Crustal structures from MT soundings in the Canadian Cordillera

Details Crustal structures from MT soundings in the Canadian Cordillera Crustal structures from MT soundings in the Canadian Cordillera

Mar 1991

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1991e%26psl.102..444m&link_type=abstract

Earth and Planetary Science Letters, Volume 102, Issue 3-4, p. 444-454.

Computer Science

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Scientific paper

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.

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