Superior Layer-cake: An Investigation of the Mid-Lithospheric Discontinuity in the Superior Craton in Quebec, Canada

Michael Klaser, Rutgers University New Brunswick, New Brunswick, NJ, United States, Vadim L Levin, Rutgers University, Earth and Planetary Sciences, Piscataway, United States, Fiona Ann Darbyshire, University of Quebec at Montreal UQAM, Centre de recherche GEOTOP, Montreal, QC, Canada and Dr. Huaiyu Yuan, Macquarie University, CCFS, Sydney, NSW, Australia

Contact First Author: Michael Klaser; michaelklaser@gmail.com

Previously Published Material: Most of the data was presented at the AGU Fall 2014 meeting, though an investigative focus of the MLD (and geologic interpretation) is new.

Abstract ID#: 34305

 

English Abstract:
The Superior craton has some of the thickest lithosphere on the planet, extending as deep as ~250 km, as suggested by elevated shear wave speed, considerations of heat flow, petrology of xenoliths, electrical resistivity etc. However, in the Superior the most significant decrease in shear wave speed with depth (over 10 m/s per 1 km of depth) does not correspond to the expected lower limit of the lithosphere. Rather, the continent-scale shear-wave velocity model of Yuan et al (2014) puts it at depths of ~125 km within the Superior. In the velocity model, the vertical extent of this region of large negative shear wave speed gradient is approximately 50 km.

P-to-S converted waves from teleseismic earthquakes offer a way to detect abrupt changes in seismic properties in the upper mantle, and may be used to determine the depth and the vertical extent of such abrupt changes.

We produced P-to-S receiver function (RF) gathers for 5 long-term seismograph stations, utilizing 5.0 and greater magnitude teleseismic earthquakes from the period 2000-2013. For each station, we constructed RF gathers arranged by back-azimuth and epicentral distance, to identify phases that can be associated with horizontal discontinuities within the crust and upper mantle. We also use newly collected data from a portable seismic array between Lake Mistassini and James Bay to augment the data from the permanent stations.

We detect clear negative P-S conversions consistent with an abrupt decrease in shear wave speed. Their delays range from 6 to 9 s, corresponding to the depths between 60 and 90 km. Similar observations are found in most cratons, and give rise to the concept of a Mid-Lithospheric Discontinuity (MLD). Conversions from the MLD in the Superior craton appear clearly in RFs with frequencies up to 0.5 Hz, suggesting a relatively abrupt boundary with a vertical extent of ~2 km or less.

We use seismic velocities from the model of Yuan et al (2014) and the timing of pulses in our receiver function time series to develop more precise estimates of the depth to the MLD in the Superior. Also, we use synthetic seismogram computations in simple layered models to evaluate vertical profiles of seismic properties within it.

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