Proterozoic tectonothermal processes imaged with magnetotellurics and seismic reflection in southern Australia

Journal Pre-proof Proterozoic tectonothermal processes imaged with magnetotellurics and seismic reflection in southern Australia Tom Wise, Stephan Thiel PII:

S1674-9871(19)30171-9

DOI:

https://doi.org/10.1016/j.gsf.2019.09.006

Reference:

GSF 888

To appear in:

Geoscience Frontiers

Received Date: 4 February 2019 Revised Date:

26 June 2019

Accepted Date: 25 September 2019

Please cite this article as: Wise, T., Thiel, S., Proterozoic tectonothermal processes imaged with magnetotellurics and seismic reflection in southern Australia, Geoscience Frontiers, https:// doi.org/10.1016/j.gsf.2019.09.006. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 China University of Geosciences (Beijing) and Peking University. Production and hosting by Elsevier B.V. All rights reserved.

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Proterozoic tectonothermal processes imaged with magnetotellurics and seismic

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reflection in southern Australia

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Tom Wisea,*, Stephan Thiela,b

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a

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5005, Australia

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b

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Australia

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*

Geological Survey of South Australia, Department for Energy and Mining, Adelaide, SA

School of Earth and Environmental Sciences, University of Adelaide, Adelaide, SA 5005,

Corresponding author. E-mail address: [email protected] (Tom Wise)

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Abstract

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Over the last two decades, co-located seismic and magnetotelluric (MT)

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profiles provided fundamental geophysical data sets to image the Australian

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crust. Despite their complimentary nature, the data are processed and often

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interpreted separately without common processes in mind. We here

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qualitatively compare 2D resistivity inversion models derived from MT and

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seismic reflection profiles across a region of Archean–Proterozoic Australia to

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address the causes of variations in seismic response and anomalous

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conductivity in the crust. We find that there exists a spatial association between

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regions of low reflectivity in seismic sections and low resistivity in co-located

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2D MT modelled sections. These relationships elucidate possible signatures of

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past magmatic and fluid-related events. Depending on their diffuse or discrete

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character, we hypothesize these signatures signify fossil melting of the crust

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due to mafic underplating, magma movement or hydrothermal fluid flow

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through the crust. The approach discussed herein is a process-oriented

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approach to interpretation of geophysical images and a significant extension to

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traditional geophysical methods which are primarily sensitive to a singular bulk

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rock property or state.

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Keywords: Magnetotellurics; Seismic reflection; Resistivity

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1. Introduction

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Imaging the continental lithosphere is pivotal to our understanding of the state and

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properties of the lithosphere and the processes that have shaped the continents. As

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an example, substantial geophysical data acquisition programs have been

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undertaken since the 1980’s in Australia and Canada (Green et al., 1988; Goleby et

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al., 1989; Cook et al., 2010). These programs commonly use seismic reflection

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profiles to interrogate the crustal component of Precambrian lithosphere, partly

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under extensive cover sequences. Increasingly, national and international programs

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incorporate other geophysical techniques, such as magnetotelluric (Jones, 1987,

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1998) and gravity/magnetics inversion models to compliment the interpreted

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reflection seismic profiles. Between 1984 and 2005, the Lithoprobe program in

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Canada established seismic reflection methods as a tool for mineral exploration

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(Clowes, 2010). In addition, mostly long-period (10–10000 s) magnetotelluric

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(MT) profiles provided insight into the crustal and mantle resistivity structure of

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the Canadian Lithosphere (Jones et al., 2014). In Australia, deep seismic reflection

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surveys have been used to provide two-dimensional profiles of the whole crust,

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informing crustal architecture studies and mineral system settings since the 1970’s

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(e.g., Goleby et al., 1989; Drummond et al., 2006; Kennett and Saygin, 2015;

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Korsch and Doublier, 2015; Kennett et al., 2016; Dentith et al., 2018).

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Previous attempts to integrate seismic and MT interpretation have largely revolved

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around correlating conductors with individual reflectors, or reflective packages.

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This has had success in sedimentary environments, including through joint

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inversions. Cook and Jones (1995), however urged caution against attributing a

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geological process to satisfy both the reflective and conductive responses,

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suggesting that interlayered sulphide-rich metasedimentary units and mafic sills

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cause the electrical and acoustic response, respectively. A common approach to

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interpretation of crustal seismic reflection profiles relies on subjective

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identification of faults, which in most cases are identified as discrete and narrow

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zones of deformation along which crustal movement was accommodated.

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However, examples of co-located magnetotelluric and seismic reflection profiles in

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Australia show that particularly in the mid to lower crust sub-vertical zones of high

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conductivity cross-cut crustal-scale faults interpreted from seismic sections

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(Korsch et al., 2010; Thiel et al., 2010, 2015; Johnson and Thorne, 2011). There is

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therefore a need to determine whether or not the results of both techniques can be

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related to the same geodynamic process (Cook and Jones, 1995), and inform on

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how different types of inter pretation may better link the two data types.

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Recent work by Heinson et al. (2018) links electrical and seismic signatures

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beneath the world class Olympic Dam mine to fluid pathways within a whole-of-

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lithosphere mineral system. We present two profiles in addition to the work of

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Heinson et al. (2018) to address the wider applicability of the joint interpretation of

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seismic and MT to mapping melt and fluid ascent processes through the crust. We

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focus specifically on the spatial association between variations in seismic

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reflectivity and magnetotelluric low resistivity that cross-cut earlier crustal fabrics.

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Through incorporation of geological and isotopic constraints, coupled with the

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spatial co-incidence of reflectivity and resistivity variations, we infer the main

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geodynamic mechanisms by which pre-existing fabrics in this Precambrian crust

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are overprinted, and suggest implications for melt/fluid ascent processes, and

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controls on their geometry.

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2. Geological background

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Central southern Australia is comprised of the Gawler Craton, a Meso-Neoarchean

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block with a protracted history of tectono-magmatic basin development and

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subsequent deformation and further magmatism throughout the Neoarchean–

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Mesoproterozoic. Surrounding the Gawler Craton are younger, poly-deformed

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Proterozoic regions, namely the Coompana, Musgrave and Curnamona provinces

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(Fig. 1), all with enigmatic relationships with the Gawler nucleus. Here, we use co-

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located reflection seismic and MT transects spanning the core of the craton, the

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eastern and western margins, and into the Coompana Province with the aim of

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characterising major syn-post tectono-thermal events affecting these regions. The

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dominant crustal fabrics imaged by seismic sections are interpreted to have been

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formed prior to (or during) the last significant thermal event to affect the region

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(e.g. Drummond et al., 2006; Doublier et al., 2015; Pawley et al., 2018; Curtis and

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Thiel, 2019). The following events are therefore likely to modify the seismic and

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electric response of the crust in the regions they affect. The ca. 1595–1575 Ma

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Hiltaba Suite and Gawler Range Volcanics were part of a widespread mantle-

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derived magmatic event and siliceous large igneous province (SLIP) (Wade et al.,

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2019) in the central, southern, eastern and western Gawler Craton (Fig. 1),

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associated with regional deformation in the western and northern Gawler (Hand et

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al., 2007). This event was also associated with significant metallogenesis, and the

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formation of the Olympic Dam, Prominent Hill and Carrapateena Iron oxide-

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Copper-Gold (IOCG) deposits in the eastern Gawler Craton, and Au-only deposits

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in the central Gawler Craton (Hand et al., 2007; Skirrow et al., 2007). The ca.

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1200–1140 Ma Moodini Supersuite of the Coompana Province (Fig. 1) represents

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part of a volumetrically-massive thermal event that affected regions between the

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Gawler, Yilgarn and North Australian cratons (Spaggiari, 2015; Wise et al., 2015a;

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Kirkland et al., 2017; Dutch et al., 2018). In the eastern Coompana Province, the

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Moodini Supersuite consists of syn-tectonic anatectic melts, and isotopically

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juvenile, high KFe, post-tectonic plutons concentrated in a NE–SW trending belt at

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a low angle to the regional grain (Wise et al., 2015a; Dutch et al., 2018).

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3. Overview of co-located seismic and magnetotelluric profiles

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The examples presented here (Fig. 1), whilst geographically limited to southern

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Australia, provide a wide variety of cross-sections through differing geological

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terranes, including: Archean–Proterozoic intra-cratonic reworking, Archean–

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Proterozoic craton margin deformation, syn- and post-orogenic magmatism, and

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the crustal architecture of a major magmatic mineral system. In addition to the

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variety in geological regions and structures covered, the MT surveys have close

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station spacing between 1 km and 5 km for all profiles, and a wide frequency range

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(0.001–1000 s). These acquisition parameters allow resolution of conductors to

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within 1–2 km width from crustal depths to the surface. For many examples

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outside Australia (e.g. Jones et al., 2014), where co-located seismic and MT

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surveys exist, significantly wider MT site spacing was used, limiting the resolution

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in the mid-upper crust, and therefore the effectiveness of joint interpretations of

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seismic and MT at all crustal levels. As such, the examples used herein represent a

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high-quality, yet spatially limited case study in the joint interpretation of crustal-

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scale seismic reflection and MT.

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3.1. Seismic reflection profiles

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Acquisition of deep seismic reflection transects in southern Australia has been

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conducted using vibrator sources since 1998, replacing earlier explosive sources

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(Kennett et al., 2016). Data was recorded to 20 s two-way time (TWT), imaging

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crustal structure and Moho geometry (Kennett and Saygin, 2015). Stacked data are

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processed by Geoscience Australia using a workflow designed to consistently

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image from the surface to 20 s TWT (Jones et al., 2005; Fomin et al., 2010;

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Holzschuh, 2015). Traditional methodologies employed for interpreting deep

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seismic reflection transects involve use of multiple forms of processed seismic

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sections, such as data stacks, dip moveout (DMO) processed images as well as

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high/low contrast filters (Doublier et al., 2015). Structural interpretation of seismic

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reflectors is carried out in a similar fashion to structural mapping of surface

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geology, with discrete structures being marked at offsets, terminations or change in

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direction of reflectors (Doublier et al., 2015). Lithostratigraphic units are

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interpreted based on seismic character and named when found outcropping or in

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drillhole intersections, whilst seismic provinces are termed for interpreted

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packages with no known surface expression or drillhole intersection, and are

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generally reserved for mid-lower crustal units (Korsch et al., 2010). Typically,

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interpretations in the style described above (e.g., Korsch et al., 2010; Fraser et al.,

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2010a) are appropriate for brittle deformation styles, whilst only limited attention

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has been paid to ductile deformation in the lower crust (Torvela et al., 2013),

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pervasive mass transfer or broad scale alteration processes. Drummond et al.

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(2006) and more recent studies endorse modification of reflectivity as a key

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characteristic enabling interpretation of post-formational processes such as magma

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transfer, hydrothermal alteration or ductile shear fabric development (Doublier et

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al., 2015; Wise et al., 2015c; Dutch et al., 2016b).

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3.2. Magnetotellurics

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MT stations have been deployed alongside seismic reflection profiles to augment

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national mineral exploration programs studying the Australian crust since the early

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to mid 2000’s.

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Practically, broadband (0.001–1000 s) MT surveys are sensitive to structures in the

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crust and upper mantle, unless significant enhanced conductivity structures are

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present near the surface in the form of deep sedimentary basins and crustal zones

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of high conductivity, reducing the penetration of the electromagnetic signal. In

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most cases, crustal resistivity structure is well resolved and inferences about

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conductivity connections into the mantle can be made reliably. As a result of MT

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surveys collected along seismic traverses, the MT data requires analysis of the

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dimensionality along the profile to determine if a predominant 2D geoelectric

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strike direction exists (Becken and Burkhardt, 2004; Caldwell et al., 2004). This

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step is necessary for modelling profile data with 2D inverse codes (Thiel and

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Heinson, 2010; Robertson et al., 2015; Heinson et al., 2018). However, if

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dimensionality analysis shows 3D data, then 3D inverse codes are required to

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model data along profiles (Robertson et al., 2017).

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The examples discussed herein have both seismic and MT surveys amenable to

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excellent data quality: MT surveys are high-resolution using broadband data with

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site spacing between 1 km and 5 km, seismic lines are generally straight,

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topography is limited (reducing curved line and topographic issues; Coltrin et al.,

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1989; Wu, 1996) and profiles are oriented at a high angle to the dominant

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geological strike of the basement. For these reasons, we suggest that variations in

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seismic reflectivity/amplitude are likely due to variations in geological structure

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and lithology, not near-surface effects reducing data quality (Lyons and Goleby,

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2005). Inclined as opposed to vertical zones also reduces the likelihood of seismic

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reflectivity variations being due to data quality problems.

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The direct comparison of seismic reflection and magnetotelluric profiles needs to

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address the differing vertical resolution possible with both techniques. The wave

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propagation nature of the seismic method achieves higher resolution of horizontal

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layers than the diffusive nature of the MT method. The MT technique has a good

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control on delineating the top of conductors but inherent lack of sensitivity to the

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bottom of conductive structures (Chave and Jones, 2012), generally leading to an

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overestimation of their depth extent. At the same time, seismic reflection profiles

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rely on accurate velocity models to convert the two-way travel time of the seismic

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wave arrival to absolute depth. The absolute depth to features may have an error

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margin of less than about 3–5 km in the lower crust.

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Vertical features are well constrained in MT and their horizontal resolution is

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mainly controlled by the station spacing. Seismic reflection does not have the same

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resolution to vertical features as to horizontal layers, but sub-vertical variations in

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reflectivity occur at similar scales as imaged in the MT, varying from just a few

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km in the upper crust to around tens of km in the lower crust.

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3.3. Eucla-Gawler (13GA-EG1)

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The Eucla-Gawler collocated seismic and magnetotelluric profile extends from

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central South Australia across the border with Western Australia (Dutch et al.,

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2015b; Fig. 1). Reflection seismic and 5 km-spaced broadband MT surveys were

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acquired along the trans-continental railway, providing part of an east-west

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transect imaging the crustal architecture across the margins of, and in-between, the

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Gawler and Yilgarn cratons. Given two distinct geo-electric strike directions along

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the ca. 800 km long profile Eucla-Gawler profile (Thiel et al., 2015, 2018), the

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entire profile was modelled in 2D along two separate sections of predominantly

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two-dimensional nature with a geoelectric strike of N27°E for the Eucla-Gawler

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East (Fig. 2), and N0°E for the Eucla-Gawler West profile (Fig. 2). The Eucla-

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Gawler East and the Eucla-Gawler West profiles correspond to the Gawler Craton

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and margins, and the Coompana Province, respectively. For details on data

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analysis and modeling for this profile, the reader is referred to the supplementary

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information.

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3.3.1. Eucla-Gawler East (13GA-EG1)

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The 370 km Eucla-Gawler East profile extends from the central Mesoarchean–

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Mesoproterozoic Gawler Craton, across the interpreted cratonic margin, and into

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the Palaeo-Mesoproterozoic Coompana Province (Fig. 2). The reader is referred to

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Reid and Dutch (2015) for a review of the geology of the western Gawler Craton,

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and Doublier et al. (2015), Dutch et al. (2015a) and Murdie et al. (2017) for

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interpretation of the seismic section. Broadly, the crust is comprised of three

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layers, a reflective upper crust, weakly reflective middle crust and highly reflective

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lower crust, changing to a largely two-layered nature to the west of the Karari

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Shear Zone, with a weakly reflective upper crust and highly reflective middle to

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lower crust (Doublier et al., 2015).

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The MT data exhibits a N27°E geoelectric strike following the NNE-trending

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magnetised zones in aeromagnetic imagery, as well as the gradients of the residual

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gravity anomalies of the region (Thiel et al., 2015). The 2D resistivity modelling

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reveals generally enhanced conductivity in the near surface due to cover sediments.

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The mid- to lower crust is generally resistive (>10000 Ωm), intersected by sub-

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vertical crustal zones of high conductivity (<100 Ωm), which correspond to major

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regional shear zones (e.g. Karari, Coorabie and Colona shear zones, Fig. 1)

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identified in potential field data sets and previously imaged using long-period MT

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(Thiel and Heinson, 2010). In Fig. 2, the conductive zone C1 corresponds to an

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arcuate magnetic anomaly related to magnetite bearing metasedimentary rocks

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(Thiel et al., 2015). The bottom of the conductor C1 is bound by a zone of

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enhanced seismic reflectivity in the mid-crust. Conductors C2 and C3 coincide

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with the surface location of the Karari and Coorabie Shear Zones, respectively

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(Thiel et al., 2015, 2018; Fig. 2).

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3.3.2. Eucla-Gawler West (13GA-EG1)

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The western half of the Eucla-Gawler profile crosses the Coompana Province, the

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Mesoproterozoic Madura Province and joins the eastern end of the Albany-Fraser

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Orogen collocated profile (Spaggiari, 2014). Here we focus on just the ∼380 km

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Coompana Province section of this line (Fig. 1). The Coompana Province is a

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region without surface exposure, and characterised by few drillhole constraints,

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and is predominantly composed of an intrusive upper crust entirely blanketed by

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stacked Neoproterozoic–Cenozoic basins (Spaggiari, 2015; Wise et al., 2015b;

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Pawley et al., 2018). In the seismic imagery, the Coompana Province shows a

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weakly reflective upper crust in the east, above a highly reflective lower crustal

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unit (Fig. 3). To the west, the reflective lower crust ramps up to the surface, with a

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wedge of non-reflective lower crust abutting the boundary with the Madura

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Province (Dutch et al., 2016b; Pawley et al., 2018). In comparison with the highly

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complex Eucla-Gawler East profile, the resistivity structure in Eucla-Gawler West

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generally displays a three-layer resistivity distribution in the sub-surface (Thiel et

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al., 2015, 2018). In the top 1 km, sedimentary and calcrete cover is electrically

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conductive and underlain by a resistive upper to middle crust. The lower crust and

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upper mantle are conductive (∼10 Ωm) over a horizontal distance of 450 km. The

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high conductance (C6) is also co-located with a zone of low reflectivity, marking

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the lower bound of a highly reflective middle crust across the Coompana Province.

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The sub-vertical conductor (C5) extends through the crust in the centre of the

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profile, subcropping beneath sedimentary cover in areas associated with high-KFe

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granites of the Moodini Supersuite (Fig. 1; Kirkland et al., 2017; Dutch et al.,

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2018; Thiel et al., 2018).

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3.4. Olympic Dam (03GA-OD1)

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Olympic Dam is the type example of iron oxide-copper-gold±uranium (IOCG±U)

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deposit styles (Williams et al., 2005; Skirrow, 2008; Skirrow et al., 2018). Two

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seismic reflection profiles were centred on the Olympic Dam deposit, with the aim

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of imaging the crustal architecture constraining the mineralisation setting (Lyons

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and Goleby, 2005), which has been reviewed in detail by Drummond et al. (2006).

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Broadly, the Olympic Dam seismic line (03GA-OD1) shows a weakly reflective

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upper crust and a highly reflective and thick middle crust overlying a weakly

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reflective lower crust (Lyons and Goleby, 2005; Drummond et al., 2006; Fig. 4). A

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significant Moho offset is interpreted beneath the middle of the north-south profile,

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(Lyons and Goleby, 2005; Drummond et al., 2006).

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We focus on the longer of the two lines, oriented approximately north-south, due to

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greater availability of MT data. The 200 km north-south Olympic Dam profile has

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been the focus of recent work in both seismic and MT. A variation in seismic

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processing methodology, partially preserved amplitude processing (Wise et al.,

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2015c) designed to emphasise variation in signal strength has been applied,

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revealing greater reflectivity contrasts in the upper crust and permitting

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interpretation of steeply dipping structures, which may reflect possible fossil fluid

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pathways (Wise et al., 2015c). Extensive MT acquisition has also been undertaken

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in recent years infilling an original site spacing of 5–10 km using long-period MT

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data to approximately 1 km using higher-frequency broadband MT equipment

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(Heinson et al., 2006, 2018). A reduction in MT site spacing has revealed greater

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narrow, conductive structures in the upper crust (C9-11), not resolved by early

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surveys, linked to a highly conductive mid-lower crustal region (C8, Heinson et al.,

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2018) At the southern end of the profile, a highly resistive crust is typical of

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regions of the Gawler Craton (e.g. the eastern Eucla-Gawler East profile, Fig. 2).

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4. Discussion

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Here we focus not on conductive and seismic character associations caused by

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stratabound lithostratigraphic packages in sedimentary and metamorphic settings,

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the topic of previous discussion (e.g. Cook and Jones, 1995; Jones, 1998; Adetunji

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et al., 2014), but focus on spatial associations interpreted to be formed by crustal

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tectonothermal events that overprint earlier crustal fabrics. Broad regions in the

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lower-middle crust on the Eucla-Gawler East (C2), Eucla-Gawler West (C5,6) and

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Olympic Dam profiles (C7; Fig. 5) display a close association between weakly

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reflective zones and conductive pathways, whilst this relationship is also observed

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in the middle-upper crust of the Olympic Dam profile (C9-11).

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4.1. Crustal conductors - diffuse, or structurally controlled?

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Establishing controls on the nature and crustal geometry of tectonic, magmatic, and

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hydrothermal modification of conductivity and reflectivity is key to understanding

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the modification processes themselves as surface exposures can only give a limited

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window into processes active in mid-lower crustal regions (Daczko et al., 2016;

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Stuart et al., 2016). Seismic data, imaging the structure of the crust within, and

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surrounding, zone of anomalous conductivity can provide constraints on the

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structural control (or lack thereof) of conductors. We suggest that the process(es)

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causing modification of conductivity and reflectivity are either diffuse in nature, or

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are structurally controlled through fractures or shear zones. Differing rheologies

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associated with the crustal level and stress regime (at the time of modification) are

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the likely controlling factors on the observed geometries.

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4.1.1. The lower crust

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A broad and diffuse signature is observed in lower crustal conductors C2, C5, C7

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with similar features being observed in other co-located profiles globally (e.g. in

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the hanging wall of the Central Metasedimentary Boundary Belt Zone of the

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Grenville Province, Canada; Adetunji et al., 2014). Such conductive zones

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continue across the Moho, and are assumed to be connected to a conductive upper

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mantle (Heinson et al., 2006; Thiel et al., 2016a). Lower to mid-crustal conductors

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are widely observed in a variety of tectonic settings, e.g. partial melts in

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compressional continent-continent collisions (Rippe and Unsworth, 2010), fluids

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released from metamorphic devolatilization or a fluid/magma infilitration in

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intraplate deformation regions (Thiel et al., 2016b; Feucht et al., 2017), subducting

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slab dewatering in active ocean-continent collision (Wannamaker et al., 2009), or

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partial melts in incipient continental rifts (Didana et al., 2014). Spatially coincident

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with such weakly reflective and conductive zones are Moho offsets, or regions of

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significant Moho topography based on interpretation of seismic reflection data

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(Fig. 3). This may imply a causative relationship. A Moho offset could provide a

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locus for episodic reworking and the establishment of fluid pathways. In the

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examples presented here, a commonality is the diffuse and wide-spread nature of

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the lower crustal conductors and zones of low reflectivity, ranging from tens of

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kilometres wide, and extending up to horizontal layers of low conductivity over

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300 km long in the case of the Coompana Province (C6, Eucla Gawler West

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profile; Figs. 3 and 6).

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4.1.2. The middle-upper crust

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In the middle-upper crust in the Eucla-Gawler East and Olympic Dam profiles,

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above zones of accumulation at the BDT, conductive pathways appear to be

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discrete (2–10 km wide). True widths however may be less, given that resolution is

336

controlled by station spacing of the MT surveys, which is 5 km and 1 km for the

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Eucla Gawler East and Olympic Dam profiles, respectively. Where syn-magmatic

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compressive tectonism is evident (Eucla-Gawler East; Dutch et al., 2015a; Dutch

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and Hand, 2009), anomalous conductivity and reflectivity is controlled by syn-

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magmatic discrete shear zones, leading to moderately dipping pathways (Fig. 7).

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Where there is less evidence for compressive tectonism synchronous with upward-

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moving volatiles (Hayward and Skirrow, 2010), discrete, sub-vertical pathways

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dominate (e.g. the Olympic Dam profile, C9-11; Figs. 4 and 7).

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4.2. Relating spatially associated features to tectonothermal processes

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Because lower crustal conductors can be traced to surface expressions of magmatic

346

suites such as the A-type granites in the Olympic Domain (Fig. 1; Skirrow et al.,

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2007; Reid et al., 2017) and the Moodini Supersuite in the Coompana Province

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(Fig. 1; Wise et al., 2015b; Dutch et al., 2018), we suggest that the process leading

349

to the high conductivity and low reflectivity also has a magmatic-hydrothermal

350

origin. The high Fe content, in the form of magnetite, in A-type granites may

351

explain elevated conductivities (Yang and Emerson, 1997). Fluorine enrichment

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associated with A-type granites (and associated alteration minerals) has also been

353

noted from Olympic Dam (McPhie et al., 2011; Xing et al., 2019), whilst drilling

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adjacent to a Moodini Supersuite pluton in the Coompana Province encountered

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alteration consistent with F-rich magmatic fluids (Dutch et al., 2018). Xing et al.

356

(2019) discuss the role of fluorine in IOCG systems as a mobilising agent for U

357

and REE, whilst increasing silica solubility, promoting porosity creation and

358

therefore increasing fluid pathway connectivity. As a possible source for the A-

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359

type granites in the Olympic Domain, Skirrow et al. (2018) have speculated that

360

phlogopite in mantle peridotites could be a way to achieve the low electrical

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resistivity <10 Ωm observed in the Gawler Craton mantle (Thiel and Heinson,

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2013) underlying the Olympic Domain. An experimental study by Li et al. (2017)

363

has endorsed fluorine as a mechanism for producing high conductivities. F/Fe-rich

364

magmatic/hydrothermal fluid flow would therefore also explain the apparent

365

destruction of seismically reflective lower crust and any associated hydrothermal

366

alteration could leave behind minor mineral phases with enhanced conductivity,

367

e.g. sulphides, F-rich silicates.

368

4.2.1. Syn-post tectonic magmatic/hydrothermal overprints

369

Though conductive and non-reflective zones across the western (beneath the KSZ

370

in the Eucla-Gawler East profile in Fig. 2) and eastern (Olympic Dam profile in

371

Fig. 4) Gawler craton margin are similar in nature to a conductive and non-

372

reflective pathway in the Eucla-Gawler West profile (Fig. 3), we attribute different

373

geological processes to their formation. Commonalities in the lower crust between

374

the two types are: a mantle source, a Moho offset providing a locus for upward

375

movement, apparent destruction of a pre-existing fabric in the lower crust resulting

376

in a reduction in reflectivity and a broad, diffuse high conductivity signature. Key

377

differences however, reside in the large extent (hundreds of kilometres) of the

378

anomalously conductive lower crust beneath the Eucla Basin compared to the

379

compact conductive pathways (tens of kilometres) observed across the western and

380

eastern margins of the Gawler craton. The geometries suggest that the

381

magmatic/fluid movement are primarily controlled by the architecture of the

382

lithospheric mantle and crust. Pre-existing cool cratonic roots, such as the Gawler

383

Craton, focus magmatic flux through the crust along its margins, whereas the

384

magmatic flux through Palaeoproterozoic crustal tract that has seen a protracted

15

385

history of melt extraction and mantle input (Kirkland et al., 2017). It likely had

386

already elevated geotherms and was more wide-spread.

387

4.2.2. Underplating and mass melt migration through the crust

388

The Eucla-Gawler West profile (Figs. 3 and 6) displays a highly conductive upper

389

mantle/lower crust, the top of which correlates in part with the base of a seismic

390

province of enhanced reflectivity (the interpreted Moho). At the top of the sub-

391

vertical mantle conductor C4, sits a highly conductive and strongly reflective lower

392

crustal unit which may signify horizontally layered magmas frozen at the base of

393

the crust, a rare example of imaging an underplate (e.g. Thybo and Artemieva,

394

2013), or perhaps, due to the lack of an associated positive density anomaly, the

395

product of metasomatic reactions between a pre-existing reflective lower crust and

396

upwelling melts. A similar highly conductive lower crust-upper mantle is observed

397

in the Lachlan Orogen, where Robertson et al. (2017) suggested enhanced

398

conductivity may be caused by enrichment due to dehydration reactions of a

399

subducted oceanic crust.

400

To the west of the sub-vertical mantle conductor C4 in the Eucla-Gawler West

401

profile sits a sub-horizontal lower crustal/upper mantle conductor C6 that shallows

402

to the west. This coincides spatially with a region of low reflectivity, with a

403

moderately sharp upper margin with a more reflective mid crust. In the central

404

Eucla-Gawler West profile, the base of a sub-vertical, broad and diffuse pathway,

405

indicated by low reflectivity and high conductivity (C5; Fig. 6) is associated with a

406

gradient or offset in the Moho. We invoke a crustal-scale structure providing a

407

focus for upward movement of melt from the base of the crust (e.g. Daczko et al.,

408

2016) to the surface, without significant crustal residence times. Upward moving

409

melts fed voluminous magmatism at ca. 1180 Ma in the Coompana Province

410

(Spaggiari, 2015; Wise et al., 2015b; Kirkland et al., 2017; Dutch et al., 2018;

411

Pawley et al., 2018). This is reflected in relatively juvenile isotopic signatures from

16

412

magmatic rocks of this age (Spaggiari, 2015; Kirkland et al., 2017; Dutch et al.,

413

2018). Diffuse porous flow (Stuart et al., 2016) and/or inter-connected, channelised

414

flow networks (e.g. Daczko et al., 2016) are likely candidates for melt migration

415

mechanisms producing the observed seismic and electrical signatures. It is likely

416

that the magmatic flux homogenised pre-existing crustal packages with strong

417

reflectivity through partial melting and resetting the acoustic impedance (e.g.

418

Drummond et al., 2006; Heinson et al., 2006; Spaggiari, 2014; Heinson et al.,

419

2018). The elevated conductivity associated with the aforementioned low-

420

reflectivity zone is likely related to a north-east trending belt of strongly magnetic,

421

high-KFe, A-type granites (Fig. 1; Dutch et al., 2018; Thiel et al., 2018; Wise et

422

al., 2015b).

423

4.2.3. Magmatic volatiles/minor phase movement along re-worked cratonic margins

424

We interpret crustal conductivity anomalies in the Olympic Dam and Eucla-Gawler

425

East profiles (C2, C7-11; Figs. 2 and 4) to be due to minor conductive phases,

426

possibly the product of de-volatilization of a metasomatized sub-continental

427

lithospheric mantle (SCLM; Skirrow et al., 2018) focussed into the crust by major

428

lithospheric faults of the pre-existing Gawler Craton margin. Steeply-dipping

429

conductive and non-reflective pathways extend from the upper mantle into the mid

430

crust, possibly controlled by the location of regions of Moho topography (Fig. 7).

431

Ponding, or accumulation, of conductive material occurs in the mid crust at what

432

we interpret to represent the brittle-ductile transition (BDT) at the time of fluid

433

movement (Fig. 7). Accumulation of conductive material at this crustal level is

434

also associated with a relative increase in conductivity, in line with suggestions for

435

self-propagating hydraulic domains beneath the mechanical barrier of the BDT

436

(Connolly and Podladchikov, 2004), which have also been observed further east in

437

South Australia in the western Curnamona Province (Thiel et al., 2016b).

17

438

The conductivity and reflectivity structure above the BDT is quite different in

439

examples from the Eucla-Gawler East and Olympic Dam profiles. In the Olympic

440

Dam profile, lateral movement or accumulation of conductive phases beneath a

441

highly reflective mid-upper crustal layer is inferred (C7, C8), before propagating

442

up and producing 3-4 narrow, sub-vertical conductive and non-reflective pathways

443

to the sub-surface (C9-11; Heinson et al., 2018). Pre-existing, sub-vertical fault or

444

shear structures in seismic imagery are not obviously controlling such pathways as

445

no offset on a reflective marker horizon in the mid-crust is observed (Figs. 4 and

446

7). Although the lack of obvious shear zones may be a function of their steep

447

nature not resolved by seismic imagery. However, the narrow conductivity

448

pathways in the upper crust coincide with sub-vertical low reflectivity zones in the

449

re-processed seismic section (Fig. 4, top; Heinson et al., 2018).

450

In contrast, in the Eucla-Gawler East example, the conductivity and reflectivity

451

structure above the BDT (C2 in the upper crust) appears to be controlled by syn-

452

fluid movement deformation along west-dipping shear zones. A similar occurrence

453

of conducting fluid pathways above a mid-crustal accumulation is noted by

454

Robertson et al. (2015), where mantle-derived fluid is interpreted to have reacted

455

with mid-crustal rocks and migrated to the surface along major fault structures.

456

5. Conclusions

457

Changes in physical properties (resistivity and acoustic impedance) where co-

458

incident, enable a more powerful inference on the physio-chemical process causing

459

the joint response than could be made by each dataset alone. Here we demonstrate,

460

from three co-located reflection seismic and MT surveys, a closer association

461

between subtle changes in the conductive/reflective character, than between

462

conductors and discrete structures interpreted from seismic. We propose reactive

463

melt and fluid flow processes modify the bulk electrical and seismic structure of

18

464

the crust. These changes occur across a range of scales and rheology, depending on

465

the geodynamic regime and temperature/structural level. The thermal state of the

466

lithosphere controls the location and geometry of melt/fluid pathways through the

467

SCLM and lower crust, whilst the geodynamic regime focuses flux within the

468

middle-upper crust. Importantly, in the examples discussed here, melt/fluid

469

pathways are not controlled by pre-existing structures in the lower crust, rather

470

they flowed in broad, diffuse zones.

471

Most importantly, we show that cross-correlation of seismic reflectivity and low

472

electrical resistivity allows a rare opportunity to map the imprints of past

473

geodynamic processes, as both properties are a primary result of minor phases,

474

such as fluid and magmas. It is a vital extension to traditional geophysical methods

475

which are primarily sensitive to a singular bulk rock property or state. Our

476

examples show, that the seismic reflectivity and electrically conductive features are

477

cross-cutting pre-existing rock fabrics and are thus a proxy for modification

478

processes such as alteration due to fluid and magma fluxes.

479

480

Acknowledgements

481

We are grateful for constructive comments from A. Reid, S. Curtis and R. Dutch

482

(all GSSA) throughout many revisions of this article, and two anonymous

483

reviewers. Geoscience Australia (GA) are thanked for providing seismic data and

484

were involved in the acquisition of the Eucla Gawler MT data. HiSeis Pty Ltd are

485

thanked for reprocessing of seismic line 03GA-OD1. Jingming Duan (GA) was

486

involved in overseeing the data acquisition of the MT data acquired by

487

Moombarriga Geoscience. TW and ST publish with the permission of the Director

488

of the Geological Survey of South Australia.

19

489

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876 877 878

Figure captions

879

Figure 1. Location of deep seismic reflection profiles across Australia (inset) and

880

selected co-located deep seismic reflection and magnetotelluric profiles in South

881

Australia discussed in text (blue) Major crustal elements, structures and key syn-

882

post tectonic magmatic suites (Stewart et al., 2013; Wise et al., 2015a, 2018; Curtis

883

and Thiel, 2018) are shown.

884

Figure 2. Eucla Gawler East 13GA-EG1 co-located seismic and magnetotelluric

885

profiles (after Holzschuh, 2015; Thiel et al., 2015, 2018), with the surface location

886

of selected major shear zones. Conductor C1 is associated with a region of high

34

887

variability in seismic character, west of the Karari Shear Zone (KSZ, C2, and Fig.

888

1). The Coorabie Shear Zone (CSZ, C3 and Fig. 1) whilst near surface expressions

889

in seismic and MT are coincident, displays a large discrepancy between a shallow,

890

west-dipping zone in the seismic imagery (see Doublier et al.. 2015 for the full

891

interpretation), and a steep, west-dipping conductive zone. There exists

892

coincidence between the high conductivity zones C2 and C3 and a break in the

893

higher reflectivity in the lower crust.

894

Figure 3. Eucla Gawler West co-located seismic and magnetotelluric profiles

895

(13GA-EG1, after Dutch et al., 2016a; Thiel et al., 2016a, 2018; Pawley et al.,

896

2018). Major west-dipping shear zones are marked at their surface intercept (Fig.

897

1). MundSZ = Mundrabilla Shear Zone, MulySZ = Mulyawara Shear Zone, BSZ =

898

Border Shear Zone, PSZ = Palinar Shear Zone. The Coompana Province is

899

dominated by a highly reflective middle crust between 15 km and 40 km. Beneath,

900

the lower crust has a low reflectivity/high conductivity character (C6). The

901

continuous lower crustal conductor extends to the surface (C5) where granitic

902

basement subcrops beneath the extensive sedimentary cover (Wise et al., 2015b).

903 904

Figure 4. Olympic Dam co-located seismic (03GA-OD1) and magnetotelluric

905

profiles. Second panel shows the original processed image (after Jones et al.,

906

2005). Top panel shows an enlarged view of a reprocessed section of the same

907

seismic data, with interpreted fluid pathways from the middle crust (after Wise et

908

al., 2015c). Third panel shows the 2D resistivity model across the same profile

909

(Heinson et al., 2018). Bottom panel, resistivity model is overlain on the seismic

910

image.

911

Figure 5. The location of crustal conductors C2, C5, C9 and the spatial

912

coincidence with zones of reduced reflectivity cross-cutting flat-lying mid-lower

35

913

crustal reflective packages, indicating the process causing enhanced conductivity is

914

related to that causing reduced reflectivity.

915

Figure 6. Profile 13GA-EG1, Eucla-Gawler West. Melt transfer pathways imaged

916

as conductive and weakly reflective pathways in the crust. The region of flat-lying

917

reflectors at the base of the crust coinciding with a mantle-derived conductor may

918

indicate an underplate. For abbreviations of major structure names refer to Fig. 3.

919

Figure 7. Top: Partially interpreted seismic profiles with 2D resistivity models

920

overlain. Mantle sourced fluid pathways characterised by a reduction in reflectivity

921

and enhanced conductivity overprint earlier fabrics. Interpreted zones of fluid

922

accumulation at the BDT are identified as highly conductive regions associated

923

with an increase in reflectivity. The location and geometry of fluid pathways is

924

controlled by shear zones above the brittle ductile transition (BDT) in the Eucla-

925

Gawler East profile (right), whilst evidence is lacking for upper-crustal shear

926

control in the Olympic Dam profile (left). Bottom: schematic model depicting

927

pathways of fluid flow, and zones of accumulation.

36

to r

uc

or

r C2

ucto

d Con

30 km

Eucla Gawler East Profile

C9

Co nd

u

ct

30 km

C5

Eucla-Gawler West Profile

nd Co

10 km

Olympic Dam Profile

MundSZ

MulySZ

BSZ

Weakly reflective and conductive crustal melt pathways? PSZ

60 km

Structure intersecting Moho providing locus for upwelling melt/fluid

Moho Weakly reflective and conductive lower crust-upper mantle region

W

E

60 km

150 km

Highly reflective and conductive lower crustal region

Highly conductive and reflective fluid ponding zone at BDT

S

KSZ

N

W

E BDT

Exhumation?

BDT

60 km

60 km

Highly reflective mid crustal layer

Conductive and non-reflective pathways through reflective mid crustal layer

60 km Conductive and weakly reflective fluid migration zones

No shear zone control on fluid flow in the upper crust

Brittle Ductile

Shear zone control on fluid flow in the upper crust Brittle Ductile

Fluid migration pathway - enhanced conductivity and low reflectivity Fluid Migration Pathway along shear zones - enhanced conductivity and high reflectivity

Moho

Moho

Fluid accumulation zone Mantle

Mantle

App. Resistivity (ohm−m)

10

10

10

10

4

Zxx

Zxy

Zyx

Zyy

3

2

1

0

10 90

Phase (deg)

75 60 45 30 15 0 0.001

0.01

0.1

1

10

Periods (s)

100

1000

10000

Length

1.0 0.8 0.6 0.4 0.2 0.0 −0.2 −0.4 −0.6 −0.8 −1.0

Real

−3

10

Imag

−2

10

−1

10

0

10

1

10

Periods (s)

2

10

3

10

4

10

N

Regional strike

20

W

5

10

15

20

25

30

35

60

E

65

70

75

10

5

15

W

80

85

20 sites

E

S

0.001−1000 s

40 45 50 55 Minimum phase

Regional strike

60 sites

S

0.001 s − 1000 s

0

40

N

Karari SZ Station ID

90

90

100

110

120

130

140

150

160

10−2

10−1

Eucla Basin Sediments

Mundrabilla Shear Zone

log Period [s]

100

101

Phases below zero

102

103

0

50

100

150

200

250

300

350

400 450 Distance [km]

500

550

600

650

700

750

800

East

West 126˚00'E 29˚30'S

126˚30'E

127˚00'E

127˚30'E

128˚00'E

128˚30'E

129˚00'E

129˚30'E

130˚00'E

130˚30'E

131˚00'E

131˚30'E

0 5 10 15 20 25 30 35 40 45 50 55 60 65 70

Phase skew

30˚00'S

-EG1 We Profile 13GA

30˚30'S

st EGC090

31˚00'S

31˚30'S −500 −250 0

250 500 750 1000 1250 1500

500 s

Total magnetic intensity 32˚00'S 130˚00'E 29˚00'S

130˚30'E

131˚00'E

131˚30'E

132˚00'E

132˚30'E

133˚00'E

133˚30'E

134˚00'E

29˚30'S 5 10 15 20 25 30 35 40 45 50 55 60

Phase skew

30˚00'S

Profile 13G A-E

G1 East

30˚30'S

EGC090 31˚00'S

500 s

31˚30'S

−500 −250

0

250 500 750 1000 1250 1500

Total magnetic intensity 32˚00'S

135˚00'E

East

West 0

134˚30'E

Highlights • • • •

We image Precambrian crustal fluid/melt pathways using seismic reflection and MT We observe coincident seismic reflectivity and electrical conductivity patterns Geological interpretation should address joint signatures using both techniques Crustal rheology controls fluid flux atop zones of lower crustal melt networks

*Declaration of Interest Statement