Marianne Mareschal

Obliquity between seismic and electrical anisotropies as a potential indicator of movement sense for ductile shear zones in the upper mantle

Teleseismic shear-wave splitting and magnetotelluric experiments across the Grenville front, between the Archean craton and the Proterozoic Grenville province in the regions of the Pontiac subprovince and northwestern Grenville province (Canada), show a consistent obliquity between the polarization direction of the fast split shear wave Φ and the most electrically conductive direction (ΦMT) in the upper mantle transcurrent shear zones. At all well-recorded stations, Φ is nearly N103°E, and ΦMT is approximately N80°E. The obliquity may be considered a potential kinematic indicator, because the seismic and electrical anisotropies are thought to be controlled by lattice-preferred and shape-preferred orientations of mantle minerals (mainly olivine), respectively. The dextral movement sense of the transcurrent shear zones in the mantle, inferred from the observed obliquity, is consistent with that inferred from surface geology of the crustal shear zones. This consistency implies that deformation of the crust and the subcrustal upper mantle in the lithosphere was largely coherent in the study region.

Magnetotelluric delineation of the Trillabelle massive sulfide body in Sudbury, Ontario

A magnetotelluric (MT) survey was carried out on the western flank of the Sudbury Structure (Ontario, Canada) over a recently discovered ore body to test the potential of the method as a reconnaissance tool for deep mineral exploration. The observed responses exhibit a large phase anomaly (exceeding 75° in some places) centered over the ore body, which is interpreted as arising from a combination of 3-D induction and current channeling. Impedance decomposition methods were used to define frequency ranges in which the response is quasi 2-D with geologically plausible strike values. Two‐dimensional rapid relaxation inversions of the data, where appropriate, generated quantitative models in good agreement with known parameters of the ore body. Two‐dimensional inversions, however, cannot replicate either the size or the peculiar nature of the observed phase anomaly. Therefore 3-D modeling trials were undertaken, which best explain the MT responses in terms of 3-D induction within the Trillabelle body coupled ...

Electrical image of the subducting Cocos plate from magnetotelluric observations

Magnetotelluric data acquired over the subducting Cocos plate in southern Mexico image the top of the oceanic plate for at least 150 km inland from the coast. Although fluid expulsion occurs in the accretionary prism, enough fluid appears to remain at the top of the subducting plate, because of sealing of pores and fractures on the underside of the continental plate, to produce the conductivity contrast necessary for electrical mapping. The results are supported by those of previous gravity and seismic refraction surveys that suggest the presence of more porous material on top of a denser, subducting oceanic crust. Earthquake epicenters also confirm the location of the top of the plate. Magnetotelluric data could, therefore, be used to map the Cocos plate along the entire Middle America Trench, where it is apparently broken into separate segments that subduct at different rates and dip at different angles.

A magnetotelluric investigation of the structural geology beneath Charlevoix Crater, Quebec

Abstract The results of ten tensor magnetotelluric soundings recorded within the Charlevoix crater are presented. The geological complexity of the region (the crater is located at the junction of two major geological provinces, i.e. the crystalline basement of the Canadian shield and the more conductive sediments of the Appalachian system) and the amount of cultural noise present in our data prevent a highly definitive interpretation of the shallow structure. A subhorizontal conductive fault related to Rondots (1971) structural sketch of the original impact structure may exist within the upper 5 km, but it is not completely constrained by the data. Pseudo 3D modelling using stacked thin sheets indicates that superficial patches of high conductivity may explain the large anisotropy observed at the long periods and suggests that our deeper structures fit well with the 2D model previously proposed by Kurtz (1982) for the region.

ELECTRICAL CONDUCTIVITY: THE STORY OF AN ELUSIVE PARAMETER, AND OF HOW IT POSSIBLY RELATES TO THE KAPUSKASING UPLIFT (LITHOPROBE, CANADA)

Possible sources of crustal conductivity are reviewed, with special emphasis on shield environments. Typical lower crustal conductivities are interpreted in terms of brines and/or graphite films precipitated at grain-boundaries under cooling after peak metamorphism. The nature of the conductive films may strongly depend on the age of the crust: grain-boundary graphite may be present in most lower crusts, while brines would be found in the more active and younger regions. Electromagnetic investigations of the Kapuskasing Uplift confirm that exposed lower crustal rocks are very resistive. Water does not seem to play any significant role in the present lower crust of the uplift, where no correlation between deep seismic reflectors and conductors can apparently be found. Interpretation is only preliminary, however, it also suggests that brines may exist deep in the upper crust (~ 5km). Other shallow conductors could be related to lithologic boundaries, faults or detachment zones.

On the use of K indices to define maximum external contributions to Magsat data at mid-latitudes

Abstract Published K indices defined from ground-based magnetic variations are shown to give reliable upper bounds to the mid-latitude, external transient field recorded by Magsat (350 km altitude). The argument relies on model calculations of three-dimensional current systems known to be representative of auroral activity and gives a simple method for estimating source contamination in the magnetic data used for main field and crustal anomalies calculations.

Magnetotelluric observations over the Rhine Graben, France : a simple impedance tensor analysis helps constrain the dominant electrical features

Abstract A simple impedance tensor analysis of four magnetotelluric soundings recorded over the ECORS section of the Rhine Graben shows that for periods shorter than about 30 s, induction dominates over channelling. For longer periods, 2-D induction galvanically distorted by surface heterogeneities and/or current chanelled in the Graben can explain the observations; the role of chanelling becomes dominant at periods of the order of a few hundred seconds. In the area considered, induction appears to be controlled by inclusions of saline water in a porous limestone layer (Grande Oolithe) and not by the limits of the Graben with its crystalline shoulder (Vosges). The simple analysis is supported by tipper analyses and by the results of schematic 2-D modelling.

Removal of near‐surface galvanic effect

Galvanic distortion of the regional electric field bY near-surface heterogeneities leads to disturbance in the measured MT impedance tensor. This tensor is expressed as the product of a local galvanic distortion tensor and a two-dimensional regional impedance tensor. In this paper, we present a method to remove the effect of galvanic distortion and extract the regional impedances. This method is based essentially on single value decomposition of the distortion tensor and can be used for two or three-dimensional surface heterogeneities. The recovered regional impedances are shifted by a real constant in such a way that neither the shape of regional apparent resistivity curve nor the phases are altered. A synthetic model with conductive hemispherical heterogeneity embedded in a two-dimensional regional structure (Groom and Bailey, 1988) demonstrates the utility of the method.