The Architecture, Kinematics, and Lithospheric Processes of A

Architecture, kinematics, and lithospheric processes of a compressional intraplate orogen | RESEARCH

The architecture, kinematics, and lithospheric processes of a compressional intraplate orogen occurring under Gondwana assembly: The Petermann orogeny, central Australia

Alan R.A. Aitken1,*, Peter G. Betts1, and Laurent Ailleres1 1SCHOOL OF GEOSCIENCES, MONASH UNIVERSITY, WELLINGTON ROAD, CLAYTON, VICTORIA 3800, AUSTRALIA

ABSTRACT

We ally aeromagnetic interpretation with constrained three-dimensional (3D) gravity inversion over the Musgrave Province in central Aus- tralia to produce a 3D architectural and kinematic model of the ca. 550 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 10–25 km. This thick wedge of mantle within the lower crust has been in isostatic disequilibrium for more than 500 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 modiﬁ cation of leading models of Gondwana assembly.

LITHOSPHERE; v. 1; no. 6; p. 343–357. doi: 10.1130/L39.1

INTRODUCTION et al., 2003) and Altai (Cunningham, 2005) inlier (e.g., Collins and Shaw, 1995; Sandi- regions of central Asia. These studies highlight ford, 2002). The architecture and kinematics of oro- the diverse range of settings for intraplate com- The initiation of these orogens has been the genic belts have been the topic of many stud- pressional deformation, and also the variety of topic of much study, and several quite different ies, mostly focused on actively deforming or lithospheric processes that can occur. How- models may explain the initiation and some of recently deformed plate margins, such as the ever, one fi nding that is common to all studies the main features of these orogens (cf. Braun Himalaya (e.g., Molnar, 1988; Yin, 2006) or the is the importance of thermal and mechanical and Shaw, 2001; Camacho et al., 2002; Neil European Alps (e.g., Bruckl et al., 2007; Ebbing heterogeneities in the continental lithosphere and Houseman, 1999; Neves et al., 2008; San- et al., 2001; Luschen et al., 2004). In contrast, as a control on crustal architecture (e.g., Cun- diford et al., 2001). A detailed regional-scale compressional intraplate orogens have been the ningham, 2005; Dickerson, 2003; Hand and model of the 3D architecture and kinematics subject of comparatively few studies. Sandiford, 1999; Sandiford and Hand, 1998; of these orogens is lacking. This is important To date, studies of the structure and kine- Ziegler et al., 1998). because it may indicate the orientation and matics of intraplate compression have concen- Late Neoproterozoic to Devonian tectonic intensity of the forces driving the system and trated on the inversion of extensional basins in reworking of central Australia is interpreted characterize the feedback processes that con- both backarc-hinterland and forearc-foreland to refl ect intraplate compressional orogenesis trol the interaction between crustal architecture settings (e.g., Dickerson, 2003; Sandiford, (e.g., Betts et al., 2002; Camacho and Fanning, and the dynamics of the orogen. 1999; Turner and Williams, 2004; Ziegler et al., 1995; Camacho et al., 2002; Hand and Sandi- In addition, these orogens provide a record 1995), with others studying the dynamic evolu- ford, 1999; Sandiford and Hand, 1998). Two of intraplate continental lithospheric deformation of the currently active intraplate compres- discrete orogens are recognized: the ca. 600– tion under the infl uence of one of Earth’s most sional orogens of the Tien Shan (Burov et al., 500 Ma Petermann orogeny that reworked the dramatic periods of tectonism associated with 1990; Tapponnier and Molnar, 1979; Zhao Mesoproterozoic Musgrave Province (e.g., the assembly of Gondwana, and their architec- Major and Conor, 1993; Wade et al., 2008) ture and kinematics may help to recognize the *Corresponding author e-mail: alan.aitken@sci. and the ca. 450–350 Ma Alice Springs orog- most (and least) credible of many competing monash.edu.au. eny that reworked the Paleoproterozoic Arunta tectonic models (e.g., Boger and Miller, 2004;

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Cawood, 2005; Collins and Pisarevsky, 2005; deus and Offi cer basins, defi ning the extent of collision of Gondwana, resulting in the suturing Jacobs and Thomas, 2004; Meert and Van Der the ca. 1200 Ma Musgravian-Albany-Fraser of Australia, east Antarctica, and the Kalahari Voo, 1997; Meert, 2003; Rino et al., 2008; orogeny (Aitken and Betts, 2008). craton, onto the remainder of Gondwana, which Veevers, 2003). Subsequent to the Musgravian orogeny, the was previously assembled during the ca. 750– A combination of aeromagnetic data and voluminous mafi c intrusions of the Giles Com- 620 Ma east African orogen (Meert, 2003). gravity data can be used to image architecture plex and coeval mafi c dikes and granitoids were However, a lack of accreted arc fragments or from the near surface to crust-mantle bound- emplaced within the Musgrave Province dur- continental blocks and the limited extent of its ary geometry (e.g., Williams and Betts, 2007), ing the extensional Giles event at ca. 1080 Ma component terranes led Squire et al. (2006) to and therefore these data provide the ideal tool (Glikson et al., 1995; Sun et al., 1996), along suggest that the Kuunga orogeny is an intracra- to unify the concepts of previous studies of with surfi cial volcanic rocks now exposed at the tonic response to the East African–Antarctic orogenic architecture at multiple scales. In margins of the Musgrave Province (Glikson et orogen, which is interpreted by several authors this paper, we combine interpretation of high- al., 1995). Although not well defi ned, the extent to record the major event in the amalgamation resolution aeromagnetic data with 3D gravity and orientation of this event may be defi ned of Gondwana (Jacobs and Thomas, 2004; Stern, inversions to produce a crustal-scale model of by east-to-east-southeast–trending shear zones 1994). A third hypothesis for Gondwana assem- the architecture and kinematics of the intra- that predate or are synchronous with the dike bly recognizes the dominance of transpressional plate Petermann orogeny in the eastern Mus- emplacement events (Aitken et al., 2008; Ait- orogenic belts, and proposes that oblique sub- grave Province. This model is constrained by ken and Betts, 2009b; Clarke et al., 1995); the duction along the Pacifi c margin of Gondwana geological observations at a number of scales, alignment of Giles Complex mafi c intrusions from ca. 560 Ma onwards led to continental including pressure-temperature-time (P-T-t) along the axis of the Musgrave Province with blocks becoming a “counter-rotating cog” in data, structural interpretations, petrophysical no geophysical evidence for buried Giles Com- Gondwana (Veevers, 2003). sampling, and macro-scale geological obser- plex plutons outside of this zone (Glikson et al., The localization of strain in the Musgrave vations, and constraint is also derived from 1995; Glikson et al., 1996); and the orientation Province during the Petermann orogeny has crustal-scale seismic refl ection lines and pas- and extent of the Warakurna large igneous prov- been the subject of some discussion. An early sive seismic models. From this architectural ince (LIP), of which the Giles Complex is a key model suggested thermal blanketing of an upper and kinematic model, we infer the most infl u- constituent, which extends from northern West- crust high in heat-producing elements by the ential lithospheric processes that have shaped, ern Australia to the Musgrave Province (Wing- thick sediments of the Centralian Superbasin as and been controlled by, the architecture and ate et al., 2004). a mechanism to create anomalously weak lith- kinematics of the Petermann orogeny. After a hiatus of ca. 200 million years, mafi c osphere beneath the deepest part of the basin, dikes were emplaced at ca. 800 Ma along east- which was interpreted to overlie the Musgrave THE GEOLOGIC SETTING OF THE southeast– and southeast-oriented structures Province (Hand and Sandiford, 1999; Sandiford PETERMANN OROGENY (Zhao et al., 1994). The inception of the Offi cer and Hand, 1998). This model has since been and Amadeus basins is broadly contemporane- disputed on the grounds of an emergent Mus- The Musgrave Province preserves a variety ous with these dikes, and probably formed as grave Province as the source for detrital zircons of Mesoproterozoic gneissic rocks of domi- part of the once contiguous Centralian Superba- in ca. 700 Ma to ca. 500 Ma Amadeus Basin nantly felsic lithology with precursors dated at sin (Walter et al., 1995). This ca. 800 Ma exten- sedimentary rocks (Camacho et al., 2002), and ca. 1600 Ma (Gray, 1978; Wade et al., 2006) sional event may represent a northwest-trending the possibility of high heat production in the that were metamorphosed at amphibolite to aulacogen, related to a mantle plume centered lithospheric mantle as a mechanism for strain granulite facies during the ca. 1200 Ma Mus- beneath the Adelaide Rift Complex to the south- localization has also been raised (Neves et al., gravian orogeny (Camacho and Fanning, 1995; east (Betts et al., 2002; Zhao et al., 1994). A 2008). Other alternatives have been suggested to Gray, 1978; Maboko et al., 1991; Sun and second hiatus of ca. 200 million years followed drive this orogenesis, including Rayleigh-Taylor Sheraton, 1992; White et al., 1999). Although, this event, before the Musgrave Province was instability of the lithospheric mantle (Neil and in outcrop, the structural trend of this event is intensely reworked during the ca. 550 Ma Peter- Houseman, 1999) and strain localization at variable throughout the province (cf. Aitken mann orogeny. the interface between regions of contrasting et al., 2008; Aitken and Betts, 2009b; Clarke Although locally derived vertical driving mechanical strength, often related to the weak- et al., 1995; Edgoose et al., 2004; Major and forces may have played a signifi cant role (Neil ening effects of previous deformation events Conor, 1993), linking these observations to a and Houseman, 1999), the Petermann orogeny (Braun and Shaw, 2001; Camacho et al., 2002). coincident structural grain in aeromagnetic is typically interpreted to represent the intra- In outcrop, Petermann orogeny deforma- data defi nes this structural trend at the regional plate response to far-fi eld stresses related to tion is characterized by the development of scale and shows that it is dominantly northeast Gondwana assembly. Due to the uncertainties mylonite, ultramylonite, and pseudotachylite trending (Aitken et al., 2008; Aitken and Betts, regarding the assembly of Gondwana, defi ning zones, varying from a few meters in width to 2009b). The emplacement of the granitic plu- a specifi c tectonic driver for this event is not several kilometers (Clarke et al., 1995; Edgoose tons of the Pitjantjatjara Supersuite occurred straightforward, and several major orogens may et al., 2004). The primary structure in outcrop, during and shortly after this orogeny (Major have contributed to the stress fi eld. The closest the Woodroffe thrust, is a shallowly south-dip- and Conor, 1993), and their emplacement pat- active plate-margin orogen may be found in the ping mylonite zone with an apparent thickness tern dominantly refl ects the northeast-trending ca. 570–530 Ma collision of India with Austra- of up to 3 km, and a strike length greater than structural grain of the Musgravian orogeny lia’s western margin, termed the Kuunga orog- 500 km (Fig. 1). The Woodroffe thrust forms (Aitken et al., 2008; Aitken and Betts, 2009a, eny (Collins and Pisarevsky, 2005; Meert et al., the boundary between the Fregon subdomain 2009b; Edgoose et al., 2004; Major and Conor, 1995; Meert and Van Der Voo, 1997; e.g., Meert, to the south, which is dominated by granu- 1993). These chains of magnetic granitoids are 2003). This orogeny was originally interpreted lite-facies metamorphic rocks, and the Mulga interpreted to be continuous beneath the Ama- to represent the ca. 570–530 continent-continent Park subdomain to the north, which contains

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# Teleseismic station Amadeus Basin Deep seismic reflection line ^ Petrophysical sample site # 1 P-T-t study location

25°S PDZ Major shear zone # Major thrust # # Musgravian Gneiss units # # Granulite facies WT WT # # Transitional amphibolite- # granulite facies MF 2 # # Wataru Gneiss

26°S # 1 ^ HF # ^ ^ Amphibolite facies ^ # MF FF ^ MYF HF # ^ # 3 # LG Greenschist facies WHL # EL # ^ Other lithological units # PL EL WL # MG # PL ^ Pitjantjatjara Supersuite # WL # WL LL # Giles Complex 27°S LL LL Volcanic/sedimentary sequences Syn-Petermann Orogeny grabens Officer Basin Kilometers 05010025 28°S

129°E 130°E 131°E 132°E 133°E 134°E

Figure 1. Map showing the locations of teleseismic stations (Lambeck and Burgess, 1992), deep seismic reﬂ ection 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: WT—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), WL—Wintiginna lineament, LL—Lindsay lineament, LG—Levenger graben, MG—Moorilyanna graben.

amphibolite-facies metamorphic rocks (Cama- Springs orogeny (e.g., Haddad et al., 2001; across this shear zone refl ects differing crustal cho and Fanning, 1995; Maboko et al., 1992). Hoskins and Lemon, 1995; Lindsay, 2002), levels of the same terrane (Camacho and Fan- Within the Fregon subdomain, several other including a major thrust complex at the south- ning, 1995). Geochronologically constrained major shear zones are recognized in outcrop, ern margin of the Musgrave Province that has P-T data defi ned fi ve metamorphic events for including the Mann fault, Marryat fault, Ferdi- deformed the Ordovician to Devonian strata of this region. The fi rst four reached up to granu- nand fault, and Hinckley fault; however, many the Offi cer Basin (Lindsay and Leven, 1996). lite facies and may represent the ca. 1200 Ma more that are not observed at the surface due to Musgravian orogeny (Maboko et al., 1991). extensive cover successions are evident in the Geochronological and Metamorphic The fi fth metamorphic event is characterized by aeromagnetic data (Fig. 1). As well as defi ning Studies of the Petermann Orogeny muscovite development in mylonite zones, and major metamorphic grade transitions, Peter- occurred under greenschist-facies conditions mann orogeny shear zones also delineate the Geochronological and metamorphic studies (~4 kbar, <400 ºC) at 540 ± 10 Ma (Maboko et margins of the Levenger and Moorilyanna gra- relevant to the Petermann orogeny have focused al., 1991). In contrast to this greenschist-facies bens, interpreted as syntectonic transtensional on defi ning the evolution of Petermann orogeny metamorphic event, P-T estimates from the grabens that formed during the Petermann shear zones and the contrast in crustal blocks Davenport shear zone, located ~10 km south orogeny (Gravestock et al., 1993; Major and across them. These studies have been undertaken of the Woodroffe thrust, contain evidence for Conor, 1993). in three regions: the Musgrave Ranges, the Tom- a subeclogite-facies event (~12 kbar, ~650 ºC) The Musgrave Province records little tec- kinson Ranges, and the Mann Ranges (Fig. 1). dated at 547 ± 30 Ma (Camacho et al., 1997; tonic activity subsequent to the Petermann In the Musgrave Ranges (Fig. 1), geochro- Ellis and Maboko, 1992). A geodynamic model orogeny, with deformation being restricted to nological studies indicate that the Woodroffe based on these P-T data proposes crustal thick- infrequently observed low metamorphic grade thrust was active during the late Neoprotero- ening in the early stages of the Petermann orog- shear zones, thought to be related to the Alice zoic to Early Cambrian (560–525 Ma) (Cama- eny, before exhumation begins, progressing to Springs orogeny (Edgoose et al., 2004; Major cho and Fanning, 1995; Maboko et al., 1992). a crustal-scale fl ower structure (Camacho and and Conor, 1993). In contrast, the Offi cer and The similar geochronological evolution either McDougall, 2000). Amadeus basins record extensive deformation side of the Woodroffe thrust is interpreted to A similar evolution is observed in the and subsidence during the 450–350 Ma Alice indicate that the metamorphic grade difference Tomkinson Ranges in the western Musgrave

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Province (Fig. 1), where geochronologically Edgoose et al., 2004; Flottmann et al., 2005) plutons, basins, and shear zones in the near sur- constrained P-T data indicate several late Meso- have defi ned the kinematics of some of these face by their magnetic character, and also the proterozoic granulite-facies events followed by shear zones in localized areas, although the lack defi nition of the principal structural trends and the development of ultramylonite and pseudo- of a regional framework makes these results their overprinting relationships (Aitken et al., tachylite zones, and a metamorphic overprint diffi cult to integrate with the lithospheric-scale 2008; Aitken and Betts, 2009a, 2009b). Major at up to eclogite facies (14 ± 1 kbar, 700–750 architecture. Models of the lithospheric-scale Petermann orogeny shear zones are defi ned by ºC). These rocks were subsequently overprinted architecture of the Musgrave Province based narrow, high-amplitude magnetic lows, and by mica-rich retrograde shear zones (Clarke et on passive seismic data are characterized by were mapped throughout the eastern Musgrave al., 1992, 1995). Although not radiometrically steep lithospheric-scale shear zones, correlated Province (Fig. 2A). In addition to defi ning the dated in this locality, these shear zones connect with the Mann fault, Wintiginna lineament, locations of these shear zones, kinematic infor- into the regional network of major Petermann and Lindsay lineament, that defi ne an upwards mation for these shear zones was interpreted orogeny shear zones, and are interpreted to be Moho offset beneath the central Musgrave Prov- from the aeromagnetic data using similar meth- Petermann orogeny aged (Clarke et al., 1995). ince (Lambeck and Burgess, 1992). ods to those used in structural geology (Betts et Analysis of Petermann orogeny overprints in Foreland basins are sensitive indicators of al., 2007). the Mann Ranges (Scrimgeour and Close, 1999) the isostatic and geodynamic processes of oro- The aeromagnetic data show a variable but showed that metamorphosed granites north of genic belts, and as a result, provide an important broadly northeast-trending magnetic grain, the Woodroffe thrust in the Mulga Park subdo- record of orogenic evolution (e.g., Berge and which is interpreted to be Musgravian orogeny main underwent amphibolite-facies metamor- Veal, 2005; Burbank, 1992; Karner and Watts, aged (Aitken et al., 2008; Aitken and Betts, phism (6–7 kbar, 650 ºC) during the Petermann 1983; Lambeck, 1983). The Offi cer and Ama- 2009a, 2009b). This magnetic grain, along with orogeny, whereas in mylonites immediately deus basins that fl ank the Musgrave Province ca. 1200 Ma Pitjantjatjara Supersuite granit- across the Woodroffe thrust granulite-facies should therefore record the evolution of the oids and ca. 1080 Ma Giles Complex plutons, conditions were observed (9–10 kbar, 700 ºC) Petermann orogeny. act as magnetic marker horizons that have been and, ~40 km farther south, subeclogite-facies Provenance studies of both the Amadeus deformed by Petermann-aged shear zones, per- conditions were observed (12–13 kbar, 700–750 and Offi cer basins have detected a large infl ux mitting a kinematic interpretation of the main ºC). These subeclogite-facies mylonites are cut of sediments between ca. 600 Ma and 500 Ma, Petermann orogeny shear zones. by high metamorphic grade migmatitic shear with “Grenville-aged” detrital zircon popula- The shallow south-dipping Woodroffe zones that have been U-Pb sensitive high-reso- tions consistent with the erosion of the Mus- thrust (Fig. 2A) is defi ned by an abrupt lution ion microprobe (SHRIMP) dated at 560 grave Province during the Petermann orogeny change in magnetic texture from more mag- ± 11 Ma (Scrimgeour et al., 1999). These were being observed in each basin (Maidment et al., netic rocks with high-amplitude magnetic then cut by mylonites at amphibolite facies (7 2007; Wade et al., 2005; Camacho et al., 2002; fabrics to the south of the thrust, to less mag- ± 2 kbar, 660 ± 50 ºC). These overprinting rela- Zhao et al., 1992). In the Amadeus Basin, the netic rocks with lower amplitude magnetic tionships are interpreted to refl ect the exhuma- Musgrave Province is considered a source of fabrics to the north of the thrust, refl ecting tion of the province from subeclogite facies to zircon throughout the evolution of the Amadeus ca. 550 Ma juxtaposition of granulite-facies amphibolite facies during the Petermann orog- Basin, providing a small to moderate contribu- and amphibolite-facies rocks (Maboko et al., eny (Scrimgeour and Close, 1999). tion prior to 560 Ma, the dominant contribution 1992). The aeromagnetic data do not reveal These sharp transitions in crustal level across during the period 540–500 Ma and progressively any kinematic information for the Woodroffe shear zones indicate the juxtaposition of crustal less infl uence in later samples (Maidment et al., thrust, but kinematic indicators within the blocks, within which P-T estimates can be fairly 2007). In the eastern Offi cer Basin, provenance shear zone consistently indicate south-over- consistent (Scrimgeour and Close, 1999). This studies have detected a large infl ux of Mus- north movement (Edgoose et al., 2004). is echoed in the mineralogy of igneous rocks grave Province–derived sediments ca. 600 Ma, The Mann fault is defi ned in the aeromag- from the Giles event, which shallow southwards indicating the onset of the Petermann orogeny, netic data by a broad (2–3 km) and intense across sharp, shear zone–related transitions. but a lack of Musgrave Province–derived sedi- aeromagnetic low, and can be traced from the Ultramafi c plutons in the northern Fregon sub- ments during the 580–540 Ma period (Wade western edge of the interpretation area, through domain were emplaced at ~6 kbar (Clarke et et al., 2005). Eastward transport of sediments the Levenger graben, to its connection with the al., 1995). Moving south, coeval rocks show a along east-trending structures was proposed as Echo lineament. Defl ection of the Musgravian transition through gabbro-pyroxenite, troctolite, the most likely reason for this lack of sediment structural trend proximal to the Mann fault indi- and ultimately surface volcanics at the margins input from the Musgrave Province (Wade et al., cates right-lateral shear sense, consistent with of the province (Glikson et al., 1995). This 2005). Subsidence analysis in the Offi cer Basin folding of the Levenger Formation within the indicates that since ca. 1080 Ma, the northern indicates a period of subsidence during the Levenger graben (Major and Conor, 1993). Fregon subdomain has been uplifted by ~20 km Petermann orogeny, followed by a brief period The Ferdinand fault extends northeast from relative to the margins of the province. of nonsubsidence, and then further subsidence the Levenger graben, and the defl ection of the until ca. 500 Ma, attributed to the Delamerian Musgravian structural trend indicates that this The Architecture of the Petermann Orogeny orogeny (Haddad et al., 2001). shear zone is left-lateral, consistent with surface mapping (Major and Conor, 1993). In the Shear zones are important in defi ning the AN AEROMAGNETIC INTERPRETATION OF aeromagnetic data, the Ferdinand fault is cut kinematics and architecture of the Petermann PETERMANN OROGENY STRUCTURES by the southeast-trending Marryat fault, which orogeny; however, very little work has been has caused large apparent right-lateral offsets done to defi ne the architecture and kinematics The high-resolution (200–400 m line spac- to magnetic granitoids and also the Woodroffe of these shear zones, either in the near surface or ing) regional aeromagnetic data covering the thrust. These two major shear zones may form at depth. Structural studies (Clarke et al., 1995; Musgrave Province permits the defi nition of a conjugate set, indicating N-S compression.

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A WT

MF MYF FF LG

EL WHL MG

WL PL WL

LL 6950000N 7000000N 7050000N 7100000N 7150000N 0 50000E 100000E 150000E 200000E 250000E 300000E 350000E 400000E

B 6950000N 7000000N 7050000N 7100000N 7150000N 0 50000E 100000E 150000E 200000E 250000E 300000E 350000E 400000E

Kilometers RTP magnetic intensity (nT) Bouguer Gravity (mGal) 0255075

GDA94 zone 53 -9000 -500 -250 0 5000 -160 -120 -100 -80 -60 -40 -30 -20 50

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.

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The Mann fault, Ferdinand fault, and Mar- estimates for the Petermann orogeny are not constraint on the upper crustal density distribu- ryat fault defi ne the northern limit of the axial suffi ciently precise to detect this multiphase tion greatly reduces the uncertainty in model- zone of the orogen, which extends south to the evolution, and the absolute age difference ing the geometry of the lower crust and upper Wintiginna lineament, a shear zone that extends between these events is therefore unconstrained. mantle (e.g., Ebbing et al., 2001). across the whole province (Fig. 1). This axial To produce density models of the eastern zone is characterized by an anastomosing net- A 3D DENSITY MODEL OF THE EASTERN Musgrave Province, three-dimensional inver- work of shear zones, many showing apparent MUSGRAVE PROVINCE sion was conducted using VPmg software (Ful- right-lateral offset of magnetic marker horizons lagar et al., 2008), which by iteratively modi- (Fig. 2A). Major shear zones within this zone The Gravity of the Eastern Musgrave fying an input geological model containing include the Wintiginna-Hinckley lineament and Province lithological units and density information, seeks Paroora lineament, neither of which show any to optimize the fi t to gravity data. With a uni- strike-slip kinematic indicators in the aeromag- The kinematics indicated in the aeromag- form target misfi t, the fi t to the gravity data is netic data, and the Echo lineament, which shows netic interpretation gives an estimate of two- defi ned by the root mean square of the residual prominent defl ection of the Musgravian struc- dimensional motion in the near surface, but anomaly (the RMS misfi t). With VPmg, inver- tural trend indicating right-lateral shear sense. constraining the depth penetration and vertical sion terminates when the RMS misfi t is less In the aeromagnetic data, the Wintiginna linea- component of motion on these shear zones is than the target misfi t (convergence), or when ment shows apparent right-lateral offset of sev- more diffi cult. If these shear zones are associ- the algorithm fails to reduce this parameter on eral magnetic marker horizons (Fig. 2A). ated with crust mantle-boundary offsets and the successive iterations (a stalled inversion). Litho- South of the Wintiginna lineament, a lack juxtaposition of crustal blocks from different logical units in the model can be either homog- of strike-slip kinematic indicators indicates that levels, then gravity modeling should indicate enous (i.e., density is the same throughout) or dip-slip movement is dominant in this region. In their deeper geometry. heterogeneous (i.e., density can be varied within particular, the major shear zone in this region, The granulite-facies Fregon subdomain is the unit). The software has two main gravity the Lindsay lineament, shows no evidence of associated with a very high amplitude regional modeling modes—heterogeneous density opti- strike-slip motion. gravity anomaly (~150 mGal) and steep regional mization and geometry optimization. Although they were active during the Alice gravity gradients (~30 eotvos). This intense For heterogeneous density optimization, the Springs orogeny in places, the margins of the high, indicating a large subsurface load, is situ- subsurface is discretized into cells of regular Musgrave Province are also interpreted to ated within a broad, subcircular gravity low in x, y, and z extent, each represented by a single have been active during the Petermann orog- central Australia, 1000 km in diameter, that cor- density value, and the inversion algorithm seeks eny (Edgoose et al., 2004; Flottmann et al., responds to a region of thick crust (Clitheroe et to replicate the gravity data by modifying the 2005; Scrimgeour et al., 1999). The prominent al., 2000) and may represent a long-wavelength density distribution represented by these cells. crustal-scale nappe complexes in the vicinity of fl exural depression. In this inversion mode, the density of homog- the Petermann Ranges (Flottmann et al., 2005; Gravity data coverage in the eastern Mus- enous units and the boundaries between units Scrimgeour et al., 1999) are characteristic of grave Province is relatively good (Fig. 2B), with cannot change. For geometry optimization, the basement-cored nappes throughout the Mulga regional grids at 7.5–10 km spacing supple- subsurface is discretized into vertical prisms, Park subdomain (Edgoose et al., 2004) that are mented by more recent high-resolution profi les within which the depths to lithological bound- interpreted to represent pervasive Petermann at ~1 km spacing (Gray and Flintoft, 2006; Gray ary intersections are recorded, and the inversion orogeny deformation. The southern margin of and Aitken, 2007; Gray et al., 2007). The main algorithm seeks to replicate the gravity data by the Musgrave Province may also have been shear zones interpreted in the aeromagnetic data varying the depths to these lithological boundar- active during the Petermann orogeny but was are broadly correlative with the major steep gra- ies. In this inversion mode, the density within extensively reactivated during the Alice Springs dients in the gravity fi eld (Fig. 2B), although each prism cannot change, although any preex- orogeny (Aitken and Betts, 2009a; Lindsay and there are signifi cant differences: The principal isting heterogeneity is maintained. Leven, 1996; Hoskins and Lemon, 1995). surface boundary, the Woodroffe thrust, is not The much-documented inverse problem Well-defi ned crosscutting relationships associated with the principal gravity gradient, (e.g., Parker, 1994; Zhdanov, 2002) means that between these shear zones (Fig. 2A) indicate that which is located farther south, whereas in other changes in density during inversion must be con- the Petermann orogeny had at least two stages areas steep gravity gradients occur with no evi- trolled to avoid an unrealistic density structure. of deformation: the fi rst phase, in which gran- dence of major Petermann orogeny shear zones During heterogeneous density optimization the ulite-facies crust was emplaced above amphib- in the near surface. A broad, northeast-trending user can impose upper and lower bounds on the olite-facies crust, is characterized by north- and low crossing the regional high (Fig. 2B) is not range of densities permissible for each lithology south-directed movement on divergent shallow- associated with any structure defi ned in the and control the maximum change in density per- dipping, crustal-scale thrust faults, principally magnetic fi eld, and may relate to a deep crustal mitted per iteration. During geometry optimiza- the Woodroffe thrust, the Lindsay lineament, or lithospheric boundary. tion, a user-defi ned parameter controlling the the Piltardi detachment zone, and possibly also maximum relative change in interface depth per the margins of the province. The second phase The Gravity Inversion Method iteration is applied. This constraint means that it is characterized by dextral transpressional move- is mathematically easier to change the geometry ment on more steeply dipping crustal to litho- Petrophysically constrained gravity model- of units at depth, and acts as depth weighting spheric-scale shear zones in the axial zone of the ing has been shown to be an effective method to counteract the loss of sensitivity with depth. orogen, principally the Mann-Ferdinand-Mar- for defi ning the architecture of the near surface More rigid constraints can also be imposed on ryat fault system and the Wintiginna lineament. (Farquharson et al., 2008; Fullagar et al., 2008; the model geometry by defi ning regions in 3D Although the relative timing of these McLean and Betts, 2003). These methods can space within which the boundaries of a litho- deformation events is clear, geochronological be extended to model the whole crust because logical unit or units cannot change.

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The Initial Model Parameters, Constraints, the crustal layering observed in the seismic 2.62 g/cm3 and 2.72 g/cm3; the Wataru gneiss and Boundary Conditions refl ection studies (Korsch et al., 1998; Lindsay was constrained to between 2.7 and 2.8 g/cm3; and Leven, 1996), a four-layer model was con- and the Ammaroodinna inlier was constrained The spatial limits of the gravity model are structed with the mantle (3.3 g/cm3), eclogitic to between 2.8 and 2.9 g/cm3. Granulite-facies broadly defi ned by the limits of high-resolution crust (3.1 g/cm3), lower crust (2.85 g/cm3), and crust was constrained to densities between gravity data (Fig. 2B). The base of the model upper crust separated by boundaries at 50 km, 2.67 and 2.87 g/cm3, 0.1 g/cm3 either side of was set at 90 km depth, and precise topography 35 km, and 25 km, respectively. the measured median density (2.77 g/cm3). from the gravity data was maintained in model- The upper crust was subdivided in accor- The densities of the homogenous units—the ing as the upper surface bound. Prior to inver- dance with the major geological boundaries Amadeus and Offi cer basins, the lower crust, sion, the free-air gravity data were minimum (Fig. 1), with units representing the Amadeus the eclogite layer, and the mantle—were held curvature gridded with 5 km cell size, upward and Offi cer basins (2.55 g/cm3), amphibolite- invariant. continued by 2500 m to remove short wave- facies crust (2.67 g/cm3), granulite or transitional From an initial RMS misfi t of 29.16 mGal, length content irresolvable with 5 km cell size, granulite-facies crust (2.77 g/cm3), and also the the inversion stalled after 21 iterations at 5.06 and detrended along a planar surface, to remove transitional granulite-amphibolite-facies Wataru mGal. Residual anomalies are mostly observed the need for density sources outside the model. gneiss (2.75 g/cm3) in the southwest of the area at the margins of the model (Fig. 4A), although This planar trend slopes from the south to the (Gray, 1978) and the Ammaroodinna inlier (2.85 there are signifi cant negative residual anomalies north over a total range of 11 mGal. g/cm3) in the southeast (Krieg, 1993). These (~15 mGal) over regions of the amphibolite- To provide constraint on the upper crustal density values are constrained by both petro- facies crust where the lower density limit of 2.62 density distribution, 146 measurements of the physical data (Fig. 3) and the density contrasts g/cm3 is too high to permit a fi t to the deep grav- density of major lithologies were made on sam- required to satisfy the short-wavelength gravity ity lows. The fi t over the granulite-facies crust is ples and core from throughout the Fregon sub- gradients revealed in high-resolution data across generally good, although the lack of Giles Com- domain (Fig. 1). These measurements showed the major density boundaries (Gray and Flintoft, plex mafi c intrusions in the model is refl ected that the density distribution is heterogeneous at 2006; Gray and Aitken, 2007; Gray et al., 2007). in short-wavelength positive residual anomalies small scales (tens of meters), and this means that over major intrusions. The density distribution individual density measurements do not refl ect A Heterogeneous Upper Crust Model within this model (Fig. 4A and Animation 1)1 the bulk density of large modeling cells, and are is generally reasonable and shows that there is not therefore used to directly constrain the den- To investigate the density distribution no inherent requirement in the gravity data for sities of measurement localities. However, the required to satisfy the gravity anomaly from the crust-mantle boundary relief beneath the Mus- statistical distribution of these measurements upper crust alone, petrophysically constrained grave Province. (Fig. 3) is important in constraining the prob- density inversion was applied within the upper However, the density distribution in this able density distribution in the near surface. crust using 5 km × 5 km × 1 km cells. The model has large areas of anomalously dense or Deep seismic refl ection studies (Korsch et maximum density change per iteration was set light upper crust, for which there is little petro- al., 1998; Lindsay and Leven, 1996) and passive at 0.02 g/cm3, and the target misfi t was set to physical evidence (Fig. 3). A particularly large seismic models (Clitheroe et al., 2000) constrain 1 mGal. The densities within lithological units density contrast is required between the Mulga the Moho depth to ~50 km beneath central Aus- were constrained as follows: amphibolite-facies Park subdomain (2.62 g/cm3 or less) and the tralia. To correspond with this constraint, and crust was constrained to densities between Fregon subdomain (2.75–2.87 g/cm3). In addition, the major surface boundary juxtaposing crustal levels, the Woodroffe thrust, is only associated with a small density contrast in this Max model, with a large near-surface density con- 3.5 3.53 trast concentrated farther south. We consider σ 3.40 Mean±1 this model to be inconsistent with the P-T and density constraints, and it also bears little resem- 3.3 Median blance to the architecture imaged in seismic ) 3 Min models (Lambeck and Burgess, 1992). Some 3.1 amount of crust-mantle boundary relief is therefore probable. 3.00±0.21 2.91 2.9 A Median Density Model 2.83

Density (g/cm 2.82±0.17 2.77 2.76±0.05 2.72±0.08 The geometry of the crust-mantle boundary 2.7 2.70 2.64 and the amount of relief required to produce 2.61 2.60 2.52 2.5 1If you are viewing the PDF of this paper or 0 10 20 30 0 4 812 0246 8 0 246 810 reading it ofﬂ ine, please visit the full-text article Granulite facies Granite/granitic Giles Complex Charnockite on http://lithosphere.gsapubs.org/ to view Anima- gneiss n = 91 gneiss n = 25 n = 17 n = 11 tions 1–5. You can also access them at the follow- ing respective links: http://dx.doi.org/10.1130/L39.S1, Figure 3. Histograms and calculated parameters showing the statistical distribution of http://dx.doi.org/10.1130/L39.S2, http://dx.doi.org/ speciﬁ c gravity measurements collected throughout the Fregon subdomain, divided 10.1130/L39.S3, http://dx.doi.org/10.1130/L39.S4, and into broad lithological groups. http://dx.doi.org/10.1130/L39.S5.

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Observed gravity A Calculated gravity 7100000

70 WL 7000000 PL 0 EL FAA FF 0 100000 200000 300000 400000 (mGal) -70 WT X=275000mE Gravity misfit (mGal)

0 -35 0 30 80 LL PL WL 10 EL WT MF 25 FAA X=170000mE (mGal) -60

50 0 Depth (km) 100 LL WL 70 25 40 WHL

2.95 FAA MF (mGal) -20 WT X=55000mE 50 Depth (km)

2.87 0 ) 3 70 2.82 25 2.77

6950000 density (g/cm density 50 7000000

2.72 Depth (km) 7050000 2.67 70 7100000 N Northing (m) 2.62 7150000

Observed gravity B Calculated gravity 7100000

70 7000000 WL 0 PL 0 100000 200000 300000 400000 FAA EL (mGal) FF X=275000mE Gravity misfit (mGal) -70 WT -35 0 30 0 80 LL WL 10 PL MF EL 25 FAA WT X=170000mE (mGal) -60

50 0 Depth (km) 100 LL 70 WL 2.95 25 40 WHL FAA X=55000mE MF (mGal) -20 WT

50 2.87 Depth (km)

) 3 2.82 70 25 2.77

Density (g/cm Density 6950000 2.72 50 7000000 Depth (km) 7050000 2.67 70 710000 0 thing (m) N Nor 2.62 7150000

Figure 4. (A) Interactive three-dimensional (3D) view of the result of the heterogeneous upper crust inversion model, showing the fl at crust-mantle boundary, the upper crust density distribution, and the fi t 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 fi t 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 fi gure please visit http://lithosphere.gsapubs.org/content/1/6/343 and click on Animations in the middle column or go directly to the fi gure at http://dx.doi.org/10.1130/L39.S6.

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the observed gravity anomaly were investigated ducted to quantify this sensitivity by running architecture, due to the omission of major intru- using a geometry optimization inversion, with geometry inversions in which the density con- sive suites and sedimentary basins and the geo- 5 km × 5 km vertical prisms of 90 km depth trast was perturbed in the initial model. For a metric and density assumptions imposed on extent and a maximum depth change per itera- variety of contrast values, the statistical variance the models. A more detailed initial model was tion of 2%. The target misfi t was 1 mGal, and the of the depth to the resulting crust-mantle bound- constructed (Fig. 6A) including Giles Complex boundaries of all units were permitted to change. ary was calculated as a measure of its fl atness and Pitjantjatjara Supersuite plutons, and syn- From an initial RMS misfi t of 29.16 mGal, (Fig. 5). Petermann orogeny grabens. The geometry of the inversion stalled after 80 iterations at 4.90 The results of the sensitivity analysis (Fig. 5 the lower crust and crust-mantle boundaries mGal. Short-wavelength residual anomalies are and Animation 4 [see footnote one]) indicate were remodeled to represent offset of crustal observed throughout the model area (Fig. 4B) as that low or negative density contrast, between layers along the plane of lithospheric-scale a result of the inability of this model to resolve −0.05 and 0.025 g/cm3, results in a broad crust- shear zones, and to more closely resemble the small near-surface features. The geometry mantle boundary high beneath the gravity high, seismic architecture of the Musgrave Province derived from this inversion (Fig. 4B and Anima- and therefore high variance. Moderate density (Fig. 6A). The source of the northeast-trending tion 3[see footnote 1]) shows that the mantle and contrast, between 0.05 and 0.125 g/cm3, pro- low in the gravity data (Fig. 2B) is not known, eclogite layers are, in general, uplifted beneath duces a fl at but undulating crust-mantle bound- but this anomaly is associated with an eastward the east-trending central gravity high, and ary surface and low variance, and high density gravity gradient. Forward modeling prior to depressed beneath the gravity lows. The amount contrast, between 0.15 and 0.25 g/cm3, pro- inversion indicated that west-up offset of the of crust-mantle boundary relief changes along duces a broad crust-mantle boundary depression lower crust and crust-mantle boundaries by strike, with the greatest relief of ~20–25 km in beneath the gravity high, and high variance. 8.5 km fi ts this gradient well. the western part of the model, and a reduction Figure 5 illustrates that a density contrast of The lithological densities in this model are to ~10–20 km of relief in the eastern part of the between 0.075 and 0.125 g/cm3 produces a low- similar to those used in the previous models, model. In this model, steep crust-mantle bound- variance crust-mantle boundary and also a low and the densities of the Amadeus and Offi cer ary gradients correlate with major Petermann RMS misfi t. This result verifi es the constraints basins, lower crust, eclogite layer, and mantle orogeny shear zones, principally the Mann from petrophysical data and high-resolution were held invariant at their initial density. Greater fault, Ferdinand and Marryat faults, Wintiginna- gravity data that indicate ~0.1 g/cm3 of density heterogeneity was incorporated into the upper Hinckley lineament, and Wintiginna lineament contrast between granulite-facies and amphibo- crust by introducing homogenous units repre- (Fig. 4B). lite-facies gneiss. senting the Levenger and Moorilyanna Forma- The geometry of the lower crust, eclogite, tions (each 2.55 g/cm3), and heterogeneous units and mantle layers in this model are very sensi- A Combined Heterogeneous Density and representing the Giles Complex (3 ± 0.1 g/cm3) tive to the density contrast between the granu- Geometry Inversion and the Pitjantjatjara Supersuite, subdivided into lite-facies gneiss and the amphibolite-facies granitic (2.7 ± 0.1 g/cm3) and charnockitic (2.8 gneiss, with small changes in density causing Neither the heterogeneous upper crust ± 0.1 g/cm3) lithologies. The location of these large changes in the offsets required to satisfy model (Fig. 4A) nor the median density model upper crustal units was defi ned in accordance the gravity data. A sensitivity analysis was con- (Fig. 4B) are a good representation of the crustal with aeromagnetic data and outcropping geology. The amphibolite-facies crust in the Mulga Park subdomain, the southern Fregon subdo- 5.50 1E+08 RMSgrav CMBvariance main, and beneath the Amadeus and Offi cer 5.45 basins was heterogeneous in this model, with 9E+07 density of 2.67 ± 0.05 g/cm3. The Wataru gneiss 5.40 (2.75 ± 0.05 g/cm3) and Ammaroodinna inlier 8E+07 (2.85 ± 0.05 g/cm3) were also heterogeneous. 5.35 The granulite-facies upper crust was subdi- 7E+07 5.30 vided into four east-trending units, bounded by the Woodroffe thrust, the Mann-Ferdinand-Mar- 5.25 6E+07 ryat fault system, the Wintiginna-Hinckley and 5.20 Paroora lineaments, the Wintiginna lineament, 5E+07 and the Lindsay lineament (Fig. 6A). The north- 5.15 ernmost (granulite facies 1) and southernmost

RMS gravity misfit (mGal) 4E+07 5.10 (granulite facies 4) of these units were hetero- Crust-mantle boundary variance boundary Crust-mantle geneous, with upper density bounds of 2.83 and 3E+07 5.05 2.87 g/cm3, respectively, and lower bounds of 2.77 g/cm3. The central units were homogenous, 5.00 2E+07 0.25 0.20 0.175 0.15 0.125 0.1 0.075 0.05 0.025 0 -0.05 with densities of 2.77 g/cm3 (granulite facies 2) Granulite facies - amphibolite facies and 2.75 g/cm3 (granulite facies 3). density contrast (g/cm3) For this inversion, by running consecutive density and geometry inversions, we ﬁ rst opti- Figure 5. The sensitivity of the root-mean-square (RMS) gravity misﬁ t and mized the densities in the heterogeneous units, the amount of crust-mantle boundary topography to the density contrast between the granulite-facies and amphibolite-facies crust. The crust-mantle before adjusting the geometry of all units. The boundary surfaces created in this process are displayed in Animation 4 (see heterogeneous density inversion was run with footnote 1). 5 km × 5 km × 1 km cells, with a maximum

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WL A MF-FF-MYF

X=275000mE

25 X=170000mE LL 0

Depth (km) 50

70 25 X=55000mE

Depth (km) 0

70 25

6950000 50 Depth (km) 7000000

70 7050000 7100000 Northing (m) N 7150000 Granulite facies 1 Granulite facies 4 Giles Complex Granulite facies 2 Amphibolite facies Basins Granulite facies 3 Pitjantjatjara Supersuite B

WL 7100000 MF-FF-MYF

WT 7000000

0 100000 200000 300000 400000 Gravity misfit (mGal) X=275000mE -35 0 30 0

25 LL X=170000mE 0

Depth (km) 50

70 25 X=55000mE 50

Depth (km) 0

70 25

6950000 50 Depth (km) 7000000

70 7050000 7100000 Northing (m) N 7150000

2.62 2.67 2.72 2.77 2.82 2.87 2.95

Density (g/cm3)

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 fi t to the data (top right). The inset (bottom left) shows the infl uence 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 fi gure please visit http://lithosphere.gsapubs.org/content/1/6/343 and click on Animations in the middle column or go directly to the fi gure at http://dx.doi.org/10.1130/L39.S7.

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density change per iteration of 0.02 g/cm3, and 1998; Lambeck and Burgess, 1992), with the Strain Accommodation and Escape a target misfi t of 1 mGal. From an initial RMS exception that neither the Lindsay lineament Tectonics misfi t of 18.43 mGal, the inversion stalled after nor the shear zone at the southern margin of the 17 iterations at a RMS misfi t of 6.62 mGal. province penetrates the crust-mantle boundary Strain during the Petermann orogeny appears Residual anomalies are concentrated above the in our model, as suggested previously (Korsch to have been accommodated in two ways: (1) granulite-facies core, which was invariant in et al., 1998; Lambeck and Burgess, 1992; pervasive ductile deformation and crustal thick- this inversion. Lindsay and Leven, 1996). The median den- ening in the Mulga Park subdomain and in the Prior to geometry inversion, the lower- sity model (Fig. 4B) and the combined model Mann Ranges and (2) vertical and lateral extru- crustal stratigraphy beneath the amphibolite- (Fig. 6B) show that a relatively thin wedge sion of relatively rigid crustal blocks along duc- facies crust was constrained so that the inversion of granulite-facies gneiss above a shallow- tile shear zones. algorithm would only modify the lower-crustal dipping Lindsay lineament satisfi es the grav- Pervasive Petermann orogeny ductile defor- stratigraphy beneath the granulite-facies crust ity anomaly here, and crust-mantle boundary mation is recorded throughout the Mulga Park (Fig. 6A). This constraint is necessary because uplift south of the Wintiginna lineament is not subdomain (Edgoose et al., 2004). The struc- the inversion algorithm will preferentially supported. tures associated with this deformation are too modify boundaries at depth with high-density small to be resolved in our model, although geo- contrast (i.e., the base of the amphibolite-facies DISCUSSION—LITHOSPHERIC logical mapping indicates that this deformation regions) and without constraint, the resulting PROCESSES OF THE PETERMANN is characterized by north-vergent recumbent iso- geometry in these regions is inconsistent with OROGENY clinal folding of both crystalline basement and deep seismic refl ection studies (Korsch et al., sedimentary cover successions accompanied by 1998; Lindsay and Leven, 1996). Probable Inversion of Ca. 1080–800 Ma a metamorphic overprint at upper-greenschist to The geometry inversion was run with 5 km Extensional Architecture upper-amphibolite facies (Edgoose et al., 2004; × 5 km vertical prisms of 90 km depth extent, Flottmann et al., 2005; Scrimgeour et al., 1999). a maximum depth change per iteration of 2%, A defi ning characteristic of intraplate oro- Signifi cant ductile deformation is also pres- and a target misfi t of 1 mGal. This inversion genesis in the upper crust is the reactivation of ent in the Mann Ranges of the Fregon subdo- stalled after 14 iterations, reducing the RMS relict architecture, typically identifi ed on the main, although the origins of this ductile defor- misfi t from 6.62 mGal to 5.58 mGal. This basis of inverted extensional basins and upthrust mation are thought to be quite different to that in inversion greatly reduced residual anoma- basement blocks (e.g., Turner and Williams, the Mulga Park subdomain, resulting from rela- lies above the granulite-facies crust, although 2004; Ziegler et al., 1995; Ziegler et al., 1998). tively rapid uplift of the crust, causing upward short wavelength fl uctuations are still observed The east to southeast orientation and moder- advection of heat and the resulting migmatiza- (Fig. 6B). ate to steep dip of the shear zones in the axial tion (Scrimgeour and Close, 1999). zone parallel the architecture interpreted to have In contrast to the Mann Ranges, much of the A Best-Fit Model developed during ca. 1080 Ma and ca. 800 Ma axial zone of the orogen lacks a pervasive meta- extensional events, which are characterized in morphic overprint in granulite-facies gneiss Of the three inversions attempted, the archi- the Musgrave Province by east-southeast– and despite intense deformation within shear zones. tecture derived from the combined density and southeast-trending shear zones, often co-located This indicates that strain during the Petermann geometry inversion (Fig. 6B; Animations 5 and with mafi c dikes (Aitken et al., 2008; Aitken and orogeny was highly partitioned onto mylonite 6 [see footnote one]) is the most consistent with Betts, 2009b; Clarke et al., 1995; Edgoose et al., structures (Camacho et al., 1997; Camacho and the magnetic interpretation, geological observa- 2004). In existing geologic maps, these two McDougall, 2000; Camacho et al., 2001; Clarke tions (Camacho et al., 1997; Clarke et al., 1995; major dike suites are generally undifferentiated, et al., 1995) possibly as a result of shear heating Ellis and Maboko, 1992; Maboko et al., 1991; and therefore it is diffi cult to make a distinction (Camacho et al., 2001). Similarly, at the crustal- Major and Conor, 1993; Scrimgeour and Close, between the geometries of these tectonic events scale, Petermann orogeny shear zones defi ned 1999), and seismic observations (Korsch et al., with confi dence. in aeromagnetic data (Fig. 2A) generally bound 1998; Lambeck and Burgess, 1992; Lindsay and The high-pressure recrystallization of dikes relatively undeformed crustal blocks, and defi ne Leven, 1996). The fi t to the gravity anomaly is in regions intensely deformed during the Peter- numerous structural features normally associ- close to that achieved in the other inversions. mann orogeny (Camacho et al., 1997; Clarke et ated with brittle deformation, including conju- This 3D density model is well constrained al., 1995; Ellis and Maboko, 1992; Major, 1967; gate shear zones and pop-up structures, despite in the vicinity of the Woodroffe thrust and the Scrimgeour and Close, 1999) indicates that they being active at lower-crustal levels. This indi- Mann, Ferdinand, and Marryat faults due to were important in partitioning strain, and prob- cates a deformation regime that at crustal scale the relatively high gravity petrophysical and ably focused this strain into the coincident struc- was characterized by the motion of competent teleseismic data resolution, and greater degree tures along which their emplacement may have crustal blocks along ductile shear zones, onto of geological constraint. Away from this zone been controlled (Aitken et al., 2008; Aitken and which strain was highly partitioned (Fig. 2A). the model becomes less well constrained as the Betts, 2009a, 2009b; Clarke et al., 1995). The major shear zones that bound the axial resolution of gravity, petrophysical and tele- Although these criteria are not completely zone are transpressional and accommodate seismic data decreases, and outcrop becomes defi nitive, we infer these similarities in orienta- strain by both the vertical and lateral extrusion sparser, although deep seismic refl ection lines tion and inferred geometry and the recrystalliza- of crustal blocks: In the west of the modeled in the Amadeus Basin and southern Musgrave tion of mafi c dikes to represent the reactivation area, 20–25 km of crust-mantle boundary offset Province provide constraint on the architecture of preexisting weaknesses in the mid-to-lower is accommodated on the Mann fault and Win- of the province margins. crust during the Petermann orogeny, resulting tiginna-Hinckley lineament. In the east of the The architecture at depth is similar to that in crustal-scale inversion of the relict geometry modeled area, there is signifi cantly less verti- proposed from seismic models (Korsch et al., from previous extensional tectonic events. cal offset (10–20 km) and signifi cant northward

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and eastward movement of lithospheric blocks some modifi cation: Kinematically speaking, these events is not constrained by radiometric accommodated by oblique reverse movement oblique subduction of the paleo–Pacifi c plate dating (Clarke et al., 1995; Scrimgeour and on the Ferdinand fault, Marryat fault, and Win- (Veevers, 2003) could generate this relative Close, 1999). In addition, the mineralogy of tiginna lineament (Figs. 2A and 6B). At the far- motion, although to do so would require either the ca. 1080 Ma Giles Complex and associated east of the model area, an extensive network of a greater degree of obliquity than suggested by volcanic rocks indicates ~20 km of exhumation splays (Fig. 2A) indicates the terminal accom- Veevers (2003), or intracontinental curvature in the axial zone relative to the margins of the modation of intraplate strain by north-directed of the stress fi eld (e.g., Hillis and Reynolds, province. The uplift of the entire crustal pile by lateral spreading of the orogen. 2000). The initiation of paleo-Pacifi c subduc- between 10 km and 25 km in the axial zone of There is therefore an along-strike transition tion, at its earliest ca. 560 Ma (Goodge, 1997) the orogen (Fig. 6B) is therefore consistent with from a large amount of vertical extrusion in the and probably younger near Australia (Boger these geologic data. west, to the east where vertical extrusion is less and Miller, 2004), may preclude this model on The architecture of the Petermann orogeny is but a greater amount of northward and eastward the basis of timing. unusual, in that with a geologically reasonable lateral extrusion is observed. This is interpreted The original model for the Kuunga orogeny density structure in the upper crust, the axial to refl ect escape tectonics with motion directed (Meert et al., 1995; Meert, 2003) and subsequent zone of the orogen is underlain by a wedge of toward the less competent lithosphere at the references to this model (e.g., Boger and Miller, lithospheric mantle in the lower crust. Isostati- eastern margin of Australia, bounded by an 2004; Collins and Pisarevsky, 2005; Goscombe cally, this load cannot be supported locally, and incipient subduction zone (Boger and Miller, and Gray, 2008) imply E-W–directed collision studies of central Australia have indicated that at 2004; Gray and Foster, 2004) and away from of India with Australia and Antarctica, which short wavelengths (~200 km), central Australia the highly competent lithosphere of the West does not readily explain the N-S shortening is not in isostatic equilibrium (Lambeck, 1983; Australian craton. In the far-eastern Musgrave within central Australia. An oblique collision of Stephenson and Lambeck, 1985). The pattern Province, motion is directed north, away from India with Australia during the Kuunga orogeny, of gravity anomalies in central Australia indi- the Archean to Paleoproterozoic Gawler craton. at least NW-SE oriented, would be a straightfor- cates that this load, and a similar one beneath Transpressional kinematics and escape tec- ward way to explain the deformation observed the Arunta inlier (Goleby et al., 1989; Goleby et tonics are interpreted to be fundamental in the in central Australia. al., 1990), may be collectively compensated by currently active intraplate orogens of central Although more distant, the East African long-wavelength (~1000 km) lithospheric fl ex- Asia where they occur as a result of the reactiva- orogen is a plausible source of ~N-S compres- ure beneath central Australia. tion of structures oblique to the ~N-S compres- sive stress in central Australia, as the collision The short-wavelength isostatic disequilib- sion driven by the Himalayan collision (Cun- was essentially parallel, and was of suffi cient rium implied by the crust-mantle boundary ningham, 2005; Tapponnier and Molnar, 1979), magnitude to have propagated signifi cant forces architecture (Fig. 6B) induces strong forces in and a similar situation may have occurred in the into the continents. However, the reactivation the lithosphere. The preservation of this archi- Petermann orogeny. of central Australia would require the transmis- tecture for more than 500 million years, despite sion of stress through the entirety of East Ant- a number of tectonic events including both east- The Gondwana Connection arctica, which is typically considered as a group west–oriented compressional events and north- of coalesced cratonic blocks (e.g., Fitzsimons, south–oriented rifting episodes (e.g., Betts et al., As noted previously, locally derived verti- 2000; Jacobs et al., 2008), and is thus prone 2002; Bryan et al., 1997; Gray and Foster, 2004; cal forces may have played a signifi cant role in to reworking. We consider the direct transmis- Miller et al., 2002), implies that the isostatic the Petermann orogeny (Neil and Houseman, sion of stress unrealistic, unless achieved by the forces are not being counteracted dynamically 1999), but the transmission of far-fi eld stress focusing of strain into a network of transpres- but that the lithosphere is signifi cantly strength- from the collisional orogens related to Gond- sional intracratonic mobile belts, including the ened, at least locally. wana assembly are likely to be the main source Kuunga orogen, in response to oblique conver- The exhumation of the entire crustal pile of driving forces. As the most proximal of the gence of the East African orogen (Squire et al., during the Petermann orogeny may be the driver potential sources of the intraplate stress fi eld, 2006). for this increase in lithospheric strength, due to a the Kuunga orogen is a compelling candidate, combination of the emplacement of a wedge of especially considering that stress need only be Crustal Uplift and Lithospheric relatively strong lithospheric mantle into the rel- transmitted through the West Australian craton, Strengthening atively weak lower crust, and the erosion of an which is underlain by a keel of Archean litho- upper crust containing abundant heat-producing spheric mantle (Simons et al., 1999), and as a In the Musgrave Ranges, amphibolite and elements (Hand and Sandiford, 1999; McLaren result is highly competent. greenschist-facies mylonites, dated at 540 et al., 2005; Sandiford and Hand, 1998). These We therefore consider the N-S compres- ± 10 Ma, crosscut subeclogite-facies shear processes would create a block of cool and sional stress in central Australia to result from zones, dated at 547 ± 30 Ma (Camacho et al., strong lithosphere relative to the surrounding SSE-directed motion of India relative to Austra- 1997; Ellis and Maboko, 1992; Maboko et al., regions, causing tectonic stability and enabling lia, as part of the Kuunga orogeny. This resulted 1991). This close coexistence in space and time the preservation of this crustal architecture in in sinistral shear on the N-S–oriented Darling of subeclogite-facies metamorphism at ~40 km the long term. fault (Harris, 1994) and dextral shear on the depth, and greenschist-facies metamorphism In addition, the exhumation of the axial ESE-SE–oriented shear zones of the Petermann at ~20 km depth requires ~20 km of exhuma- zone of the province by 10–20 km implies that orogeny (Fig. 2A). This also explains sinis- tion during the Petermann orogeny. The meta- a large amount of material has been eroded tral shear on the NE-oriented Ferdinand fault morphic evolution of the Mann Ranges and from the surface. Although these sediments (Fig. 2A). the Tomkinson Ranges defi ne a similar evolu- are distributed across a huge area (Lindsay, This relative motion can be achieved within tion, with deep-crustal mylonites overprinted 2002), a large proportion of this material was the aforementioned models of Gondwana with by mid-crustal mylonites, although the age of deposited in the foreland Amadeus and Offi cer

354 www.gsapubs.org | Volume 1 | Number 6 | LITHOSPHERE

basins, which contain up to 3 km of late Neo- We note, however, that within this paradigm orogeny: The cause and effect of an Early Cambrian reconfi guration of plate motions: Earth and Planetary proterozoic to Early Cambrian sedimentary the Petermann orogeny is remarkable for several Science Letters, v. 219, no. 1–2, p. 35–48, doi: 10.1016/ rocks (Gravestock et al., 1993; Lindsay, 2002; reasons: (1) deformation is focused to a very S0012-821X(03)00692-7. Lindsay and Leven, 1996). This erosion-depo- discrete region far from the plate margin, within Braun, J., and Shaw, R., 2001, A thin-plate model of Palaeo- zoic deformation of the Australian lithosphere: Impli- sition feedback is likely to have amplifi ed the which intense lithospheric-scale deformation cations for understanding the dynamics of intracra- uplift of the axial zone relative to the foreland, has occurred; (2) although none can be excluded tonic deformation: Geological Society, London, similar to other regions (e.g., Avouac and on the basis of this study, existing models of this Special Publication 184, p. 165–193. Bruckl, E., Bleibinhaus, F., Gosar, A., Grad, M., Guterch, A., Burov, 1996; Burov, 2007). region during Gondwana assembly (Collins and Hrubcova, P., Keller, G.R., Majdanski, M., Sumanovac, Pisarevsky, 2005; Meert, 2003; Squire et al., F., Tiira, T., Yliniemi, J., Hegedus, E., and Thybo, H., 2007, CONCLUSIONS 2006; Veevers, 2003) require modifi cation to Crustal structure due to collisional and escape tectonics in the Eastern Alps region based on profi les Alp01 be consistent with the architecture and kinemat- and Alp02 from the ALP 2002 seismic experiment: A combination of aeromagnetic interpreta- ics of the orogen; and (3) orogenesis has led to Journal of Geophysical Research, Solid Earth, v. 112, B06308, doi: 10.1029/2006JB004687. tion and 3D gravity inversion has enabled the signifi cant lithospheric strengthening, causing Bryan, S.E., Constantine, A.E., Stephens, C.J., Ewart, A., derivation of an architectural and kinematic the stabilization of a previously much reworked Schon, R.W., and Parianos, J., 1997, Early Cretaceous model of the intraplate compressional Peter- terrane and the preservation of its remarkable volcano-sedimentary successions along the eastern Australian continental margin: Implications for the mann orogeny in central Australia. This model crustal architecture to the present. break-up of eastern Gondwana: Earth and Planetary is characterized by the development in the Science Letters, v. 153, no. 1–2, p. 85–102, doi: 10.1016/ early stages of the orogeny of divergent crustal- ACKNOWLEDGMENTS S0012-821X(97)00124-6. Burbank, D.W., 1992, Causes of recent Himalayan uplift scale thrusts at the margins of the province that deduced from deposited patterns in the Ganges basin: accommodated crustal thickening, and the jux- This work was supported by Primary Industry and Nature, v. 357, p. 680–683, doi: 10.1038/357680a0. Burov, E., 2007, Coupled lithosphere-surface processes in col- taposition of granulite-facies and amphibolite- Resources South Australia (PIRSA) and Austra- lision context, in Lacombe, O., Roure, F., Lavé, J., and facies crustal blocks. These thrusts are cut by an lian Research Council Linkage grant LP0560887. Vergés, J., eds., Thrust Belts and Foreland Basins: Ber- axial zone of steep, crustal to lithospheric-scale Aitken was also supported by a Monash Univer- lin, Springer, p. 3–40, doi: 10.1007/978-3-540-69426-7_1. Burov, E.B., Kogan, M.G., Lyon-Caen, H., and Molnar, P., transpressional shear zones that accommodated sity Faculty of Science Postgraduate Publication 1990, Gravity anomalies, the deep structure, and the exhumation of the axial zone, causing crust- Award. The Musgrave Province aeromagnetic dynamic processes beneath the Tien Shan: Earth and mantle boundary offsets of up to 25 km. dataset was supplied by PIRSA. Gravity data Planetary Science Letters, v. 96, no. 3–4, p. 367–383, doi: 10.1016/0012-821X(90)90013-N. The architecture of this orogen permits the were obtained under license from Geoscience Camacho, A., and Fanning, C.M., 1995, Some isotopic con- characterization of several lithospheric pro- Australia and PIRSA. We thank Rick Squire, straints on the evolution of the granulite and upper amphibolite facies terranes in the eastern Musgrave cesses, including the probable reactivation of James Evans, and an anonymous reviewer for block, central Australia: Precambrian Research, v. 71, relict extensional architecture, strain accom- their constructive comments on the manuscript. p. 155–181, doi: 10.1016/0301-9268(94)00060-5. modation by the vertical and lateral extrusion Camacho, A., and McDougall, I., 2000, Intracratonic, strike- slip partitioned transpression and the formation of of competent crustal blocks along major duc- REFERENCES CITED eclogite facies rocks: An example from the Musgrave tile shear zones, and escape tectonics directed block, central Australia: Tectonics, v. 19, p. 978–996, toward the east. 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