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

Abstract

Magnetotelluric (MT) responses at the Proterozoic Grenville Front in Canada have been interpreted as being caused by lithospheric electrical anisotropy, and the area is often noted as a classic example of lithospheric anisotropy. This study reevaluates evidence for the electrical anisotropy using 56 MT stations. The spatially uniform MT responses noted at the Grenville Front extend to~200 km southeast and for at least 400 km along strike and are associated with rocks at less than 150 km depth. Examination of induction arrows at longer periods shows arrows at high angle to the MT conductive direction consistent with the presence of macroscopic resistivity structures. New 2-D anisotropic inversions show that electrical anisotropy is not required to fit the MT data. The results indicate that in the resistive mantle lithosphere beneath the Grenville Front, and in conductive lithosphere in adjacent areas, the maximum horizontal resistivity anisotropy is <10%, much less than the factor of 15 determined in earlier 1-D studies. The results suggest that the upper lithospheric mantle in the area is devoid of significant electrical anisotropy and that the observed MT response directionality is due to large-scale resistivity structure. We interpret the spatially consistent MT responses observed at the Grenville Front as being associated with the resistive Archean lithosphere extending southeast beneath the Grenville Front. The obliquity between seismic and MT responses arises because the Superior fabric is oblique to the seismic fast direction. If dextral shearing occurred, it appears to have not caused any significant shape preferred electrical anisotropy.

Heterogeneity and anisotropy in the lithospheric mantle

“…A major difficulty in these measurements is that azimuthal variation in the MT response may also result from larger-scale variations in electrical conductivity (heterogeneity). For instance, recent re-examination of MT data in the Grenville concluded that the observed signal records extension of the resistive Archean lithosphere beneath the Greenville front rather than conductivity anisotropy in the lithospheric mantle, which is constrained to be <10% (Adetunji et al, 2015).…”

Electrical Conductivity of Clinopyroxene‐NaCl‐H2O System at High Temperatures and Pressures: Implications for High‐Conductivity Anomalies in the Deep Crust and Subduction Zone

“…Electrical conductivity of Cpx‐NaCl‐H 2 O systems with various salinities calculated based on HS + and Cube models and high‐conductivity layers in southern Tibetan Plateau (Xie et al, 2016), Dabie Orogen (Xu et al, 2016), Grenville province (Adetunji et al, 2015), and central New Zealand (Wannamaker et al, 2009). Colored solid and dashed lines represent the electrical conductivities of Cpx‐NaCl‐H 2 O systems calculated based on the HS + and Cube models, respectively.…”

Magnetotelluric imaging of anisotropic crust near Fort McMurray, Alberta: implications for engineered geothermal system development

“…It should also be noted that a given resistivity structure may not necessarily produce its MT and IV responses at the same period range (Wannamaker et al 1984). Adetunji et al (2014Adetunji et al ( , 2015 showed that both the MT and induction vector responses could be modelled successfully with macroscopic 2-D resistivity structures. Further, Weckmann (2006) showed that phase splits from off-profile 3-D conductors could be misinterpreted as evidence for electrically anisotropic resistivity structure.…”