Kate Selway

Magnetotelluric constraints on subduction polarity: Reversing reconstruction models for Proterozoic Australia

Two-dimensional, lithospheric-scale magnetotelluric imaging in the central Australian Proterozoic has constrained the large-scale architecture of terrane assembly during Paleoproterozoic accretion and collision. The comparatively conductive North Australian craton, consisting of rocks between ~2500 and 1730 Ma in age, has been imaged to extend for 150 km under the 1690–1620 Ma Warumpi Province, which forms part of a large, comparatively juvenile terrane in central-southern Australia. Collision between the North Australia craton and Warumpi Province occurred ca. 1640 Ma. The boundary between these domains is modeled to be subvertical at crustal scale, but dips south at ~45° in the mantle to depths of 150 km. We interpret this geometry to reflect lithospheric-scale underthrusting of the North Australian craton beneath the Warumpi Province, and suggest that it provides a first-order constraint on subduction polarity during collision ca. 1640 Ma. In contrast, most contemporary models for the evolution of Paleoproterozoic Australia propose that the North Australian craton was located on the overriding plate of a long-lived (ca. 1800–1550 Ma) north-directed subduction system. The polarity of these models is not consistent with the lithospheric-scale geophysical architecture.

Magnetotelluric constraints on the tectonic setting of Grenville-aged orogenesis in central Australia

Abstract: Magnetotelluric (MT) data were collected over a 480 km long profile in central Australia, extending from the Mesoarchaean–Palaeoproterozoic Gawler Craton into the Mesoproterozoic–Neoproterozoic Musgrave Province. The two regions have contrasting geological histories and many reconstruction models of Proterozoic Australia propose that they were juxtaposed during the Mesoproterozoic. Despite the significant differences in geological background between the Gawler Craton and the Musgrave Province, the MT data did not image an electrical structure, or any change in electrical character, between them. MT models suggest that southern–central Australia is electrically separated into two parts. The first, comprising the northernmost Gawler Craton and the Musgrave Province, has a 5 km thick layer of moderate resistivity overlying highly resistive crust. The second, comprising the central–northern Gawler Craton, displays moderate resistivity to the base of the crust. The lithospheric mantle underlying both electrical regions is uniformly highly resistive. Our results suggest that the Musgrave Province and Gawler Craton are more closely related than is currently recognized. We propose that the Gawler Craton forms basement to the Musgrave Province and that the Grenvillian Musgrave Orogeny was an intracratonic reworking event that overprinted the older basement.

Acquisition of a Unique Onshore/Offshore Geophysical and Geochemical Dataset in the Northern Malawi (Nyasa) Rift

The Study of Extension and maGmatism in Malawi aNd Tanzania (SEGMeNT) project acquired a comprehensive suite of geophysical and geochemical datasets across the northern Malawi (Nyasa) rift in the East Africa rift system. Onshore/offshore active and passive seismic data, long‐period and wideband magnetotelluric data, continuous Global Positioning System data, and geochemical samples were acquired between 2012 and 2016. This combination of data is intended to elucidate the sedimentary, crustal, and upper‐mantle architecture of the rift, patterns of active deformation, and the origin and age of rift‐related magmatism. A unique component of our program was the acquisition of seismic data in Lake Malawi, including seismic reflection, onshore/offshore wide‐angle seismic reflection/refraction, and broadband seismic data from lake‐bottom seismometers, a towed streamer, and a large towed air‐gun source.

Constraints on volumes and patterns of asthenospheric melt from the space-time distribution of seamounts

Although partial melt in the asthenosphere is important geodynamically, geophysical constraints on its abundance remain ambiguous. We use a database of seamounts detected using satellite altimetry to constrain the temporal history of erupted asthenospheric melt. We find that intraplate volcanism on young seafloor (<60 Ma) equates to a ~20 m thick layer spread across the seafloor. If these seamounts tap partial melt within a ~20 km thick layer beneath the ridge flanks, they indicate extraction of an average melt fraction of ~0.1%. If they source thinner layers or more laterally restricted domains, larger melt fractions are required. Increased seamount volumes for older lithosphere suggest either more active ridge flank volcanism during the Cretaceous or additional recent melt eruption on older seafloor. Pacific basin age constraints suggest that both processes are important. Our results indicate that small volumes of partial melt may be prevalent in the upper asthenosphere across ocean basins.

Two-dimensional magnetotelluric analysis of three-dimensional bodies: a case study from South Australia

Magnetotelluric (MT) tensors have significantly different forms depending on whether the subsurface is one-dimensional (ID), two-dimensional (2D), or three-dimensional (3D). In subsurface geological structures that are not ID, two-dimensionality is often assumed, as inversion routines for 2D earth models are computationally more tractable than those for full 3D media. In 2D, the MT tensor decouples into two independent modes, the transverse electric (TE) mode and the transverse magnetic (TM) mode. Often only one of these modes is acquired during commercial operations. Field data were collected over an elongate magnetic anomaly of a type that would normally be approximated as 2D but which has a finite strike length and is therefore a 3D body. With this in mind, the applicability of interpreting data defined as TE and TM were assessed using (a) Mohr circles galvanic distortion analyses, (b) determination of strike of local and regional geology, and (c) comparison of 2D inversion techniques. The data were collected with the Mount Isa Mines Distributed Acquisition System (MIMDAS) in the Deep Well prospect of the Curnamona Province in South Australia. We show that the TM mode accurately delineates boundaries and that since boundary-charges are included in the inversion formulation, it also provides accurate values of apparent resistivity. The TE mode provides poor boundary delineation and underestimates the resistivity of the 3D body. Joint inversions provide only a small improvement upon TM-only inversions, but determination of dimensionality, strike, and detection of galvanic distortion mean that collection of both data modes is still preferable.

Author Correction: Uplift of the central transantarctic mountains

The original version of this Article incorrectly referenced the Figures in the Supplementary Information. References in the main Article to Supplementary Figure 7 through to Supplementary Figure 20 were previously incorrectly cited as Supplementary Figure 5 through to Supplementary Figure 18, respectively. This has now been corrected in both the PDF and HTML versions of the Article.