SummaryA magnetotelluric survey, comprising 40 stations, has been completed in the southern Yilgarn Craton. The preferred resistivity cross section through the crust and upper mantle shows the local lithosphere comprises three distinct units separated by steep boundaries. The central unit, interpreted as equivalent to the Southern Cross Domain has a resistive crust overlying a more conductive mantle. The two units on either side comprise a conductive lower crust overlying a resistive mantle. Dipping narrow zones of increased conductivity in the crustal part of the model correlate with known surface structures. The eastern margin of the Southern Cross Domain as inferred from deep crustal and mantle resistivity occurs about 50 km to the west of the Ida Fault, the margin of the domain at the surface. The three fold subdivision of the local lithosphere is consistent with the geologically and geochemically defined terranes and domains in this part of the Yilgarn.Current models for regional mineral exploration targeting emphasize the significance of major geological structures and the edges of cratonic blocks as areas of greatest prospectivity. The South Yilgarn MT dataset demonstrate that such features can be located based on variations in the electrical conductivity of the lower crust and mantle, which can be measured in a cost effective manner using the magnetotelluric method.
A 1400 km-long, 2-D magnetotelluric (MT) profile across the Archaean Kaapvaal Craton, the Proterozoic Rehoboth Terrane and the Late Proterozoic/Early Phanerozoic Ghanzi-Chobe/Damara Belt reveals significant lateral heterogeneity in the electrical resistivity structure of the southern African lithosphere. The profile indicates the following present-day average lithospheric thicknesses, to a precision of about ± 20 km, for each of the terranes traversed (inferred conductive geotherms in brackets): Eastern Kimberley Block of the Kaapvaal Craton 220 km (41 mWm-2), Western Kimberley Block 190 km (44 mWm-2), Rehoboth Terrane 180 km (45 mWm-2) and Ghanzi-Chobe/Damara Belt 160 km (48 mWm-2). Previously published mantle xenolith pressure-temperature (P-T) arrays from the Gibeon, Gordonia and Kimberley fields, however, suggest that the Rehoboth Terrane had equilibrated to a cooler conductive palaeo-geotherm (40 – 42 mWm-2 ) very similar to that of Eastern Kimberley Block of the Kaapvaal Craton, at some (unconstrained) time prior to the Mesozoic eruption of the kimberlites. A model consisting of the penetration of heat transporting magmas into the lithosphere, with associated chemical refertilisation, at an early stage of Mesozoic thermalism appears to be the most plausible model at present to account for both the present-day lithospheric structure of the Rehoboth Terrane and an earlier, cooler palaeo-geotherm. Some problems, however, remain unresolved in terms of the isostatic response of the model. Based on a compilation of xenocryst Cr/Ca-in-pyrope barometry observations, the extent of depleted mantle in the Rehoboth Terrane is found to be significantly reduced with respect to the Eastern Kimberley Block: 117 km versus 138 – 167 km. It appears most likely that the chemical depletion depth in both terranes, at least in the vicinity of kimberlite eruption, is accounted for by refertilisation of the lower lithospheric mantle.
As a part of the Central Baffin Multidisciplinary Project (a collaborative effort of theGeological Survey of Canada, The Canada-Nunuvut Geoscience Centre, and the Polar Continental Shelf Project), a 45 station, 500 km long regional-scale magnetotelluric profile was acquired. The profile crosses the northern margin of the Trans-Hudson Orogen and extends northward into the Archean Rae Craton. To the south, the profile crosses the Paleoproterozoic Piling Group. The primary goal of the experiment was to determine major geological boundaries by delineating regional electrical structures. Preliminary analysis shows that the conductive Astarte River Formation can be mapped and used as a proxy for the base of the Piling Group. Analysis has also revealed a high conductivity contrast between the Piling Group metasedimentary rocks and the northern Archean granite and gneissic complexes. Laboratory results indicate that the conductivity in the Astarte River Formation is due to the high content of interconnected graphite.
Summary An integrated geophysical investigation of geological structure in the east Kimberley, northern Western Australia, was performed to identify structures and features important for the mineral potential of the region. Subsurface structure was constrained through the use of magnetic, gravity and magnetotelluric (MT) data along an E-W transect. Significant crustal-scale structures were interpreted and investigated to determine their influence on the development of regional structure, the emplacement of magma, and circulation of hydrothermal fluids. Some newly interpreted features include a north-trending structure that intersects the region, orogen-normal structures and a large mafic magma chamber at 20km depth.
[1] A regional-scale magnetotelluric (MT) experiment across the southern African Kaapvaal craton and surrounding terranes, called the Southern African Magnetotelluric Experiment (SAMTEX), has revealed complex structure in the lithospheric mantle. Large variations in maximum resistivity at depths to 200–250 km relate directly to age and tectonic provenance of surface structures. Within the central portions of the Kaapvaal craton are regions of resistive lithosphere about 230 km thick, in agreement with estimates from xenolith thermobarometry and seismic surface wave tomography, but thinner than inferred from seismic body wave tomography. The MT data are unable to discriminate between a completely dry or slightly “damp” (a few hundred parts per million of water) structure within the transitional region at the base of the lithosphere. However, the structure of the uppermost ∼150 km of lithosphere is consistent with enhanced, but still low, conductivities reported for hydrous olivine and orthopyroxene at levels of water reported for Kaapvaal xenoliths. The electrical lithosphere around the Kimberley and Premier diamond mines is thinner than the maximum craton thickness found between Kimberley and Johannesburg/Pretoria. The mantle beneath the Bushveld Complex is highly conducting at depths around 60 km. Possible explanations for these high conductivities include graphite or sulphide and/or iron metals associated with the Bushveld magmatic event. We suggest that one of these conductive phases (most likely melt-related sulphides) could electrically connect iron-rich garnets in a garnet-rich eclogitic composition associated with a relict subduction slab.
Profiling electromagnetic distributed acquisition systems (DAS), such as MIMDAS and Titan-24, are commonly used for near-surface exploration. Such systems allow more rapid acquisition compared to standard magnetotelluric (MT) approaches. Instead of recording horizontal electric and magnetic fields at every site, DAS acquire a single (along-profile) component of the electric field at every site, but the perpendicular component is measured at every second or so site, and the magnetic field sensors are normally positioned only at a couple of locations within the study area. It is common practice to apply standard MT inversion algorithms to invert such DAS data, despite the lack of full four component measurements at each site. This is only valid in the strictly two-dimensional (2D) case with the geoelectric strike perpendicular to the acquisition profile and for the transverse magnetic (TM) mode, and also under certain assumptions on frequency range and resistivities. In case of the 2D transverse electric (TE) mode and in the general 3D case, employing standard MT inversion software will lead to erroneous results. Therefore, we have developed a new 3D inversion algorithm that considers the correct positioning of all sensors. We investigate the ability of the new inversion to recover the subsurface resistivity and compare the results to standard MT inversion of DAS data. The newly developed inversion code is also useful for conventional MT surveys when data from some channels is lost. With the new approach missing fields can be easily substituted by fields from another site. Presentation Date: Wednesday, October 17, 2018 Start Time: 1:50:00 PM Location: 213A (Anaheim Convention Center) Presentation Type: Oral
'/%3e%3c/defs%3e%3cpath fill='%23012169' d='M0 0h10080v5040H0z'/%3e%3cpath stroke='%23fff' stroke-width='.6' d='m0 0 6 3m0-3L0 3' clip-path='url(%23b)' transform='scale(840)'/%3e%3cpath stroke='%23e4002b' stroke-width='.4' d='m0 0 6 3m0-3L0 3' clip-path='url(%23c)' transform='scale(840)'/%3e%3cpath stroke='%23fff' stroke-width='840' d='M2520 0v2520M0 1260h5040'/%3e%3cpath stroke='%23e4002b' stroke-width='504' d='M2520 0v2520M0 1260h5040'/%3e%3cg fill='%23fff'%3e%3cuse xlink:href='%23d' x='2520' y='3780'/%3e%3cuse xlink:href='%23a' x='7560' y='4200'/%3e%3cuse xlink:href='%23a' x='6300' y='2205'/%3e%3cuse xlink:href='%23a' x='7560' y='840'/%3e%3cuse xlink:href='%23a' x='8680' y='1869'/%3e%3cuse xlink:href='%23e' x='8064' y='2730'/%3e%3c/g%3e%3c/svg%3e)
'/%3e%3cpath d='M200 752.362h200v300H200z' style='fill:%23fff;fill-opacity:1;stroke:none' transform='translate(0 -752.362)'/%3e%3cpath d='M400 752.362h200v300H400z' style='fill:%23ff883e;fill-opacity:1;stroke:none' transform='translate(0 -752.362)'/%3e%3c/svg%3e)
'%3e%3cpath fill='%23001489' d='M0 0v6h9V0z'/%3e%3cpath fill='%23e03c31' d='M0 0v3h9V0z'/%3e%3cg stroke='%23fff' stroke-width='2'%3e%3cpath id='d' d='m0 0 4.5 3L0 6m4.5-3H9'/%3e%3cuse xlink:href='%23b' stroke='%23ffb81c' clip-path='url(%23c)'/%3e%3c/g%3e%3cuse xlink:href='%23d' fill='none' stroke='%23007749' stroke-width='1.2'/%3e%3c/g%3e%3c/svg%3e)