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Investigation of the electrical resistivity structure of the crust and upper mantle at Lake Nipigon, Ontario using the magnetotelluric method



Investigation of the electrical resistivity structure of the crust and upper mantle at Lake Nipigon, Ontario using the magnetotelluric method



University of Manitoba, Winnipeg, MB, Canada (CAN)



Magnetotelluric data collected in the Proterozoic Nipigon Embayment and surrounding western Archean Superior Province are used to investigate the geoelectric dimensionality and structure in the area of Lake Nipigon in northwestern Ontario, Canada. The objective of the study is to better define the bulk crustal and mantle electrical resistivity of the area by first understanding the complexity of the structures influencing the MT responses and secondly by applying appropriate modelling methods to the data. The western Superior Province is characterized by subparallel, east-west trending, fault-bounded terranes each with distinct rock types, structures, ages, and metamorphic conditions. The Nipigon Embayment consists of Proterozoic rocks that uncomformably overlie and intrude the basement rock of the western Superior Province and well as older flat lying sedimentary rocks of the Sibley Group. The magnetotelluric method is a passive electromagnetic technique that uses natural, time-varying electric and magnetic fields in order to examine the electrical resistivity structure of the subsurface. The spatial distribution of the resistivity is referred to as the geoelectric dimensionality and can be described as predominantly one-dimensional, two-dimensional, or three-dimensional. The magnetotelluric tensor contains frequency-domain transfer functions that relate different components of the horizontal electric and magnetic fields. Transfer functions can be used to investigate the dimensionality of the data and determine the geoelectric strike. These parameters were determined using methods based on magnetotelluric tensor decomposition and mathematical invariants of the tensor such as evaluation of the phase-skew and the WALDIM method. Long period dimensionality results from the western Superior Province around Lake Nipigon indicate that the general behaviour is two-dimensional. In contrast, the long period dimensionality results from the Nipigon Embayment indicated the general behaviour is three-dimensional. Although the data deviates from two-dimensionality, a two-dimensional modeling approach is justified for large parts of the study area and should provide reliable resistivity models. Two-dimensional models were determined for two north-south profiles using a non-linear conjugate gradient inversion method. The resulting models show a number of significant resistivity variations in both the crust and upper mantle. Observation of a north-dipping resistive slab adjacent to the North Caribou Superterrane as well as a highly conductive anomaly located at mantle depths support the favoured hypothesis that the Superior Province formed as an accretionary orogen. Many of the mantle features observed in the Nipigon Embayment resistivity model are distinct from those observed in the surrounding Archean Superior Province. In particular, regions of mantle beneath the Nipigon Embayment are observed to be anomalously resistive and contribute to the phase anomaly observed in this region.

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