Publication 175

Imaging the mantle lithosphere of the Precambrian Grenville Province: large-scale electrical resistivity structures

Adetunji, A., I.J. Ferguson, and A.G. Jones

Abstract

The resistivity structure of the lithospheric mantle beneath the Proterozoic Grenville Province in southern Ontario, Canada is investigated using 84 MT sites divided into four profiles. Depth-based regional geoelectric dimensionality analyses of the MT responses indicate that the mantle lithosphere north of Lake Ontario can be subdivided into upper (45-150 km) and deeper (>200 km) lithospheric mantle layers with regional strike azimuths of N85 degE (+/- 5 deg) and N65 degE (+/- 5 deg) respectively. MT responses from the Grenville Front and the northwest part of the Central Gneiss Belt are compatible with the presence of 2-D resistivity structures but farther to the southeast, in the southeast part of the Central Gneiss Belt and Central Metasedimentary Belt, they suggest the presence of localized 3-D structures. 2-D inversion of distortion-free MT responses images a large scale very resistive (>20,000 ohm.m) region that extends 300 km southeast of the Grenville Front and for at least 800 km along-strike in the lithospheric mantle beneath the Grenville Province. This feature is interpreted to be Superior Province lithosphere and the corresponding N85 degE geoelectric strike to be associated with the fabric of the Superior Province. The base of the resistor reaches depths of 280 km on two of the three MT profiles north of Lake Ontario and this depth is interpreted to be the base of the lithosphere. A large region of enhanced conductivity in the lower lithosphere, spatially correlated with decreased seismic velocity, is bounded to the northwest by a sub-vertical resistivity anomaly located near the Kirkland Lake and Cobalt kimberlite fields. The enhanced conductivity in the lower lithosphere is attributed to refertilization by fluids associated with Cretaceous kimberlite magmatism and can be explained by water content in olivine of 50 wt ppm in background areas with higher values in a localized anomaly beneath the kimberlite fields. Farther to the southeast the resistivity models include a lithospheric conductor between 100 and 150 km depth beneath the Central Metasedimentary Belt. The enhanced conductivity is attributed to grain boundary graphite films, associated with the Cretaceous kimberlitic magmatic process, or to water and carbon, introduced into the mantle during the pre-Grenvillian tectonism.

Source

Geophysical Journal International, 201, 1038-1059, doi:10.1093/gji/ggv060. [PDF]

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Alan G Jones / 25 March 2015 / alan-at-cp.dias.ie