Mapping lithospheric alteration using AusLAMP MT data in the Ikara-Flinders Ranges and Curnamona Province
A total of 74 long-period magnetotelluric (MT) stations were deployed across the Ikara-Flinders Ranges and the adjacent Curnamona Province as a part of the Australian Lithospheric Architecture Magnetotelluric Project (AusLAMP; Fig. 1). The intention is to cover Australia with a grid of long-period MT stations at half a degree spacing (~55 km), with the aim of imaging the resistivity of the entire lithosphere of Australia. To date, Victoria, Tasmania and the lower two-thirds of South Australia have been completed, along with parts of New South Wales and Western Australia.
AusLAMP provides a means to address the UNCOVER initiative (Australian Academy of Science 2012), which aims to increase the exploration success of Australia for new mineral deposits under cover. One of the four main themes of UNCOVER is to understand the lithospheric framework of the continent to improve our understanding of the transition of heat, fluids and metals from sub-lithospheric depths through to the crust. MT is one of only two techniques capable of imaging the entire lithosphere (the other being seismic tomography), and is particularly sensitive to minor interconnected conducting phases such as fluids, graphite and sulfides. The MT technique records the naturally occurring magnetic and electric fields at earth’s surface which are generated from interactions of solar wind with earth’s magnetosphere and from lightning strikes around the globe. Once the data have been transformed from the time domain to the frequency domain, impedance values of ratios of these electric and magnetic fields are derived.
This article provides a summary of modelling parameters and outlines some of the key findings to date. See Robertson, Heinson and Thiel (2016) for more detail.
The survey region is of geoscientific interest with abundant mineral occurrences (Fig. 2) and diamondiferous kimberlites (found mainly in the Nackara Arc around Monks Hill and Eurelia; Fig. 3), as well as hosting copper and uranium mines and the world-class zinc–silver–lead Broken Hill Mine (Fig. 2).
The Ikara-Flinders Ranges forms an intraplate deformation region (Sandiford and Quigley 2009; Thiel et al. 2016), co-located with the South Australian Heat Flow Anomaly (Neumann, Sandiford and Foden 2000), and has elevated levels of seismicity down to lower crustal depths (Holford et al. 2011). It is difficult to explain these observations, and various theories of rift-related mechanical weaknesses and thermal weaknesses are called upon. Recently, suggestions were made that high pore fluid pressure in the lower crustal rocks may be triggering earthquakes (Balfour et al. 2015). Further, a major conductivity anomaly, the Flinders Conductivity Anomaly (Fig. 1), has been observed from many lower resolution prior studies over the last 50 years, with poor depth constraints mainly derived from forward modelling and little idea on the cause of the anomaly (e.g. Chamalaun 1985).
Impedance values, along with the tipper (ratios of horizontal and vertical magnetic fields) were inverted using the 3D ModEM algorithm of Egbert and Kelbert (2012).The effect of varying model parameters was investigated in detail, with many inversions run while changing the model covariance (a smoothing factor that penalises deviations from a prior resistivity model; see Kelbert et al. 2014), half-space starting resistivity, model dimensions, cell size, number of layers and the method of error calculation. For the presented model, a starting resistivity of 100 Ωm was used with a model covariance factor of 0.3, and error floors of 5% for the impedance tensor and 3% for the tipper error. AusLAMP involves the collection of long-period MT data, ideal for investigating the entire lithosphere. We inverted the full impedance tensor at 23 periods from ∼2 to 17,000 s, and the tipper at only 21 periods ranging from ∼2 to 8,000 s, resulting in an electrical resistivity model of the lithosphere beneath the 400 x 600 km survey region.
Observations and interpretations
Modelling of the new MT data shows that the Ikara-Flinders Ranges are mostly quite resistive (1,000–5,000 Ωm). There are three notable exceptions with the Flinders Conductivity Anomaly now thought to be three separate conductive anomalies rather than one continuous 400+ km conductor: the Western Nackara Arc Conductor, the Eastern Nackara Arc Conductor and the Curnamona Conductor.
The large Curnamona Conductor within the Curnamona Province occurs at depths of 5–40 km (Robertson, Heinson and Thiel 2016) in a region that is thought to have been unaffected by tectonic events since the Olarian Orogeny (1.6 Ga), with the exception of the Delamerian Orogeny in the Cambrian which has modified only the edges of the province. The pervasive presence of the conductor at lower crustal depths indicates that previous interpretations of sediments as the cause of the Flinders Conductivity Anomaly (e.g. Lilley and Tammemagi 1972; Chamalaun 1985) cannot be an explanation on their own. We interpret the conductor as widespread fluid alteration which may present an example of the UNCOVER (Australian Academy of Science 2012) goal of imaging fluid pathways throughout the entire lithosphere to potentially highlight prospective regions for mineral deposits in regions of sedimentary cover and regolith.
Within the Nackara Arc, two parallel arcuate conductors were observed extending from depths of about 20–80 km. These conductors straddle the previous best estimate for the Flinders Conductivity Anomaly, as determined by induction arrows. This indicates that the previous estimation may have been a result of a superimposition of two conductive bodies giving the anomalous signature in the induction arrows.
The Nackara Arc is a region that has been weakened by numerous rift cycles since the Neoproterozoic. Of interest is the coincidence of the conductors and discovered diamonds (Fig. 3). We interpret these conductors to be a result of pre-existing trans-lithospheric structures previously exploited by carbon-rich kimberlite hosting magmas on their ascent prior to their eruption during the Jurassic period. Carbon can be precipitated from magma and can form thin grain boundary graphite films, known to enhance conductivity. These kimberlites are thought to have been expelled from subducted remnants of the proto-Pacific plate (Tappert et al. 2009). In addition, the Eastern Nackara Arc Conductor is situated at a significant lithospheric structure – the approximate location of the transition from Proterozoic to Phanerozoic lithosphere.
A question that can be addressed with the AusLAMP data is the cause of the intraplate seismicity within the Ikara-Flinders Ranges. These earthquakes occur down to depths where the earth would be ductile. Balfour et al. (2015) suggest high pore fluid pressure in the lower crust. These pore fluids may enhance conductivity if these fluids are interconnected. However, there is no obvious correlation of regions of high conductivity and earthquake locations (Fig. 3) indicating that either pore spaces are not connected or that pore fluids are not the cause of the earthquakes.
AusLAMP has proved successful in highlighting anomalous regions of conductivity associated with fluid or magma alteration of the lithosphere in the Ikara-Flinders Ranges that may extend to upper crustal depths and highlight potentially prospective zones for mineral exploration. The collection and modelling of AusLAMP data will continue in other regions, providing crucial insight into the lithospheric architecture and its relationship to tectonic events and mineral potential.
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