The crustal scale architecture of the Eastern Succession, Mount Isa: The influence of inversion

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Precambrian Research 163 (2008) 31–49

The crustal scale architecture of the Eastern Succession, Mount Isa: The influence of inversion T.G. Blenkinsop a,∗ , C.R. Huddlestone-Holmes a , D.R.W. Foster a , M.A. Edmiston a , P. Lepong a , G. Mark b , J.R. Austin a , F.C. Murphy c , A. Ford a , M.J. Rubenach a a

Predictive Mineral Discovery Cooperative Research Centre and School of Earth Sciences, James Cook University, Townsville, Queensland 4811, Australia b Predictive Mineral Discovery Cooperative Research Centre and School of Geosciences, Monash University, Melbourne, Victoria 3168, Australia c Predictive Mineral Discovery Cooperative Research Centre and School of Earth Sciences, University of Melbourne, Victoria 3010, Australia Accepted 13 August 2007

Abstract The three-dimensional crustal architecture of the eastern part of the Mount Isa Inlier is investigated from serial cross-sections constructed using geological map data, revised chronostratigraphy, gravity, magnetics, worms (multiscale wavelet edges of potential field data) and seismic data. The top part of the crust consists of rift and platform type metasediments that were deposited in three cover sequences from 1850 to 1610 Ma. These rocks constitute the Mount Isa Eastern Succession, and they were intruded by mafic–felsic plutons, dykes and sills of various ages before and during the Isan Orogeny (ca. 1.6–1.5 Ga). The Eastern Succession overlies a felsic metamorphic basement, which in turn sits on a tonalitic–gabbroic lower crust. The depositional basin architecture for the Eastern Succession was controlled by major N–S trending structures that penetrated the lower crust, and accommodated E–W extension. These structures also underlie major upper crustal structures such as the Mitakoodi Culmination and Snake Creek Anticline that were formed by contraction in the Isan Orogeny. Positive inversion may therefore have been a key process in the evolution of the eastern part of the inlier, and governs its architecture at the crustal scale. Inversion involved reactivation of basement-penetrating structures, which localised contractional structures in the cover sequences above, as well as influencing pluton emplacement. The felsic metamorphic basement may have been penetratively deformed during inversion. The spatial association between the basin-controlling and contractional structures suggests that either early extensional displacements were completely reversed by later contraction, or that much of the Eastern Succession has remained essentially parauthochthonous relative to the basement. © 2007 Elsevier B.V. All rights reserved. Keywords: Mount Isa Inlier; Crustal architecture; Inversion tectonics; Positive inversion; Extension; Isan Orogeny

1. Introduction The Eastern Succession of the Proterozoic Mount Isa Inlier consists of rift and platform type metasedimentary/metavolcanic rocks deposited in three major “cover sequences” that have experienced complex, locally high strain deformation and metamorphism up to amphibolite facies (Blake and Stewart, 1992). The good exposure of a variety of rock types that preserve a detailed record of deformation and metamorphism means that the Eastern Succession is an attractive target for studies of upper crustal tectonic processes (cf. Betts and Goleby, 2006). The apparently intracontinental setting for the major phase of defor-

∗

Corresponding author. Tel.: +61 7 4781 4318. E-mail address: [email protected] (T.G. Blenkinsop).

0301-9268/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.precamres.2007.08.011

mation, the Isan Orogeny from ∼1.6 to 1.5 Ga (Giles et al., 2002), is a particular feature of interest. A strong motive for understanding crustal deformation in the Mount Isa Inlier is also provided by its exceptional mineral endowment in a variety of commodities including Cu, Au, Pb, Zn, and Ag. For example, the Eastern Succession hosts Ernest Henry and Osborne Iron Oxide Copper Gold (IOCG) deposits, as well as numerous smaller IOCG deposits and occurrences, and the Cannington Ag–Zn–Pb deposit. Structural controls on mineralization are evident where detailed studies have been carried out (e.g. Baker and Laing, 1998; Williams et al., 2005; Mark et al., 2006). Structural studies of the Eastern Succession have concentrated on the exposed rocks, and, given greenschist–amphibolite facies metamorphism, have focused on upper crustal processes in selected parts of the inlier (e.g. Passchier, 1986; Passchier and Williams, 1989; Loosveld, 1989a,b; Pearson et al., 1992;

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Reinhardt, 1992a,b; Oliver, 1995; Giles and MacCready, 1997; Adshead-Bell, 1998; Laing, 1998; Mares, 1998; Rubenach and Barker, 1998; Oliver et al., 1999; Betts et al., 2000; Sayab, 2005, 2006; Giles et al., 2006; O’Dea et al., 2006; Potma and Betts, 2006). MacCready and coworkers (Drummond et al., 1998; Goleby et al., 1998; MacCready et al., 1998; MacCready, 2006) analysed upper crustal structure on the basis of the Mount Isa seismic transect, which crossed the central part of the inlier. However, as in many terranes that lack much deep seismic data, relationships between upper and lower crustal structure of the eastern part of the Mount Isa Inlier are poorly understood, and the profound changes in structure that occur along strike from the position of the Mount Isa seismic transect have not been explained. The aim of this contribution is to investigate the threedimensional architecture of the Eastern Succession and underlying structure down to the Moho in order to understand crustal scale structure better, focusing on the relationships between the upper and lower crust. A major implication to be drawn from the architecture is the importance of positive inversion in the evolution of the crust from basin formation through to convergent orogenesis. This study may have general relevance to other examples of intracontinental orogeny (e.g. Holdsworth et al., 2001; Hand et al., 2005; McClaren et al., 2005), as well as implications for large-scale patterns and processes of mineralization. 2. Background The study area comprises most of the exposed Eastern Succession (Fig. 1), and includes the Kalkadoon–Leichhardt and Eastern Fold Belts (Blake, 1987b). 2.1. Geology of the Eastern Succession The oldest rocks exposed in the study area, the Plum Mountain Gneiss (basement in Fig. 1, restricted to the SW of the study area), were deformed and metamorphosed in the Barramundi Orogeny at ca. 1900–1870 Ma (Etheridge et al., 1987). These rocks are overlain by the first cover sequence, the Leichhardt volcanics (1850–1875 Ma), and intruded by the Kalkadoon granite (∼1850 Ma) (Blake, 1987b; Blake and Stewart, 1992; Bierlien et al., 2008). Cover sequence 1 rocks are identified as Kalkadoon–Leichardt in Fig. 1, and are exposed only in the west of the study area.

Most of the rocks of the Eastern Succession were formed between 1780 and 1500 Ma. Sedimentary and volcanic rocks are described in terms of two further cover sequences (CS2 and CS3) deposited at 1780–1690 and 1680–1610 Ma, respectively (Blake, 1987b; Blake and Stewart, 1992; Page and Sun, 1998; Foster and Austin, 2008). CS2 rocks may have been deposited from ca. 1780 to 1690 Ma diachronously from west to east (Foster and Austin, 2008). In this study, the Tewinga Group (excluding the Leichhardt Volcanics), the Marraba Volcanics, the Ballara and Mitakoodi Quartzite, the Corella Formation and the Doherty Formation are included in CS2 (Fig. 1). The ca. 1680–1610 Ma CS3 units consist of quartzites, pelites, carbonates and volcanic rocks and display important lateral facies changes from west to east (Foster and Austin, 2008). These rocks include the Soldiers Cap, Young Australia and Mount Albert Groups (Fig. 1). The first major deformation to affect the cover sequences in the Eastern Succession was the extensional Wonga event of Holcombe et al. (1991) and Pearson et al. (1992) (see also Passchier, 1986; Passchier and Williams, 1989; Scott et al., 2000; Southgate et al., 2000). Ages in the range 1750–1735 Ma are associated with this event, which affected the Corella Formation and correlatives (Doherty Formation, Corella Beds), but not the Soldiers Cap Group, as this was not deposited at this time. The amount of deformation, kinematics and timing of this event have been debated (e.g. Stewart, 1987; Oliver et al., 1991; Blake, 1992; Bell et al., 1992). The Isan Orogeny began at ca. 1600 Ma and was dominated by EW compression that lasted at least episodically until ca. 1500 Ma. A summary of the Isan deformation and metamorphism sequence in the Eastern Succession is given in Table 1, based on Rubenach and Barker (1998), Rubenach and Lewthwaite (2002) and our own observations. The nature of the Isan D1 deformation is problematic: N to NW directed thrusting has been suggested by Betts and coworkers (e.g. Betts et al., 2006; MacCready et al., 2006) on the basis of field studies in two parts of the inlier. Sayab (2005) proposed N over S thrusting for D1 , while Giles et al. (2006) were not able to determine whether D1 was contractional or extensional. The major crustal structures produced during the Orogeny were D2a km scale upright folds and steep faults. From ca. 1550 to 1500 Ma, voluminous mafic and felsic potassic magmatism was emplaced at mid-crustal levels (the Williams and Naraku batholiths, abbreviated to the Williams Batholith: Fig. 1, Mark et al., 2005a). Despite having A-type geochemical signatures, the granites are

Table 1 Summary of deformation events and metamorphism in the Isan Orogeny in the Eastern Succession Event

DAb

D1

D2a

D2b

Timing Kinematics/major structure Fabrics

1660–1630 Extensional basin in CS3 times

Pre 1595 EW folding, thrusting

1600–1580 NS upright folding

Vertical shortening

EW axial planar foliation

NS axial planar upright cleavage Pervasive, dominant

Gently dipping crenulations Very localised

Distribution, effects

Local

DAb refers to an albitisation event (Rubenach et al., 2008).

D3a,b 1527 N to NE upright folding Crenulations, reactivation of S2 Local

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Fig. 1. Geology of the eastern part of the Mount Isa inlier (modified as discussed in text from NWQMPR). The approximate surface traces of the Cloncurry and Pilgrim worms are shown as dotted lines. The dashed lines show the Mount Isa seismic survey. Inset shows location of the inlier in NE Australia. Grey box shows the outline of the study area, with the lines of section shown on Fig. 6. Figs. 2 and 3 are of the same area, bound by 7,781,000 and 7,529,000 mN and 350,000 and 535,000 mE.

syn-tectonic and derived from high temperature crustal melting at pressures not exceeding 1000 MPa (Mark et al., 2005a). Microstructures suggest that there are probably more events in D1 , D2 , and D3 than those listed in the table, but these extra events appear not to have had a significant effect on the mesoand macroscopic structures. The large scale of this study does not allow resolution of the details of structures produced by different deformation events. P-T-t paths were complex, showing an initial anticlockwise segment, followed by a clockwise segment and then heating events corresponding to granite intrusion (Rubenach, 1992, 2005; Foster and Rubenach, 2006; Rubenach et al., 2008). Metamorphism is interpreted to has been caused by advec-

tive heating in the lower-middle crust, due to melt migration, and conductive heating in the upper crust (Rubenach et al., 2008). 2.2. Positive inversion: general aspects Positive inversion is the reversal of extensional fault movement during contractional tectonics (e.g. Williams et al., 1989). If the extensional fault has been associated with pre- and syn-rift deposition, these sequences may show extensional or contractional movements after inversion, but post-rift sequences will always show reverse movements. The position on the fault surface at which contractional movements change to extensional

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papers in Butler et al., 2006), as well as analogue models (e.g. Yamada and McClay, 2003a,b; Del Venitisette et al., 2006) show that crustal geometries may be complex as a consequence of inversion. A critical factor that distinguishes different types of inversion is the extent to which basement is involved. Examples indicate four possibilities (Fig. 2): Type 1. Inversion occurs on faults exclusively within sedimentary or cover sequences (Fig. 2b; e.g. Hayward and Graham, 1989; Glen et al., 2005). This mechanism requires decollement between the basement and cover; the latter is commonly folded (e.g. Bailey et al., 2002). Type 2. Faults cutting into basement are passively reactivated (Fig. 2c; e.g. Coward et al., 1989; Mason, 1997; Marshak et al., 2000). Basement blocks may form buttresses (e.g. Butler, 1989; Bailey et al., 2002; Betts et al., 2004). Type 3. New faults are formed in the basement, but there is little penetrative basement strain (Fig. 2d; e.g. Noll and Hall, 2005). Type 4. Basement is penetratively deformed (Fig. 2e; e.g. Butler, 1989; Ghisetti and Sibson, 2006). By analogy with terms commonly used in contractional deformation, type 1 could be called “thin-skinned” inversion, and types 3 and 4 “thick-skinned inversion”; type 2 is an intermediate case. More than one type of inversion may occur progressively in an inversion event (e.g. Williams et al., 1989), as shown in the schematic progression of Fig. 2. 2.3. Inversion in the Mount Isa Inlier—previous examples

Fig. 2. Inversion geometries: a schematic summary. Active faults indicated by half-arrows; shortening in cover and basement shown by full black and white arrows, respectively. (a) Pre-, Syn-, and Post-rift sequences after a rifting event. (b) Type 1 inversion: contractional deformation is confined to cover sequence rocks, including reverse faulting and folding. This requires decollement along the basement–cover interface (Hayward and Graham, 1989; Bailey et al., 2002; Glen et al., 2005). The null point is where there is no stratigraphic separation on the fault: extension is exactly balanced by contraction. (c) Type 2: basement normal faults are reactivated as reverse faults, but there is no penetrative deformation in the basement (Coward et al., 1989; Mason, 1997; Marshak et al., 2000). (d) Type 3: New reverse faults are formed, which may cut the basement. The basement is still not penetratively deformed. A footwall shortcut fault is shown (Noll and Hall, 2005). (e) Type 4: penetrative deformation of the basement occurs, including the basement–cover interface (Butler, 1989). Inversion may involve a progression from types 1 to 4 (cf. Williams et al., 1989).

is called the null point (Fig. 2b). Folds are commonly localised above faults, and strain, including folds, may be concentrated in the hanging wall of normal faults because of the presence of a relatively rigid footwall. This is known as the buttressing effect (e.g. Coward et al., 1989). Fault dip directions may be reversed during reactivation (Hayward and Graham, 1989). Published examples of positive inversion (e.g. Cooper and Williams, 1989; Buchanan and Buchanan, 1995; Coward, 1994;

The geological evolution of the Mount Isa Inlier described above involves basin formation followed by contractional deformation; this virtually requires positive inversion, and the process has been identified in the Mount Isa Inlier by a number of previous studies. Most of this research has concentrated on the Western Succession because the generally lower states of strain and metamorphic grade allow much finer resolution of the stratigraphy, which facilitates definition of inversion (e.g. O’Dea and Lister, 1995; Broadbent et al., 1998; Lister et al., 1999; Betts, 1999, 2001; Betts et al., 2004). Some recent studies, however, have elucidated details of positive inversion in the Eastern Succession. A study around the Mitakoodi Culmination by Potma and Betts (2006) showed how normal faults around the Culmination were inverted during the Isan Orogeny, resulting in buttressing effects and complex fold geometries involving the cover sequences. This is type 1 inversion in the above classification. O’Dea et al. (1997) proposed that the Eastern Fold Belt, containing much of the Eastern Succession, may be an inverted backarc basin, created by stacking of younger, hotter basin phases over older basins in a thin-skinned thrust style, involving a major detachment (the Argylla detachment) over which upright folds were formed above ramps, and as fault propagation folds. The basement, which is beneath the detachment, is largely uninvolved in this inversion (type 1), except in the final stages when some folding occurs. A similar

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evolution, from extension to thin-skinned, NW directed thrusting and thick-skinned EW shortening is proposed for the SE part of the inlier by Giles et al. (2006), also implying inversion tectonics for the Eastern Succession. The structures described in the latter study are all within the cover sequence rocks, so that it is unclear whether basement is involved. 3. Data sources 3.1. Geology map data The 1:100,000 geology maps compiled by the Bureau of Mineral Resources (BMR) are available for all of the area in which Proterozoic rocks crop out (Derrick et al., 1977; Derrick and Little, 1979; Derrick, 1980; Wilson et al., 1980; Blake et al., 1982, 1983, 1984; Donchak et al., 1984; Ryburn et al., 1988). These data provide a good representation of the surface geology at the 1:100,000 scale, despite some inconsistencies between map sheets. The 1:500,000 map by Blake (1987a) is valuable in providing a broad scale overview of the geology of the region, and Blake (1987b) contains useful information such as formation thicknesses. 3.2. North West Queensland Mineral Province Report The North West Queensland Mineral Province Report (NWQMPR) (Queensland Department of Mines and Energy et al., 2000) contains an interpreted solid geology map/GIS at 1:250,000 scale and an attributed fault GIS dataset. The map units are based on an updated “Time-Space Chart” that corrected some of the problems in Blake’s (1987a) map. This interpreted geology provides the base geology from which the architecture is analysed. 3.3. Gravity data Bouguer anomaly gravity data (Murray, 2001; reference density 2670 kg m−3 ) has a grid cell size of 0.5 min of arc (ca. 800 m), giving 1600 m for the lowest wavelength of anomaly that can be seen. However, the observations used to generate the grid in the study area have highly variable spacings (Fig. 3) that range from hundreds of metres to over 10 km. This further limits the scale at which the anomalies can be resolved. A false colour Bouguer anomaly map is produced for the area (Fig. 3) as well as anomaly profiles at various levels of upward continuation. These profiles help to separate different wavelength anomalies. 3.4. Magnetic data Aeromagnetic data from MIM Holdings Ltd. (now Xstrata) are used. The survey was flown at a height of 70 m with flight and tie line spacings of 70 and 400 m, respectively. The total magnetic intensity dataset is grided at a 50 m cell size, providing a high quality dataset that shows good detail. A greyscale total magnetic intensity map is produced for the area (Fig. 4) as well as anomaly profiles at various levels of upward continuation.

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3.5. Worms Both gravity and magnetic worm datasets (multiscale wavelet edges of potential field data, cf. Archibald et al., 1999; Holden et al., 2000) provide key data for this project. The worm processing was done by Fractal Geosciences using the MIM aeromagnetics and gravity data. Both maximum and enhanced first vertical derivative worm types were produced. The magnetic data were regrided to a 200 m cell size and upwardly continued to just under 30 km. The gravity data were regrided to 500 m and upwardly continued to just over 60 km. Worms can be used in the interpretation of the 3D geology. The worms show maximum gradients in potential field data and resolve small wavelength anomalies (low levels of upward continuation) from large wavelength anomalies (higher levels of upward continuation). Worms are particularly useful for defining the dips of structures and their continuity both at depth and along strike. It is important to note that worms will be present only where there is a contrast in the petrophysical properties on either side of a fault or lithological contact, and that a very strong anomaly at the surface may result in high levels of upward continuation that do not represent a deep source. The magnetic worms provide information on structures near the surface to approximately 10 km depth because of the fine scale of the original data and the maximum height of upward continuation. On the other hand, the gravity worms provide information on structures in the 2–20 km depth range. 3.6. Seismic data The Mount Isa seismic survey (Goleby et al., 1998; Goncharov et al., 1998: Fig. 1) consists of a 250 km seismic reflection line and a coincident but longer (500 km) seismic refraction line. In the study area, the seismic line runs E–W at approximately 7,690,000 mN. Interpretations of the seismic data for the Eastern Succession have been presented in Goleby et al. (1998), MacCready et al. (1998) and MacCready (2006). The seismic reflection line has recently been reprocessed in an attempt to improve the resolution (Lepong and Blenkinsop, 2005). The reprocessing made improvements in signal-to-noise ratio, continuity of reflectors, and collapsed some diffractions. However, the section below approximately 3 s two-way travel time (approximately 9 km) remained almost devoid of information. The refraction data was collected to provide regional velocity data. Because of its coarse shot and receiver spacing it reveals little structural data and is more useful as an indicator of lithology and crustal composition at depth. Unfortunately it has not been possible to source a digital version of the velocity data. 3.7. Stratigraphic data A revised chronostratigraphy constructed by Foster and Austin (2008) is used for this study (Fig. 5). The revisions are made on the basis of all available age dates and reinterpretation of stratigraphic relationships, and are justified in Foster and Austin (2008). The main changes to the chronostratigraphy of

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Fig. 3. False colour Bouguer gravity anomaly image. Points show observation locations. The box and lines show the study area and cross-section lines. Data from Murray (2001).

the NWQMPR are • including the Kuridala Formation with the Soldiers Cap Group; • moving the Lewellyn Creek Formation out of and below the Soldiers Cap Group; • assigning an age of 1725 Ma for the Doherty Formation (younger than the Corella Formation, the Wonga Batholith and Mount Fort Constantine Volcanics); • grouping the siliciclastic rocks in some of the eastern units of the Mary Kathleen Group (Answer Slate, Marimo Slate, Stavely Formation) together as the informal Young Australia Group (as lateral facies variations to the Soldiers Cap Group and Mount Albert Group);

• grouping the Ballara and Mitakoodi Quartzites as age equivalent units. These changes are reflected in a revised version of the NWQMPR geology map (Fig. 1). Some other modifications made to the geology map of the NWQMPR based on newer age data and reinterpretation of stratigraphy are given below and justified in Foster and Austin (2008): • Removing the double-crossing metamorphics and replacing them with Ballara/Mitakoodi Quartzite. • Assigning Wonga Batholith ages to the Levian Granite (southern end of the Naraku Batholith) and interpreted granites to the south of the Gin Creek Granite.

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Fig. 4. Greyscale total magnetic intensity (TMI) image. The grey box and lines show the study area and cross-section lines. Data from MIM Holdings Ltd. (Now Xstrata).

• Dividing the Capsize Creek Granite into the Dipvale Granite (southern end-Wonga Batholith) and the Mavis Granite (northern end-Williams Batholith). • Separating the Doherty Formation from the Corella Formation. • Including the Tommy Creek Beds and Roxmere Quartzite in the Young Australia Group. 4. Methods Seven E–W cross-sections are constructed from 7,570,000 to 7,750,000 mN (the 7,570,000 mN section is referred as 757, etc.) at 30-km spacing, using the worm data for a 600 m wide

swath centred on the section northing, the 1:100,000 geology GIS, the NWQMPR geology map and a strip map of the magnetic worms (Fig. 6). A Fracsis database containing all available digital data is used to provide a 3D context while constructing the sections. This is very important for tracing structures and maintaining consistent interpretations between sections. The sections show a “conservative” interpretation of the geology: shapes of contacts are kept simple and faults are not extrapolated to form linked networks. Once the cross-sections had been drawn they were digitised and imported into GoCad and Fracsis. Surfaces were built in GoCad for major faults and the lower crust. Surfaces for lithological contacts and intrusions were constructed.

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Fig. 5. Revised, simplified stratigraphy of the Eastern Succession, based on Foster and Austin (this issue).

5. Assumptions The biggest problem in interpreting the structure in 3D is the paucity of data in the vertical dimension. This problem requires some assumptions to be made, of which the

most basic was the present composition of the crust, down to the Moho. An iterative process was used to derive internally consistent interpretations. The forward modelling of gravity data discussed below is a key part of this process.

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The crustal geometry assumes that a top layer of upper crust is composed of siliciclastic and calcareous metasedimentary rocks, granitic intrusions and minor mafic material of the three cover sequences. This layer has a maximum thickness of up to 10 km and a density that ranges between 2.7 and 2.8 × 103 kg/m3 (Table 2). A basement of felsic composition, with a density of 2.9 × 103 kg/m3 (Table 2) is interpreted to underlie these rocks,

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which crops out in the SW of the area as the Plum Mountain Gneiss and Kuribaya Migmatite. These or similar rocks are interpreted to be present between the cover sequence 1 and the denser rocks of the mid-lower crust. The mid to lower crust, under this felsic basement, is interpreted to be composed of mixed tonalitic–gabbroic rocks and melt residual material (cf. Wyborn, 1998; Mark et al., 2005a). These rocks are interpreted

Fig. 6. Seven, true-scale geological cross-sections to the lower crust along lines indicated in Fig. 1. Cross-sections labelled by abbreviated UTM Northing.

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Fig. 6. (Continued ).

to have densities that range between 3 and 3.1 × 103 kg/m3 (Table 2). Combined whole-rock data and geobarometric constraints on intrusive phases of the Williams Batholith show that they were sourced from this mid-lower crustal material 10–20 km below the current level of exposure (cf. Pollard et al., 1998; Mark, 2001; Mark et al., 2005a,b). This depth control, combined with the maximum thickness of units in the three cover sequences, provides a constraint on the thickness of the felsic basement. The mid-lower crustal material is interpreted to extend to the transition zone described by Goncharov et al. (1998), which sits immediately above the Moho at a depth of 50–55 km.

6. Validation The three central cross-sections (772, 769 and 766) are forward modelled against gravity data to validate the interpreted cross-sections (Fig. 7). An average density value for each unit is obtained from a compilation of open source petrophysical data (Hone et al., 1987) and density measurements of characteristic rock samples for individual units (Table 2). The 2D models are constructed from the cross-sections using ModelVision Pro software and the average density values are assigned to each unit. The modelled gravitational responses are compared to the measured gravity data, extracted from the regional gravity grid

T.G. Blenkinsop et al. / Precambrian Research 163 (2008) 31–49 Table 2 Density ranges of units used in the forward models Stratigraphic unit

Density range (×103 kg/m3 )

Young Australia Group Soldiers Cap Group Doherty Formation Corella Formation BM Quartzite Marraba Volcanics Tewinga Group Kalkadoon/Leichhardt Williams Batholith Wonga Granite Ernest Henry Diorite Basement Lower crust

2.76–2.78 2.79–2.81 2.75–2.83 2.76–2.81 2.73–2.77 2.76–2.82 2.68–2.72 2.67–2.73 2.65–2.68 2.65–2.68 2.84–2.86 2.85–2.91 2.98–3.10

along each section line. Densities of units in the model were then varied, within the range of values measured for each unit, in order to achieve a better fit between the modelled and measured gravitational response. The fit of the modelled curve to the measured gravity data is generally very good (Fig. 7). While the geometries of the smaller near-surface units are poorly constrained, the geometry of the basement–lower crust interface (with a general density contrast of 0.1 × 103 kg/m3 ) plays an important part in achieving a good model fit. The forward modelling process has demonstrates that the central cross-sections are valid solutions of source distributions using the densities shown. Two-dimensional forward modelling of gravity data based on the Mt Isa seismic refraction section (Goleby et al., 1998) has also been performed. Seismic velocities from the section were

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converted to densities based upon the Nafe-Drake relationship between P-wave velocity and density (Ludwig et al., 1971). The modelling demonstrated that the high seismic velocity layer in the upper crust identified in the refraction survey (Goncharov et al., 1998) is incompatible with the gravity data, and hence this body was not used as an input to interpreting the cross-sections. 7. Results and interpretations of crustal structure The seven E–W cross-sections of the study area (Fig. 6) show the crustal architecture from the top of the transition zone to the current land surface; the inferred geometry shown above surface on lines 769, 766 and 763 emphasises the importance of the major Mitakoodi culmination by showing its pre-erosional geometry. 7.1. Basement architecture Basement architecture has been interpreted on the basis of the long-wavelength gravity features and the gravity worms, which define major contrasts in gravity. There are three major gravity worms referred to as the KL (Kalkadoon–Leichhardt), Pilgrim and the Cloncurry worms (Fig. 8), all trending generally NS. The KL worm implies a steeply dipping structure close to the surface near the western edge of outcrop of Eastern Succession rocks. The Pilgrim worm is readily defined at high levels of upward continuation, but is less apparent as a discrete surface near ground level. No structure that corresponds to the Pilgrim worm can be identified in exposed rocks, although it is nearly coincident with the Pilgrim fault in its central section. The Pilgrim worm indicates a generally east dipping density contrast. In the south-

Fig. 7. Forward models of gravity on the three central sections (766, 769 and 772). Model assumptions and methods discussed in the text.

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Fig. 8. Bouguer gravity anomaly as a 3D surface overlain by surfaces corresponding to the three major gravity worms. Oblique view towards the NW. Data from Murray (2001).

ern part of the model area, although the worm appears to be a continuous structure, it may step to the east as an en echelon segment. The Cloncurry worm indicates a structure that dips steeply either to the west or east. It does not persist to the highest levels of

upward continuation south of about 7,700,000 mN (Austin and Blenkinsop, 2008). Gravity data indicate that the structure corresponding to the Cloncurry worm cuts and displaces basement with an east side up sense. The major gravity worms are interpreted as steps in the lower crust because they separate long-wavelength anomalies of different magnitudes. Elevated gravity anomalies are interpreted as basement highs. An alternate hypothesis would be that gravity highs are caused purely by the intrusion of large volumes of mafic rocks in the regions that are now gravity highs. This is considered unlikely for two reasons. Firstly, the area of mafic outcrop is not high enough to increase the average density sufficiently; secondly, the seismic refraction study suggests the density of the crust down to 18–20 km is “granitic” (i.e. metasedimentary rocks, felsic volcanic rocks, granite) in terms of its silica content (Goncharov et al., 1997). However, it is possible that increased volumes of unexposed mafic rocks could play some role in the generation of the gravity highs. Additional alternative interpretations of the worms as contrasts in alteration or metamorphism are unlikely given the lack of evidence for these variations in rocks at surface anywhere along strike of the worms. The major gravity (Fig. 8) and magnetic worms (Fig. 9) are approximately coincident in several places, showing that the lower crustal steps that give rise to the gravity worms have a shallower expression that causes the magnetic worms.

Fig. 9. Greyscale total magnetic intensity (TMI) image as a 3D surface overlain by surfaces corresponding to the three major aeromagnetic worms. Oblique view towards the NE. Data sources as discussed in the text.

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At the latitude of Cloncurry, a subtle but consistent change in strike occurs in the gravity and magnetic anomalies. These anomalies are deflected to more easterly strikes over a zone approximately 20 km wide striking NE, which is referred to as the Cloncurry Flexure. This feature is most pronounced in the magnetic image (Fig. 10). The Cloncurry worm cannot be recognised as a discrete structure in this zone. To the north of the Cloncurry Flexure, a north-striking worm with similar continuity to the Cloncurry worm and higher levels of upward continuation can be seen. If this worm is the northern continuation of the Cloncurry worm, it indicates a dextral deflection or displacement of approximately 20 km. The distribution of gravity highs and lows changes across the Cloncurry Flexure, with a broad central high to the north and the two ridges to the south: this change may reflect the presence of mafic intrusive rocks of the Ernest Henry Diorite in the north. The lack of a direct surface expression of this flexure, combined with the above observations, suggest that it is a deep and fundamental crustal structure, which may have been a transfer zone during basin formation that was active from early stages of basin formation through to orogenesis. 7.2. Depositional basin architecture and stratigraphy Fig. 10. TMI aeromagnetic image superimposed with surfaces representing the major magnetic worms showing a change in strike of major features at Cloncurry (the Cloncurry Flexure), represented by the ENE trending vertical surface.

The basin architecture is established by compiling stratigraphic thickness estimates on 1: 100,000 sheets and analysing their geographical variation. The results are discussed in detail in Foster and Austin (2008). Fig. 11a is a schematic cross-section

Fig. 11. (a) Cartoon of the stratigraphic relationships of cover sequences 2 and 3 (CS2 and CS3) in the Eastern Succession, based on Foster and Austin (2008). LCF, Lewellyn Creek Formation; MFC, Mount Fort Constantine Volcanics. Scales in km. (b) Simplified true-scale cartoon of deformed cross-section corresponding approximately to the undeformed section shown in part a, based on lines 766 and 769, and omitting faults except those interpreted to cut the basement. The cartoon shows approximately the same shortening as calculated for the Mitakoodi Culmination, the largest fold in the Eastern Succession, shown localised over the Pilgrim worm structure. The null point where reverse separation changes to normal separation is located around the double arrow that indicates inversion.

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Fig. 12. Reprocessed, migrated seismic section over the Mitakoodi Culmination (a) (Lepong and Blenkinsop, 2005) and interpreted cross-section (b). The major insights offered by the seismic section are reflectors interpreted to represent the base of cover sequence 2 under the Bulonga anticline and the Duck Creek anticline, and the lack of a consistent sub-horizontal surface representing a detachment. Other features are similar to the large-scale structure suggested by MacCready (2006).

showing the stratigraphic relationships in the central northing of the study area. It shows two structures that continue into the lower crust that may have influenced the deposition of the three cover sequences. The western most structure is interpreted to have controlled the location of a growth fault during the deposition of early CS2 rocks, as well as the relationships between the Mount Albert and Young Australia Groups of CS3. The Eastern Succession is interpreted to represent the western margin of a sedimentary basin that had a depocentre to the east of where Proterozoic rocks now crop out. This basin formed as the result of E–W extension, and the Pilgrim and Cloncurry worms and the Cloncurry Flexure are interpreted as having formed by basement–cutting structures that were active during basin formation. CS1 rocks were deposited early during basin formation and crop out only on its western margin. The lateral extent of CS1 and CS2 to the east is unknown. As basin formation progressed in CS2, the depocentre migrated to the east. 7.3. Upper crustal structure Large-scale aspects of the upper crustal structure of the Eastern Succession have been examined, using aeromagnetic worms to constrain dips of contacts, and the reprocessed Mount Isa seismic section to assist in extrapolation of the surface geological data. The interpretation of the reprocessed section (Fig. 12) is broadly similar to the previous interpretations of MacCready et al. (1998) and MacCready (2006), although the new processing does not suggest the presence of a sub-horizontal surface, interpreted as a detachment by MacCready et al. (1998), across the whole section. The largest upper crustal folds (Fig. 1) are the Mitakoodi Culmination, the Snake Creek Anticline, the Weatherly Creek

Syncline, the Middle Creek Anticline and a feature to the east of Eloise referred to as the Holy Joe Creek structure (cf. Edmiston et al., 2008). Fold shapes in the cross-sections were constrained from surface geology, and the reprocessed seismic section along line 769. All the folds identified above have upright to steeply dipping, approximately NS axial surfaces (Figs. 1 and 6). The Mitakoodi Culmination, which is the largest fold, plunges gently N to NE and consists in the north of the Wakeful Syncline between the Bulonga and Duck Creek Anticlines (Fig. 6). The Pilgrim worm at surface follows approximately the axial surface of the Mitakoodi Culmination in its northern part. The Snake Creek Anticline hinge plunges subvertically in the northern part of the structure, and much of the structure consists of overturned beds. The Cloncurry worm structure coincides with part of the Snake Creek Anticline, which is separated by the tight angular Weatherly Creek Syncline from the Middle Creek Anticline; the latter appears to have a similar geometry to the Snake Creek Anticline. The Holy Joe Creek structure is delineated on the aeromagnetic interpretation by almost continuous high susceptibility thin units, commonly in a pair or triplet, which are readily interpreted as mafic sills by analogy with their signature around the Snake Creek and Middle Creek folds (Edmiston et al., 2008). The structure also appears to be similar to the Snake Creek Anticline. Significant faults identified from surface geology and worms include the Fountain Range Fault, the Pilgrim Fault, the Cloncurry Fault (Fig. 1) and a number of N–S faults inferred to be under cover in the eastern part of the area. All the major faults are subvertical at surface on the basis of dips given by worms, and strike approximately NS. Fault depth can be inferred to some extent by the levels of upward continuation in worms corresponding to the faults. NE trending faults of generally shorter

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strike length appear to offset the Cloncurry Fault and worm dextrally in places.

Typical features of inversion that can be seen in the crosssections (Figs. 6 and 11b) are

7.4. Intrusions

1. a change from extensional displacements at depth to contractional at surface; 2. the existence of a null point somewhere above the mid-lower crust at which the change from extensional to contraction occurs; and 3. the localisation of large-wavelength folds above the normal fault.

The largest intrusions are all bodies of the Williams and Naraku batholiths (including the Squirrel Hills, Wimberu, Mount Angelay, Naraku, Saxby and Yellow Water Hole granites, and the unnamed interpreted granites south of the Wimberu granite and under the Holy Joe Creek structure). Geophysical modelling of shallow crustal cross-sections indicate that the edges of these granites generally dip steeply, but that the granites are no more than a few kilometers thick (Barlow, 2005). These intrusive masses are represented as tabular bodies in the cross-sections. Both radiometrics and aeromagnetic patterns show clearly that the larger bodies such as the Wimberu and Squirrel Hills granites are composite, containing several discrete intrusions that may have approximately internal concentric zones, and they contain a wide range of compositions (Wyborn, 1998). The presence of intrusions in the cores of the Mitakoodi, Snake Creek, Middle Creek and Holy Joe Creek structures is a notable feature (Figs. 1 and 6). The first three are all phases of the Williams Batholith, which is also likely for the granite in the core of the Eloise structure. 8. Discussion: positive inversion as an explanation of relationships between basement, basins and upper crustal structures 8.1. Inversion in the Eastern Succession A striking feature on the largest scale brought to light by this study is the spatial coincidence between the major gravity worm (the Pilgrim worm) and the major fold structure, the Mitakoodi Culmination. Cross-sections 769, 766 and 763 (Fig. 6) show how the Mitakoodi Culmination (comprising the Bulonga Anticline, the Wakeful Syncline and the Duck Creek Anticline in line 769) overlies a basement-penetrating fault. Stratigraphic evidence shows that the Pilgrim worm structure served as a major locus of sedimentary thickening to the east from CS2 to CS3 times (ca. 1760–1610 Ma). This structure is likely to have been of fundamental significance since before the Isan Orogeny, because it also marks a contrast in lower crustal isotopic signatures (Mark et al., 2005b). The Pilgrim worm corresponds to a structure with a normal separation of basement. These observations may be used to suggest that the Pilgrim worm structure represents a basement fault that controlled the architecture of the depositional basins in CS2 and CS3, with thickening to the east in its hanging wall (Fig. 11a). The coincidence in the location of the Pilgrim worm with the Mitakoodi Culmination (Fig. 11b) suggests the hypothesis that the fault has been inverted, and that the Mitakoodi Culmination may have developed in response to reverse movement on a fault corresponding to the Pilgrim worm. In places the surface expression of the Pilgrim worm may be the Pilgrim fault itself.

The cross-sections suggest type 4 inversion in which the felsic basement represented at surface by the Plum Mountain Gneiss has been penetratively strained during inversion: the contact between this basement and the cover sequences appears to be folded. Although the age of the Plum Mountain Gneiss is not known, the degree of deformation and the orientation of the foliation in the Gneiss are similar in the adjacent Argylla formation, strongly suggesting an Isan age for this deformation. The cross-sections do not show folding of the felsic/mafic basement contact. However, the detailed shape of this contact is not constrained by the data, and it may be penetratively deformed. The other major gravity worm in the Eastern Fold Belt is the Cloncurry worm, which coincides with the Snake Creek Anticline. Cross-section 769 (Fig. 6) shows the association between a basement penetrating structure corresponding to the Cloncurry worm, and the Snake Creek Anticline (the fold to the east of the Cloncurry Fault). Inversion is also compatible with some features of the structure inferred to cause the Cloncurry worm, which marks a considerable thickening of the Soldiers Cap Group to the east. The coincidence of the Snake Creek Anticline with the Cloncurry worm may be due to the localising effect of a basement structure. The dip direction of the Cloncurry worm reverses along its length, effecting normal and reverse separations of the basement. The possibility of reorientation of the Cloncurry worm structure during positive inversion can be raised to explain the variability in dip direction and change in separation. Reorientation of the structure also implies type 4 inversion with basement deformation. Other structures have been inferred in the basement between the Cloncurry and Pilgrim worms (Fig. 6). Because of their more limited strike extent, it has not been possible to link these so clearly with surface structures. These structures may have played a role in the facies changes observed in CS2 and 3 (i.e. the change from Corella to Doherty Formation in CS2 and from Young Australia to Soldiers Cap Groups in CS3), and also in the overall thickening of CS3 to the east. In summary, the relationship between major basement structures, stratigraphy and contractional structures suggests that positive inversion is a key process in the evolution of the inlier, not only on the regional scale as recognised by Potma and Betts (2006) and O’Dea et al. (1997), but also at the scale of the whole crust. The dominant role of NS structures was established at least as early as CS2 deposition and possibly before the time of CS1, and continued throughout the crustal evolution. NE-SW orientated structures, such as the Cloncurry Flexure, may also have

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been important in basin formation and subsequently (Austin and Blenkinsop, 2008). 8.2. Implications of inversion for crustal structure, basement–cover relations, plutonism and metallogenesis in the Eastern Succession The spatial alignment between stratigraphic changes, extensional and contractional structures has an important implication for the relationship between the basement and Paleoproterozoic cover sequence rocks over the Pilgrim worm structure. This alignment would not be preserved if large-scale nappe structures or d´ecollement had occurred either in extension or contraction, unless the contraction was equal and in the opposite direction to the extension. A simpler alternative is that the Paleoproterozoic cover is broadly autochthonous (parautochthonous) relative to the Pilgrim worm structure in the mid-lower crust. In either case, significant constraints are placed on both early extensional and all contractional phases of the Isan deformation. If the cover is parautochthonous to the basement, this implies that shortening must also have occurred in the basement. A simple line length balance over the Mitakoodi Culmination as drawn in cross-section 768 indicates a stretch of 0.44 (or 56% shortening), which must also have occurred in the basement. The present thickness of the crust is approximately 40 km, but evidence from the Ce/Y ratios in mafic rocks indicates that it was much thinner at 1686 Ma (Butera et al., 2005). Underplating, thrust stacking (possibly on reactivated normal faults) and penetrative deformation are all ways in which the crust may have thickened, and thickening may have occurred in more than one event. The type 4, thick-skinned inversion described here certainly contributed to the crustal thickening from 1686 Ma to post-Isan Orogeny times. The existence of Williams Batholith plutons in the cores of the major structures of the Eastern Succession is striking and suggests that inversion may have had a role to play in the intrusion of these plutons. Inversion may have directly controlled the plutons by providing magma ascent pathways via reactivated faults, or the control may be indirect if pluton emplacement was primarily influenced by the contractional structures such as the Mitakoodi Culmination and Snake Creek Anticline. The identification of long-lived, crustal scale structures in the Eastern Succession would appear to have significant metallogenic implications. Such structures are able to act as fluid pathways and may have played a role in locating mineral deposits: for example Ernest Henry mine is located on a bend in a N–S structure that may have been a basin margin fault; this was reactivated in contraction during copper–gold mineralization (Mark et al., 2005c, 2006). This structural setting is very similar to that at Eloise IOCG deposit (Baker et al., 2001). The existence of reactivated structures may have played a role in the exceptional mineral endowment of the Eastern Succession, although the relationship between specific structures and copper–gold mineralization is complex because the Pilgrim worm does not appear to be associated with major mineral deposits. The study has highlighted a number of aspects of Eastern Succession geology that need further investigation. Studies of

exposed features that may link with the major lower crustal structures, and thus preserve some evidence for reactivation, are called for. One example is the area where the structure corresponding to the Pilgrim worm coincides with the Pilgrim Fault. The magnetic worm on the margin of the Wonga belt requires a geological explanation. The Starra–Selwyn area, which has evidence for strong localised deformation (Adshead-Bell, 1998), partly coincides with a significant gravity worm: this relationship may be similar to the Pilgrim worm. The study also emphasises the need for a better understanding of the stratigraphy of the Eastern Succession based on high calibre new geochronological work. For example, the age and significance of the double-crossing metamorphics is very important to understanding the deeper architecture of the Eastern Succession, because these rocks have a maximum age of ca. 1815 Ma from detrital zircons and are interpreted as Ballara/Mitakoodi equivalents (ca. 1755 Ma) for the purposes of the current model. However, they may be considerably older than this and, if so, would place a major constraint on the 3D crustal architecture. More age dating for CS3 rocks is needed to test the suggested correlations made in this study. 9. Conclusions The upper crustal structure of the eastern part of the Mount Isa Inlier is dominated by kilometre- to tens of kilometrewavelength upright folds, and north to NE trending steeply dipping faults such as the Fountain Range, Pilgrim and Cloncurry faults. These structures are developed in three cover sequences of platform type metasediments, that accumulated from 1850–1610 Ma, and which were deformed in the Isan Orogeny from ca. 1600–1500 Ma, and intruded by tabular bodies of the Williams Batholith. They overlie a deformed felsic basement of variable thickness, which in turn overlies a mafic lower crust. The major structure in the lower crust is a depression in the central part of the area between basement highs to the west and east. The boundaries of this lower crustal feature are structures that correspond to worms known as the Pilgrim and Cloncurry worms. A clockwise deflection in strike of all crustal features around Cloncurry occurs along a 20 km wide NE zone, called the Cloncurry Flexure. There is an important relationship between the structure corresponding to the Pilgrim gravity worm and the depositional basin architecture: major thickness increases occur to the east over this structure in some of the cover sequence 2 and 3 rocks. The Mitakoodi Culmination and Snake Creek Anticline occur over the structures corresponding to the Pilgrim and Cloncurry worms, respectively. The relationships between changes in depth to the lower crust (which cause the major gravity worms), depositional basin architecture, and major folds suggest that basin margin faults exerted a fundamental control on both sedimentation and contractional deformation during the geological history of the inlier from 1780 Ma to at least the major deformation during the Isan Orogeny. The Pilgrim worm structure can be interpreted as a basin margin fault that was positively inverted to localise the largest fold structure in the inlier—the Mitakoodi Culmination. Likewise the Snake Creek Anticline overlies the

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structure corresponding to the Cloncurry worm and appears to have been localised by it. Inversion may also have had a role in pluton intrusion. The coincidences between major stratigraphic changes, the Mitakoodi Culmination and the basement structure, mean that either any early large extensional deformation was completely reversed by later contraction, or that the cover is parautochthonous with respect to the basement. Acknowledgments The Predictive Mineral Discovery Cooperative Research Centre supported publication of the printed colour figures. We acknowledge the pmd*CRC for supporting this project and thank Xstrata for the use of their magnetic worm data. Fractal Graphics Pty Ltd. (now Geoinformatics Exploration Australia Ltd) is thanked for supplying Fracworms to the pmd*CRC. Two constructive reviews by Andy Barnicoat and Laurie Hutton are acknowledged. References Adshead-Bell, N.S., 1998. Evolution of the Starra and Selwyn high-strain zones, Eastern Fold Belt, Mount Isa Inlier: implications for Au–Cu mineralization. Econ. Geol. 93, 1450–1462. Archibald, N.J., Gow, P., Boschetti, F., 1999. Multiscale edge analysis of potential field data. Explor. Geophys. 30, 38–44. Austin, J.R., Blenkinsop, T.G., 2008. The Cloncurry Lineament: geophysical and geological evidence for a deep crustal structure in the Eastern Succession of the Mount Isa Inlier. Precambrian Res. 163, 50–68. Bailey, C.M., Girogis, S., Coiner, L., 2002. Tectonic inversion and basement buttressing: an example from the central Appalachian Blue Ridge province. J. Struct. Geol. 24, 925–936. Baker, T., Laing, W.P., 1998. Eloise Cu–Au deposit. East Mount Isa Block; structural environment and structural controls on ore. Aust. J. Earth Sci. 45, 429–444. Baker, T., Perkins, C., Blake, K.L., Williams, P.J., 2001. Radiogenic and stable isotope constraints on the genesis of the Eloise Cu–Au deposit, Cloncurry district, northwest Queensland. Econ. Geol. 96, 723–742. Barlow, M., 2005. Potential field modelling of geological cross-sections Duchess-Urandangi region Mount Isa basin. In: Blenkinsop, T.G. (Ed.), Final Report, Total Systems Analysis of the Mt Isa Eastern Succession. Predictive Mineral Discovery CRC (Digital Appendices). Bell, T.H., Reinhardt, J., Hammond, R.L., 1992. Multiple foliation development during thrusting and synchronous formation of vertical shear zones. J. Struct. Geol. 14, 791–805. Betts, P.G., 1999. Palaeoproterozoic mid-basin inversion in the northern Mount Isa terrane, Queensland. Aust. J. Earth Sci. 46, 735–748. Betts, P.G., 2001. Three-dimensional structure along the inverted Palaeoproterozoic Fiery Creek Fault System, Mount Isa terrane, Australia. J. Struct. Geol. 23, 1953–1969. Betts, P.G., Goleby, B.R., 2006. Mount Isa tectonics. Aust. J. Earth Sci. 53, 1–3. Betts, P.G., Ailleres, L., Giles, D., Hough, M., 2000. Deformation history of the Hampden Synform in the Eastern Fold Belt of the Mount Isa terrane. Aust. J. Earth Sci. 47, 1113–1125. Betts, P.G., Giles, D., Lister, G.S., 2004. Aeromagnetic patterns of halfgraben and basin inversion: implications for sediment-hosted massive sulfide Pb–Zn–Ag exploration. J. Struct. Geol. 26, 1137–1156. Betts, P.G., Giles, D., Mark, G., Lister, G.S., Goleby, B.R., Aill`eres, L., 2006. Synthesis of the proterozoic evolution of the Mount Isa Inlier. Aust. J. Earth Sci. 53, 187–211. Bierlien, F., Black, L.P., Hergst, J., Mark, G., 2008. Evolution of Pre-1.8 Ga Basement Rocks in the Western Mt Isa Inlier, northeastern Australia—insights

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