From Isostasy to Proterozoic Underplating: New Inferences from Crustal Thickness and Vp/Vs Ratio beneath the Western Canada Sedimentary Basin

Abstract:

The rapid expansion of regional broadband seismic arrays in Alberta provides an unprecedented opportunity to map the subsurface seismic structure. In this study, we compute the P-to-S receiver functions from 5 regional seismic networks and 62 stations to determine the crustal properties beneath southern-central Alberta, a boundary zone between the Mesozoic Canadian Cordillera and the Precambrian craton(s). The optimal Moho depth and Vp/Vs ratio at each receiver are determined by tracing the direct P, converted (Pms) and reverberated (PpPms and PpSms+PsPms) phases through a three-dimensional shear velocity model. The resulting Moho depth-Vp/Vs ratio (H-K) image shows more coherent stacking amplitudes than the traditional H-K method. The map of Moho depth shows an increase of crustal thickness from an average of 41 km in Precambrian domains to ~50 km beneath the foothills of the Rocky Mountains, which indicates a strong crustal gradient across the craton-domain boundary in the Western Canada Sedimentary Basin (WCSB). The surface elevations along the foreland belt of the Rockies are isostatically compensated by a thickened crustal root, which is consistent with thick-skinned deformation likely sustained during the Laramide phase of the Farallon subduction. To the west of the foreland belt, however, a relatively thin crust with an average thickness of ~36 km is mapped beneath the extreme topography of southern Canadian Rockies. The lack of a deep crustal root is inconsistent with the Airy isostasy model, implying very high temperatures beneath the Cordillera upper mantle. Ratios of average crustal velocities (Vp/Vs) vary from 1.61 to 1.91 across the study region. The smallest values are observed near the northern Snowbird Tectonic Zone (STZ) and Taltson magmatic arc, which suggest a felsic crust resulted from crustal thickening and melting during the early Proterozoic. On the other hand, the Vp/Vs ratio shows the highest value (~1.9) in the vicinity of the Vulcan structure, a proposed suture zone between the Medicine Hat Block and Archean Hearne province. This observation, in combination with a southward-thickening crust, implies extensive Post-collisional mafic magmatism (i.e. underplating) in the Paleoproterozoic eon.