Teleseismic studies of the Superior craton, eastern Canada

Category Geophysic
Group GSI.IR
Location International Geological Congress,oslo 2008
Author Darbyshire, Fiona
Holding Date 23 September 2008

The Superior craton of eastern Canada and its surrounding mobile belts are the subject of a number of recent and current teleseismic studies, primarily through the auspices of the POLARIS (Portable Observatories for Lithospheric Analysis and Research Investigating Seismicity) project and related initiatives. The Superior is the largest Archean craton on Earth, and ongoing studies indicate that the crust and lithosphere are characterised by significant structural heterogeneity on a sub-provincial scale.
Recent studies using both travel-time and surface-wave tomography in northern and eastern Ontario suggest significant structural differences between the western Superior craton and regions further to the east. The subcratonic mantle is everywhere characterised by a high-velocity lithospheric ’lid’, but its thickness and seismic velocity vary across the study area. Surface wave tomography indicates the presence of multiple layers of azimuthal anisotropy in the upper mantle. In the western Superior, fast-propagation directions are nearly coincident from periods of 40 to 170 seconds, suggesting that ‘frozen’ lithospheric anisotropy aligns with sublithospheric anisotropy associated with present-day mantle flow. The large SKS splitting delay times in this region, with a similar fast-propagation direction to that inferred from the surface wave tomography, support this observation. In contrast, further east, two distinct fast-propagation directions are resolved from the surface wave tomography, with the deeper layer aligned approximately with present-day plate motion.
Current studies related to the Superior craton focus on two geographical regions: (i) the Abitibi-Grenville region, which spans the south-central Superior craton and part of the Proterozoic Grenville Province, thought to have been affected by the passage of a hotspot beneath North America in Mesozoic times. P wave travel-time residual measurements are made at seismograph stations across the region, and azimuthal dependence of residual patterns is examined. (ii) the Hudson Bay intracratonic basin and the surrounding Archean and Proterozoic provinces. Rayleigh wave phase velocity dispersion analysis, using a two-station method, provides information on the average upper mantle structure along selected inter-station paths through the region. Of particular interest are variations in lithospheric characteristics between paths crossing Hudson Bay and paths sampling purely the subcratonic mantle on either side of the basin.