Phanerozoic reactivation along a fundamental Proterozoic crustal fault, the Darling River Lineament, Australia: constraints from apatite fission track thermochronology

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Earth and Planetary Science Letters 164 (1998) 451–465

Phanerozoic reactivation along a fundamental Proterozoic crustal fault, the Darling River Lineament, Australia: constraints from apatite fission track thermochronology Paul B. O’Sullivan Ł , Barry P. Kohn, Melinda M. Mitchell Australian Geodynamics Cooperative Research Centre, School of Earth Sciences, La Trobe University, Bundoora, Victoria, 3083, Australia Received 23 June 1998; revised version received 7 October 1998; accepted 7 October 1998

Abstract The Darling River of eastern Australia flows along a relatively straight course for much of a ¾2000 km linear feature extending from South Australia to the offshore continental margin of Queensland. This crustal-scale feature, the Darling River Lineament (DRL), is considered to have originally been a part of the Tasman Line, a major Late Proterozoic fracture zone which was active during the breakup of the Rodinia supercontinent. Even though extensive Phanerozoic orogenic contraction has since affected the Darling River region, the present surface expression of the DRL indicates relatively recent reactivation. Time–temperature modelling of apatite fission track data from rocks straddling the southwestern portion of the DRL suggests that Phanerozoic cooling along this part of the lineament principally occurred during three major episodes. Each of these can be tentatively linked to tectonism at the time; Late Silurian to Middle Devonian (¾420–380 Ma) deformation during either the Lachlan Orogeny or the Alice Springs Orogeny, Late Permian to Early Triassic (¾260–240 Ma) deformation during the Hunter–Bowen Orogeny and Early to middle Tertiary (¾60–40 Ma) deformation in response to Pacific Plate rearrangement and north Australian collision in Papua New Guinea. Moreover, the results reveal that the magnitude of cooling differed significantly across the lineament during each episode, indicating that this major Late Proterozoic feature has been reactivated several times during the Phanerozoic.  1998 Elsevier Science B.V. All rights reserved. Keywords: tectonics; crust; lineaments; geochronology; fission-track dating; faults; reactivation; Proterozoic

1. Introduction The Australian continent is transected by continental-scale sutures and lineaments that can be correlated with aligned regional and local-scale geological, geophysical and geomorphological features [1,2]. One such feature in eastern Australia is the Ł Corresponding

author. Tel.: C61 3 9479 3517; Fax: C61 3 9479 1272; E-mail: [email protected]

Tasman Line (Fig. 1), a major fundamental suture along which the breakup of the Rodinia supercontinent is alleged to have occurred during the Neoproterozoic at ¾700–725 Ma [1,3–5]. Today, the Tasman Line defines the boundary between the Proterozoic craton to the west and the younger Tasman fold belt system to the east. In western New South Wales, in the region of Wilcannia and Bourke (Fig. 1), a major segment of the Tasman Line is made up by the east-northeast-

0012-821X/98/$ – see front matter  1998 Elsevier Science B.V. All rights reserved. PII: S 0 0 1 2 - 8 2 1 X ( 9 8 ) 0 0 2 3 8 - 6

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Fig. 1. Generalized map showing the main structural entities of eastern Australia and the location of the Darling River Lineament (DRL), which follows the Darling River drainage throughout much of Queensland and New South Wales. Structural entities include from east to west: NEFB, New England Fold Belt; SBB, Sydney–Bowen Basin; TFB and LFB, Thomson and Lachlan fold belts, which combined make up the Tasman Fold Belt System; KFB, Kanmantoo Fold Belt; TL, Tasman Line; and AFB, Adelaide Fold Belt.

erly-trending Darling River Lineament (DRL) [1,6]. Scheibner [1,6] referred to it as “an approximately 50 km wide zone of linear features reflecting faults and fractures of a fossil fracture zone (transform fault), oriented east-northeasterly in western and northern New South Wales” (Fig. 1). A deep-crustal feature which follows a similar course to the DRL has subsequently been identified by O’Driscoll [2] using gravity data. Powell [4] and Scheibner [7] proposed that during Rodinian breakup, the DRL was initiated as a deep-crustal right-stepping transform fault as part of the Tasman Line. However, there is little evidence to suggest any transform motion along the fault since Rodinia breakup (e.g., [8–10]). For instance, Glen et al. [10] reported that the only manifestation of the DRL in the region of Cobar (see Fig. 2), is the presence of transfer=tear faults parallel to its trace and lying within or close to its boundaries. Furthermore, while these tear faults might be high-level manifestations of the DRL, the absence of pull apart or en echelon basins or folds suggests that it had only minimal effect during Middle to Late Palaeozoic regional extension or contraction [10].

Subsequent to proposed Rodinian breakup, during the early to middle Phanerozoic several orogenic events have accreted terranes onto the eastern edge of the Australian craton resulting in the preservation of the Adelaide, Kanmantoo, Lachlan, and New England fold belts (Fig. 1). However, the position of the DRL is still evident at the surface today, as suggested by the straight course of the Darling River and its tributaries, and extends over 2000 km eastward from the Gawler Craton in South Australia through to eastern Queensland [7]. Furthermore, offshore and along strike with the DRL at its northeastern extremity is the Cato fracture zone along which the continental shelf appears to be offset (Fig. 1). North of the Cato fracture zone the sea floor is at shallow depths of between 0–2000 m, while south of the fracture zone the sea floor quickly drops to depths of ¾4000 m. Therefore, this deep crustal transform fault, originally active during the Proterozoic and subsequently affected by multiple accretionary events, evidently: (1) still exerts some structural control on the presentday land surface, and (2) must have been reactivated in some form other than by transform motion. To test whether the DRL has been reactivated since the Proterozoic and if so to constrain the time– space framework of such reactivation, a series of samples for apatite fission track (AFT) analyses were collected from outcrop localities spanning the lineament in the Wilcannia=Bourke region of northwest New South Wales (Fig. 2). This approach potentially provides a powerful tool for elucidating the reactivation history of major crustal lineaments, especially in areas where crosscutting relationships are lacking. Using the AFT data, this paper aims to: (1) place constraints on the timing of post-Proterozoic reactivation along the DRL, and (2) provide previously lacking details on the Phanerozoic cooling history of the region and suggest how that history may be interpreted in terms of specific tectonic events.

2. Geologic setting Proterozoic and Phanerozoic sedimentary and granitic rocks characterize the Darling River region of northwest New South Wales (e.g., [7,10– 12]). These are distributed into: (1) structural blocks comprising Proterozoic cratonic rocks, the Adelaide,

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Fig. 2. Generalized geologic map of the Darling River region showing locations for surface samples as well as well locations.

Kanmantoo, and Lachlan fold belts, and (2) Jurassic and Cretaceous sedimentary rocks of the Eromanga Basin, which unconformably overlie basement rocks in northwestern New South Wales (Fig. 1) [7,10]. The Proterozoic cratonic rocks are exposed as inliers within the overlying Neoproterozoic to Early Cambrian Adelaide fold belt rocks, which accumulated in a continental rift as a cover sequence over the eastern margin of the then Australian craton [7]. The Neoproterozoic to Ordovician Kanmantoo fold belt and the Early to Middle Palaeozoic Lachlan fold belt are orogenic belts consisting of metamorphosed volcanic and cratonic-derived deep-marine sedimentary rocks, and extensive suites of granitic rocks, which were accreted by overthrusting the eastern margin of the Australian craton [13]. Phanerozoic compressional deformation within

the region has occurred during multiple episodes (e.g., [7]). The first of these, the Early to Late Cambrian Delamerian Orogeny, resulted in reactivation of earlier structures within the Proterozoic cratonic rocks [14], and severely deformed both the Adelaide and Kanmantoo fold belts. The boundary between the accretionary Kanmantoo rocks and the Adelaidean cratonic cover sequence is along the Tasman Line [7,15,16]. Subsequently, much of the interior of continental Australia, including the region of Broken Hill (Fig. 2), was severely deformed due to north–south compression during the Early Devonian to Late Carboniferous (¾300–400 Ma) Alice Springs Orogeny [17,18]. Contraction in the region also occurred during: (1) the Late Ordovician to Late Devonian Lachlan Orogeny [19], which combines the Late Ordovician to Early Silurian Benambran

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Orogeny and the middle Devonian Tabberabberan Orogeny, and (2) the Early to middle Carboniferous Kanimblan Orogeny. These resulted in major north– south-trending structures throughout the Kanmantoo and Lachlan fold belts in response to a subducting slab along a convergent plate boundary towards the east [20,21]. Located to the northeast of the study region is the New England fold belt (Fig. 1). It has been proposed that the Lachlan and New England fold belts are intrinsically related and that both orogens were affected during the Devonian to middle Carboniferous orogenic events [22]. The New England fold belt also experienced compressional deformation as the result of oblique convergence associated with the Permo-Triassic (255–230 Ma) Hunter–Bowen Orogeny [21,22]. Recently it has been proposed that the Lachlan fold belt in both eastern New South Wales and Victoria also experienced km-scale denudation during the Late Permian to Early Triassic, and that perhaps the effects of the Hunter–Bowen Orogeny are much more widespread than previously thought [23,24].

3. Fission track thermochronology and methodology To constrain the reactivation and denudation history of the DRL, AFT data was collected from 21 outcrop samples in the Wilcannia=Bourke region (Fig. 2). Sample location information and results are presented in Table 1. 3.1. Thermochronology The spontaneous nuclear fission of 238 U over geological time in uranium-bearing minerals such as apatite and zircon causes the accumulation of linear zones of radiation damage known as fission tracks. When formed in apatite, the tracks have a fairly constant mean length of ¾ 16 š 1 µm [25]. The tracks can be made visible by a chemical etching procedure on a polished surface of the mineral so that they can be observed and measured by optical microscopy. The number of tracks that have accumulated in a mineral can be related to the uranium content and time over which they have been quan-

titatively retained, i.e. a measure of geological age. Fission tracks possess a fundamental property, which forms the basis of their use in thermochronology. When exposed to elevated temperatures the radiation damage making up the tracks is progressively repaired or annealed, over a temperature interval which is characteristic for each particular mineral. In the case of apatite, this temperature interval occurs at <¾110ºC for geological heating times of the order of 107 years. Some annealing in apatite occurs even down to ambient surface temperatures, but below ¾60ºC this is relatively insignificant. Hence, the temperature interval between ¾60º and ¾110ºC is often referred to as the AFT partial annealing zone (PAZ). In the PAZ, increasing temperatures cause progressive annealing which results in shorter tracks, reduced track density, and a reduction in the AFT age [25,26]. It follows that at temperatures >¾110ºC total annealing results in the reduction of the fission track age to zero. Because new tracks continuously form throughout geologic time, the AFT age and the distribution of track lengths in an apatite sample reflects the integrated thermal history of the host rock [25,26]. In this study, our data have been interpreted using the understanding of AFT system response described by Green et al. [26]. This understanding is based on an empirical kinetic description of laboratory annealing data in Durango apatite [27,28]. Thermalhistory interpretations are based on a quantitative treatment of annealing achieved by forward computer modelling [26], of track shortening and age evolution through likely thermal histories for an apatite composition equal to that of Durango (0.4 wt% Cl). Though the Cl composition was not determined directly for any of the apatites analyzed, etching characteristics of the grains suggested that they were less than that of Durango, and therefore, palaeotemperatures estimated by applying the Laslett et al. [28] model for track annealing are interpreted to be maximum values. Gallagher [29] has automated the modelling procedure to give a forward modelling approach, which combines a Monte-Carlo simulation of numerous possible thermal histories with statistical testing of the outcome against the observed fission track measurements. A genetic algorithm is also used to provide rapid convergence to an acceptable fit.

Table 1 Fission track analytical results: surface samples from the Darling River Lineament region Sample number Lat. (# grains) (ºS)

Long. (ºE)

Unit name Elevation Standard track Fossil track Induced track Chi square Fission track (m) density density density probability age (ð106 cm 2 ) (ð106 cm 2 ) (ð106 cm 2 ) (%) (Ma š 1¦ / 1.391 (2701) 1.370 (5344) 1.437 (5632) 1.394 (2701)

1.326 (30) 1.192 (1339) 0.532 (117) 7.660 (7710)

2.077 (47) 1.230 (1381) 1.063 (234) 7.666 (7717)

94POS130 (8) 94POS131 (20) 94POS135 (25) 94POS136 (25) 94POS137 (9)

31.57º 31.70º 31.36º 31.23º 31.19º

145 210 215 270 300

1.335 (2597) 1.328 (2597) 1.322 (2597) 1.370 (5344) 1.316 (2597)

3.064 (648) 5.133 (1474) 1.103 (1018) 1.645 (498) 3.940 (953)

2.483 (525) 28.0 3.810 (1094) 83.4 6.738 (622) 84.1 1.232 (373) 100.0 2.877 (696) 3.3

TBG-1 (21)

29.43º 142.03º PG

150

1.090 (3854)

2.723 (1356) 2.177 (1084)

12.8

Surface samples (south of the Darling River) 94POS101 (24) 29.99º 146.84º PG 150

1.381 (2701)

2.880 (1355) 2.527 (1189)

0.0

94POS103 (9)

140

1.385 (2701)

0.610 (119)

0.0

94POS109 (25) 30.43º 146.56º PG

178

1.398 (2701)

1.832 (1042) 2.384 (1356)

4.4

94POS110 (25) 30.32º 146.65º PG

145

1.433 (5632)

2.073 (1336) 2.935 (1892)

3.6

94POS111 (25) 94POS113 (17) 94POS114 (25) 94POS123 (16) 94POS124 (16) 94POS125 (9) 94POS128 (16)

130 155 180 260 230 230 220

1.401 (2701) 1.429 (5632) 1.366 (2597) 1.353 (2597) 1.347 (2597) 1.453 (5667) 1.341 (2597)

1.933 (991) 1.626 (368) 1.091 (442) 5.177 (2642) 1.878 (762) 1.763 (370) 6.844 (3028)

143.23º 142.79º 142.64º 142.68º 142.69º

MDG WB WB WB WB

30.12º 146.27º MDG

30.20º 30.37º 30.67º 31.62º 31.09º 31.55º 31.71º

146.89º 146.85º 146.40º 145.90º 145.91º 145.22º 143.84º

PG PG PG CS PG MDG MDG

0.738 (144)

2.497 (1280) 2.457 (556) 2.482 (1005) 6.049 (3087) 2.593 (1052) 2.130 (447) 8.076 (3573)

47.2 88.7 46.8 0.0

14.9 93.5 16.6 5.5 37.0 66.6 0.0

Standard deviation

166.2 š 39.0 247.1 š 10.2 134.8 š 15.4 258.8 š 6.8 (258.4 š 9.6) a 305.1 š 19.0 330.6 š 14.9 397.7 š 21.9 337.8 š 23.7 332.9 š 18.0 (327.3 š 25.8) a 256.3 š 11.4

16.6 11.2 9.2 61.0

12.41 š 1.0 (2) 13.16 š 0.1 (130) 12.26 š 0.8 (3) 12.23 š 0.1 (125)

1.41 1.41 1.46 1.29

20.6 31.8 5.7 11.2 24.3

11.58 š 0.3 (42) 11.76 š 0.2 (81) 11.91 š 0.2 (113) 11.38 š 0.1 (104) 11.35 š 0.2 (52)

2.09 1.95 2.51 1.69 1.74

22.2

12.18 š 0.2 (76)

1.90

291.7 š 20.3 (276.2 š 20.3) a 213.4 š 26.8 (173.3 š 44.2) a 200.5 š 9.3 (195.2 š 10.9) a 189.1 š 7.4 (189.1 š 8.9) a 202.4 š 9.5 176.9 š 12.2 112.9 š 6.9 215.9 š 7.3 182.4 š 9.5 224.1 š 16.1 211.9 š 6.9 (214.5 š 11.3) a

20.3

12.60 š 0.1 (76)

1.30

5.9

12.93 š 0.4 (8)

1.10

18.9

12.93 š 0.1 (141) 1.42

25.6

13.12 š 0.1 (120) 1.43

19.8 21.5 20.2 49.6 21.4 18.3 66.8

11.95 š 0.2 (100) 13.01 š 0.2 (26) 12.87 š 0.1 (101) 12.29 š 0.1 (102) 12.16 š 0.3 (24) 12.32 š 0.4 (16) 12.74 š 0.1 (101)

1.68 1.41 1.14 1.33 1.18 1.29 1.34

455

Standard and induced track densities measured on mica external detectors .g D 0:5/, and fossil track densities on internal mineral surfaces. Ages for apatite samples calculated using  D 379:2 š 3 for dosimeter glass CN5 (analyst: P. O’Sullivan; 94 POS samples) or  D 383:5 š 3 for dosimeter glass CN5 (analyst: B. Kohn; TBG-1 sample). Sampled units include: CS D Cobar Supergroup (Silurian); MDG D Mulga Doens Group (Devonian); RDG D Rolling Downs Group (Cretaceous); PG D Granite (Palaeozoic–Silurian); WB D Wonominta Beds (Precambrian). a Central age used when  2 test fails at <5%.

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Surface samples (north of the Darling River) 94POS104 (3) 29.74º 145.95º RDG 125 94POS105 (25) 29.65º 145.76º RDG 115 94POS106 (4) 29.60º 145.21º RDG 125 94POS108 (15) 29.00º 144.45º PG 155

Uranium Mean track length

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3.2. Methodology

4. Fission track results and interpretations

Samples were crushed and ground, and apatites concentrated by conventional heavy liquid and magnetic techniques. The separates were mounted in epoxy resin on glass slides, ground and polished to an optical finish to expose internal surfaces of the grains, then etched in 5 M HNO3 for 20 seconds at room temperature to reveal the fossil fission tracks. Irradiations were carried out in a well-thermalized neutron flux in the X-7 position of the HIFAR reactor at Lucas Heights, NSW. Fission tracks in each mount were counted in transmitted light using a dry objective at a magnification of 1600ð. Where possible, 25 grains were counted on each mount. AFT ages were calculated using the zeta calibration method and standard fission track age equation [30]. Errors were calculated using the techniques of Green [31]. In samples with a significant spread in single grain ages, the ‘conventional analysis’ (e.g., [31]), based purely on Poissonian variation, is not valid. In such cases, the ‘central age’ [32], which is essentially a weighted-mean age, is reported. The observed age spread is determined statistically using the chi2 test which indicates the probability that all grains counted belong to a single population of ages. A probability of <5% is evidence of an asymmetric spread of single grain ages. Such a spread in individual grain ages can result either from inheritance of detrital grains from mixed detrital source areas, or from differential annealing in apatite grains of different compositions [26]. Lengths of confined tracks were measured using the procedure outlined by Green [33]. Measurements were made under similar conditions to those employed for age determinations. Suitable track lengths were measured using a projection tube and a digitizing tablet calibrated using a stage micrometer (with µm divisions). As many tracks as possible, up to ¾100, were measured from each sample.

4.1. Results The AFT ages of these samples range between 113 š 7 and 398 š 22 Ma and the mean track lengths range between 13:1 š 0:1 µm and 11:4 š 0:1 µm with standard deviations between 1.1 and 2.5 µm (all uncertainties presented below are š1¦ unless otherwise stated). All AFT ages from Proterozoic through Devonian rocks were much younger than their depositional or emplacement ages, indicating a reduction in grain ages in response to thermal annealing since the rocks were crystallized or deposited. AFT ages from three Cretaceous rocks north of the DRL, however, were much older or equal to their depositional age. These ages indicate the Cretaceous rocks have not been exposed to elevated palaeotemperatures necessary to significantly anneal apatite since deposition and that the AFT data from these samples essentially represent provenance information rather than constraining the time of regional cooling from elevated palaeotemperatures. In Fig. 3, all AFT ages as well as representative single grain age and confined track length data are presented. Two obvious trends emerge from the regional results, suggesting that rocks on opposite sides of the DRL have experienced significantly different thermal histories. Firstly, with few exceptions, rocks collected south of the DRL gave young AFT ages (¾113–215 Ma), while rocks to the north yielded much older ages (247–398 Ma) (Table 1, Figs. 3 and 4). This is true even for rocks of similar stratigraphic ages and present-day elevations. Furthermore, samples south of the DRL included a large number of grains with ages <125 Ma, but very few grains with ages >275 Ma. Samples north of the lineament included a significant number of grains with ages >300–400 Ma and minimum grain ages of ¾220–240 Ma (Fig. 3). These relationships sug-

Fig. 3. Representative AFT data from surface samples from the Darling River region. Shown for each representative sample are: sample #; apatite fission track age (Ma š 1¦ /; ML, mean confined track length (µm); SD, standard deviation of track length distribution (µm); N, number of confined track lengths measured in sample. Also shown are representative single grain age results, in the form of radial plots [34]. Radial plots show apatite fission track ages for each grain from a sample. A unit standard error is assigned to each grain on the y-axis, its actual precision is indicated on the x-axis, and extrapolating a line from the 0-point through the plotted point indicates age. Geologic map is same as shown in Fig. 2, with final apatite ages shown for each sample instead of the sample number. DRL marks approximate position of Darling River Lineament. Other locations include B, Bourke; BH, Broken Hill; C, Cobar; W, Wilcannia.

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Fig. 4. AFT age vs. mean track length for outcrop samples in the Wilcannia=Bourke region of the DRL.

gest that rocks now at the surface north of the DRL cooled from elevated palaeotemperatures long before rocks now at the surface south of the DRL, and that rocks south of the DRL have cooled relatively recently from elevated palaeotemperatures. Exceptions to this trend include: (1) two Cretaceous samples north of the DRL (94POS106 and 94POS104; 135 and 166 Ma respectively), and (2) a Palaeozoic granite (94POS101; 276 Ma) collected south of the DRL (Figs. 3 and 4). In the first case, the two Cretaceous samples have not been significantly thermal annealed since deposition, and therefore the AFT ages from these represent ages preserved from their provenance. Next, even though 94POS101 has an anomalously old age and contains a unique spread of single grain ages, the distribution includes older grains with ages >300–400 Ma similar to those seen in most of the samples from north of the DRL, as well as young grains with ages <125 Ma characteristic of samples south of the DRL (Fig. 3). Interpretation of the AFT data from this sample suggests it has experienced a thermal history similar to that of samples south of the DRL, however, cooling has occurred from much lower palaeotemperatures. Secondly, the mean track lengths and track length distributions from samples south of the DRL are distinctly different from those to the north (Fig. 3). Samples to the south typically contain: (1) narrow distributions with standard deviations <1.5 µm, (2)

mean track lengths of ¾12.3–12.9 µm, and (3) few tracks with lengths <10 µm or >14 µm. Furthermore, long tracks >14 µm were primarily located in the older grains, while the short tracks were found in the younger grains. These relationships are indicative of samples which have experienced a major episode of cooling prior to recent reheating necessary to partially anneal the confined tracks and explain the shift in mean track length from values >14 µm to the present values of <13 µm. Relatively recent cooling has brought the samples to surface conditions. In contrast, samples north of the DRL contain: (1) much broader distributions with standard deviations >1.5 µm and a noticeable ‘tail’ of shorter tracks, (2) mean track lengths <12 µm, and (3) a significant number tracks with lengths <10 µm. In these samples the long tracks were primarily located in the younger grains, while the short tracks were found in the older grains. These AFT data, including the presence of ‘tails’ of short tracks, suggest that rocks to the north of the DRL did not experience the same degree of recent cooling suggested by AFT data for rocks south of the DRL. 4.2. Interpretation Interpretation of the AFT ages and track length data suggests that all except the Cretaceous samples resided at palaeotemperatures ½¾110ºC prior to

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Fig. 5. Proposed time=temperature histories based on AFT data for Neoproterozoic–Palaeozoic rocks exposed on either side of the DRL. PAZ represents the AFT partial annealing zone (¾60–110ºC) in which fission tracks in apatite undergo accelerated annealing. At temperatures below ¾60ºC, fission tracks in apatite anneal but at a slow enough rate that little reduction in the apparent apatite age occurs, whereas at temperatures of ¾110ºC at the base of the PAZ tracks will anneal rapidly resulting in a zero apparent age. See text for further details.

rapid cooling starting during the middle Palaeozoic. Applying the track annealing model of Laslett et al. [28] and using the procedure described by Gallagher [29], the young ages (215 Ma), short mean track lengths and narrow length distributions from the samples south of the DRL indicate rapid cooling from palaeotemperatures ½¾110ºC to ¾70ºC (in <5 m.y. to preserve the narrow distributions) during the Late Permian to Early Triassic at some time between ¾260–240 Ma (Fig. 5). Subsequently, the rocks have been reheated to palaeotemperatures between ¾70–90ºC, probably in response to deposition of Jurassic and Cretaceous sedimentary rocks of the Eromanga Basin. This resulted in the systematic reduction of the track length and the pronounced shift of the track length distribution towards shorter values of <13 µm (Fig. 3). Based on modelling and the single grain age data, these samples also experienced a significant cooling episode from palaeotemperatures between ¾70–90ºC to temperatures <50–60ºC during the Early to middle Tertiary between ¾60–40 Ma. The low numbers of tracks with lengths >14 µm and the presence of young grains in many samples indicates relatively recent cooling. If cooling to surface temperatures had occurred prior to the Tertiary, there would be a significantly greater proportion of long tracks preserved. Protracted cooling has brought the rocks to present-day temperatures.

Modelling the old ages (½247 Ma), short mean track lengths and broad length distributions from the samples north of the DRL suggest that they cooled slowly from palaeotemperatures ½¾110ºC to ¾80ºC (over ¾30–40 m.y.) during the Late Silurian to Middle Devonian between ¾420–380 Ma (Fig. 5). This occurred prior to deposition of some of the rocks sampled south of the DRL and long before recorded cooling south of the DRL. To retain their significantly older ages and the shapes of their length distributions, these samples must have remained at temperatures ¾80ºC for some time prior to a second episode of relatively rapid cooling. Modelling and minimum grain ages from these samples suggest cooling to <60ºC occurred during the Late Permian to Early Triassic, at a time similar to that suggested for rocks to the south (between ¾260–240 Ma). Since the Late Permian to Early Triassic, the northern rocks have remained at palaeotemperatures <60ºC so the more recent time–temperature history is difficult to constrain. Based on modelling, and the low numbers of tracks with lengths >14 µm, it is possible that these samples were also reheated due to deposition of Cretaceous sediments and subsequently experienced the Early to middle Tertiary cooling episode between ¾60–40 Ma. However, it is also possible they have since experienced protracted cooling to present-day conditions. Though

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Fig. 6. Schematic illustration of the thermal history experienced by rocks presently exposed both north and south of the DRL based on the AFT thermochronology data. The trends are discussed in the text.

these later two cooling events may have affected the entire region, the results suggest that rocks to the south experienced significantly more cooling during each of these episodes relative to rocks to the north of the DRL (Figs. 5 and 6). 4.3. Cooling and denudation rates The AFT results indicate that rocks on both sides of the DRL were cooled from elevated palaeotemperatures >¾110ºC during the Palaeozoic; rocks to the north during the Late Silurian to Middle Devonian, and rocks to the south during the Late Permian to Early Triassic. With the present data it is not possible to constrain the maximum palaeotemperatures to which these rocks were exposed prior to cooling, nor the maximum geothermal gradient at the time of cooling. However, by assuming the rocks were subjected to geothermal gradients of ¾25ºC=km, equivalent to present-day gradients within the local basin (D. Alder, NSW Dept. of Mineral Resources, pers. comm., 1996), it is possible to estimate the total amount of denudation which has occurred in response to cooling below ¾110ºC, as well as the

amount which occurred during each episode of cooling. The results indicate that rocks throughout the region have cooled a minimum of 90ºC, from ½¾110ºC to an estimated present-day surface temperature of ¾20ºC (Fig. 6). To account for the required amount of cooling, >3.6 km of overburden must have been removed from the present-day surface since cooling began. This occurred at average rates of ¾9 m=m.y. and ¾14 m=m.y. for rocks north and south of the DRL respectively. The AFT data also indicate that cooling occurred episodically rather than at a constant rate. For rocks north of the DRL, the major cooling event occurred during the Late Silurian to Middle Devonian. At that time, cooling=denudation from palaeotemperatures ½¾110ºC to ¾80ºC over ¾30–40 m.y. defines minimum rates of cooling and denudation of ¾0.75ºC=m.y. and ¾30 m=m.y. respectively. Rocks south of the DRL experienced two major episodes, during the Late Permian to Early Triassic, and during the Early Tertiary. During the early episode, cooling=denudation from palaeotemperatures ½¾110ºC to ¾70ºC in <5 m.y. de-

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fines minimum rates of cooling and denudation of ¾8ºC=m.y. and ¾320 m=m.y. respectively. Whereas rates of ¾1.5ºC=m.y. and ¾60 m=m.y. occurred during the Early Tertiary episode (Fig. 6). Importantly, during the Tertiary, rocks south and north of the DRL have cooled ¾60ºC and <30ºC respectively, which equate to ¾2.4 km and ¾1.2 km of denudation.

5. Tectonic implications Geologic relationships and the AFT data from the Wilcannia=Bourke region (Fig. 3), indicate that significant reactivation of structures along the DRL has occurred long after: (1) transform motion along the fossil fracture zone occurred during the alleged mid-Neoproterozoic break-up of Rodinia, and (2) multiple orogenic events affected the region by accreting Palaeozoic rocks onto the original Australian craton. Furthermore, the AFT results indicate that the region has experienced a complicated thermal history with distinct episodes of cooling during the Late Silurian to Middle Devonian, Late Permian to Early Triassic, and the Early Tertiary. It is likely that the major Palaeozoic compressional events in both the Kanmantoo and Lachlan fold belts (e.g., [7,19,35–37]), resulted in reactivation along the DRL. Therefore these cooling episodes occurred in response to kilometre-scale denudation resulting from either compressional reactivation of earlier structures or from regional rock uplift. The tectonic events that could have caused these periods of reactivation are discussed below. 5.1. Late Silurian to Middle Devonian cooling=denudation During the Late Silurian to Middle Devonian, it is evident from the AFT data that rocks located north of the DRL were moving upwards relative to those south of the lineament. Furthermore, it is evident from the regional mapping in the Wilcannia=Bourke region that a thick accumulation of Lower Devonian sediments is still preserved south of the DRL, while there are few Lower Devonian deposits north of the DRL (Fig. 2). Therefore, it is likely that the sediments accumulating south of the DRL, at the same time as denudation was occurring to the north,

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were themselves sourced from the uplifted rocks to the north (Fig. 7). The available AFT and geological results are consistent with both: (1) a simple thermal history involving relatively slow regional rock uplift (¾30 m=m.y.) of rocks north of the DRL during the Devonian, while the area to the south remained buried under >4 km of Devonian clastics, as well as (2) one involving compressional tectonics responsible for major north–south-trending structures within the Kanmantoo and Lachlan fold belts in the Wilcannia=Bourke region. The major periods of known contraction occurred during: (1) the Early Devonian to Late Carboniferous Alice Springs Orogeny (interior of Australia), (2) the Late Ordovician to Late Devonian Lachlan Orogeny (eastern margin of Australia), which combines the Late Ordovician to Early Silurian Benambran Orogeny and the Middle Devonian Tabberabberan Orogeny, and (3) the Early to middle Carboniferous Kanimblan Orogeny. Due to the similarity in timing, we propose Late Silurian to Middle Devonian reactivation in the DRL region was related to major compression during either=both the Alice Springs Orogeny or one of the episodes (Tabberabberan) of the Lachlan Orogeny (Fig. 5). 5.2. Late Permian to Early Triassic cooling=denudation During the Late Permian to Early Triassic, it is evident from the AFT data that the entire region experienced some degree of denudation. However, rocks located south of the DRL experienced significantly greater amounts of denudation relative to those north of the lineament (Fig. 7). This episode of reactivation is difficult to explain as Permo-Triassic denudation has not been previously reported in western New South Wales. For instance, work within the Lachlan fold belt (Fig. 1), has suggested that deformation ceased during the middle Carboniferous [13,37]. Thus compressional reactivation within or west of the fold belt since the Late Carboniferous would seem an unlikely option. Compressional deformation within the New England fold belt, however, continued into the Triassic as the result of oblique convergence associated with the Hunter–Bowen orogeny [21,22]. As a result, peak deformation of the Andean-style continental mar-

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Fig. 7. Schematic diagram of the Phanerozoic reactivation history of the DRL in the Wilcannia=Bourke region. (A) During the Late Silurian to Middle Devonian, rocks to the north moved upward relative to those south of the lineament, likely in response to compression during the Lachlan Orogeny. Sediments accumulating south of the DRL were probably sourced from the uplifted rocks to the north. (B) During Late Permian to Early Triassic Hunter–Bowen orogenesis, the entire region experienced some amount of denudation, however, rocks to the south moved upwards relative to those north of the lineament. (C) From the Early Triassic to Late Cretaceous the cooling=denudation history is difficult to constrain, however results suggest that rocks south of the DRL were reheated ¾15–20ºC, probably in response to deposition of Jurassic and Cretaceous sedimentary rocks of the Eromanga Basin. (D) During the Tertiary, the entire region experienced significant amounts of cooling=denudation, primarily during the Early Tertiary between ¾60–40 Ma. It is likely that this occurred in response to intraplate stresses. See text for further details.

gin occurred in the Late Permian at ¾255 Ma, and continued into the Early Triassic at ¾230 Ma. Furthermore, sediments within the Bowen and Sydney basins were also deformed. To date, due to lack of crosscutting relationships, no reported evidence exists in the geologic record to suggest that the Hunter– Bowen compressional episode affected the Lachlan fold belt [21]. Furthermore, recent AFT results from the Lachlan fold belt indicate that deformation occurred within the fold belt since the Carboniferous, and likely during the Late Permian to Early Triassic [23]. As a result of this deformation, O’Sullivan et al. [23,24] suggested that kilometre-scale denudation related to Hunter–Bowen orogenesis occurred within the Lachlan fold belt during the Late Permian to Early Triassic. The similarity in timing suggests that Late Permian to Early Triassic reactivation of the DRL could also be related to the Hunter–Bowen compressional event (Fig. 7).

5.3. Early Tertiary cooling=denudation It is evident from the AFT data that during Early Tertiary, the entire Wilcannia=Bourke region experienced some degree of denudation, with rocks located south of the DRL experiencing significantly greater amounts of denudation than those to the north (Fig. 7). Movement at this time is once again difficult to explain, as crosscutting relationships do not exist to constrain any post-Cretaceous motion (Fig. 2). There is however, a significant amount of evidence to suggest that intraplate stresses have resulted in a significant amount of deformation within Australia throughout the Tertiary (e.g., [38,39]). There are two major Tertiary events preserved in the structural record which are readily related to important plate tectonic events. The most significant global plate rearrangement in the Tertiary occurred during the middle Eocene, between ¾45–40 Ma [39,40]. This event is ascribed to a global redistribution of

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stresses due to the onset of continental collision along the Alpine–Himalayan mountain belt. Evidence for this is preserved both regionally in the northern Pacific Ocean as evidenced by the sharp bend in the Hawaii–Emperor ocean island chain, and locally in a number of Australia’s marginal sedimentary basins, including the formation of significant hydrocarbon traps in the Gippsland Basin of southeastern Australia. The second major event to affect the Australian plate during the Tertiary was the subduction=collision of the Australian plate in Papua New Guinea [39,41]. The northern leading edge of the Australian continent may have been the locus of subduction as early as the Palaeocene. However, the Solomon Sea plate may have insulated the Australian continent from most plate boundary stresses up until ¾25 Ma when partial obduction of the Solomon Sea plate onto the northern Australian passive margin occurred. Collision of the northwestern Australian continental margin with Indonesia starting in the Middle Miocene clearly led to the propagation of compressional stresses well into the continent and reactivating intraplate structures. We propose that the episode of Early Tertiary denudation suggested by the AFT data from along the DRL in the Wilcannia=Bourke region occurred in response to major intraplate reactivation in response to compressional stresses. While the majority of the recorded cooling=denudation occurred during the Early Tertiary, possibly in response to the Eocene plate reorganization, it is likely that denudation along the DRL has continued to the present in response to subduction=collision along the northern Australian plate margin.

6. Concluding remarks Apatite fission track data indicate that the DRL has been tectonically active during the Phanerozoic, long after the breakup of the Neoproterozoic supercontinent Rodinia and the accretion of kilometres of overburden onto the Australian Craton. Three episodes of denudation associated with reactivation are suggested by the results, and each episode can be associated with tectonics along the margins of Australia during the Phanerozoic. Reactivation along

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the deep-crustal feature occurred in response to distal tectonic forces but was nonetheless significant. These deformational events were previously undetected because traditional stratigraphic and structural markers are lacking, thus highlighting the usefulness of low-temperature thermochronology in elucidating the reactivation history of major crustal lineaments.

Acknowledgements This work was made possible with funding by the Australian Geodynamics Cooperative Research Centre, the Australian Institute of Nuclear Science and Engineering, and La Trobe University. Sample TBG-1 was provided by John Webb. Work reported here was conducted as part of the Australian Geodynamics Cooperative Research Centre (AGCRC) and this paper is published with the permission of the Director, AGCRC. The ideas presented here were improved and refined greatly through discussions with many members of the Fission Track Research Group at La Trobe University, including Rod Brown, Dave Foster, Andrew Gleadow, Kevin Hill, Dan Kendrick, Wayne Noble, Andrea O’Sullivan, Asaf Raza, Richard Spikings and Edy Sutriyono. The quality of the presentation was greatly improved through helpful comments and reviews by Mary Roden-Tice and an anonymous reviewer. [RV]

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