Nature of the Moho transition in NW Canada from combined near-vertical and wide-angle seismic-reflection studies

Abstract

Although the year 2009 marked one century since the discovery of the Moho, or crust-mantle boundary, the exact nature of that boundary and the manner in which it formed remain major uncertainties in lithospheric studies. In northwestern Canada, sharp Moho reflections at both near-vertical and wide-angle incidence have been imaged beneath the Great Bear magmatic arc. They show a remarkably flat Moho and do not reflect the complex tectonic history of the Wopmay orogen, of which the Great Bear arc is one part. In order to understand the origin of these reflections and the nature of the Moho, we calculated near-vertical and wide-angle synthetic seismograms for a number of crust-mantle transition models using one- and two-dimensional wave propagation algorithms. Only laterally and vertically heterogeneous models can properly simulate the observed seismic signature recorded on both near-vertical and wide-angle reflection data. The heterogeneity is achieved by either laterally discontinuous layering or a lamellae structure with randomly distributed ellipses. These models suggest that the Moho represents a thermal/metamorphic front, a regional décollement, or both.