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

Abstract

S U M M A R YThe resistivity structure of the lithospheric mantle beneath the Proterozoic Grenville Province in southern Ontario, Canada is investigated using 84 magnetotelluric (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 • E (±5 • ) and N65 • E (±5 • ), 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 m) region that extends 300 km southeast of the Grenville Front and for at least 800 km alongstrike in the lithospheric mantle beneath the Grenville Province. This feature is interpreted to be Superior Province lithosphere and the corresponding N85 • E 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 subvertical 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.

Conductivity structure of the lithosphere‐asthenosphere boundary beneath the eastern North American margin

“…This conductive anomaly is attributed to lithospheric refertilization by fluids associated with kimberlite fields magmatism. Additionally, a subvertical conductor located 50 km along strike from the Mesozoic Kirkland Lake and Cobalt kimberlite fields is interpreted as refertilization of an old mantle scar [Adetunji et al, 2014[Adetunji et al, , 2015. Thus, there is substantial evidence for kimberlite intrusions at the proximity of our study region.…”

“…Further North along the Great Meteor hot spot track, numerous kimberlitic melts have intruded the Canadian Slave Craton from deep within the lithosphere to the surface, over the last 500 Ma [Selway, 2014]. An MT study performed at the Grenville Province in southern Ontario, Canada, identified a conductive anomaly in the lower lithosphere that is spatially associated with low seismic velocities [Adetunji et al, 2015]. This conductive anomaly is attributed to lithospheric refertilization by fluids associated with kimberlite fields magmatism.…”

“…We propose that a paleo-intrusion of kimberlites may explain the observed LACZ (Figures 6e and 10a). Apart from the thermal erosion hypothesis, alternative processes such as mantle scar refertilization [Adetunji et al, 2014[Adetunji et al, , 2015 or enrichment of the lithospheric keel by plume material [VanDecar et al, 1995] might also explain the presence of the LACZ.…”

Conductivity Anomalies in Central Europe

“…Thereby, the anisotropy interpretation would share the fate of another case study: Perhaps the most widely known example of lithospheric anisotropy is that seen in the Superior and Grenville Provinces of eastern Canada by Mareschal et al (1995), which was compared to seismic anisotropy across the Grenville Front by Ji et al (1996). However, a recent re-examination of data from the region by Adetunji et al (2015) concluded that the spatially homogeneous anisotropy observed in the region of the Grenville Front is consistent with isotropic 2D structures.…”

Reexamination of magnetotelluric responses and electrical anisotropy of the lithospheric mantle in the Grenville Province, Canada

“…More recently, Adetunji et al . [, ] show that the MT responses in the Grenville Front area can be accurately modeled by two‐dimensional (2‐D) models without any anisotropy. As with the earlier results, these 2‐D models indicate the presence of resistive mantle lithosphere between 50 km and 150 km depth in the Grenville Province.…”