Crustal signature of Late Archaean tectonic episodes in the Yilgarn craton, Western Australia: evidence from deep seismic sounding

Tectonophysics 329 (2000) 193±221

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Crustal signature of Late Archaean tectonic episodes in the Yilgarn craton, Western Australia: evidence from deep seismic sounding B.J. Drummond a,*, B.R. Goleby a, C.P. Swager b a

Australian Geodynamics Cooperative Research Centre, Australian Geological Survey Organisation, G.P.O. Box 378, Canberra, Australian Capital Territory 2601, Australia b Geological Survey of Western Australia, Mineral House, 100 Plain Street, East Perth, Western Australia 6004, Australia Received 3 May 1999; accepted 17 November 1999

Abstract Deformation in the greenstone supracrustal rocks of the Eastern Gold®elds Province of the Archaean Yilgarn Craton in Western Australia is delaminated from the underlying basement along a regional detachment surface presently at 3±7 km depth. This might suggest that the history of crustal deformation cannot be inferred with any certainty from that of the greenstones. However, seismic images of the crust below the greenstones show structures that can be interpreted in terms of a series of tectonic events similar to those within the greenstones. The upper crust (below and to the west of the greenstones) is largely unre¯ective, with interpreted west-dipping reverse faults. The middle crust is re¯ective, with re¯ector geometry implying thickening by west directed thrust stacking, and the lower crust has a fabric indicative of ductile deformation. These re¯ection fabrics imply crustal shortening, probably during the Late Archaean regional D2 ENE±WSW shortening event. They were subsequently overprinted and disrupted by structures consistent with regional NNW±SSE strike slip D3 faulting, and probably younger, more localised D4 faulting. The seismic images of the crust therefore show that the crust suffered tectonic events in which both the order and direction of deformation are similar to those of the greenstones. This is evidence that the whole crust deformed when the greenstones deformed. However, the scale and style of deformation vary with depth through the crust, and include thrusting and probably folding in the upper crust, thrust stacking in the middle crust, and ductile deformation in the lower crust. The length scale (wavelength) of structures in the middle and lower crust is greater than that in the greenstones. q 2000 Elsevier Science B.V. All rights reserved. Keywords: Archaean tectonics; deep seismic sounding; Yilgarn craton; detachment surfaces

1. Introduction The greenstone belts of the Archaean Yilgarn Craton in Western Australia, and particularly those near Kalgoorlie, are host to world class gold and nickel deposits. Many of the gold deposits have a large degree * Corresponding author. Tel.: 161-262-499381; fax: 161-262499972. E-mail address: [email protected] (B.J. Drummond).

of structural control (Witt, 1993, 1997), so knowledge of the geological structure at depth should play an important role in exploration models. Consequently, the Australian Geological Survey Organisation recorded a regional seismic traverse across the eastern Yilgarn Craton in 1991 in order to map the major structures in the region at depth. The traverse was 213 km long, and oriented approximately east±west (Fig. 1). Early results of the seismic re¯ection pro®ling were discussed by Drummond et al. (1993), Drummond and Goleby (1993), Goleby et al. (1993, 1994) and Swager

0040-1951/00/$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S 0040-195 1(00)00196-7

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Fig. 1. Simpli®ed geological map of the survey area, showing the locations of terrane boundaries and the seismic transect.

et al. (1997). These papers described the thickness and internal structure of the greenstones and their intrusive granitoid plutons, which form the upper 3±7 kmthick layer of supracrustal rocks along the eastern part of the traverse. The seismic data show a regional decollement at the base of the greenstones. Deformation within the greenstones was therefore detached

from lower crustal deformation, and this could imply that few constraints on the crustal scale tectonics can be inferred with any certainty from surface mapping in the greenstones. This paper examines the deep crustal structure in more detail than the preliminary studies of Drummond et al. (1993) and Goleby et al. (1994). We show that the lower crust

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Gindalbie SOUTHERN CROSS PROVINCE (Barlee Terrane)

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Fig. 2. (a) Un-migrated seismic data with the interpreted structures superimposed. Terrane boundaries are marked at the top of the section. (b) Post stack wave equation migrated data. Inset in both (a) and (b) shows the positions of Figs. 4 and 5. Depth on right hand axis (and in subsequent figures) calculated assuming an average crustal velocity of 6.0 kms ⫺1. V=H ˆ 1 (approx.). DD 0 — basal detachment of greenstones, S — ?sills, AA 0 , BB 0 , CC 0 , TT 0 — see text.

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Gindalbie SOUTHERN CROSS PROVINCE (Barlee Terrane)

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contains structures that can be interpreted in terms of major tectonic episodes equivalent to those mapped in the greenstone belts. The seismic data therefore allow concepts of tectonic evolution developed from the geology of the greenstone supracrustals to be extended to the whole crust. 2. Geology along the seismic traverse The seismic line crossed two granite±greenstone provinces, the Eastern Gold®elds and Southern Cross Provinces, which contain greenstone successions of different ages, and are separated by the Ida Fault. The Eastern Gold®elds Province contains a number of discrete greenstone belts, including the Kalgoorlie, Gindalbie and Kurnalpi Terranes (Fig. 1). These are de®ned on the basis of differences in their volcano-sedimentary sequences, but they have the same ages of volcanism, deformation, metamorphism and granite plutonism, with the main events occurring between 2.71 and 2.62 Ga (Nelson, 1997; Swager et al., 1992; Swager, 1997). The seismic re¯ection results showed the greenstones to be 3±7 km thick and underlain by a regional sub-horizontal re¯ector Ð interpreted as a basal decollement to the greenstones (Swager et al., 1997). Several terrane and domain boundary faults mapped within the greenstones are imaged as prominent shallowly east- or west-dipping seismic re¯ectors. The greenstones have been affected by a complex deformation history (Archibald et al., 1981; Hammond and Nisbet, 1992; Swager et al., 1992). The nomenclature of Hammond and Nisbet (1992) and Swager (1997) is adopted here. D1 thrusting and recumbent folding, most likely with south-tonorth movement, was followed by ENE±WSW D2 shortening. Swager et al. (1992) recorded that D2 caused upright regional folds, but Hammond and Nisbet (1992) reported thrusting to form regional antiforms. This latter view may be more consistent with the seismic data (Drummond et al., 1997). Strike slip (D3) and oblique slip (D4) faulting followed and may re¯ect relatively small changes in the east±west regional stress regime from D2 onwards. Several episodes of extensional faulting have recently been proposed and appear to punctuate the main stages of contractional deformation. The most obvious in the seismic data is the late stage down-to-the-east normal

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displacement on the Ida Fault. Granites intruded the greenstones throughout this deformation history, with the bulk of the granite volume emplaced around ca. 2.66 Ga after initial D2 shortening. The Ida Fault separates the Kalgoorlie Terrane from the Southern Cross Province (Barlee Terrane) which contains a much older (ca. 2.9 Ga) greenstone succession. These greenstones are exposed mostly in narrow, high grade belts, such as the Watt Hills at the western end of the seismic traverse, within extensive areas of granite and granitic gneiss. They were deformed by the same Late Archaean 2.7±2.6 Ga orogeny as the Eastern Gold®elds Province. 3. The interpretation approach The seismic re¯ection data are shown in Fig. 2a (unmigrated data) and Fig. 2b (migrated). Deep crustal seismic re¯ection data can be dif®cult to migrate because the re¯ection segments are often short. This can lead to a smearing of the data laterally along the migration hyperbola, in some cases contaminating the migrated section by migration `smiles', and a loss of relative strong amplitudes. Therefore, the initial interpretation was performed on the un-migrated data. During the interpretation, attention was given to identifying places in the seismic section where groups of re¯ections with a similar trend were cut off by other re¯ections with a different trend. The interpretation also identi®ed zones with different re¯ection strength. The interpretation is shown as solid lines in Fig. 2a, and mostly highlights boundaries between blocks of crust. Little attention was paid to following individual re¯ections, although some trend lines are shown as dashed lines. Particular care was taken to ensure that diffractions were identi®ed and not included in the interpretation. The positions of the boundaries between blocks and the trend lines were then migrated and superimposed on the migrated seismic data (Fig. 2). In almost all cases the migrated boundaries corresponded to changes in the re¯ection character of the migrated data; for example, changes in re¯ection trend or intensity. 4. Crustal geometry The orientation of the seismic traverse is approximately orthogonal to the main NNW trend of the

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granite±greenstone belts. Interpretation of the re¯ection data must consider the evidence for possible substantial out-of-the-section movement, e.g. during the D1 and D3±D4 deformation stages. Drummond et al. (1993) discussed the composition and structure of the crust based on its seismic re¯ection character, and on the velocity of seismic waves through the crust measured during a previous seismic refraction survey located immediately to the west of the re¯ection pro®le (see also Drummond, 1988). The crust in the region can be divided into a number of layers on the basis of seismic re¯ection character (Fig. 2a and b). The greenstone belts in the central and eastern parts of the section form the uppermost layer. The upper crystalline crust and the middle and lower crust throughout the region each has a distinctive re¯ection character. Below the crust, the mantle is mostly unre¯ective. Laterally, the western (Southern Cross Province), central (the western half of the Kalgoorlie Terrane between the Ida Fault and Bardoc Shear) and eastern parts of the seismic pro®le (eastern part of Kalgoorlie Terrane, plus the Gindalbie and Kurnalpi Terranes) have different signatures in the seismic re¯ection data of the midddle crust, as discussed below. 4.1. Greenstones (Fig. 3) Swager et al. (1997) described the seismic image of the greenstone rocks. Only their conclusions are summarised here. The greenstone sequences are mostly highly re¯ective throughout the Kalgoorlie Terrane, but are less re¯ective in the Gindalbie and Kurnalpi Terranes. The seismic data show the greenstone strata truncated with angular relationships by a laterally continuous and very strong re¯ector (DD 0 in Figs. 2 and 5). Below the re¯ector DD 0 , the upper crust is mostly transparent to two way re¯ection times of approximately 5 s; the greenstone re¯ection signature cannot be interpreted in this zone, and Swager et al. (1997) interpreted DD 0 as a decollement that forms the base of the greenstones. This re¯ector is at 2±3 s two way travel time (TWT), or at ,5±7 km depth along most of the seismic line. However, at the eastern end of the seismic line, the strong re¯ector shallows to only about ,3 km (1 s TWT) just west of and underneath the Arcoona Granite.

D1 involved NNW directed thrusting. Earlier interpretations of the seismic data did not address the D1 thrust surface because the transport direction was approximately orthogonal to the transect and the thrust surface was not expected to be imaged. However, Drummond et al. (1997) reported that D1 can be traced in the seismic data from its position in outcrop with an apparent east dip until it intersects and appears sheared out by or soles into the regional basal decollement to the greenstones. D2 emplaced broad regional NNW±SSE trending antiforms or domes, probably above antiformal stacks which formed above the basal decollement (Drummond et al., 1997), suggesting that the decollement was active at the time of D2. Most D2 and D3 faults and shear zones link into the basal decollement of the greenstones, suggesting that they are either older than the basal decollement, or that D3 shearing in the upper crust was in many places detached from the lower crustal response at the regional decollement. However, the gently west-dipping Bardoc and Avoca Faults appear to cut through the decollement and can be traced into the middle crust. The Bardoc Shear is apparently displaced by, and displaces the basal decollement. It links at depth with the gently eastdipping, crustal scale Ida Fault (Fig. 2a and b). In contrast to the Bardoc and Avoca Faults, the Ida Fault has normal displacement, probably throughout the entire crust. The Moho deepens from 33±35 km depth over a distance of 40±50 km from the west of the Ida Fault to 37±38 km depth farther east. The geometry of the greenstone sequences abutting the Ida Fault suggests that the Ida Fault is not a growth fault; this and other geological observations suggest a late stage extensional (post D2±D3, pre-D4) displacement, but do not rule out an earlier, more prolonged movement history (Swager et al., 1997). Swager et al. (1997) argued that this allows for greenstones to have once extended farther west of the Ida Fault, and to have been uplifted and eroded. Gravity modelling shows that several imbricate wedges of material above the basal detachment are likely to be felsic (Fig. 3). They are re¯ective in places, suggesting a foliated character, and are interpreted to be similar to highly foliated felsic gneisses found elsewhere within the Yilgarn Craton. Archibald et al. (1981) considered the felsic gneisses to be the basement to the greenstones.

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Late granite

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Greenstone sequence (in Bardoc Shear Zone)

Fig. 3. Cross section through the greenstones of the Eastern Gold®elds Province based on the interpretation of the seismic data (from Swager et al., 1997). V=H ˆ 1 (approx.).

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Granitoids crossed by the traverse are thin, tabular bodies with steep sides and probably ¯at bottoms, and have clearly intrusive relationships with the surrounding greenstones. The base of the Arcoona Granite at the eastern end of the seismic line is poorly imaged. However, on the basis of the seismic data, no granitoid sheets within the greenstones would appear to be more than 3±4 km thick. 4.2. The Southern Cross Province (western part of the seismic traverse) (western end, Figs. 2 and 4) Although the greenstone rocks of the Southern Cross Province are older than those of the Eastern Gold®elds Province, they have the same style and ages of deformation, and also experienced the thermal event which emplaced the granites at 2.6 Ga. Crust in the eastern Southern Cross Province might therefore be representative of the basement across which the greenstones of the Eastern Gold®elds Province were formed or emplaced. Structures in the crust of the Southern Cross Province should be similar to the older structures in the crust under the supracrustal greenstone rocks of the Eastern Gold®elds Province. The crust in the Southern Cross Province consists of three parts: a mostly unre¯ective upper crust, a re¯ective middle crust with a mostly east-dipping re¯ection fabric, and a lower crust in which the re¯ectivity is sub-horizontal. The Moho is interpreted at ,11 s, at the base of the re¯ective lower crust. Only occasional, weak re¯ections are observed below the Moho. The upper crust is about 9 km (3 s TWT) thick across all of the terrane traversed by the seismic pro®le. It is mostly non-re¯ective, with several exceptions. At the western end of the line, (e.g. BB 0 , Figs. 2 and 4; TT 0 , Fig. 2), bands of re¯ections dip west. Some (e.g. BB 0 ) appear to ¯atten and become subhorizontal just above the top of the stronger re¯ectivity in the middle crust. At the extreme western end of the seismic section, a small block of re¯ective rocks sits above the re¯ector TT 0 , and is interpreted as re¯ective middle crust thrust east and upwards along TT 0 . These west-dipping re¯ectors are therefore interpreted as east directed thrusts. The transition from mostly non-re¯ective upper crust to highly re¯ective middle crust corresponds with an increase in seismic velocities from 5.8±

6.15 km s 21 in the upper crust to 6.35±6.5 km s 21 in the middle crust (Drummond, 1988). The stronger, east-dipping re¯ections in the middle crust are interpreted clearly in un-migrated data (Fig. 2a), and are shown for comparison in more detail in Fig. 4. They can be grouped into curved, lensoid bodies which resemble horses within a stacked duplex. This geometry suggests shortening by thrusting from east to west. The boundaries of the duplexes and the individual horses are de®ned in a number of ways (Figs. 2a and 4). The tops are concave downwards. The western limbs of the horses truncate against the eastern limb of the underlying horse. For example, a package of rocks has been thrust over another along a surface marked AA 0 , de®ned by high angular discordance of re¯ections across it. This is seen in both the migrated data (main image, Fig. 4) and the un-migrated data (Fig. 4 inset, bottom left, and Fig. 2a). The re¯ections immediately below AA 0 , i.e. forming the foot wall, are mostly sub-parallel to AA 0 . Those in the hanging wall are at a high angle to AA 0 , and suggest hanging wall cutouts above the AA 0 thrust fault. Other boundaries between duplexes have less angular discordance. Some are de®ned by differences in re¯ection character within the duplexes. The duplexes have no apparent regular pattern in their internal re¯ectivity. The duplexes thin towards their base, where most re¯ections are concave upwards. Both the individual re¯ections and the boundaries between duplexes sole into a zone of sub-horizontal to gently east-dipping re¯ections between 7 and 11 s TWT (21±33 km depth, approx.) at the western end of the seismic line. In this zone, seismic velocities increase from 6.8 km s 21 to over 8.2 km s 21 (Drummond, 1988), which is typical of mantle rocks. The re¯ection geometry suggests that the basal detachment to the duplexed middle crust is a highly ductile detachment zone 10±12 km thick and corresponding to the density gradient between the crust and mantle, rather than a single, thin decollement surface. 4.3. The eastern Kalgoorlie, Gindalbie and Kurnalpi Terranes (the eastern third of the seismic pro®le) (eastern end, Fig. 2a and b) The seismic re¯ection signature of the crust under the greenstone belts at the eastern end of the seismic

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Fig. 4. Migrated seismic data from the western end of the seismic transect (Southern Cross Province) (main image). The main boundaries between discrete blocks of crust are marked with lines; see Fig. 2 for their regional positions. Some shorter re¯ections have over-migrated into smiles, e.g. at the left hand end between 4 and 5 s two way time, and could be confused as cross cutting structures. Inset: portion of the data, un-migrated. V=H ˆ 1 (approx.). 215

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line has many similarities to that in the Southern Cross Province at the western end of the seismic line. The upper crust under the greenstone belts is largely non-re¯ective, except for a few, mostly west-dipping re¯ectors. One of the west-dipping groups of re¯ections (CC 0 ) cuts through the greenstone belts and projects to the surface at the Avoca Fault near the western edge of the Arcoona Granite. This set of re¯ections may be associated with an approximately 1 s offset of the basal greenstone detachment (higher to the east). It soles at depth into the boundary between the non-re¯ective upper crust and the middle crust, similar to re¯ectors with a similar attitude in the upper crust of the Southern Cross Province (BB 0 , Figs. 2 and 4; TT 0 , Fig. 2). However, whereas TT 0 and BB 0 were interpreted above as thrust faults, the Avoca Fault has a geometry suggesting normal displacement, at least in its latest stages of movement. The geometry of the re¯ectors in the un-migrated data from the middle crust at the eastern end of the seismic line (Fig. 2a) is consistent with duplexes, as in the Southern Cross Province, but they appear to be structurally disrupted. The degree of disruption increases to the west. In the region where re¯ector CC 0 soles out near the top of the re¯ective middle crust (between stations 7000 and 8000), lateral changes in the dip, continuity and amplitude of the middle crustal re¯ectors could be interpreted to include ¯ower structures similar to those attributed to wrench tectonics at a smaller scale in sedimentary basins. The structure of the ¯ower structures in the migrated data is not as clear, although the migrated data in this region contain many migration smiles, consistent with the presence of mostly short, and by inference, disrupted, re¯ectors. The tops of the ¯ower structures are interpreted to be near the boundary between the non-re¯ective upper crust and the re¯ective middle crust, near the position where re¯ector CC 0 becomes sub-horizontal. However, the generally non-re¯ective nature of the upper crust might preclude any re¯ection signature of these structures should they continue sub-vertically into the upper crust. The base of the ¯ower structures is near the transition from the middle crust with its generally east-dipping re¯ections and the lower crust that contains sub-horizontal re¯ections. Very few diffractions can be identi®ed in the

middle and lower crust at the eastern end of the pro®le, even near the interpreted ¯ower structures where abrupt lateral changes in structure might be expected. Nor are there many in the middle to lower crust in the Southern Cross Province at the western end of the pro®le. This could imply that lateral changes in the velocity ®eld at depth are gradual rather than abrupt (Goodwin and Thompson, 1988). However, in the Kalgoorlie Terrane, between stations 7000 and 7500, the un-migrated data have many diffractions. The presence of diffractions implies disrupted, laterally discontinuous structures. 4.4. Western Kalgoorlie Terrane (central part of the seismic pro®le) (Figs. 2a,b and 5) The seismic re¯ection signature of the crust beneath the western Kalgoorlie Terrane is different from both the Southern Cross Province to the west and the eastern Kalgoorlie, Gindalbie and Kurnalpi Terranes to the east. The disrupted, laterally discontinuous structures in the middle crust between stations 7000 and 7500 appear to mark the eastern edge of zone in which the strong middle crustal re¯ectivity is absent. The Ida Fault forms the western edge of the zone. Instead, the region is mostly non-re¯ective except for several groups of faults. This zone of re¯ectivity occupies a region of crust up to 40 km wide immediately beneath the greenstones, and extends eastwards from the Ida Fault approximately to beneath the surface trace of the Bardoc Shear and the western side of the Scotia Kanowna Granite. The Bardoc Shear dips west from its outcrop position, extends through the basal detachment of the greenstones (DD 0 ) and into the middle crust where it links with the Ida Fault (Fig. 2a and b) (Goleby et al., 1993; Drummond and Goleby, 1993; Swager et al., 1997). The Bardoc Shear appears to offset DD 0 slightly (western side of DD 0 downward). However, the Bardoc Shear is also affected by movement on the basal detachment, because it is much wider above the detachment than below. This suggests that the Bardoc Shear was active when the greenstones were moving above the detachment. The upper crust above and below the Bardoc Shear is crossed by a number of east-dipping re¯ectors. They are shown in more detail in Fig. 5, where they are labelled D4. Drummond and Goleby (1993)

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Fig. 5. Migrated seismic data from the region between the Ida Fault and Bardoc Shear. See Fig. 2 for location. DD 0 Ð basal detachment to the greenstones, D4 Ð late stage faults, S Ð ?sills. V=H ˆ 1 (approx.). 217

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interpreted these re¯ections as shear zones. They have the same attitude as the Ida Fault, and they offset and are also offset by the Bardoc Shear (Fig. 5). This implies that they were active at the same time as or later than the Bardoc Shear, and therefore also at the latest stage of movement on the detachment DD 0 . With the Bardoc Shear, they appear to form a conjugate set of shears within the upper and middle crust. The re¯ection character of the lower crust in the central part of the seismic section is similar to that in the western and eastern parts of the seismic section. 4.5. Other re¯ections (Fig. 2a and b) Swarms of sub-horizontal re¯ections marked with an `S' occur between 5 and 7 s TWT (15±20 km depth) in the eastern Southern Cross Province and western Kalgoorlie Terrane; i.e. in the western and central parts of the seismic pro®le. In the western Southern Cross Province, they appear to cross duplex boundaries without being offset and are therefore interpreted as younger than the duplex structures. Similar re¯ectors in the western Kalgoorlie Terrane are assumed to be the same age as those farther west. They would probably be the youngest re¯ectors imaged in the seismic section. Proterozoic dolerite dykes are the youngest features in the region, and sub-vertical dykes elsewhere have been known to cause re¯ected refractions that appear as sub-horizontal events in seismic sections (e.g. Zaleski et al., 1997). Several dykes adjacent to and sub-parallel to the seismic section have a high magnetic susceptibility contrast with many of the surrounding rocks, and some have reversed remanent magnetism. Those of suf®cient thickness to cause re¯ected refractions can therefore be mapped reliably using regional aeromagnetic data. None lies in a position likely to cause these events, which are therefore interpreted to indicate sub-horizontal sills. 5. Discussion The duplexes interpreted in the middle crust in the western part of the seismic section represent signi®cant shortening with a major east±west component. Out of plane movements cannot be estimated from the seismic section. The orientation of the mid-crustal duplexing suggests that the shortening episode

would be D2. Duplexes can also be interpreted in the east. The tops of the duplexes are concave downwards, rather than planed off at a sub-horizontal roof detachment. Because signi®cant shortening occurred both in the middle crust and in the greenstones, the non-re¯ective upper crust underneath the greenstone belts was probably also shortened. The backthrusts interpreted in parts of the sections, e.g. BB 0 , TT 0 in the western part of the section would not accommodate the amount of shortening in the upper crust implied for the lower crust by the duplex structures. They must therefore be accommodation structures rather than the primary shortening fabric, leading to a further conclusion that the non-re¯ective crust contains rocks with few impedance contrasts. This would be consistent with a predominantly felsic crust, as implied by the seismic velocities from the adjacent seismic refraction experiment. Alternatively, the upper crust might be folded on a scale that could not be imaged in the regional-scale seismic data. Some of the horses in the middle crust can be subdivided using their internal re¯ection fabric into smaller segments in which the re¯ectivity either is of different amplitudes or has different dips. Where segments in which the re¯ectivity is at different dips abut each other, a tectonic episode older than the duplexing, i.e. older than D2, is suggested. It may be the out of the plane D1 event. Alternatively, multiple stages of movement during D2 may be the explanation. Where the adjacent segments have different re¯ection strength, rocks of different composition, or different rheological properties are suggested. The duplex horses interpreted in the middle crust sole into a zone of sub-horizontal re¯ectors that is continuous across the entire section. This geometry suggests that the horses formed above lower crust that remained ductile during the D2 shortening event. The disruption of the re¯ection character along the central part of the seismic pro®le, particularly in the eastern Kalgoorlie and the Gindalbie and Kurnalpi Terranes, indicates at least two tectonic episodes younger than D2. The wrench structures interpreted in the eastern part of the Kalgoorlie Terrane (Fig. 2a and b) are younger than the thrust duplexes because the fabric of the duplexes is disrupted across them. They link with the west-dipping re¯ections CC 0 , which caused offsets on the basal detachment of the greenstones

B.J. Drummond et al. / Tectonophysics 329 (2000) 193±221 0

(DD ). They are therefore probably post D2. Wrench structures usually imply out of plane movement, and they may be the mechanism by which strike slip motion during D3 was transmitted through the middle crust and into the ductile lower crust. The Ida Fault lies along the top of a middle crustal duplex in the eastern Southern Cross Province and most likely represents late stage D3 reactivation of the upper boundary of the duplex. However, the last major movement along the Ida Fault was extensional, and probably younger (Swager et al., 1997). The Bardoc Shear has the same attitude as the re¯ectors seen in the non-re¯ective upper crust elsewhere along the section (BB 0 , Figs. 2 and 4; TT 0 and CC 0 , Fig. 2). They are interpreted above as backthrusts from the top of the zone of duplex structures in the middle crust, which are believed to be D2. The Bardoc Shear and re¯ector CC 0 discussed above, would therefore also be D2 faults reactivated during D3. The middle crust in the central part of the seismic section between the Ida Fault and Bardoc Shear does not have the same duplex-style re¯ection fabric as that seen in the western and eastern parts of the section. The lack of pervasive re¯ectivity throughout this part of the crust might indicate that the crust has always had a different structural fabric from that seen to the west and east, and it behaved as a thick, possibly rigid crustal block during crustal shortening, rather than sub-horizontally strati®ed rocks that could be thrust into duplexes. However, the lower crust has the same re¯ection character along the entire seismic section, implying that the lower crust was continuous throughout the region. Alternatively, therefore, this region of the crust might once have had the same fabric as the middle crust elsewhere, which has since been overprinted. End member candidates of mechanisms for doing this might include intense ductile deformation, cataclasis and brittle fracture (e.g. McCarthy and Thompson, 1988), intensive metasomatism, dehydration, or metamorphism. The seismic data do not discriminate between these models. The weak, eastdipping re¯ectors in this zone (D4, Fig. 5) offset the Bardoc Shear and are therefore either D3 or D4. Goleby et al. (1997) suggested from geomechanical modelling that they are D4. Short, sub-horizontal re¯ectors interpreted as sills in the middle crust cut all of these features and are the youngest features. These sills may correlate in time

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with the post-tectonic, Early Proterozoic dolerite dyke swarms in the region. The relative order of events in the crust beneath the greenstone belts would therefore be: (1) Emplacement of the angular relationships between groups of re¯ectors within individual horses in the duplexes, especially in the middle crust of the Southern Cross Province. This could involve out of plane movement. These surfaces may be evidence for D1 deformation in the middle crust, or alternatively evidence for different scales and episodes of D2 thrust stacking. (2) Predominant east±west shortening leading to duplexes throughout the western and eastern parts of the pro®le. The non-re¯ective upper crust (underneath the greenstones where present) was probably folded, and possible back thrusts at several places along the seismic section developed late in this phase. This would be consistent with D2 seen in the greenstones. (3) Strike slip faulting in and out of the plane of the section, particularly in the east, leading to wrench structures in the middle crust, and possible strike slip reactivation of some faults such as CC 0 (Fig. 4) and the Bardoc Shear. This would be D3. (4) Formation of the east-dipping re¯ectors between the Ida Fault and the Bardoc Shear. These would be D3 or D4. (5)Movement (late stage, extensional) on the Ida Fault, faults parallel to the Ida Fault in the middle crust in the central part of the seismic section (D4, Fig. 5), and the Bardoc Shear during the last stages of movement on detachment DD 0 . Swager (1997) argued that normal displacement along the Ida Fault occurred between the D3 and D4 transpressional events. (6) The intrusion of post-tectonic sills. Many of the structures interpreted in the seismic data imply several episodes of crustal shortening. These were probably the events that thickened the original, thin protocrust so that it became stable, buoyant and therefore able to survive later tectonics. Because they were the last events to have affected the region, their seismic signatures have overprinted those of earlier events, for example, any lithospheric extension to form the basins in which the greenstones might have originally formed. However, deformation within the greenstone supracrustals was detached from that in the crust below, and the length scale of the shortening deformation, and the means of deformation, differed

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between the greenstones and the upper, middle and lower crust. For example, D2 domes in the greenstones have wavelengths of 20±25 km, whereas duplexes within the middle crust, particularly in the Southern Cross Province, have wavelengths greater than 40 km. Deformation in the middle crust was by thrust stacking, and in the lower crust by ductile shear. The amount of shortening could have been considerable, and this cannot be accommodated by the short amount of back thrusting interpreted in the upper crust on TT 0 in Fig. 2a and b. Deformation therefore must have occurred by some other mechanism in the non-re¯ective upper crust, e.g. folding, and may be the reason for its lack of re¯ectivity. The absolute ages of the structures interpreted in the seismic data cannot be constrained by the seismic data alone. However, relative ages for some of the structures can be de®ned. If the fabric in the middle crust of the Southern Cross Province (Fig. 4) is indicative of the crust in the Eastern Gold®elds Province before the effects of D3 and D4 deformation, the seismic images of the crust can be interpreted in terms of tectonic events which have the same apparent relative ages, and senses of deformation, as those mapped in the surface geology. This series of events provides compelling evidence that the deformation mapped in the surface geology of the greenstone belts affected the whole crust, on different scales at different depths, and by different means in the upper, middle and lower crust. Despite the range in age of the protocrust across the region of the seismic pro®le (approximately 300 Ma), the seismic data imply a coherent tectonic history across the region of the pro®le. The crustal fabric seen in the seismic data is also similar to that seen in seismic data from the western part of the Yilgarn Craton, which show east-dipping fabric in a region of high metamorphic-grade surface rocks that were once buried to mid-crustal levels. There the east-dipping fabric is interpreted as evidence for west directed thrusting at ca. 2.64 Ga (Middleton et al., 1995). The seismic data presented here and those of Middleton et al. (1995) therefore indicate the craton-scale effect of the Late Archaean 2.7±2.6 Ga orogeny. Acknowledgements B.J.D. and B.R.G. publish with the permission of

the Executive Director of the Australian Geological Survey Organisation and the Director of the Australian Geodynamics Cooperative Research Centre. Joe Mifsud drew the ®gures. We thank Th. Flottmann, R.J. Durrheim and C. Juhlin for constructive reviews. References Archibald, N.J., Bettenay, L.F., Bickle, M., Groves, D.I., 1981. Evolution of Archaean crust in the Eastern Gold®elds province of the Yilgarn Block, Western Australia. In: Glover, J.E., Groves, D.I. (Eds.). Archaean Geology Second International Symposium, Perth, 1980. Geological Society of Australia, Special Publication, vol. 7, pp. 491±504. Drummond, B.J., 1988. A review of crust/upper mantle structure in the Precambrian areas of Australia and implications for Precambrian crustal evolution. Precambrian Research 40/41, 101±116. Drummond, B.J., Goleby, B.R., 1993. Seismic re¯ection images of the major ore-controlling structures in the Eastern Gold®elds Province, Western Australia. Explor. Geophys. 24, 473±478. Drummond, B.J., Goleby, B.R., Swager, C.P., 1997. Crustal signature of the major tectonic episodes in the Yilgarn Block, WA: evidence from deep seismic sounding. In: Cassidy, K.F., Whittaker, A.J., Liu, S.F. (Compilers). An International Conference on Crustal Evolution, Metallogeny and Exploration of the Yilgarn Craton Ð an Update. Australian Geological Survey Organisation Record, 1997/41, pp. 15±20. Drummond, B.J., Goleby, B.R., Swager, C.P., Williams, P.R., 1993. Constraints on Archaean crustal composition and structure provided by deep seismic sounding in the Yilgarn Block. Ore Geol. Rev. 8, 117±124. Goleby, B.R., Rattenbury, M.S., Swager, C.P., Drummond, B.J., Williams, P.R., Sheraton, J.W., Heinrich, C.A., 1993. Archaean crustal structure from seismic re¯ection pro®ling, Eastern Gold®elds, Western Australia. Australian Geological Survey Organisation Record, 1993/15. Goleby, B.R., Drummond, B.J., Korsch, R.J., Willcox, J.B., O'Brien, G.W., Wake-Dyster, K.D., 1994. Review of recent results from continental deep seismic pro®ling in Australia. Tectonophysics 232, 1±12. Goleby, B.R., Drummond, B.J., Owen, A.J., Yeates, A.N., Jackson, J., Swager, C., Upton, P., 1997. Structurally controlled mineralisation in Australia ± how seismic pro®ling helps ®nd minerals: recent case histories. In: Gubins, A.G. (Ed.), Proceedings of Exploration '97: Fourth Decennial International Conference on Mineral Exploration, Toronto, pp. 409±420. Goodwin, E.B., Thompson, G.A., 1988. The seismically re¯ective crust beneath highly extended terranes: evidence for its origin in extension. Geol. Soc. Am. Bull. 100, 1616±1626. Hammond, R.L., Nisbet, B.W., 1992. Towards a structural and tectonic framework for the central Norseman±Wiluna greenstone belt, Western Australia. In: Glover, J.E., Ho, S.E. (Eds.), The Archaean: Terrains, Processes and Metallogeny, 22. Geology Department (Key Centre) and University Extension, The University of Western Australia, pp. 39±49.

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