D. Ian Gough

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.

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.

Electromagnetic exploration for fluids in the Earth's crust

Abstract Electromagnetic geophysical techniques yield information on the spatial distribution of electrical conductivity, a parameter sensitive to fluids in the rocks. The paper describes electromagnetic techniques of use through the Earths crust, including magnetovariation (MV) arrays, magnetotelluric (MT) sounding and DC resistivity sounding. Field studies using these techniques lead to a discussion of various causes of high crustal conductivity, including aqueous fluids, silicate partial melt and films of graphite or magnetite on grain surfaces. Water in the crust is proposed as the preferred cause of high conductivity in the juvenile crust of tectonically active regions, in particular where the anomalous conductivity is correlated with high heat flow and other geophysical and geological parameters.

Crustal structures from MT soundings in the Canadian Cordillera

Abstract Magnetotelluric soundings have been made across the Intermontane and Omineca tectonic belts of the Canadian Cordillera between latitudes 51.5 and 53.5°N. The frequency range, 0.016 to 130 Hz, gives penetration into the middle crust. In this part of the Cordillera the upper crust has very low resistivities, ranging from 3 to 300 ohm m, compared with continental shields and stable platforms. The most resistive rocks (100–300 ohm m) rise to the surface as the Coast Plutonic Complex is approached, and we identify them with confidence as granodiorites and similar plutonic rocks (hereafter “plutonics”). Phase pseudosections and resistivity-depth sections are used to infer that these plutonics continue northeastward from the Coast Plutonic Complex, across more than half of the width of the Intermontane Belt, with a sharp edge well located in the phase pseudosections. The Miocene basalts have extremely low resistivities (3–30 ohm m) and form a sheet 0–2 km thick covering the plutonics. The very low resistivities in all rocks are probably caused by saline hot water in connected spaces, with low fracture density giving relatively high resistivities in the plutonic rocks, and much greater fracture densities giving extremely low resistivities in the volcanics. This is consistent with the lower mechanical strength of basalt as against granodiorite; the volcanics may have accommodated most of the post-Miocene extension of the upper crust. Off the edge of the plutonic rocks beneath the basalts, very low resistivities extend at least into the middle crust; this deep extension of the highly conductive rock may mark the feeder channel of the Miocene basalt to the surface. Three resistivity-depth sections show a fall of resistivity with depth, to values of 10 ohm m or even less, at a depth of only 8 km. Heat flow is high in the region, and the temperature at 8 km may be as high as 350°C. The increase in conductivity may be due in part to the temperature effect on NaCl solutions, and in part to release of water from hydrated minerals. All crustal features disclosed in this work support the hypothesis advanced earlier, that the interior Canadian Cordillera lie above an elongated upflow in the mantle inland from the currently active subduction.

Electrical conductivity and temperature in the Canadian Cordilleran crust

Abstract High electrical conductivity and high heat flow coexist in much of the Canadian Cordillera, a region of recent accretion of crustal terranes to North America. The relationship between temperature and electrical conductivity in the crust is investigated by means of results from 251 heat flow determinations and 301 magnetotelluric (MT) soundings. We first compared maps of MT phase at period 10 s and of heat flow. Larger phases correlate with higher heat flows in some areas, but the relationship is complicated by other causes of variation in these two parameters. To give a more focussed comparison, we map the depth d 450 to the 450°C isotherm for two assumed distributions of reduced heat flow, and the depth d c to the top of the lower crustal conductor at MT sites away from major fracture zones. The map of d c visually resembles both of the maps of d 450 , and readings at 76 points of a grid give correlation coefficient 0.80. This supports a relationship between high conductivity in the crust and temperatures above 450°C. Both the lower crustal conductor and the 450°C isotherm occur at depths near 30 km above the subducting oceanic plate under Vancouver Island, at 15–20 km in the Intermontane Belt, and of 10–15 km in the Omineca Belt. If there is an upcurrent in the mantle beneath the continent, behind the subduction, as proposed elsewhere, its axis may now be under the Omineca belt. This location is in harmony with evidence of recent extension and uplift there.