JOINT ANALYSIS OF SKS-SPLITTING AND MT DATA ACROSS AN ANCIENT TRANSCURRENT 
FAULT SYSTEM: GREAT SLAVE LAKE SHEAR ZONE, NORTHERN CANADA

DAVID W. EATON (University of Western Ontario), Alan Jones (Geological Survey 
of Canada), Ian Ferguson, Xianghong Wu (University of Manitoba), Isa Asudeh 
(Geological Survey of Canada)

Observations of seismic and electrical anisotropy provide complementary 
approaches to estimate the degree and orientation of strain in the 
subcontinental mantle. Seismic anisotropy in the mantle, generally believed 
to be controlled by strain-induced lattice-preferred orientation of olivine 
crystals, can be characterized by measurements of shear-wave splitting 
parameters for teleseismic phases such as SKS. Similarly, magnetotelluric 
methods can be used to determine electrical anisotropy of the mantle, which 
has been attributed to preferred interconnection of a highly conducting 
mineral phase (such as graphite) within foliation planes.

The Great Slave Lake shear zone (GSLsz) is a major continental transcurrent 
fault, linked to the Paleoproterozoic convergence and collision between the 
Slave and Rae provinces, Canada. It is exposed along the southeast shore of 
Great Slave Lake within a 25-km wide, northeast-trending subvertical mylonite 
corridor that produces one of the most spectacular linear magnetic anomalies 
in North America. Based on its magnetic expression, the GSLsz can be traced 
for at least 1300 km along strike, mostly in the subsurface. In its early, 
ductile phase (ca. 1.97 Ga), the GSLsz accommodated up to 700 km of dextral 
strike-slip motion in a regime of non-coaxial strain. In the present study, 
shear-wave splitting measurements from a 1999 portable teleseismic experiment 
across the Great Slave Lake shear zone (GSLsz) are compared with electrical 
anisotropy parameters from an earlier magnetotelluric study. The SKS data 
are generally consistent with a simple two-layer model. According to this 
model, the deep layer is characterized by a fast polarization direction of 
71 +/- 5 deg, whereas the shallow layer has a fast polarization of 40 +/- 11 
deg. The fast splitting direction of the upper layer is consistent with the 
inferred maximum conductivity orientation at crustal periods (0.1-10 s), and 
that of the lower layer consistent with the maximum conductivity orientation 
at upper mantle periods (100-1000 s), . The inferred splitting times of both 
layers range from about 0.3 s to 1.3 s and are anticorrelated, suggesting that 
the shallow layer thickens at the expense of the deeper layer towards the 
central axis of the shear zone. The fast polarization of the deep layer is 
oblique to the direction of absolution plate motion (225 deg). Our two-layer 
model is similar to one proposed to explain SKS splitting observations near 
the San Andreas Fault in California. The two layers may be a simplified 
representation of strain-dependent rotation of olivine crystalline axes, 
in which an extensive but mildly deformed layer in the lower lithosphere 
underlies a localized zone of higher strain beneath the fault zone.