major density boundaries (Gray and Flintoft, 26; Gray and Aitken, 27; Gray et al., 27). A Heterogeneous Upper Crust Model To investigate the density distribution required to satisfy the gravity anomaly from the upper crust alone, petrophysically constrained density inversion was applied within the upper crust using 5 km 5 km 1 km cells. The maximum density change per iteration was set at.2 g/cm 3, and the target misfit was set to 1 mgal. The densities within lithological units were constrained as follows: amphibolite-facies crust was constrained to densities between ± ± ± ± Granulite facies gneiss n = 91 Granite/granitic gneiss n = Giles Complex n = 17 Max Mean±1σ Median Charnockite n = 11 Figure 3. Histograms and calculated parameters showing the statistical distribution of specific gravity measurements collected throughout the Fregon subdomain, divided into broad lithological groups. Min 2.62 g/cm 3 and 2.72 g/cm 3 ; the Wataru gneiss was constrained to between 2.7 and 2.8 g/cm 3 ; and the Ammaroodinna inlier was constrained to between 2.8 and 2.9 g/cm 3. Granulite-facies crust was constrained to densities between 2.67 and 2.87 g/cm 3,.1 g/cm 3 either side of the measured median density (2.77 g/cm 3 ). The densities of the homogenous units the Amadeus and Officer basins, the lower crust, the eclogite layer, and the mantle were held invariant. From an initial RMS misfit of mgal, the inversion stalled after 21 iterations at 5.6 mgal. Residual anomalies are mostly observed at the margins of the model (Fig. 4A), although there are significant negative residual anomalies (~15 mgal) over regions of the amphibolitefacies crust where the lower density limit of 2.62 g/cm 3 is too high to permit a fit to the deep gravity lows. The fit over the granulite-facies crust is generally good, although the lack of Giles Complex mafic intrusions in the model is reflected in short-wavelength positive residual anomalies over major intrusions. The density distribution within this model (Fig. 4A and Animation 1) 1 is generally reasonable and shows that there is no inherent requirement in the gravity data for crust-mantle boundary relief beneath the Musgrave Province. However, the density distribution in this model has large areas of anomalously dense or light upper crust, for which there is little petrophysical evidence (Fig. 3). A particularly large density contrast is required between the Mulga Park subdomain (2.62 g/cm 3 or less) and the Fregon subdomain ( g/cm 3 ). In addition, the major surface boundary juxtaposing crustal levels, the Woodroffe thrust, is only associated with a small density contrast in this model, with a large near-surface density contrast concentrated farther south. We consider this model to be inconsistent with the P-T and density constraints, and it also bears little resemblance to the architecture imaged in seismic models (Lambeck and Burgess, 1992). Some amount of crust-mantle boundary relief is therefore probable. A Median Density Model The geometry of the crust-mantle boundary and the amount of relief required to produce 1 If you are viewing the PDF of this paper or reading it offline, please visit the full-text article on to view Animations 1 5. You can also access them at the following respective links: /L39.S3, and LITHOSPHERE Volume 1 Number

8 Density (g/cm 3 ) density (g/cm 3 ) AITKEN et al. A Observed gravity Calculated gravity FAA (mgal) - FF EL PL X=275mE Gravity misfit (mgal) FAA (mgal) FAA (mgal) MF EL PL MF WHL -35 X=1mE 3 X=55mE Northing (m) N 695 B Observed gravity Calculated gravity FAA (mgal) FAA (mgal) FF FAA (mgal) EL PL MF EL 71 PL X=275mE Gravity misfit (mgal) X=1mE MF WHL X=55mE Northing (m) N 695 Figure 4. (A) Interactive three-dimensional (3D) view of the result of the heterogeneous upper crust inversion model, showing the flat crust-mantle boundary, the upper crust density distribution, and the fit to the gravity data (top right); see also Animation 1 (see footnote 1). (B) Interactive 3D view of the result of the median density inversion model, showing the geometry of the lower crust and crust-mantle boundaries, the upper crust density distribution, and the fit to the gravity data (top right); see also Animations 2 and 3 (see footnote 1). Annotated shear zone locations are independently derived from the aeromagnetic data, and follow the nomenclature used in Figure 1. To view the interactive version of this figure please visit and click on Animations in the middle column or go directly to the figure at Volume 1 Number 6 LITHOSPHERE

9 RMS gravity misfit (mgal) Granulite facies - amphibolite facies density contrast (g/cm 3 ) the observed gravity anomaly were investigated using a geometry optimization inversion, with 5 km 5 km vertical prisms of 9 km depth extent and a maximum depth change per iteration of 2%. The target misfit was 1 mgal, and the boundaries of all units were permitted to change. From an initial RMS misfit of mgal, the inversion stalled after 8 iterations at 4.9 mgal. Short-wavelength residual anomalies are observed throughout the model area (Fig. 4B) as a result of the inability of this model to resolve small near-surface features. The geometry derived from this inversion (Fig. 4B and Animation 3[see footnote 1]) shows that the mantle and eclogite layers are, in general, uplifted beneath the east-trending central gravity high, and depressed beneath the gravity lows. The amount of crust-mantle boundary relief changes along strike, with the greatest relief of ~2 km in the western part of the model, and a reduction to ~1 2 km of relief in the eastern part of the model. In this model, steep crust-mantle boundary gradients correlate with major Petermann orogeny shear zones, principally the Mann fault, Ferdinand and Marryat faults, Wintiginna- Hinckley lineament, and Wintiginna lineament (Fig. 4B). The geometry of the lower crust, eclogite, and mantle layers in this model are very sensitive to the density contrast between the granulite-facies gneiss and the amphibolite-facies gneiss, with small changes in density causing large changes in the offsets required to satisfy the gravity data. A sensitivity analysis was conducted to quantify this sensitivity by running geometry inversions in which the density contrast was perturbed in the initial model. For a variety of contrast values, the statistical variance of the depth to the resulting crust-mantle boundary was calculated as a measure of its flatness (Fig. 5). The results of the sensitivity analysis (Fig. 5 and Animation 4 [see footnote one]) indicate that low or negative density contrast, between.5 and. g/cm 3, results in a broad crustmantle boundary high beneath the gravity high, and therefore high variance. Moderate density contrast, between.5 and.1 g/cm 3, produces a flat but undulating crust-mantle boundary surface and low variance, and high density contrast, between.15 and. g/cm 3, produces a broad crust-mantle boundary depression beneath the gravity high, and high variance. Figure 5 illustrates that a density contrast of between.75 and.1 g/cm 3 produces a lowvariance crust-mantle boundary and also a low RMS misfit. This result verifies the constraints from petrophysical data and high-resolution gravity data that indicate ~.1 g/cm 3 of density contrast between granulite-facies and amphibolite-facies gneiss. A Combined Heterogeneous Density and Geometry Inversion Neither the heterogeneous upper crust model (Fig. 4A) nor the median density model (Fig. 4B) are a good representation of the crustal RMSgrav CMBvariance E+8 9E+7 8E+7 7E+7 6E+7 5E+7 4E+7 3E+7 2E+7 Figure 5. The sensitivity of the root-mean-square (RMS) gravity misfit and the amount of crust-mantle boundary topography to the density contrast between the granulite-facies and amphibolite-facies crust. The crust- mantle boundary surfaces created in this process are displayed in Animation 4 (see footnote 1). Crust-mantle boundary variance architecture, due to the omission of major intrusive suites and sedimentary basins and the geometric and density assumptions imposed on the models. A more detailed initial model was constructed (Fig. 6A) including Giles Complex and Pitjantjatjara Supersuite plutons, and syn- Petermann orogeny grabens. The geometry of the lower crust and crust-mantle boundaries were remodeled to represent offset of crustal layers along the plane of lithospheric-scale shear zones, and to more closely resemble the seismic architecture of the Musgrave Province (Fig. 6A). The source of the northeast-trending low in the gravity data (Fig. 2B) is not known, but this anomaly is associated with an eastward gravity gradient. Forward modeling prior to inversion indicated that west-up offset of the lower crust and crust-mantle boundaries by 8.5 km fits this gradient well. The lithological densities in this model are similar to those used in the previous models, and the densities of the Amadeus and Officer basins, lower crust, eclogite layer, and mantle were held invariant at their initial density. Greater heterogeneity was incorporated into the upper crust by introducing homogenous units representing the Levenger and Moorilyanna Formations (each 2.55 g/cm 3 ), and heterogeneous units representing the Giles Complex (3 ±.1 g/cm 3 ) and the Pitjantjatjara Supersuite, subdivided into granitic (2.7 ±.1 g/cm 3 ) and charnockitic (2.8 ±.1 g/cm 3 ) lithologies. The location of these upper crustal units was defined in accordance with aeromagnetic data and outcropping geology. The amphibolite-facies crust in the Mulga Park subdomain, the southern Fregon subdomain, and beneath the Amadeus and Officer basins was heterogeneous in this model, with density of 2.67 ±.5 g/cm 3. The Wataru gneiss (2.75 ±.5 g/cm 3 ) and Ammaroodinna inlier (2.85 ±.5 g/cm 3 ) were also heterogeneous. The granulite-facies upper crust was subdivided into four east-trending units, bounded by the Woodroffe thrust, the Mann-Ferdinand-Marryat fault system, the Wintiginna-Hinckley and Paroora lineaments, the Wintiginna lineament, and the Lindsay lineament (Fig. 6A). The northernmost (granulite facies 1) and southernmost (granulite facies 4) of these units were heterogeneous, with upper density bounds of 2.83 and 2.87 g/cm 3, respectively, and lower bounds of 2.77 g/cm 3. The central units were homogenous, with densities of 2.77 g/cm 3 (granulite facies 2) and 2.75 g/cm 3 (granulite facies 3). For this inversion, by running consecutive density and geometry inversions, we first optimized the densities in the heterogeneous units, before adjusting the geometry of all units. The heterogeneous density inversion was run with 5 km 5 km 1 km cells, with a maximum LITHOSPHERE Volume 1 Number

10 AITKEN et al. A MF-FF-MYF X=275mE 5 X=1mE 5 X=55mE Northing (m) N Granulite facies 1 Granulite facies 4 Giles Complex Granulite facies 2 Granulite facies 3 Amphibolite facies Pitjantjatjara Supersuite Basins B MF-FF-MYF Gravity misfit (mgal) X=275mE X=1mE 5 X=55mE Northing (m) N Density (g/cm 3 ) Figure 6. (A) Three-dimensional (3D) view of the input model to the combined property and geometry inversion showing the distribution of geologic units, and the geometry of major shear zones and the lower-crustal and crust-mantle boundaries. The inset (bottom left) shows the location of constraints imposed on the crust-mantle boundary geometry. (B) Interactive 3D view of the inversion result, showing the upper-crustal density distribution, the geometry of major shear zones, the lower-crustal and crust-mantle boundaries, and the fit to the data (top right). The inset (bottom left) shows the influence of the applied constraints in controlling crust-mantle boundary geometry changes. See also Animation 5 (see footnote 1). To view the interactive version of this figure please visit and click on Animations in the middle column or go directly to the figure at Volume 1 Number 6 LITHOSPHERE

11 density change per iteration of.2 g/cm 3, and a target misfit of 1 mgal. From an initial RMS misfit of mgal, the inversion stalled after 17 iterations at a RMS misfit of 6.62 mgal. Residual anomalies are concentrated above the granulite-facies core, which was invariant in this inversion. Prior to geometry inversion, the lowercrustal stratigraphy beneath the amphibolitefacies crust was constrained so that the inversion algorithm would only modify the lower-crustal stratigraphy beneath the granulite-facies crust (Fig. 6A). This constraint is necessary because the inversion algorithm will preferentially modify boundaries at depth with high-density contrast (i.e., the base of the amphibolite-facies regions) and without constraint, the resulting geometry in these regions is inconsistent with deep seismic reflection studies (Korsch et al., 1998; Lindsay and Leven, 1996). The geometry inversion was run with 5 km 5 km vertical prisms of 9 km depth extent, a maximum depth change per iteration of 2%, and a target misfit of 1 mgal. This inversion stalled after 14 iterations, reducing the RMS misfit from 6.62 mgal to 5.58 mgal. This inversion greatly reduced residual anomalies above the granulite-facies crust, although short wavelength fluctuations are still observed (Fig. 6B). A Best-Fit Model Of the three inversions attempted, the architecture derived from the combined density and geometry inversion (Fig. 6B; Animations 5 and 6 [see footnote one]) is the most consistent with the magnetic interpretation, geological observations (Camacho et al., 1997; Clarke et al., 1995; Ellis and Maboko, 1992; Maboko et al., 1991; Major and Conor, 1993; Scrimgeour and Close, 1999), and seismic observations (Korsch et al., 1998; Lambeck and Burgess, 1992; Lindsay and Leven, 1996). The fit to the gravity anomaly is close to that achieved in the other inversions. This 3D density model is well constrained in the vicinity of the Woodroffe thrust and the Mann, Ferdinand, and Marryat faults due to the relatively high gravity petrophysical and teleseismic data resolution, and greater degree of geological constraint. Away from this zone the model becomes less well constrained as the resolution of gravity, petrophysical and teleseismic data decreases, and outcrop becomes sparser, although deep seismic reflection lines in the Amadeus Basin and southern Musgrave Province provide constraint on the architecture of the province margins. The architecture at depth is similar to that proposed from seismic models (Korsch et al., 1998; Lambeck and Burgess, 1992), with the exception that neither the Lindsay lineament nor the shear zone at the southern margin of the province penetrates the crust-mantle boundary in our model, as suggested previously (Korsch et al., 1998; Lambeck and Burgess, 1992; Lindsay and Leven, 1996). The median density model (Fig. 4B) and the combined model (Fig. 6B) show that a relatively thin wedge of granulite-facies gneiss above a shallowdipping Lindsay lineament satisfies the gravity anomaly here, and crust-mantle boundary uplift south of the Wintiginna lineament is not supported. DISCUSSION LITHOSPHERIC PROCESSES OF THE PETERMANN OROGENY Probable Inversion of Ca Ma Extensional Architecture A defining characteristic of intraplate orogenesis in the upper crust is the reactivation of relict architecture, typically identified on the basis of inverted extensional basins and upthrust basement blocks (e.g., Turner and Williams, 24; Ziegler et al., 1995; Ziegler et al., 1998). The east to southeast orientation and moderate to steep dip of the shear zones in the axial zone parallel the architecture interpreted to have developed during ca. 18 Ma and ca. 8 Ma extensional events, which are characterized in the Musgrave Province by east-southeast and southeast-trending shear zones, often co-located with mafic dikes (Aitken et al., 28; Aitken and Betts, 29b; Clarke et al., 1995; Edgoose et al., 24). In existing geologic maps, these two major dike suites are generally undifferentiated, and therefore it is difficult to make a distinction between the geometries of these tectonic events with confidence. The high-pressure recrystallization of dikes in regions intensely deformed during the Petermann orogeny (Camacho et al., 1997; Clarke et al., 1995; Ellis and Maboko, 1992; Major, 1967; Scrimgeour and Close, 1999) indicates that they were important in partitioning strain, and probably focused this strain into the coincident structures along which their emplacement may have been controlled (Aitken et al., 28; Aitken and Betts, 29a, 29b; Clarke et al., 1995). Although these criteria are not completely definitive, we infer these similarities in orientation and inferred geometry and the recrystallization of mafic dikes to represent the reactivation of preexisting weaknesses in the mid-to-lower crust during the Petermann orogeny, resulting in crustal-scale inversion of the relict geometry from previous extensional tectonic events. Strain Accommodation and Escape Tectonics Strain during the Petermann orogeny appears to have been accommodated in two ways: (1) pervasive ductile deformation and crustal thickening in the Mulga Park subdomain and in the Mann Ranges and (2) vertical and lateral extrusion of relatively rigid crustal blocks along ductile shear zones. Pervasive Petermann orogeny ductile deformation is recorded throughout the Mulga Park subdomain (Edgoose et al., 24). The structures associated with this deformation are too small to be resolved in our model, although geological mapping indicates that this deformation is characterized by north-vergent recumbent isoclinal folding of both crystalline basement and sedimentary cover successions accompanied by a metamorphic overprint at upper-greenschist to upper-amphibolite facies (Edgoose et al., 24; Flottmann et al., 25; Scrimgeour et al., 1999). Significant ductile deformation is also present in the Mann Ranges of the Fregon subdomain, although the origins of this ductile deformation are thought to be quite different to that in the Mulga Park subdomain, resulting from relatively rapid uplift of the crust, causing upward advection of heat and the resulting migmatization (Scrimgeour and Close, 1999). In contrast to the Mann Ranges, much of the axial zone of the orogen lacks a pervasive metamorphic overprint in granulite-facies gneiss despite intense deformation within shear zones. This indicates that strain during the Petermann orogeny was highly partitioned onto mylonite structures (Camacho et al., 1997; Camacho and McDougall, 2; Camacho et al., 21; Clarke et al., 1995) possibly as a result of shear heating (Camacho et al., 21). Similarly, at the crustalscale, Petermann orogeny shear zones defined in aeromagnetic data (Fig. 2A) generally bound relatively undeformed crustal blocks, and define numerous structural features normally associated with brittle deformation, including conjugate shear zones and pop-up structures, despite being active at lower-crustal levels. This indicates a deformation regime that at crustal scale was characterized by the motion of competent crustal blocks along ductile shear zones, onto which strain was highly partitioned (Fig. 2A). The major shear zones that bound the axial zone are transpressional and accommodate strain by both the vertical and lateral extrusion of crustal blocks: In the west of the modeled area, 2 km of crust-mantle boundary offset is accommodated on the Mann fault and Wintiginna-Hinckley lineament. In the east of the modeled area, there is significantly less vertical offset (1 2 km) and significant northward LITHOSPHERE Volume 1 Number