Imaging the mantle lithosphere of the Precambrian Grenville Province: large-scale electrical resistivity structures
Adetunji, A., I.J. Ferguson, and A.G. Jones
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.
Geophysical Journal International, 201, 1038-1059, doi:10.1093/gji/ggv060. [PDF]
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Alan G Jones / 25 March 2015 /