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

I. J. Ferguson, Alan G. Jones, Yu Sheng, X. Wu, & I. Shiozaki
A Lithoprobe magnetotelluric (MT) survey across the Palaeoproterozoic Trans Hudson Orogen in 1992 included 34 sites within the Flin Flon Belt and adjacent geological domains to the north and south. The Flin Flon Belt comprises a collage of arc and back-arc tectonostratigraphic assemblages accreted early in the orogenic process. Later tectonic activity included overthrusting by the Kisseynew Domain from the northeast, and late-faulting which juxtaposed Flin Flon Belt rocks against higher grade metamorphic rocks from the Namew Gneiss Complex to the south. The MT impedance tensors and induction vector responses reveal four distinct geoelectric zones along this section of the Lithoprobe transect. To the east, near Cormorant Lake, the geoelectric responses are dominated by the contrast between the resistive Cormorant Batholith and more conductive rocks of the Namew Gneiss Complex. The responses to the south of the profile, for sites near Athapapuskow Lake, are dominated by a strong upper crustal conductor. To the southwest, at sites near Amisk Lake, the responses are dominated by a north-south striking geoelectric structure probably associated with the contrast between resistive felsic-intrusive rocks to the east and more conductive ocean-floor assemblages to the west. Finally, the responses at sites near the northern boundary of the Flin Flon Belt with the southern flank of the Kisseynew Gneiss Belt are related to this tectonic boundary. Geoelectrical structures defined by the shorter period (<1 s) responses are associated with east-west striking geological units, whereas the deeper-penetrating longer period responses indicate the presence of a more complex structures associated with relatively conductive rocks in the upper and mid-crust. A significant characteristic of the MT response across the Flin Flon Belt is very strong galvanic distortion, which reflects the complexity of the near-surface geological structure in the greenstone belt and suggests the presence of numerous small-scale resistivity structures distributed through the upper crust. This distortion precludes good resolution of deepest crust and uppermost mantle structures. The MT observations show that the conductive feature producing the Athapapuskow Lake conductivity anomaly (ALCA) extends for at least 40 km along-strike (~N36oE), and is roughly two- dimensional in form. Modelling shows that the top of the body dips southeast at an angle of 20-50 degrees from a vertical western edge coincident with the Athapapuskow Lake Shear Zone. The top of the conductor is at ~3 km depth on this boundary. The ALCA lies in the eastern part of the Namew Gneiss Complex. The MT method cannot resolve the exact spatial distribution of conductive rocks but it is probable that the ALCA is due to a series of isolated conductors (with resistivity <1 ohm.m) associated with subordinate graphitic and sulphidic, supracrustal pelitic and psammitic gneisses. Airborne EM anomalies in the gneiss complex are associated with this sub-unit.