ESCI Seminar: Jeffrey Gu
A broadband seismological perspective on the crust and mantle lithosphere beneath the western margin of Laurentia
The mantle lithosphere beneath the western Canada sedimentary basin and northern Montana contain vital records of the Precambrian tectonic development of western Laurentia. While much of this region is inaccessible to direct geological sampling due to extensive Phanerozoic sedimentation, the combination of potential field, active-source seismic and geochemical observations have provided early evidence for major episodes of Precambrian collisional events that resulted in an intricate regional network of tectonic domains.
In this talk I will examine the latest broadband seismic constraints in this region using P- and S-wave finite-frequency tomography, receiver functions and ambient noise correlation. Our new models suggest the presence of distinct, vertically continuous high velocity perturbations above 200 km depth beneath the Precambrian Buffalo Head Terrane, Hearne craton and Medicine Hat Block. The Hearne lithosphere is anomalously (>300 km) thick, which may have resulted from Archean/Proterozoic plate convergences between the Hearne province and its neighboring domains. Common links (e.g., a thickened crust) could be identified between these cratons, though the robustness and associated tectonic processes (e.g., melting and underplating) during the Precambrian are markedly different among major boundary zones such as the Snowbird and Great Falls tectonic zones. Our broadband data further reveals a sharp mantle transition from Precambrian cratons to the Phanerozoic Cordillera. The morphology of the craton margin is highly complex, showing significant along-strike variations in velocity, dip, fabric/anisotropy and lithospheric thickness. These distinctive structures may reflect secular tectonic modifications since the late Neoproterozoic.