James A. Craven

Electrical conductivity and Paleo-Proterozoic foredeeps

Sedimentary facies associated with Paleo-Proterozoic foredeeps are apparently highly conductive. The conductive lithology appears to be a euxinic carbonaceous shale deposited on preforedeep continental margin facies. The shale is overlaid by axial turbidites and often incorporated into the succeeding fold and thrust belt. Carbonaceous shales can be made highly conducting if graphitized by exposure to temperatures exceeding 400°C. Although foredeep facies develop diachronously, the most conductive unit is usually found in the core of the fold and thrust belt, suggesting that deformation may be essential for the complete graphitization of the carbon. The foredeep hypothesis offers an attractive explanation for several highly conductive features detected in the upper and middle crust in Laurentia, and possibly throughout the world. The euxinic facies represent highly specific, tectonic marker horizons that are readily delineated by using electromagnetic methods.

Exploration for unconformity-type uranium deposits with audiomagnetotelluric data: A case study from the McArthur River mine, Saskatchewan, Canada

Unconformity-type deposits supply a significant amount of the world’s uranium and consist of uranium that is generally codeposited with graphite in a fault zone. The low resistivity of the graphite produces a significant contrast in electrical resistivity, which can be located with electromagnetic (EM) methods. The Athabasca Basin in Western Canada hosts significant uranium deposits, and exploration in deeper parts of the basin has required the application of new EM methods. This paper presents an evaluation of the audiomagnetotelluric (AMT) exploration method at the McArthur River mine in the Athabasca Basin. AMT data were collected at 132 stations on a grid, and two-dimensional (2D) and three-dimensional (3D) inversions were used to generate resistivity models. These models showed two major results: (1) a significant conductor coincident with a major basement fault (P2) and the uranium deposits (this conductor begins at the unconformity at a depth of 550 m and extends to a depth of at least three km) an...

The North American Central Plains conductivity anomaly and its correlation with gravity, magnetic, seismic, and heat flow data in Saskatchewan, Canada

Abstract The North American Central Plains conductivity anomaly (NACP) lies, virtually in its entirety, within the Trans-Hudson Orogen. Accordingly, should these two features prove to be contemporaneous, then the geometrical relationship between these two is of foremost importance to any evolutionary tectonic model proposed to explain the collision of the Superior and Churchill Provinces in the Hudsonian, and a model that does not include a mechanism for the generation of this anomaly is obviously untenable. In order to map better the trend of the NACP in the Province of Saskatchewan, Canada, two magnetotelluric (MT) profiles were conducted over the NACP previously defined by magnetometer arrays and profiles. Data from these two profiles, along with an earlier MT profile just north of the U.S./Canadian border, suggest that the NACP is not a continuous feature, but rather that it exhibits a definite break at latitude 51° N. Other geophysical evidence examined herein is concordant with this characteristic. If a second MT anomaly mapped to the northwest of this break is, in fact, a manifestation of the same geological structure, then one possible interpretation is of a major NW-SE trending sinistral fault in the deep crust, previously undetected, with a movement of some 100–150 km along strike. The MT data from the southernmost profile, after correction for static shift, are modelled in a 2D manner, and it is shown that the NACP is consistent with an arcuate structure in vertical section of high conductivity (> 2 S m −1 ) beginning at a depth of 10 km centred on 103° W dipping down to the west reaching possibly the base of the crust. It is also shown that such an arcuate shape in section can explain an observed gravity high of 40 mgal. Such high conductivities cannot be explained in terms of connected fluids, as it would require implausibly high porosities (12–20%). A comprehensive interpretation of all the geophysical data would have to account for the observed characteristics of the anomalous region; which are, high electrical conductivity, positive density contrast, no magnetization, and high heat flow in the basement, in terms of a single causative body. One possible explanation is presented in terms of a zone of lithospheric weakness along which was emplaced low-density differentia from a mantle-derived body. An alternative explanation could be the phase transformation of obducted material into eclogite or serpentinite. However, difficulties exist with both of these explanations and further data are required.

Electromagnetic constraints on strike-slip fault geometry—The Fraser River fault system

Magnetotelluric data from four profiles crossing the Eocene strike-slip Fraser River fault in southwestern British Columbia suggest that it penetrates the entire crust. This conclusion is supported by seismic reflection observations of a 2-3 km step in the Moho slightly to the east of the surface expression of the fault, but is at variance with an interpretation of the seismic data in which the fault soles into mid-crustal reflectors that seem to be continuous across the fault trace. A crustal-penetrating geometry supports the proposal that the Fraser River fault forms part of a 2500-km-long intracontinental transform fault system in northwestern North America. Modeling studies resolve a thin, highly conducting mid-crustal zone that is connected electrically to the conducting lower crust beneath the Coast belt. Low δ 13 C values close to the Fraser fault system suggest that the electromagnetic signature of this zone may be due to the presence of organic carbon.

North American Central Plains conductivity anomaly within the Trans-Hudson orogen in northern Saskatchewan, Canada

Magnetotelluric data acquired across the Paleoproterozoic Trans-Hudson orogen, northern Saskatchewan, image one of the world9s longest crustal features, the North American Central Plains conductivity anomaly. Modeling shows the anomaly at this latitude to comprise two distinct, westward-dipping bodies of high conductivity lying structurally above a late collisional feature, the Guncoat thrust. The shallower of these bodies correlates with the western part of the La Ronge belt; the deeper body at mid-crustal depths underlies the Wathaman batholith, and its western boundary is close to the inferred subsurface extension of the sub-vertical Needle Falls shear zone. The anomaly is identified with interleaved, biotitic, metasedimentary rocks of the Nemeiben zone and Cree Lake belt and is interpreted to have been thrust beneath the margin during collision of the La Ronge arc with the Rae-Hearne continent via westward-directed subduction.

Electromagnetic images of regional structure in the southern Canadian Cordillera

As part of Lithoprobes Southern Cordilleran transect investigations, magnetotelluric (MT) soundings were made at 160 sites providing unprecedented coverage from the Rockies to the west coast. Striking lateral variation, which spatially correlates with the morphogeological belt boundaries, is apparent at periods sensing the lower crust (≈10 s). For the Rockies, MT phases are around 35°, indicative of a moderately resistive (100s – 1000s Ω·m) North American Basement. Foreland belt phases are transitional and increase from 60° in the east to 70° in the west. Omineca and Coast belt phases are high (75°), implying a conductive (10–30 Ω·m) lower crust, whereas Intermontane belt phases are more than 10° lower (equivalent to ≈150 Ω·m). The regional variation in conductivity correlates to first order with surface heat flow changes along the profile and is also correlative with coincident seismic reflection sections in some aspects.

Buried Proterozoic foredeep under the Western Canada Sedimentary Basin

Electromagnetic studies of the Precambrian basement beneath the Western Canada Sedimentary Basin in Alberta indicate a narrow linear conductivity anomaly spatially correlated with a strong positive magnetic feature, the Red Deer high. The conductor is located below sedimentary cover near the top of the crystalline basement and has limited depth extent. We propose that this anomalous feature represents graphitic metasedimentary rocks in the euxinic-flysch facies of a Proterozoic foredeep sequence. The strong magneticanomalyresultsfromanassociatedironformationdepositedontheouterrampofthe foredeep. This model explains the geophysical anomalies, has analogues on the exposed shield, and is consistent with the timing, deformation history, and known geology of the Precambrian basement.

Magnetotelluric investigations of the lithosphere beneath the central Rae craton, mainland Nunavut, Canada

New magnetotelluric soundings at 64 locations throughout the central Rae craton on mainland Nunavut constrain 2-D resistivity models of the crust and lithospheric mantle beneath three regional transects. Responses determined from colocated broadband and long-period magnetotelluric recording instruments enabled resistivity imaging to depths of > 300 km. Strike analysis and distortion decomposition on all data reveal a regional trend of 45–53°, but locally the geoelectric strike angle varies laterally and with depth. The 2-D models reveal a resistive upper crust to depths of 15–35 km that is underlain by a conductive layer that appears to be discontinuous at or near major mapped geological boundaries. Surface projections of the conductive layer coincide with areas of high grade, Archean metasedimentary rocks. Tectonic burial of these rocks and thickening of the crust occurred during the Paleoproterozoic Arrowsmith (2.3 Ga) and Trans-Hudson orogenies (1.85 Ga). Overall, the uppermost mantle of the Rae craton shows resistivity values that range from ~3000 Ω m in the northeast (beneath Baffin Island and the Melville Peninsula) to ~10,000 Ω m beneath the central Rae craton, to >50,000 Ω m in the south near the Hearne Domain. Near-vertical zones of reduced resistivity are identified within the uppermost mantle lithosphere that may be related to areas affected by mantle melt or metasomatism associated with emplacement of Hudsonian granites. A regional decrease in resistivities to values of ~500 Ω m at depths of 180–220 km, increasing to 300 km near the southern margin of the Rae craton, is interpreted as the lithosphere-asthenosphere boundary.

Lithospheric architecture of the Slave craton, northwest Canada, as determined from an interdisciplinary 3-D model

Regional-scale geologic structures characteristic of mantle lithosphere within cratons found in continent interiors are interpreted using geo-registered diverse data sets from the Slave craton of northwest Canada. We developed and applied a new method for mapping seismic discontinuities in three dimensions using multiyear observations at sparse, individual broadband receivers. New, fully 3-D conductivity models used all available magnetotelluric data. Discontinuity surfaces and conductivity models were geo-registered with previously published P-wave and surface-wave velocity models to confirm first-order structures such as a midlithosphere discontinuity. Our 3-D model to 400 km depth was calibrated by “drill hole” observations derived from xenolith suites extracted from kimberlites. A number of new structural discontinuities emerge from direct comparison of coregistered data sets and models. Importantly, we distinguish primary mantle layers from secondary features related to younger metasomatism. Subhorizontal Slave craton layers with tapered, wedge-shaped margins indicate construction of the craton core at 2.7 Ga by underthrusting and flat stacking of lithosphere. Mapping of conductivity and metasomatism in 3-D, the latter inferred via mineral recrystallization and resetting of isotopic ages in xenoliths, indicates overprinting of the primary layered structures. The observed distribution of relatively conductive mantle at 100–200 km depths is consistent with pervasive metasomatism; vertical “chimneys” reaching to crustal depths in locations where kimberlites erupted or where Au mineralization is known.

Feasibility of virtual source reflection seismology using interferometry for mineral exploration: A test study in the Lalor Lake volcanogenic massive sulphide mining area, Manitoba, Canada

Approximately 300 hours of ambient noise data were recorded on a grid of receivers covering an area of 4 km2 over the Lalor Mine, Canada, to test the capability of seismic interferometry to image ore deposits in the crystalline rock environment. Underground mining activities create the main source of ambient noise in the area. Alongside the ambient noise survey, a larger three-dimensional active-source seismic survey was also acquired and used to evaluate the interferometry results. Power spectral density calculations show random ambient noise with a frequency range of 2 Hz–35 Hz. A beamforming analysis identified body waves arriving from the west–northwest (pointing towards the mine) and surface waves propagating from the northeast. The calculated virtual shot gathers retrieved by cross-correlating ambient noise at all receivers were processed following both two-dimensional and three-dimensional approaches using a sequence similar to the one applied to the activesource three-dimensional data. The dip-moveout stacked section reveals a number of events similar to those observed on the processed active seismic sections. In particular, the passive seismic interferometry method is capable to partly image shallowly dipping reflections but did not produce convincing images of steeply dipping reflections. Dip-moveout stacked sections obtained with different cross-correlation time windows indicate that the strength and number of reflections generally increase with longer noise records. However, a few reflections at depth show reduced coherency with longer noise time windows. The passive seismic interferometry results over the Lalor mining area are encouraging, but image quality of the passive survey is lower than the acquired active three-dimensional survey at the area. Future ambient noise surveys with longer offsets, shorter receiver spacing, and wider azimuth distribution are needed in crystalline rock environment to address the potential of the method for mineral exploration.