Seismic and electrical anisotropies in the lithosphere across the Grenville Front, Canada

Abstract

The Grenville Front is a prominent tectonic feature of the Canadian Shield which has been the focus of a LITHOPROBE transect. A strong electrical anisotropy has been detected in a frequency range of 0.1–0.01 Hz (≈ 50 to 150 km), the most conductive direction being oriented N80E. In order to compare electrical and seismic anisotropies, ten seismometers have been deployed across the Grenville Front to record teleseismic events. S‐wave splitting measurements show an average delay of 1.54±0.21 s between the fast and the slow components. The fast axis direction is approximately E‐W at all stations, showing a strong coherence between the stations along the complete transect. This azimuth correlates well with that of the electrical anisotropy. The electrical anisotropy originates in the subcrustal lithosphere between 50 and 150 km. The similarity of the magnetotelluric and seismic anisotropy directions across the Grenville Front is an argument in favor of an origin of the seismic anisotropy in the subcrustal lithosphere as advocated by Silver and Kaneshima (1993).

Teleseismic studies of the lithosphere below the Abitibi-Grenville Lithoprobe transect

“…The direction of maximum conductivity is generally east-west and parallel to the lineations defined by the major deformation zones that have affected the area during Precambrian time. Along the corridor traversed by the teleseismic arrays, the electrical anisotropy shows a fairly uniform orientation of N80°E (Kellett et al 1994;Sénéchal et al 1996). These results are presented in Fig.…”

“…This paper presents a review of the teleseismic studies that have been performed along the Abitibi-Grenville transect, where two separate arrays were deployed (one in 1994 and the other in 1996). Data from the former array were used for shear-wave splitting analysis and published in Sénéchal et al (1996) and Ji et al (1996); a review of their results and interpretations is presented here. Other studies, involving receiver function analysis and traveltime inversion and shear-wave splitting on the 1996 dataset, are still in progress.…”

Rheological heterogeneity, mechanical anisotropy and deformation of the continental lithosphere

“…Electrical anisotropy measurements in the Canadian shield (Sénéchal et al, 1996) and the eastern US (Wannamaker et al, 1996) show a very good agreement between the directions of conductivity anisotropy in the upper mantle and the crustal tectonic fabric, suggesting that the mantle and the crustal fabrics are similar.…”

“…Although few measurements are available, they display an internally consistent anisotropy pattern, and a good correlation with shear wave splitting and surface geology. Sénéchal et al (1996), for instance, through coupled shear wave splitting measurements and magnetotelluric soundings along a transect in the Canadian shield, retrieved similar directions of anisotropy from both datasets and, considering that the measured electrical anisotropy originates between 50 and 150 km, they concluded that the seismic anisotropy also originates from a well-defined fabric in the lithospheric upper mantle fabric. Similar preliminary results have been obtained in the Appalachians (Wannamaker et al, 1996) and the Pyrenees (Pous et al, 1995;M.…”

“…Electrical conductivity anisotropy evidenced by magnetotelluric soundings is interpreted as resulting from the existence of conducting graphite films along grain boundaries, with the highest conductivity parallel to the foliation trend (e.g., Sénéchal et al, 1996). It therefore records the existence of a shape-preferred orientation in the upper mantle.…”