Regional correlation of Mesoproterozoic structures and deformational events in the Albany–Fraser orogen, Western Australia

Precambrian Research 116 (2002) 129– 154 www.elsevier.com/locate/precamres

Regional correlation of Mesoproterozoic structures and deformational events in the Albany–Fraser orogen, Western Australia Ernest M. Duebendorfer * Department of Geology, Northern Arizona Uni6ersity, Flagstaff, AZ 86011, USA Received 2 August 2001; received in revised form 7 February 2002; accepted 11 February 2002

Abstract Geologic mapping and meso- and microstructural analysis of a previously unstudied area in the western part of the Mesoproterozoic Albany–Fraser orogen, Western Australia, leads to a correlation of deformational events along the exposed length of the orogen. The earliest deformational event (D1) accompanied granulite-facies metamorphism and is characterized by a locally preserved subhorizontal foliation and associated recumbent folds. This event is interpreted to record northwest–southeast contraction. The second event (D2) produced a subvertical, east-northeaststriking foliation and variably plunging, upright folds. Northwest-vergent folds (toward 333°) and sparse kinematic indicators suggest southeast-up tectonic transport with a dextral component of movement. A third deformational event (D3) is manifested by a conjugate set of mylonitic to cataclastic shear zones with a dominant, dextral set oriented at 105° and a subordinate, sinistral set oriented at 010°. The bisector of the average orientations for the shear zones is 325°, consistent with northwest–southeast shortening. Like the D2 structures, the D3 structures may represent deformation in a dextral transpressive setting. D4 structures include two subvertical joint sets, one oriented at 355° and the other at 105°. All structures are similar in geometry and kinematics to structures near Albany, which have been constrained to 1190–1170 Ma (Precambrian Res., 59 (1992) 95), as well as structures east of Esperance that are associated with a major tectonothermal event at 1345– 1260 Ma (Precambrian Res., 102 (2000) 155). Additional geochronological data, particularly from the Western Albany– Fraser orogen, are needed to resolve this apparent discrepancy. There is a near one-to-one correlation in lithology, structural style, metamorphic grade, kinematics, and possibly timing of deformational events between the Albany– Fraser orogen and the Bunger Hills of East Antarctica, supporting the previously suggested ties between these two areas. Marked dissimilarities, however, in model TDM ages, U–Pb zircon dates, kinematics and timing of deformation, and timing of metamorphism between the Albany–Fraser orogen and the Oaxacan Complex of southern Mexico raise serious doubts regarding the viability of Rodinia reconstructions that juxtapose those two Mesoproterozoic terranes. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Albany– Fraser orogen; Australia; Mesoproterozoic; Tectonics; Rodinia

* Fax: + 1-520-523-9220. E-mail address: [email protected] (E.M. Duebendorfer). 0301-9268/02/$ - see front matter © 2002 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 1 - 9 2 6 8 ( 0 2 ) 0 0 0 1 7 - 7

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1. Introduction The Albany–Fraser orogen is a 100– 200-kmwide zone of Mesoproterozoic (ca. 1.3– 1.1 Ga) deformation that lies outboard of the southern and southeastern margins of the Yilgarn craton in southwestern Australia (Fig. 1). The orogen trends nearly east–west from near Windy Harbor to Bremer Bay, where structural trends curve toward the northeast. The Albany– Fraser orogen is generally considered to be a single, though

complex, structure (e.g. Myers, 1993; Myers et al., 1996). The Albany –Fraser orogen appears to have orogenic counterparts on two, and possibly three, continents. Several workers have suggested a correlation between the Albany– Fraser orogen and the Bunger Hills and Windmill Islands regions of East Antarctica based on similarities in structure, metamorphism, and age of rocks and deformational events (e.g. Lovering et al., 1981; Oliver et al., 1983; Black et al., 1992; Harris, 1993, 1995;

Fig. 1. Simplified map of the Albany–Fraser orogen (shaded) showing complexes, principal structures, and location of areas mapped in detail (marked by stars). Areas mapped in detail, BB, Bremer Bay; CC, Conspicuous Cliffs; GP, Green’s Pool, HP, Herald Point; LGP, Ledge Point; LP, Long Point; MR, Mt. Ragged; SI, Salisbury Island; PI, Point Irwin– Peaceful Bay area; PR, Pallinup River traverse. MBG, Mount Barren group; SRF, Stirling Range Formation; WB, Woodline beds. Inset map shows location of Albany– Fraser orogen in Australia and reconstructed position of East Antarctic craton. Inset, BH, Bunger Hills; WI, Windmill Islands. Map modified from Myers (1993).

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Myers, 1995a; Sheraton et al., 1995; Clark et al., 2000) and these areas are juxtaposed in several Gondwana/Rodinia reconstructions (e.g. Veevers and Eittreim, 1988; Hoffman, 1991; Rogers, 1996; Weil et al., 1998). Myers (1993), Clarke et al. (1995), Sheraton et al. (1995), White et al. (1999) suggested that the Musgrave Block, the Albany– Fraser orogen, and the Bunger Hills form part of a once-continuous Grenville-age orogen (Fig. 1) that may represent a collisional plate boundary during the formation of Rodinia. Alternatively, the Albany –Fraser– Musgrave belt has been interpreted as an intracratonic orogenic belt that preceded assembly of Rodinia at ca. 1.0 Ga (Li et al., 1995) or an ‘exterior thrust belt’ associated with a Grenville-age suture landward from coastal Antarctica (Rogers, 1996). Harris (1995) proposed 300 km of dextral transcurrent to transpressional displacement between the East Antarctica shield and the Yilgarn craton and rejected a collisional origin for the belt. Katz (1989), Harris (1993, 1995) suggested that the Central Indian Tectonic zone represents the westward extension of the Albany – Fraser orogen in Rodinia. Finally, in the Australia–Western US (AUSWUS) reconstruction of Rodinia, Karlstrom et al. (1999) (their Figure 2B), Karlstrom et al. (2001) (see also Burrett and Berry, 2000) propose that the Albany–Fraser orogen may be the western extension of the Grenville orogen in North America, in direct conflict with the Rodinia reconstructions of Hoffman (1991), Dalziel (1992), Li et al. (1995), and many others. Clearly, a detailed understanding the deformational history of the Albany– Fraser orogen, as well as that of potentially correlative terranes, is central to resolving some of these controversies regarding timing and nature of amalgamation of the Rodinian supercontinent. This paper reports new structural data from previously unstudied localities within the Albany– Fraser orogen west of Albany, Western Australia (Fig. 1). The goal of the study is to compare the structural geometry, kinematics, and sequence of deformational events to well-studied localities farther east in the orogen (Pallinup River transect, Beeson et al., 1988; Ledge and Herald Points east of Albany, Holden, 1994; Bremer Bay, Harris, 1995; Esperance and Mt. Ragged regions, Myers,

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1995b; Clark et al., 2000). This study, coupled with synthesis of published data, demonstrates that at least three Mesoproterozoic deformational events may be correlated along the entire strike length of the Albany–Fraser orogen. Significant questions remain, however, regarding the absolute timing of the events in the western part of the orogen relative to those in the eastern part (discussed below). In addition, this study supports ties between the Albany–Fraser orogen and the Bunger Hills area by proposing correlations between specific deformational events in the two areas, although exact timing is yet to be constrained. Finally, the results of this study conflict with Rodinian models that place the Oaxacan Complex of southern Mexico between the Grenville of Eastern North America and the Albany–Fraser orogen of Australia.

2. Geologic setting The Albany–Fraser orogen has been divided into two orogen-parallel complexes based on aeromagnetic data and distinctive structural and lithological characteristics (Fig. 1; Myers, 1993). The inboard Biranup Complex is dominated by heterogeneous late Archean and Paleoproterozoic orthogneisses that have been tectonically interleaved by thrusting (Myers, 1993). The outboard Nornalup Complex consists of both ortho- and paragneisses that are intruded by voluminous felsic granitoids. West of Bremer Bay, these complexes coincide, respectively, with the Central and Southern domains of the Western Albany– Fraser orogen defined by Beeson et al. (1988) (compare Figure 1 in Beeson et al., 1988, with Figure 8 of Myers, 1993). To facilitate discussion in this paper, the term ‘Western Albany– Fraser orogen’ refers to the area west of Bremer Bay, and ‘Eastern Albany– Fraser orogen’ refers to the area east of that locality. The timing of deformational events in the Albany–Fraser orogen is controversial, largely due to inadequate spatial coverage of modern radiometric age data, especially the western part. Based on structural and geochronological data, Black et al. (1992) proposed that polyphase deformation

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and plutonism affected the Western Albany– Fraser orogen between about 1190 and 1170 Ma. These authors found no evidence for pre-1200 Ma deformation in the Western Albany– Fraser orogen, although Pidgeon (1990) obtained a U– Pb zircon date of 1289910 Ma from an enderbitic orthogneiss near Albany. Most of the geochronological data of Black et al. (1992) comes from the Bremer Bay–Pallinup Estuary areas. There is only one published U–Pb zircon date (11899 9 Ma, Mount Franklin Granite, Black et al., 1992) in the 200 km stretch of the orogen west of Albany, which includes the area of this study. In contrast, Clark et al. (2000) recognize two discrete tectonothermal events in the Eastern Albany –Fraser orogen (Esperance– Mt. Ragged areas; Myers, 1995b). Stage I, constrained to ca. 1345–1260 Ma, comprises three deformational episodes that collectively record collision between the Yilgarn and South Australian cratons (Myers et al., 1996; Clark et al., 2000). Stage II, which is bracketed between 1214 and 1140 Ma, comprises two deformational episodes that record intracontinental reactivation of the suture zone by thrusting of the Salisbury Gneiss (Salisbury Island, Fig. 1) over the Nornalup Complex (Clark et al., 2000). Clark (1995), Clark et al. (2000) obtained U– Pb SHRIMP dates of 13049 5 and 116797 Ma on metamorphic zircons from a granulite-facies metasedimentary migmatitic gneiss sampled near the Albany Enderbite. These gneisses were already deformed and metamorphosed prior to intrusion of the Albany Enderbite (J.S. Myers, written communication, 2000). These observations suggest that both orogenic stages are recorded at least as far as west of Albany. Due to the paucity of geochronological data west of Albany, however, it is unclear whether Stage I deformation is recorded in the westernmost part of the Albany– Fraser orogen. Intriguingly, the geometry and kinematics of structures documented in this study are strikingly similar to both the ca. 1190– 1170 Ma structures reported by Black et al. (1992) as well as the Stage I structures described from the Eastern Albany–Fraser orogen (Myers, 1995a,b; Clark et al., 2000). Geochronological data from the Western Albany–Fraser orogen are needed to deter-

mine whether the Western Albany–Fraser orogen experienced both, or only the latter, stages of deformation. The present study focuses on the structure and lithology of previously unstudied coastal exposures within the Nornalup Complex in the Western Albany–Fraser orogen between Long Point and Point Irwin Bay (Figs. 1–3). The mapped areas contain a lithologically heterogeneous succession of granulite-grade metasedimentary and metavolcanic rocks that are intruded by a diverse suite of granitoid rocks at ca. 1190 Ma. At least four deformational events are recognized in the Nornalup Complex of the Western Albany– Fraser orogen (D1 –D4). D1 and D2 record regional ductile deformation at the granulite facies that resulted in the formation of penetrative fabrics. D3 is manifested by nonpenetrative, ductile and brittle fabrics that may reflect a late phase of D2. D4 is characterized by a regional set of conjugate(?) fractures.

3. Lithology

3.1. Metasedimentary and meta6olcanic rocks (unit psg, Figs. 2 and 3) The oldest rocks in the western part of the Albany–Fraser orogen are granulite-grade schist, paragneiss, and minor orthoquartzite that are interpreted as metamorphosed siliciclastic sedimentary and volcanic rocks. These rocks are best exposed at Long Point (Fig. 2), Conspicuous Cliffs, and Point Irwin (Fig. 3). The metasedimentary rocks consist dominantly of interlayered, migmatitic sillimanite–garnet–biotite–potassium feldspar gneiss, biotite–quartz schist, and garnet 9 biotite-bearing quartzite. These rocks are interpreted as an original sequence of interbedded shale, siltstone, and feldspathic sandstone and may, in part, represent turbidite sequences. Pelitic schists and gneisses are locally migmatitic with well-developed restitic melanosomes and anatectic leucosomes. Leucosomes define an early S1 foliation that is folded by F2 folds and transposed into the regionally dominant east-northeast-striking S2 foliation, indicat-

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Fig. 2. Simplified geologic map of the Long Point area, Albany – Fraser orogen, Western Australia. See Fig. 1 for location of Long Point.

ing that partial melting either accompanied or preceded development of the S1 foliation (see Section 4 below). Some garnets exhibit inclusionrich cores and inclusion free rims that could reflect more than one period (D1 and D2?) of garnet growth. Sillimanite occurs as both prismatic grains and as fibrolite masses that locally define intrafolial (F1?) folds. The metasedimentary rocks are intercalated with amphibole schists that typically contain the assemblage cummingtonite– hornblende – plagioclase–biotite– quartz9 pyroxene 9garnet. These rocks probably represent volcanic flows and volcaniclastic rocks of intermediate composition.

3.2. Biotite orthogneiss (unit bog, Fig. 2) Biotite quartzofeldspathic orthogneiss is present only near Long Point. It is dark gray, medium to coarse grained, and contains up to 20% biotite. The gneiss is cut by numerous centimeter- to

decimeter-scale leucocratic sills that do not cut the augen gneiss (see below), thus establishing the age relations between the two units. The contact between the biotite orthogneiss and the augen gneiss is a mixed zone at least 10–20 m wide, which could be due to isoclinal folding, diking, or both. The biotite orthogneiss locally contains a subhorizontal fabric (S1) that is folded by F2 folds and transposed into the subvertical, northeast-striking S2 foliation.

3.3. Augen gneiss (unit agn, Figs. 2 and 3) Augen gneiss is most abundant at Long Point but limited exposures are also present near Peaceful Bay (2 km north of Point Irwin). In weakly foliated augen gneiss, feldspar megacrysts (2–4 cm) are euhedral suggesting an igneous origin for the gneiss, an interpretation supported by the presence of pelitic schist xenoliths within the augen gneiss. With increasing deformation, feldspars

Fig. 3. Simplified geologic map of the Point Irwin –Peaceful Bay area, Albany – Fraser orogen, Western Australia. See Fig. 1 for location of Point Irwin area.

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become flattened and the rock grades into a streaked or layered gneiss. Biotite constitutes 10– 15% of the rock and xenocrystic(?) garnet is present locally. The augen gneiss exhibits two penetrative fabrics, a subhorizontal to gently dipping foliation (S1) overprinted by a northeaststriking, steeply dipping S2 fabric (Fig. 4), as well as later nonpenetrative, ductile dextral and sinistral shear bands. The age of the augen gneiss is not known, but the presence of three ductile fabrics suggests that it is probably older than the ‘late tectonic’ (Black et al., 1992), 11899 9 Ma Mount Franklin Granite, the only rock within the Albany –Fraser orogen west of Albany that has been dated by the U– Pb system.

3.4. Leucocratic granite gneiss (unit lgr, Fig. 3) Gray, variably foliated, fine- to mediumgrained, locally biotite-bearing, leucogranite gneiss crops out mainly in the Point Irwin area. The gneiss contains a wispy foliation, lacks compositional layering, contains both mafic and metasedimentary xenoliths, and locally is injected

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as sills into paragneisses. These observations indicate that the unit is an orthogneiss. Near Peaceful Bay, the leucocratic granite gneiss grades into an unfoliated, biotite granite (unit bg, Fig. 3) that may represent a different facies of the intrusion.

3.5. Intermediate to mafic plutonic rocks (unit di, Fig. 3) At least two thin bodies (B100 m across) of dark gray, medium- to coarse-grained, weakly foliated biotite–hornblende diorite are present approximately 1–2 km south of Peaceful Bay. This unit locally contains biotite-bearing, two pyroxene gabbroic rocks. The relation between the two units is not clear, however, contacts between them are generally gradational. The diorite exhibits a weak, but definite, S1 fabric that is folded by F2 folds. Where in contact with augen gneiss, the diorite forms boudins with necks occupied by the augen gneiss. The diorite contains abundant felsic stringers that are not present in the augen gneiss suggesting that the diorite is older.

Fig. 4. View to southwest at outcrop of augen gneiss of Long Point showing transposition of subhorizontal S1 fabric (left of hammer) into steeply dipping, northeast-striking S2 fabric (right of hammer). View is up the plunge of open F2 folds (e.g. directly left of hammer) on outcrop that slopes gently toward the viewer.

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3.6. White schist (unit wh, Fig. 2) At Little Long Point and in a 10– 15 m wide exposure east of Long Point is a muscovite– quartz schist informally referred to as white schist (Fig. 2). At Little Long Point, it occurs as a 50 m-wide band within augen gneiss. East of Long Point, it occurs within biotite orthogneiss. The schist ranges from nonfoliated to foliated in outcrop and locally contains prismatic, pale green liddicoatite (tourmaline group, Ca(Li,Mg)3Al6 B3Si6(O,OH)30(OH,F)), identified by XRD. The white schist contains the assemblage quartz+ sericite+muscovite9 tourmaline 9 clay minerals. XRD analysis also revealed the presence of minor amounts of andalusite, margarite, clinochlore, and natrolite. Contacts with the adjacent augen gneiss and biotite gneiss are commonly gradational suggesting that the white schist may be highly metasomatized orthogneiss.

3.7. Megacrystic granite Exposures of megacrystic granite are present at Green’s Pool and in a restricted area between Little Long Point and Long Point. At both localities, the megacrystic granite contains euhedral and tiled potassium feldspar megacrysts (5– 10 cm long). Feldspars are mesoscopically undeformed and there is no macroscopic evidence for solidstate deformation suggesting magmatic flow. The megacrystic granite has larger feldspars, a greater abundance of feldspars, and less biotite and quartz than the augen gneiss. In addition, the feldspar megacrysts are aligned parallel to the regional S2 foliation suggesting (late?) syn-D2 intrusion of the granite. These observations suggest that the megacrystic granite postdates, and is therefore a different body than, the augen gneiss, which contains the S1 fabric.

4. Structure

4.1. Deformation 1 Structures associated with the earliest deformational event are preserved in small domains that

have escaped complete transposition by the regionally dominant, east-northeast-striking, subvertical S2 foliation. S1 generally strikes northwest and ranges in dip from subhorizontal (Long Point), to gently northeast (Conspicuous Cliffs), to steeply southwest (Point Irwin; Fig. 5A). S1 is defined by compositional, and locally migmatitic, layering in pelitic rocks and by the development of flattened potassium feldspar megacrysts in the augen gneiss. No lineation was observed associated with S1 foliation. Rootless and refolded isoclinal F1 folds (Fig. 6) are preserved locally in the limbs of F2 folds. Type 3 and transitional Type 2–3-fold interference patterns dominate indicating nearly coaxial refolding of F1 by F2 (Ramsay and Huber, 1987).

4.2. Deformation 2 The dominant fabric in the study area is a penetrative, northeast-striking, subvertical compositional layering (S1/S2) that is axial planar to upright F2 folds (Fig. 5B, Figs. 7 and 8). Mineral elongation lineations associated with the S2 foliation were observed in only a few localities where they plunge moderately (40–55°) northeast. F2 folds are tight to isoclinal, show no consistent asymmetry, plunge moderately to steeply northeast and southwest, and may be markedly noncylindrical within a single outcrop (Fig. 5C). Where F2 folds are more tightly appressed, they plunge more steeply. This observation suggests that early formed F2 folds were rotated progressively into the finite elongation direction (subvertical). The presence of granitic material parallel to the axial surfaces of F2 folds suggests that melt was present during D2 deformation. Boudinaged dikes indicate stretching in two directions, subvertical and subhorizontal in a northeast–southwest plane (‘pseudo-chocolate tablet boudinage’). In general, stretching in the subvertical direction is slightly greater than that in the subhorizontal direction. Locally, the S2 foliation becomes more strongly developed, is accompanied by grain-size reduction, and localizes syn-D2 pegmatites. These high-

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Fig. 5. Lower-hemisphere equal-area projection of D1 –D4 structures. (A) Poles to S1 foliation. Stars, Conspicuous Cliffs area, n = 31; boxes, Point Irwin area, n= 36; dots, Long Point area, n=9. (B) Lower-hemisphere equal-area projection of poles to S2 foliation. Boxes, Point Irwin area, n= 42; plus signs, Little Long Point, n = 58; dots, Long Point, n = 55. Due to strong disharmonic folding and chaotic foliation orientations, S2 foliations were not measured in the Conspicuous Cliffs area. Mean shortening direction is approximately subhorizontal and 333°. (C) Lower-hemisphere equal-area projection of F2 fold hinge-line orientations from a single outcrop in the Conspicuous Cliffs area. No northwest-trending cross folds were observed that could have reoriented originally subhorizontal F2 fold hinge lines. This scatter in fold hinge-line orientations may reflect the marked noncylindricity of folds formed in a single deformational event (D2); n= 16. (D) Lower-hemisphere equal-area projection of poles to S3 shear bands. All data are from the Long Point – Little Long Point area. Boxes, dextral shear bands, n = 73; plus signs, sinistral shear bands, n =14. Mean shortening direction is approximately horizontal and 325°. Arrows show relative strike-slip movement on average shear-band orientation. (E) Lower-hemisphere equal-area projection of poles to joints. All data are from the Long Point – Little Long Point area. Mean joint orientations are 354°87%E and 285°74%S. Mean shortening direction (if these joints are interpreted as a conjugate set) is approximately 320°.

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Fig. 6. View of horizontal outcrop near Point Irwin showing F1 isoclinal fold (at hammer point) refolded by tight F2 fold. Arrow points to F2 fold hinge.

strain zones do not separate rocks of appreciably different lithology or structural character indicating that, although they have accommodated significant local strain, the shear zones do not represent regional-scale features. Due to the paucity of lineations, definitive determination of kinematics was not possible; however, southeastside-up separation is more common that northwest-side-up separation. The presence of recrystallized quartz ribbons and dynamically recrystallized feldspar indicate that deformation occurred at temperatures \ 450 – 500 °C (e.g. Tullis and Yund, 1987). Quartz locally shows a ‘striped gneiss’ fabric (Fig. 9) indicative of very high temperature deformation (Passchier and Trouw, 1996).

typically oriented at about 105° (Fig. 5D, Fig. 10). A subordinate set of shear bands, typically oriented near 010°, exhibits sinistral displacement. Individual shear bands range in width from 1 to 30 cm and can be traced for meters to tens of meters along strike. No lineation is evident on shear band surfaces, but three-dimensional expo-

4.3. Deformation 3 Structures associated with a third phase of deformation are variable in structural style and are best developed in the Long Point– Little Long Point areas. The most common D3 structures are nonpenetrative, subvertical, mylonitic to cataclastic, locally melt-filled, dextral shear bands that are

Fig. 7. Block diagram showing relation between S1 and S2 fabrics. Subhorizontal S1 foliation (light solid lines) is folded by upright F2 folds and ultimately transposed into parallelism with the northeast-striking, subvertical S2 foliation (heavy dashed lines).

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nent of dextral shear. In this framework, the 105° dextral shear bands may represent Riedel shears, with the sinistral zones representing antithetic shears. The range in fault rocks, from mylonitic to cataclastic, suggests that D3 deformation spanned a range of thermal conditions, although other factors such as the presence of H2O and strain rate may have played a role in determining the dominant deformation mechanism.

4.4. Deformation 4 Structures associated with a fourth deformational event include two distinct joint sets that are present throughout the study area but are best developed at Long Point. Mean orientations for the joint sets are 354°87%E and 105°74%S (Fig. 5E). Joints range in width from less than 1 to more than 20 mm and some are filled with quartz, chalcedony, or hematite. Fractures only rarely exhibit evidence for shear or cataclasis and therefore appear to be Mode I features (tensile fractures). There are no consistent crosscutting relations between the two fracture sets, suggesting that they formed contemporaneously. Despite the lack of evidence for shear, the joint sets have the geometry of a conjugate set. The horizontal bisector of the two joint sets is oriented at about 320°. Fig. 8. Oblique view of outcrop east of Long Point showing S1 compositional layering, defined by white leucosomes, folded by upright F2 folds, and incipiently transposed into the regionally dominant, northeast-striking subvertical S2 foliation orientation (parallel to hammer handle and at bottom left side of photograph). View is to south.

sures suggest that these zones underwent nearly horizontal movement. Steeply plunging mesoscopic folds with Z- and S-shaped profiles that deform the S2 foliation are probably related to D3 deformation. D3 structures appear to define a set of conjugate shear bands, based on their orientation, sense of displacement, and similarity in structural style. The bisector of the average orientations for dextral and sinistral shear bands is 325°, suggesting subhorizontal, northwest– southeast shortening. The predominance of dextral over sinistral shear bands, however, is more consistent with a kinematic framework involving a significant compo-

4.5. Discussion of structures in the study areas Due to intense overprinting of D1 structures, the significance of the D1 event is uncertain. The generally subhorizontal orientation of the S1 foliation, flattened feldspars, and the absence of lineations suggest that subvertical coaxial strain dominated during D1. The presence of isoclinal, recumbent (restored orientation), intrafolial folds suggests that S1 formed as a result of thrusting and associated recumbent folding. The sense of vergence of the recumbent folds is not known due to intense overprinting by D2 fabrics. The observation that leucosomes locally define the S1 fabric suggests that deformation occurred at temperatures above the granite solidus (\ 650°C). The subsequent deformational events (D2 –D4) record deformation under a variety of thermal conditions that may reflect the different crustal

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levels at which these events occurred. Intriguingly, structures associated with these events record uniformly subhorizontal, northwest– southeast-oriented shortening with the mean shortening direction for D2 at 333°, for D3 at 325°, and for D4 at 320°. The kinematic compatibility of D2 and D3 structures suggests that they may represent different phases of a protracted deformational event involving northwest contraction with a significant component of dextral displacement (especially during D3). The principal differences between D2 and D3 structures are that D2 structures are exclusively ductile, are more penetrative than D3 structures, and the D2 structures record a dominantly flattening component of strain. The difference in structural style between D2 and D3 may be due to deformation partitioning in a zone of oblique collision as been documented from many obliquely convergent plate boundaries (e.g. Ellis and Watkinson, 1987; Tikoff and Teyssier, 1994; Teyssier et al., 1995). It is possible that initial orogen-normal contractional deformation (i.e. D2 fabrics) may have preceded the dextral component of oblique convergence (i.e. D3 fabrics). In this scenario, crustal thickening during

D2 may have resulted in uplift such that D3 structures were formed at somewhat shallower crustal levels. This interpretation is consistent with the penetrative nature of D2 deformation and the nonpenetrative, ductile-to-brittle, character of D2 structures. Despite the strain compatibility between D4 fracture patterns and the D2 and D3 structures discussed above, the wholly brittle character of D4 structures suggests that this event records a later period of northwest–southeast contraction and northeast–southwest extension at substantially shallower crustal levels, or at least lower temperatures. These fracture patterns could be related to a Cambrian(?) phase of northwest–southeast contraction that may represent a far-field response to stresses associated with the collision between East and West Gondwanaland (e.g. Harris and Li, 1995).

5. Regional correlation of structures Table 1 is a proposed correlation of major structures and deformational events in the West-

Fig. 9. Photomicrograph of recrystallized quartz ribbons in striped gneiss. Mineralogy of gneiss is sillimanite +garnet + microcline+ biotite+quartz with minor secondary sericite. Photomicrograph is 3.5 mm across. Striped gneiss fabric is indicative of high-temperature deformation.

D1

Deformational event (this study)

Ledge Point (Holden, 1994)

Herald Point (Holden, 1994)

Bremer Bay (Harris, 1995)

S1, subhorizontal, composition a1 layering, anatectites folded. S tectonite.

None

S1

L1

None

S1, gently NE dipping compositional layering, anatectites folded. S tectonite.

None

S1, steeply SW dipping compositional layering, anatectites folded. S tectonitc. May be reoriented toward steeper dips due to F2 folding

No correlative structures. (Mineral elongation lineation pitches 20–27°E and SE)

C1a, Cla, composi- compositional tional layering layering sparsely sparsely preserved preserved in hinges in hinges of meso- of scopic mesoscopic (F2) (F2) folds; folds; generally generally transposed transposed into into parallelism parallelism with S2 with S2 None None

No correlative structures (ENE – WSW dextral ductile shear zones)

None

S1, weak, subhorizontal, compositional banding. S tectonite

S1a, foliation, axial planar to F1 isoclinal folds S1b, ductile extensional shear zones

No None correlative reported structures. (L2, 20–25°, 230 [90° pitch])

S 2, subhorizontal to gently SW dipping. L, to LS tectonite. (S1 is enigmatic foliation only locally preserved)

Variably oriented

S 1, layer-parallel migmatitic foliation, slightly precedes subhorizontal Sl/S2which is axial planar to recumbent folds

Esperance and Mt. Ragged area (Myers, 1995a; Clark et al., 2000)

Southern domain (Beeson et al., 1988)

Central domain (Beeson et al., 1988)

Long Point Conspicuous Point Irwin (this study) Cliffs (this study) (this study)

Northern domain (Beeson et al., 1988)

Eastern Albany –Fraser orogen

Western Albany–Fraser orogen

Table 1 Correlation of Mesoproterozoic structures and deformational events in the Albany–Fraser orogen

E.M. Duebendorfer / Precambrian Research 116 (2002) 129–154 141

Deformational event (this study)

Southern domain (Beeson et al., 1988)

Ledge Point (Holden, 1994)

Herald Point (Holden, 1994)

Bremer Bay (Harris, 1995)

None observed

Subvertical shortening, subhorizontal extension, regional shortening.

Interpretation

Subvertical shortening, subhorizontal extension, regional shortening.

Rootless isoclinal folds, recumbent when F2 folding is removed

NE–SW shortening in present orientation. Coaxial deformation. S1 may be reoriented toward steeper dips due to later folding

Rootless isoclinal folds, recumbent when F2 folding is removed

No correlative structures (F1, overturned to NNW; plunge 5–25°E, steeper near thrusts Dextral transcurrent with NW shortening component None reported

None reported

Dextral transcurrent (based on boudin orientations)

None reported

Subvertical shortening NW–SE extension or possible shortening

None reported

Subvertical shortening; NE–SW extension or possible shortening

Northwest thrusting, followed by extension (orogenic collapse), (11909 8 Ma interpreted as D1 = M1 (Black et al., 1992)

F2. Fla, recumbent mesoscopic, folds isoclinal folds, F1b, recumbent, SE verging tight-isoclina l folds

NW–SE shortening; first phase of Stage I deformation of Clark et al. (2000)

Rootless, intrafolial, isoclinal with subhorizontal axial surfaces

Esperance and Mt. Ragged area (Myers, 1995a; Clark et al., 2000)

Central domain (Beeson et al., 1988)

Long Point Conspicuous Point Irwin (this study) Cliffs (this study) (this study)

Northern domain (Beeson et al., 1988)

Eastern Albany –Fraser orogen

Western Albany–Fraser orogen

F1

Table 1 (continued)

142 E.M. Duebendorfer / Precambrian Research 116 (2002) 129–154

D2

Deformational event (this study)

Bremer Bay (Harris, 1995)

S2, ENEstriking, subvertical; S tectonite

L2, very sparse mineral elongation, plunging moderately SW

L2

None observed

None observed

S2, ENEstriking, subvertical to steeply NW dipping; S tectonite

No correlative structures. (L2, , sparse mineral elongation, 25–45°E pitch)

S2a, ENE striking, steep S dipping

L2, None mineral elongation; 25–35°E pitch; only near thrust contact with Northern domain

S2a, ENE striking, steeply S dipping

S2a, ENE striking; axial planar to variably plunging F2 folds; S tectonite. (S2b, LS tectonite, localized at granite contacts) L2, subhorizontal, ENE trending, localized at granite contacts None reported

No penetrative. ENEstriking fabric reported in contrast to all other localities

None reported

S1c, none reported. S1d, NW-striking, normal shear zones

None reported

S3 of Clark et al. (2000). NE striking, steeply SE dipping. Axial planar to regional-scale folds

S2a, ENE striking, steeply S dipping. (EW sinistral shears, dip 70°S)

Herald Point (Holden, 1994)

S2, ENEstriking, subvertical, but variable due to strong disharmonic folding

Ledge Point (Holden, 1994)

Esperance and Mt. Ragged area (Myers, 1995a; Clark et al., 2000)

Southern domain (Beeson et al., 1988)

Northern domain (Beeson et al., 1988)

Long Point Conspicuous Point Irwin (this study) Cliffs (this study) (this study)

Central domain (Beeson et al., 1988)

Eastern Albany –Fraser orogen

Western Albany–Fraser orogen

S2

Table 1 (continued)

E.M. Duebendorfer / Precambrian Research 116 (2002) 129–154 143

Deformational event (this study)

Southern domain (Beeson et al., 1988)

Ledge Point (Holden, 1994)

Tight to isoclinal, plunge moderately to steeply NE and SW; markedly noncylindrical; suggest subvertical stretching direction

Broadly coaxial NNW –SSE shortening

Interpretation

Broadly coaxial NNW–SSE shortening

Tight to isoclinal, plunge moderately to steeply NE and SW. Where significant melt present, folds are strongly disharmonic

Broadly coaxial NNW–SSE shortening

Tight to isoclinal, plunge moderately to steeply NE and SW; markedly noncylindrical

NNW–SSE shortening with significant dextral component

F2, mesomacroscopic (km-scale) folds overturned to NNW

F2, F2, mesomesoscopic, scopic, overturned overturned to NNW; to open to NNW; isoclinal; plunge variably variably plunge ENE – WSW; axes steepen to north near contact with Northern domain Dextral Dextral transcur- transcurrent rent with with NNW NNW–SSE – SSE shortening shortcomponent ening component NW–SE contraction, followed by NE–SE extension (?), 1190–1170 Ma (Black et al., 1992)

F1c, upright, open, NE–SW, trending folds. F1d, open folds

None F2, upright, reported open folds with gently plunging to locally subvertical fold axes

Dextral None shearing; reported NNW shortening

Bremer Bay (Harris, 1995)

Herald Point (Holden, 1994)

NW–SE bulk shortening. Last phase of Stage I deformation of Clark et al. (2000), 1315–1260 Ma (Clark et al., 2000)

F3 of Clark et al. (2000). Upright, km-scale, NE trending, variably plunging. NE, striking, subvertical axial surfaces. Some dextral asymmetry

Esperance and Mt. Ragged area (Myers, 1995a; Clark et al., 2000)

Central domain (Beeson et al., 1988)

Long Point Conspicuous Point Irwin (this study) Cliffs (this study) (this study)

Northern domain (Beeson et al., 1988)

Eastern Albany –Fraser orogen

Western Albany–Fraser orogen

F2

Table 1 (continued)

144 E.M. Duebendorfer / Precambrian Research 116 (2002) 129–154

D3

Deformational event (this study)

F3 folds, steeply plunging; near shear zones

None reported

None reported

None reported

None reported

None reported

None reported

None reported

Isoclinal; fold axes plunge moderately NE in NE-striking, steeply dipping axial surface. NW vergence

Steeply plunging

S4 of Clark et al. (2000). Discrete NE-striking, subvertical to steeply SE-dipping shear zones

None observed

No correlative structures. L3, ENE striking and subhorizontal None reported

Conjugate brittle –ductile shears

F3 foldssteeply plunging; restricted to areas near shear zones

None reported

S3, shear bands. WNW dextral; NNE sinistral; associated with EW dextral shear zones

F3

None observed

S3, shear zones and shear bands; EW to WNW dextral; NNE sinistral

None observed

S2h, E striking, plastic shear dextral bands (interpreted as P shears). C2b, conjugate plastic shear bands. NNE sinistral; WNW dextral None reported

None observed

C2a, plastic shear bands. WNW sinistral; ENE dextral). C2b, conjugate plastic shear bands. NS sinistral; WNW dextral

L3

S3, plastic (locally occupied by melts) to brittle shear zones and shear bands, WNW dextral dominate; NNE sinistral subordinate

(C2a, Brittle-plastic shear bands. E striking sinistral; N striking sinistral; WNW striking dextral). C2b, mainly brittle shear bands. NS sinistral dominate; WNW dextral subordinate None reported

Bremer Bay (Harris, 1995)

S3, plastic None (locally observed occupied by melts) to brittle shear zones and shear bands, WNW dextral dominate; NNE sinistral subordinate

Herald Point (Holden, 1994)

S3

Ledge Point (Holden, 1994)

Esperance and Mt. Ragged area (Myers, 1995a; Clark et al., 2000)

Southern domain (Beeson et al., 1988)

Northern domain (Beeson et al., 1988)

Long Point Conspicuous Point Irwin (this study) Cliffs (this study) (this study)

Central domain (Beeson et al., 1988)

Eastern Albany –Fraser orogen

Western Albany–Fraser orogen

Table 1 (continued)

E.M. Duebendorfer / Precambrian Research 116 (2002) 129–154 145

D4

Deformational event (this study)

Subhorizontal, NW–SE shortening direction associated with regional dextral transpression

Joints/extension fractures, conjugate at NS and WNW. Consistent with NW–SE shortening

S4 fractures only

Interpretation

Southern domain (Beeson et al., 1988)

Ledge Point (Holden, 1994)

Herald Point (Holden, 1994)

Bremer Bay (Harris, 1995)

None

None

None

Joints/extension fractures, conjugate at NS and WNW Consistent with NW–SE shortening

Subhorizontal, NW–SE shortening direction associated with regional dextral transpression

None reported

None reported

Dextral transcurrent motion with NNW shortening component

None reported

Dextral transcurrent motion with NNW shortening component. (minor and localized late NNE shortening) None reported

None reported

None reported

NNW shortening with significant dextral component

Dextral transcurrent shearing

NNW None shortening reported

Extension None fractures; reported dominant set at 316°

Dextral transcurrent shearing

None reported

None reported

NW–SE shortening. Minimum age: 1182 9 12 Ma (Black et al., 1992)

NW–SE shortening. Early phase of Stage II deformation of Clark et al. (2000)

Esperance and Mt. Ragged area (Myers, 1995a; Clark et al., 2000)

Central domain (Beeson et al., 1988)

Long Point Conspicuous Point Irwin (this study) Cliffs (this study) (this study)

Northern domain (Beeson et al., 1988)

Eastern Albany –Fraser orogen

Western Albany–Fraser orogen

Interpretation

Table 1 (continued)

146 E.M. Duebendorfer / Precambrian Research 116 (2002) 129–154

E.M. Duebendorfer / Precambrian Research 116 (2002) 129–154

147

Fig. 10. Horizontal outcrop surface showing dextral shear band (upper right to lower left) in augen gneiss of Long Point. Long dimension of scale card, slightly left-of-center, is about 9 cm.

ern Albany–Fraser orogen. Structures reported in Beeson et al. (1988), Holden (1994), Harris (1995), Myers (1995b), Clark et al. (2000) are placed into the deformational framework established in this study (i.e. D1 –D4 terminology). Unfortunately, absolute age constraints for each deformational event in each locality are not available. The correlation is based on the similarity in structural style, kinematics, conditions of deformation, and relative chronology of events in each area. It is possible that similar structures, and even similar structural chronologies, from different areas could be diachronous.

5.1. Western Albany–Fraser orogen Structures associated with D1 are preserved in eight of the nine areas in the Western Albany– Fraser orogen. D1 fabrics include subhorizontal foliations and recumbent folds consistent with NW –SE shortening (NW-directed thrusting where kinematics are evident). Beeson et al. (1988) interpret D1 fabrics along the Pallinup River to reflect dextral transcurrent motion with a significant component of NW– SE shortening. Holden (1994) documented a subhorizontal foliation in

his Granite I at Herald Point and attributed this fabric to subvertical, coaxial deformation, possibly related to recumbent folding. At Bremer Bay, Harris (1995) attributes D1 deformation to northwest–southeast contraction. This event appears to have been accompanied by peak metamorphic conditions at 11909 8 Ma (Black et al., 1992) in the Bremer Bay–Pallinup Estuary areas; however, no constraints on the age of this event are available for regions farther west in the Albany– Fraser orogen. Fabrics associated with the second deformational event (D2) are similar in seven of the nine areas. S2 foliation is universally northeast striking and subvertical. D2 was characterized by dominantly flattening strains with some areas exhibiting evidence for a dextral shear strain component (four of the nine areas). Folds in all areas plunge variably northeast or southwest. In five of the study areas, folds are highly noncylindrical with fold axes steepening in high strain zones. Folds verge uniformly north-northwest where determinations are possible. D2 structural data from all areas are best explained by dextral, transpressional tectonism. Differences in structural style (e.g. presence or absence of lineations) and strain

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history (i.e. coaxial vs. noncoaxial) between areas may be due to partitioning of oblique-convergent strain during D2. In the context of the age data of Black et al. (1992), this event would be part of a continuum of events bracketed between ca. 1190 and 1170 Ma. There is a striking similarity of D3 structures throughout the studied length of the Western Albany–Fraser orogen. The dominant structures include mylonitic to cataclastic conjugate shear zones and shear bands. East- to west-northweststriking dextral and north- to north-northeaststriking sinistral shear bands were documented in seven of the nine areas studied (Table 1). The orientation and kinematics of the conjugate shear bands are consistent with north-northwest shortening associated with regional dextral transpression (Beeson et al., 1988; Holden, 1994; Harris, 1995; this study). The minimum age for this deformation near Albany is 1182912 Ma based on a U –Pb zircon date on a posttectonic aplite dike from the Porongroup Range that crosscuts these structures (Black et al., 1992). It is important to recognize that the only published U–Pb zircon date from the Albany– Fraser orogen west of Albany is the 11899 9 Ma date on the Mount Franklin Granite, which is described as ‘late tectonic’ by Black et al. (1992). The preservation of three ductile fabrics in the granitoids from the Long Point– Point Irwin areas, suggests that granitoids in these areas, as well as the schists and the gneisses they intrude, may have experienced multiple phases of deformation prior to ca. 1190 Ma.

5.2. Eastern Albany– Fraser orogen The three deformational episodes that correspond to Stage I of Clark et al. (2000) from the Eastern Albany–Fraser orogen bear striking similarities to the first three deformational episodes reported in this study (Table 1). For example in the Esperance and Mt. Ragged areas, Myers (1995b), Clark et al. (2000) recognized two related deformational events (their D1 and D2) that resulted in the development of a subhorizontal S1/S2 foliation and associated recumbent folds that were interpreted to have formed during collision be-

tween the Yilgarn and South Australian cratons between 1345 and 1260 Ma. These structures are geometrically and kinematically similar to the D1 structures reported in this paper. Fabrics associated with D3 of Clark et al. (2000) include a northeast-striking, steeply southeast-dipping foliation that is axial planar to upright, variably plunging folds. This event, which is attributed to northwest–southeast shortening by Clark et al. (2000), is similar in geometry and kinematics to D2 structures documented in this study (Table 1). In the Eastern Albany–Fraser orogen, these structures (D3 of Clark et al., 2000 and D2, present paper) appear to have formed between ca. 1315 and 1260 Ma (Clark et al., 2000) whereas in the Western Albany–Fraser orogen they have been attributed to regional deformation between 1190 and 1170 Ma (Black et al., 1992). If the latter interpretation is correct, events D1 –D3 (terminology of this paper) may correlate with the intracratonic stage of deformation (Stage II) recognized in the Albany–Fraser orogen (Clark et al., 2000). Clark et al. (2000) report a D4 event in the Eastern Albany–Fraser orogen involving northwest thrusting and dextral strike slip. Structures associated with D4 of Clark et al. (2000) formed at greenschist to amphibolite facies conditions. This event may speculatively correlate with D3 structures reported herein for the Western Albany–Fraser orogen, which also record northwest shortening with a component of dextral shear. The similarity of geometry and kinematics of structures in both the western and eastern parts of the Albany–Fraser orogen, coupled with the apparent discrepancies in timing, underscores the need for more detailed U–Pb geochronology, particularly in the area west of Albany.

5.3. Bunger Hills, Antarctica Lithologic and structural similarities between the Albany– Fraser orogen and the Bunger Hills, Antarctica, have been noted by several previous workers (e.g. Sheraton et al., 1995, and references therein) and has led to the juxtaposition of these two regions in many Gondwana/Rodinia reconstructions (e.g. Oliver et al., 1983; Sheraton et al., 1992; Nelson et al., 1995). Sheraton et al. (1995)

E.M. Duebendorfer / Precambrian Research 116 (2002) 129–154

presented a preliminary correlation of events between the Bunger Hills area and the Albany– Fraser orogen, however, these authors correlated only D1 structures in the two areas. In this section, we present a preliminary correlation that includes all major deformational events in the two areas.

5.3.1. Lithology The Bunger Hills consist of a layered gneiss series and a massive gneiss series that are intruded by two large charnockite bodies (Stu¨ we and Wilson, 1990; Sheraton et al., 1995). The layered gneiss is composed of migmatized, highly aluminous metapelites, psammitic gneisses, and intermediate and mafic orthogneisses. Typical mineral assemblages in the metapelites are quartz– plagioclase –K feldspar– garnet 9sillimanite 9cordierite 9orthopyroxene. Psammites include garnet quartzites and garnet–biotite rocks (Stu¨ we and Powell, 1989; Stu¨ we and Wilson, 1990). These lithologies are similar to those documented in the Western Albany–Fraser orogen (Holden, 1994; this study). Peak metamorphism at T = 750 – 800 °C and P= 5–6 kbar (Sheraton et al., 1995) is similar to that documented from the Albany– Fraser orogen near Albany (T = 700 – 750 °C and P = 3.8–5.8 kbar, Holden, 1994) that occurred at 1190915 Ma (Sheraton et al., 1992). The massive orthogneiss unit includes garnetbearing granite gneisses and two-pyroxene charnockitic, intermediate, and mafic gneisses. Garnet-bearing granite gneisses and intermediateto-mafic gneisses are also abundant in the Western Albany–Fraser orogen (Holden, 1994; this study). Therefore, with the exception of large ‘charnockite’ bodies, which appear to be absent in the Western Albany– Fraser orogen, the principal lithologies in the Bunger Hills are similar in composition and metamorphic grade to those documented in the Western Albany– Fraser orogen. 5.3.2. Deformation and metamorphism Stu¨ we and Wilson (1990) recognized three major ductile deformation phases in the Bunger Hills that are herein correlated to structures in the Albany–Fraser orogen (Table 2). Structures associated with the first deformational event (D1) in-

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clude a subhorizontal compositional banding defined in part by anatectites (S1), intrafolial, recumbent folds, and a local and variably oriented mineral alignment (L1). According to Sheraton et al. (1992), D1 was accompanied by granulite-facies metamorphism and partial melting at 11909 15 Ma. Nelson et al. (1995), however, raise the possibility that this age may be too young. Ding and James (1991), Sheraton et al. (1995), Wilson (1997) interpret D1 as recording regional contraction. This interpretation is consistent with the interpretation for D1 structures in the Western Albany–Fraser orogen. D2 fabrics in the Bunger Hills include a southeast-dipping S2 foliation, and tight to isoclinal folds that verge generally northward (Stu¨ we and Wilson, 1990; Sheraton et al., 1995). Stu¨ we and Wilson (1990) interpret D2 as representing north– south crustal shortening with a component of dextral transcurrent deformation. This interpretation is consistent with kinematics of D2 deformation documented from all studied localities in the Western Albany–Fraser orogen (Table 2). In the Bunger Hills, D2 is bracketed between 1190915 and 11709 4 Ma (Sheraton et al., 1992; but see above reference to Nelson et al., 1995); in the Albany area, this event appears to be bracketed between 11909 8 and 11849 12 Ma (Black et al., 1992). D3 in the Bunger Hills involved regional doming and folding about dominantly east-trending axes at ca. 1170 Ma (Stu¨ we and Wilson, 1990; Sheraton et al., 1995). Sheraton et al. (1995) attribute this event to northwest–southeast shortening. Regional structural doming has not been documented from the Albany–Fraser orogen, but the shortening direction inferred for the Bunger Hills during this event is compatible with the shortening directions for all deformational events in the Albany–Fraser orogen. D3 fabrics of Stu¨ we and Wilson (1990), Sheraton et al. (1995) may either have not been recognized in the Albany– Fraser orogen, may not be present, or may be in part correlative with conjugate D3 (terminology of this paper) structures in the Western Albany– Fraser orogen. In the latter case, the differences in structural style of D3 could reflect their different positions within the origin, with the Albany–

Conjugate shear zones and shear bands; plastic to brittle; EW to WNW dextral; NNE sinistral

Uncommon steeply plunging folds near shear zones No regional lineation observed Dextral transpression with NNW shortening component. Deformation over by 1184 912 Ma (Black et al., 1992)

S3

F3 L3 Interpretation

Interpretation

L2

F2

S2

Variable due to presence of charnockite bodies. Dip moderately to steeply SW Tight to isoclinal folds, NE–SW plunge, that verge north to north or northeast (Stu¨ we and Wilson, 1990). F3 of Ding and James (1991)-upright, regional ENE–WSW trending antiforms Weak mineral elongation lineation; subvertical to moderately west plunging North–south shortening with a dextral component (Stu¨ we and Wilson, 1990); NNW–SSE shortening between 1190 915 and 1170 9 4 Ma (Sheraton et al., 1992; Wilson, 1997). NNW–SSE to NW–SE shortening Ding and James (1991) D4 dextral shear bands and shear zones, WNW- and NNW-striking, plastic to brittle, dextral shear zones; not conjugates (Stu¨ we and Wilson, 1990). WNW dextral shear zones and NNW shears with uncertain kinematics; possible conjugates (Wilson, 1997) None reported None reported Bulk NW–SE shortening (Wilson, 1997). Pre-1140 Ma (Wilson, 1997, using age data from Sheraton et al., 1990; Sheraton et al., 1995). ca. 1170 Ma (Stu¨ we and Wilson, 1990; Sheraton et al., 1995)

Subhorizontal compositional layering, partial melts. Flattening fabric Rootless isoclinal folds, some recumbent in areas of little D2 overprint Uncommon, variably oriented mineral alignment Regional extension (Stu¨ we and Wilson, 1990) or NW–SE (Sheraton et al., 1995; Wilson, 1997) shortening, coaxial deformation. Coincident with peak granulite-facies metamorphism at 1190 9 15 Ma (Sheraton et al., 1992)

Bunger Hills (Stu¨ we and Wilson, 1990; Ding and James, 1991; Sheraton et al., 1995; Wilson, 1997)

Albany–Fraser orogen notation is used for deformational events and fabric elements. Note that there are no structures in the Albany–Fraser orogen that correspond to D3 dome and basin structures in the Bunger Hills. These structures have been interpreted to represent NNW–SSE shortening (Sheraton et al., 1995) and by Wilson (1997) to be associated with intrusion of two large charnockite bodies.

D3

D2

L1 Interpretation

F1

Subhorizontal compositional layering, partial melts. Flattening fabric Rootless isoclinal folds, recumbent when F2 folding is removed Rare mineral elongation lineations Subvertical or NW–SE shortening, subhorizontal extension, coaxial deformation. Regional extension or NW–SE shortening. Coincident with peak granulite-facies metamorphism at 1190 9 8 Ma for Western Albany–Fraser orogen (Black et al., 1992), 1345–1260 Ma for Eastern Albany–Fraser orogen (Clark et al., 2000) ENE-striking, subvertical to steeply south dipping. S to LS tectonite Tight to isoclinal, plunge gently to steeply NE and SW; markedly noncylindrical in places; suggest subvertical stretching direction. In Eastern Albany-Fraser, folds verge toward NNW Sparsely distributed mineral elongation lineation; moderately to steeply east and southeast plunging Dextral transcurrent with NNW–SSE shortening component. ca. 1190–1184 Ma (Black et al., 1992). ca. 1214 and 1140 (Clark et al., 2000)

D1

S1

Albany–Fraser orogen (Beeson et al., 1988; Holden, 1994; Harris, 1995; Myers, 1995b; Clark et al., 2000 (this study)

Deformational events and fabric elements

Table 2 Comparison of structure and suggested correlation of deformational events between Albany–Fraser orogen and Bunger Hills

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E.M. Duebendorfer / Precambrian Research 116 (2002) 129–154

Fraser orogen representing the more foreland part of the orogenic belt. Stu¨ we and Wilson (1990) documented two sets of late semi-brittle and brittle shear zones (their D4) that may correlate to D3 shear zones in the Western Albany–Fraser orogen. These strike 110 and 160°, dip steeply, and show dextral displacement. As in the Western Albany– Fraser orogen (D3 fabrics), the 110° striking set dominates. The late shear fabrics in both regions formed prior to 1140 Ma (Black et al., 1992; Sheraton et al., 1995). Based on similarities in structural style, metamorphic grade, and kinematics it appears that all major penetrative structures are correlative between the Bunger Hills area and the Western Albany–Fraser orogen.

6. Comparison with the Oaxacan Complex (AUSWUS reconstruction) In their the AUSWUS reconstruction of Rodinia, Karlstrom et al. (1999) (their Figure 2B); Karlstrom et al. (2001) (their Figure 1C) show a

151

continuous Grenville-age orogen extending from the Albany–Fraser–Musgrave province/belt in the east, through the displaced Oaxacan Complex of Southern Mexico, and connecting to the Grenville belt of Eastern North America. Table 3 summarizes deformational, metamorphic, and selected age data for the two terranes and shows that there is little correlation between the Albany–Fraser– Musgrave province and the Oaxacan Complex. Although lithologic, structural, age, and isotopic characteristics can vary along strike of a continental-scale orogen, the markedly dissimilar geologic histories recorded in the two blocks seem to preclude any simple correlation between the two terranes (Table 3). Furthermore, based on geologic evidence, Ortega-Gutierrez and Keppie (1997) suggest that the closest correlatives of the Oaxacan basement lie in the Adirondack Highlands and the basement massifs of Columbia and Peru, regions that are commonly juxtaposed in Rodinia reconstructions. This interpretation is consistent with paleomagnetic evidence (Van der Voo and Urrutia-Fucugauchi, 1987; Ballard et al., 1989), that places the Oaxacan Complex adjacent to Quebec and On-

Table 3 Comparison of events and age data between the Albany Mobile belt and the Oaxacan Complex, southern Mexico Event/age data

Albany–Fraser orogen

Oaxacan Complex

Kinematics of deformation

NW-vergent folds and thrust faults. Component of dextral shear (e.g. Beeson et al., 1988; many others cited in text) 1190 98 to 1184 9 12 Ma for Western Albany–Fraser orogen (Black et al., 1992), 1345–1260 Ma for Eastern Albany–Fraser orogen (Clark et al., 2000) Granulite facies, T= 700–800 °C, P= 4–6 kbar (Holden, 1994) Granulite facies (ca. 1190 Ma); uplift to 10 km (greenschist facies) by 1100 Ma (Black et al., 1992) ca. 300 °C at 1100 Ma (Black et al., 1992; Nelson et al., 1995) 2.25–3.05 Ga (Black et al., 1992); 3.30 Ga (Fletcher et al., 1983) 2917–3110 Ma (zircon cores; Black et al., 1992). Oldest crystallization age Western Albany–Fraser orogen= 12899 10 Ma (Pidgeon, 1990). Oldest crystallization age Eastern Albany–Fraser orogen = 1330914 Ma (Clark et al., 2000)

SE-vergent recumbent folding (Solari et al., 1998)

Timing of deformation

Metamorphism Timing of metamorphism Cooling history Model TDM ages Oldest U–Pb zircon date

ca. 1125–976 Ma (Solari et al., 1998)

Granulite facies, T = 730 950 °C, P =7 9l kbar (Mora and Valley, 1985) Migmatization at ca. 1100 Ma (Solari et al., 1999); granulite facies at ca. 980 Ma (Solari et al., 1998) Below 600 °C at 940 Ma; below 300 °C at ca. 800 Ma (Patchett and Ruiz, 1987) 1.6–1.35 Ga (Patchett and Ruiz, 1987; Ruiz et al., 1988) ca. 1185 Ma (Solari et al., 1998). No Archean zircons documented

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tario at 950 Ma. Finally, although they favor the AUSWUS reconstruction, Burrett and Berry (2000) reject the placement of Oaxacan Complex between Laurentia and Australia on the basis of lack of any evidence for the Grenville orogeny in Tasmania. Although the comparison between the Albany– Fraser orogen and the Oaxacan Complex presented here does not invalidate the AUSWUS reconstruction for Rodinia, it does suggest that the Oaxacan Complex did not occupy a position intermediate between Australia and Laurentia in Rodinia.

7. Conclusions

1. New structural data from the Long Point– Pt. Irwin area allow structural correlations and relative structural chronology to be proposed for the length of the Albany– Fraser orogen. 2. The earliest deformational event (D1), was synchronous with peak granulite-facies metamorphism and is characterized by subhorizontal foliation and recumbent folds, probably recording northwest– southeast shortening. D2 resulted in the formation of generally upright, northwest-vergent folds and a subvertical northeast-striking foliation. This event involved deformation in a dextral transpressive tectonic environment. D3 is manifested by both plastic and brittle conjugate west-northwest-striking dextral and north-northeaststriking sinistral shear zones. The predominance of dextral over sinistral shear is most consistent with formation in an overall tectonic regime of dextral transpression. 3. D1 –D3 structures in the Long Point–Pt. Irwin area have counterparts in the Albany– Pallinup Estuary area (Western Albany– Fraser orogen) that appear to be constrained to ca. 1190–1170 Ma (Black et al., 1992). Structures in the Long Point–Pt. Irwin area are geometrically and kinematically similar to structures documented from the Esperance and Mt. Ragged areas (Eastern Albany– Fraser orogen), which correspond to Stage I deformation (1345– 1260 Ma) of Clark et al.

(2000). This apparent inconsistency underscores the need for more geochronological data from more widely dispersed areas within the Albany–Fraser orogen, particularly its western parts. 4. There is near one-to-one correlation in structural style, metamorphic grade, kinematics, and possibly timing of deformational events between the Albany– Fraser orogen and the Bunger Hills, East Antarctica. 5. Rodinia reconstructions that place the Oaxacan Complex of Southern Mexico between the Grenville orogen of Laurentia and the Albany–Fraser orogen of Western Australia are rejected on the basis of marked dissimilarities between model TDM ages, U–Pb zircon dates, the kinematics and timing of deformation, and timing of metamorphism. Acknowledgements I thank Lyal B. Harris of the Tectonics Special Research Center, Department of Geology and Geophysics, University of Western Australia for introducing me to the Albany–Fraser orogen and for valuable discussions regarding its history. I thank Chris and Rosemary Powell, also of the Tectonics Special Research Center, for their hospitality during my sabbatical visit to the University of Western Australia. Special thanks to Carl Beck and Lanny Blakely of the Walpole office of Conservation and Land Management (CALM) for their assistance, support, and permission throughout the course of the project. I am grateful to J.S. Myers and M.J. Van Kranendonk for very helpful reviews. The above-cited individuals may not necessarily agree with the conclusions of this paper. References Ballard, M.M., Van der Voo, R., Urrutia-Fucugauchi, J., 1989. Paleomagnetic results from Grenvillian-aged rocks from Oaxaca, Mexico: evidence for a displaced terrane. Precambrian Research 42, 343 – 352. Beeson, J., Delor, C.P., Harris, L.B., 1988. A structural and metamorphic traverse across the Albany – Fraser orogen, Western Australia. Precambrian Research 40, 117 – 136.

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