Chapter 6 Three-dimensional imaging of the Gawler Craton 6.1 Introduction The late Archaean to Palaeoproterozoic Gawler Craton spans an area of roughly 530000 km2 across much of South Australia. While the 2-D survey, shown in Chapter 5, Figure 5.1, offered some insights into the Fowler and Nuyts Domain through close site spacing, it obviously only covers a small part of the craton. In order to better understand the larger-scale tectonic framework of the Gawler Craton, a much larger area needs to be covered with MT sites. This can be achieved through the deployment of a number of 2-D profiles starting from the central part of the Gawler Craton and crossing the margins. 2-D inversion of such data sets has become standard among the MT community and results can be obtained quickly. However, 2-D inversion routines are still reliant on the underlying assumption of 2-D geology, and in case of more complex 3-D structures, 2-D inversion should be carried out with caution. Often, subsets of data show 2-D behaviour (see Chapter 4), which can be inverted without neglecting information stored in the diagonal components of the impedance tensor. In the present case, successful 2-D inversions would still pose the problem of interpolation between the profiles, which can be doubtful in case of 3-D bodies. Recent developments in the field of 3-D inversion (Siripunvaraporn et al., 2005a) have moti- vated a 3-D modelling approach for the resistivity distribution of the Gawler Craton. A smaller 2-D array of eight sites across the Gawler Range Volcanics was deployed in 2005 (Maier et al., 2007) and Heinson et al. (2006) conducted a 2-D MT profile across the IOCG Olympic Dam deposit in the north-eastern part of the craton. These surveys, together with the data presented in Chapter 5, already cover a large portion of the Gawler Craton. In 2006, another 15 sites were deployed across the north-western part of the craton to create a rectangular grid of sites 78 6.2. Geology 79 130˚E 140˚E 120˚E 150˚E 110˚E 10˚S 10˚S Kimberley 180 Arunta 20˚S 20˚S Pilbara 130 Musgrave 80 g r a 30 v Yilgarn i 30˚S t 30˚S −20 y Gawler u −70 n i t −120 s 40˚S 40˚S −170 130˚E 140˚E −220 120˚E 150˚E 110˚E Figure 6.1: Location of MT stations of the 3-D survey (marked as black triangles) on top of the Australian gravity map with a resolution of 0° 5 ′. Areas bounded by dashed lines show Archaean and Proterozoic terraines. 2 approximately 100 km apart. The final grid spans an area of around 400000 km or about 1/20th of the Australian continent (Figure 6.1). 6.2 Geology The oldest rocks in the Gawler Craton are the contemporaneous Archaean Sleaford and Mul- gathing Complexes, with the Mulgathing Complex residing in the Christie Domain (Figure 6.2). Swain et al. (2005) report emplacement ages of 2850-2510 Ma from U-Pb zircon ages. The 2480 2420 Ma Sleafordian Orogeny ended the emplacement of those complexes and led to − granulite facies metamorphism (Teasdale, 1997; Tomkins and Mavrogenes, 2002). The rem- nants of the Sleafordian Orogeny have been extensively reworked during later events. After a period of 400 million years of no tectonic activity, Proterozoic events between 2000 and 1500 Ma have largely shaped the Gawler Craton as seen today. In this time span, sedimentary processes appear to dominate in the 2000 1690 Ma interval and are followed by dominantly magmatic − processes in 1690 1500 Ma (Hand et al., 2008). − During the 2000 1690 Ma interval a number of large rift-basins developed, i.e. the now de- − formed Hutchinson Group along the eastern margin of the Gawler Craton (Parker and Lemon, 6.2. Geology 80 130˚E 132˚E 134˚E 136˚E 138˚E 130˚E 132˚E 134˚E 136˚E 138˚E 28˚S 28˚S gvd09 20˚S Pilbara gvd08 gvd11 Gawler gvd12 gvd13 Tor OFFICER gvd10 Yilgarn 40˚S BASIN ren cpd02 s 120˚E 140˚E gvd07 ari FZ CHRISITIE rox39 Kar 30˚S Moondrah gvd16 gvd15 gvd14 cpd01 30˚S Gneiss Hinge gvd06 WILGENA gvd03 rox16 ie SZ lacootra SZ rab Tal gvd01 EUCLA Coo Yerda SZ RGLE ? RPIM roxe11 BASIN ? FOWLER SZ fow25 nibba FZ fow18 NUYTS Koo GAWLER RANGE lbrinda Zon fow11 Yar RMAH RKOB 32˚S VOLCANICS e 32˚S fow06 fow01 RMTI RUNO km 0 100 200 300 34˚S 34˚S 130˚E 132˚E 134˚E 136˚E 138˚E 130˚E 132˚E 134˚E 136˚E 138˚E Station locations Hiltabe Suite Mount Woods Inlier Bathymetry intrusives gravity units Munjeela Granite Nawa Domain Mable Creek Ridge −60 −550 −50 −45 −40 −35 −30 −25 −20 −15 −10 St. Peters Suite Hutchinson Group −50 −900 −750 −600 −450 −300 −150 0 150 300 450 Peake Metamorphics Granitoids and equivalents 00 00 00 00 00 00 00 00 00 00 0 0 0 Sleaford and Fowler Domain Donington Suite and equivalents Mulgathing Complex m Coober Pedy Ridge Wallaroo Group Moondrah Gneiss paragneisses Tunkilia Suite Figure 6.2: Left: MT stations on interpreted geology map of the Gawler Craton. Right: The gravity image of the same geographical coordinates. Many of the domain boundaries were derived from potential field data sets, such as gravity and Total Magnetic Intensity information. Blue triangles – MT sites collected in 2005 (see Chapter 5); red triangles – MT sites collected in 2006; green triangles – 3-D survey conducted by Maier et al. (2007); brown triangles – subset of MT sites collected by Heinson et al. (2006); black triangles – collected by Matthew Scroggs in 2004 and re-processed by Selway (2006). 1982). At 1850 Ma, short-term compression during the Cornian Orogeny (Reid et al., in press) led to the emplacement of the Donington Suite granitoids to the east of the Hutchinson Group (Mortimer et al., 1988). Following the Cornian Orogeny was another extended period of rifting, leading to the deposition of several sediment packages along the eastern to northern margin, among which are the Wallaroo Group, the sediments within the Mt. Woods Inlier and Peake Metamorphics (Fanning et al., 1988, Figure 6.2). The deposited sedimentary basins have bi- modal magmatic suites associated with them. The Fowler Domain along the western margin of the Gawler Craton also contains both pelitic metasediments (Daly et al., 1998), and 1726 9 Ma ± old mafic metagabbros (Fanning et al., 2007). In the north-west of the Gawler Craton, metased- iments in the Nawa Domain are thought to originate from a different source than the Archaean Gawler Craton, i.e. the Arunta Block to the north of the Craton (Payne et al., 2006). The 1730 1690 Ma Kimban Orogeny had a profound metamorphic influence on deposited metased- − iments. The Kalinjala Shear Zone is a major remnant of the deformation processes and is situated between the Donington Suite and Hutchinson Group in the south-eastern part of the Gawler Craton (Vassallo and Wilson, 2002; Thiel et al., 2005). Deformation of sedimentary se- quences were reported in the Mt Woods Inlier and the Peake Metamorphics in the northern part of the Gawer Craton (Betts et al., 2003). To the west of the Craton, metamorphism reached 6.3. Two-dimensional magnetotelluric array across the Gawler Craton 81 amphibolite facies in the Fowler Domain (Teasdale, 1997). During the 1690 1500 Ma interval, igneous events were dominant in the Gawler Craton, − much more than sedimentary processes. The oldest intrusion within this period is the 1690 − 1670 Ma Tunkilia Suite in the central Gawler Craton, which forms an arcuate belt around the Nuyts Domain, while smaller discrete intrusions have also been reported in the Fowler Domain (Ferris and Schwarz, 2003). At 1630 Ma the alkaline, porphyritic rhyodacite of the Nuyts Volcanics erupted and were subsequently intruded by the 1620 1610 Ma St. Peter Suite − (Flint et al., 1990). Between 1595 and 1575Ma a large-scale magmatic event formed the Hiltaba Suite and the Gawler Range Volcanics, and form one of the largest felsic volcanic systems in the world (Daly et al., 1998). The Gawler Range Volcanics have a maximum thickness of about 1.5 km. The Hiltaba Suite comprises strongly fractionated granites to granodiorites (>70 wt % SiO2), which consist of more mantle-derived material than the host rock in which they reside (Stewart and Foden, 2003). Several major shear zones, such as the N-S trending Yarlbrinda Shear Zone and the E-W trending Yerda Shear Zone (Figure 6.2), are believed to have been reactivated during the emplacement of the Hiltaba Suite granites and a general NW- SE shortening at 1590 1570 Ma. At 1585 Ma the St. Peter Suite was subsequently intruded − by the unfractionated Munjeela Granite, as indicated by potential field data (see Figure 5.2). The Karari Fault Zone in the north-western part of the Gawler Craton was formed during the Karari Orogeny (1570 1540 Ma) and has reworked parts of the Coober Pedy Ridge and acted − within a shear-zone bounded domain within a transpressional belt of anastomosing shear zones (Teasdale, 1997). During the active time span 2000 1500 Ma, other events have been reported, at times based − on only a few samples. This shows the difficulty of obtaining a coherent picture of the processes in that time frame. A more detailed analysis of the events is described in Hand et al. (2008), however in the context of a large-scale MT survey presented in this chapter, those events are not discussed in detail.