Evolution of the Precambrian Crust and Sub-Continental Lithsophere in Eastern Canada: Constraints from Probabilistic Inversion and H-k Stacking of Receiver Functions

Abstract:

Cratons are continental nuclei that have been tectonically quiescent for at least a billion years. They show distinct geological and geophysical signatures from younger continental regions. Some Precambrian crustal generation models suggest a change in tectonic processes at the end of the Archean, marking the onset of modern-style plate tectonics and different lithospheric growth mechanisms.

Eastern Canada comprises Archean, Proterozoic and Phanerozoic terranes, recording tectonic events spanning ~3 Ga of Earth history. It is a natural laboratory to test hypotheses concerning craton genesis and Precambrian plate tectonics. To constrain structural variations across this region, a new broadband seismograph network has been deployed from southernmost Hudson Bay to coastal Nova Scotia. The main profile crosses major tectonic boundaries between the Superior craton, the Proterozoic Grenville and Phanerozoic Appalachian provinces.

A profile of crustal structure across the major tectonic terranes was constructed from transdimensional Bayesian receiver function inversions. 1D structure beneath individual stations is described in terms of two parameters: shear wavespeed variation with depth, and likelihood of discontinuity, both defined probabilistically. A clear Moho can be identified at about 36-42 km with variable transition width. Moreover, a persistent sub-continental lithospheric impedance contrast is detected at 52-57 km beneath most stations. The ubiquity of this feature across a profile spanning 3 Ga of lithospheric processes could imply that post-cratonization chemical modification may have homogenized the uppermost mantle.

Small age-dependent variations in mean bulk crustal Vp/Vs ratios are revealed from receiver function H-k stacking: ~1.71, ~1.77 and ~1.75 for the Archean, Proterozoic and Phanerozoic respectively. Crustal thickness also varies systematically with age, with Moho depths of ~35 km for the Archean and Phanerozoic but up to 10 km thicker in some parts of the Proterozoic. These results are consistent with a non-plate tectonic origin for the predominantly felsic and thin Archean crust, as opposed to more mafic Proterozoic terranes with deeper crustal roots that do not reciprocate the low-relief surface topography.