Previous authors have identified six major deformation events that have affected the Etheridge Group (D1-D6; Withnall et al., 1997). The first two folding events (D1 &
D2) were inferred to have occurred close to each other at about 1550 Ma, and are both
associated with metamorphism. The metamorphism ranges from lower-greenschist to upper-amphibolite and locally granulite facies, with peak metamorphic conditions reached during D2 (Withnall, 1996). D1 is best preserved in the western, lower grade
part of the region, where later overprinting effects are at their weakest. Withnall (1996) indicated that D1 appears to be of a relatively uniform intensity over the entire
region. D1 generally produced tight to isoclinal, upright to overturned folds, with a
strong axial-plane foliation. F1 wavelengths are generally of the order of 1-10 km
(Withnall et al., 1997).
The age of D1 remains uncertain. It was directly dated by Black et al. (1979) using
Rb-Sr whole-rock and mineral techniques to 1570 ± 20 Ma. However, the Rb-Sr whole-rock technique for dating metamorphic rocks has long been considered obsolete (Black & Withnall, 1993). Subsequent dating of post-D2 granitoids (Black &
McCulloch, 1990; Black & Withnall, 1993) has shown that the age of D1 obtained
using the Rb-Sr technique was anomalously young. Recent dating of leucogneiss from the Einasleigh Metamorphics by Black et al. (2005) showed that a significant metamorphic event occurred at 1562 ± 4 Ma. However, it remains unclear whether this age is representative of D1, or alternatively represents the onset of the second
major episode of regional deformation and metamorphism. If the latter is correct, it would indicate that D2 lasted for at least 12 million years, and that D1 is likely to have
occurred ~20 million years earlier at ca. 1580 Ma, when peak tectonothermal events were drawing to a close in the Broken Hill Block and the Mt Isa Eastern Succession (Black et al., 2005). EMPA dating of monazites from amphibolite facies metasedimentary rocks of the Robertson River Subgroup by Cihan et al. (2006) suggest the latter is true and they give a weighted average age for the peak metamorphic conditions associated with D1 of 1586 ± 6 Ma.
The effects of D2 are best seen in the eastern half of the Georgetown Inlier, as both the
intensity of folding and deformation increase eastwards. In westernmost outcrops of the Etheridge Group D2 forms local, weak, northerly trending crenulations that lie
1996). In easternmost outcrops D2 forms generally north trending tight to isoclinal
folds with wavelengths up to 2 km, which are associated with either a strong crenulation cleavage, or schistosity (Withnall, 1996). Where this deformation is strongest, D2 overprints the earlier metamorphic fabric as well as primary features
including sedimentary layering.
Using Rb-Sr techniques Black et al. (1979) obtained an age of 1469 ± 20 Ma for D2.
However, subsequent dating of Proterozoic felsic granites and volcanic rocks that postdate D2 using SHRIMP and conventional multi-grain zircon techniques (Black &
McCulloch, 1990; Black & Withnall, 1993) has shown that this age is obsolete, and that D2 is about 80 Ma older, probably occurring at around 1550 Ma (Black &
Withnall, 1993). Recent dating of leucogneiss from the Einasleigh Metamorphics by Black et al. (2005) returned an age of 1555 ± 3 Ma, which they correlated to the peak of metamorphism associated with the D2 event. Monazites analysed by Cihan et al.
(2006) yielded a weighted average age of 1542 ± 8 Ma, which they correlate to marking the beginning of D2 in the Robertson River Subgroup.
D3 produced folding of varying intensity across the Georgetown Inlier. In the central
part of the inlier near ‘Robinhood’, D3 produced generally open to tight, upright to
overturned folds with axial planes striking approximately south-easterly to easterly (Withnall, 1996). Further east, and to the south of the Gilberton Fault, D3 produced
generally weak open folds with wavelengths of less than 1 km to over 10 km (Withnall, 1996). In these areas, D3 is also associated with small-scale folds, weak
crenulations, and low-grade retrogressive metamorphism, although due to the weak development of D3 foliations, it is difficult to distinguish these features from later
events.
The age of D3 was dated at 967±28 Ma using Rb-Sr techniques from samples of
outcrops supposedly possessing well-developed S3 schistosities (Black et al., 1979).
Subsequent studies of the same outcrops have suggested that the foliation S3 may in
fact be S2, and that the age obtained by Black et al. (1979) may represent a partially
reset D2 age (Withnall, 1996). Bell (pers. comm., 1984 in Withnall, 1996), on the
basis of shallowly dipping axial planes in some F3 folds, suggests that D3 may be
early Palaeozoic in age, and related to deformation in the Greenvale Subprovince of Withnall (1989). Cihan et al. (2006) noted a large grouping of monazite ages in their study which gave a weighted mean age of 1512 ± 5 Ma. They postulated that this age
may represent a thermal pulse or hydrothermal activity that may or may not have been associated with the later stages of D2, and could therefore represent the age of D3.
D4 has had little effect on the western and southern parts of the Etheridge Group, and
the event is best recorded in the eastern, higher-grade parts of the inlier. F4 folds are
rare; however, Withnall (1996) suggests open, mesoscopic F4 folds are common near
Einasleigh and further north. Black et al. (1979) suggested an age of about 400 Ma for D4 based on K-Ar and Rb-Sr mica ages, but did not directly attempt to date the event.
McNaughton (1980) obtained K-Ar muscovite ages of 410 ± 12 Ma and 414 ± 12 Ma from the Einasleigh Metamorphics east of the study area, which he interpreted to have crystallised syn-D4. Other geochronological studies from areas surrounding the
Georgetown Inlier have also returned ages around 400 Ma (e.g. Withnall et al., 1991) indicating that this age is of regional significance; however, it is yet to be determined conclusively that this age corresponds to D4.
Later deformation events (D5-D6) produced open folding and weak crenulations in the
Etheridge Group. D5 is represented by open folds with east-striking axial planes
(Withnall, 1996). Minor crenulations have also been recognised in the Robertson River area. D6 is mainly represented by crenulations. The ages of D5 and D6 have not
been determined, but are believed to be of late-Palaeozoic age (Withnall, 1996).
2.6 Faulting
Three major fault systems can be found in the central part of the Georgetown Inlier (Fig. 2.6). The Robertson, Gilberton and Delaney fault systems all or partly occur within the study area, and are described in detail below.
2.6.1 Robertson Fault System
The Robertson Fault System has been mapped for approximately 45 km northwest from the northern end of the Agate Creek Volcanics to 2 km south of the Gongora Granodiorite (Withnall & Blight, 2003; Withnall et al., 2003b). The fault system splits into two branches north of the Robertson River (Fig. 2.6; Withnall et al., 2003b). The eastern branch and main stem of the Robertson Fault both show a sinistral movement of 5 km, while at least 2 km of dextral movement has occurred on the western branch (Withnall, 1996). Vertical displacement involving west-block-down
movement of up to 150 m also occurred along the western and main stems of the fault during the Pliocene (Withnall, 1996). This displacement is observable along the section of the fault which runs parallel to the Robertson River, where the Jurassic Hampstead Sandstone is now in fault-contact with metasediments of the Etheridge Group (Fig. 2.6; Withnall et al., 2003b).
The age of the Robertson Fault is uncertain. It is thought to be at least Pliocene in age, but could possibly be as old as late Palaeozoic, based on late Palaeozoic volcanics and intrusives lying along the south-easterly projection of the fault system (Withnall, 1996).
2.6.2 Gilberton Fault System
The Gilberton Fault System has been mapped for approximately 55 km northeast from its southern contact with Jurassic sediments 20 km southwest of Gilberton, to its intersection with the Ballynure Fault, east of ‘Gum Flats’ station (Fig. 2.6; Withnall et al., 2003a).
Considerable dextral movement of at least 50 km along the Gilberton Fault has been hypothesised (Withnall et al., 1980), as along the southern half of the fault high grade rocks of the Einasleigh Metamorphics are in contact with low grade rocks of the Robertson River Subgroup. The juxtaposition of rocks of different metamorphic grades can also be explained by vertical movement; however, the distance over which faulting is thought to have occurred makes this unlikely (Withnall et al., 1980).
In outcrop, the Gilberton Fault ranges from narrow zones of fault gouge (Withnall, 1996), through to zones of intense shearing and fracturing up to 100 m wide. Mylonitic leucogranite is recognised close to the southern side of the fault south of ‘Gum Flats’, indicating that some ductile deformation was involved during movement along parts of the fault.
The age of the Gilberton Fault is thought to be late Ordovician or early Silurian (Withnall, 1989), based on the presence of mylonite pebbles in early Silurian conglomerates in the Broken River Province to the southeast of the Georgetown Inlier (Withnall et al., 1988b). Mylonitisation along the Gilberton Fault is believed to have occurred at this time.
2.6.3 Delaney Fault System
The Delaney Fault is discontinuous, and forms a series of north-striking, narrowly offset segments that continue for at least 100 km (Fig. 2.2; Withnall, 1996). The northern limit of the fault has been mapped to 3 km south of the Yataga Granodiorite, and the southern limit is mapped to the Gilberton Fault, approximately 5 km southwest of Gilberton (Fig. 2.6; Withnall et al., 2003a). A number of small, mafic dykes occur along the length of the fault, and are interpreted to be Carboniferous to early Permian in age (Withnall & Blight, 2003).
In most areas, the fault has caused no observable displacement of stratigraphy. One notable exception is in the central part of the region, where the fault has displaced the contact of the Robin Hood Granodiorite by approximately 1 km (Withnall, 1996; Withnall & Blight, 2003). Small vertical displacements have otherwise been suggested to account for the lack of lateral movement (Withnall, 1996).
The age of the Delaney Fault is uncertain. Withnall (1996) suggests that it may have formed between the early Devonian and Permian, based on the displacement of the Robin Hood Granodiorite and the intrusion of mafic dykes.
Chapter 3.
Geology and Structure of key lithologies within the lower Etheridge Group