Geoelectric response and crustal electrical-conductivity structure of the Flin Flon Belt, Trans-Hudson Orogen, Canada

Abstract

A Lithoprobe magnetotelluric survey across the Palaeoproterozoic Trans-Hudson Orogen included 34 sites within the Flin Flon Belt and adjacent geological domains. The magnetotelluric impedance tensors and geomagnetic induction vectors reveal four distinct geoelectric zones along this segment of the Lithoprobe transect. In the east and west, the geoelectric responses are dominated by the contrast between intrusive rocks and more conductive ocean-floor assemblages. A significant characteristic of the responses throughout the Flin Flon Belt is the very strong galvanic distortion of the electric field, which reflects the complexity of the upper crustal geological structure in the greenstone belt, requiring careful application of distortion removal methods. The responses at sites near the north of the Flin Flon Belt are related to the boundary with the southern flank of the Kisseynew gneiss belt. To the south, at sites near Athapapuskow Lake, the responses are dominated by a strong upper-crustal conductor. The magnetotelluric observations show that the Athapapuskow Lake conductivity anomaly extends for at least 40 km along strike (similar to N36%E), and is roughly two-dimensional in form. Numerical modelling shows that the top of the body dips southeast at 20-50% from a western edge coincident with the Athapapuskow Lake shear zone. The conductor lies in the eastern part of the Namew gneiss complex. The magnetotelluric method cannot resolve the exact spatial distribution of conductive rocks but it is probable that the anomaly is caused by a series of isolated conductors (with resistivity < 1 Ohm.m) associated with subordinate graphitic and sulphidic supracrustal gneisses.