Seismic imaging of the upper mantle beneath the Rocky Mountain foreland, southwestern Alberta

The Vibroseis Augmented Listen Time (VAuLT) experiment is a special seismic -reflection survey designed to image the fine-scale structure of the contin ental upper mantle of the Rocky Mountain foreland in southwestern Alberta t o depths of 200 km or more. Two mutually perpendicular profiles were acquir ed across and within the Vulcan structure, a roughly east-west-trending tec tonic belt in the crystalline basement beneath the Western Canada Sedimenta ry Basin that separates the Medicine Hat block from the Loverna block. Rela tive-amplitude-preserving processing procedures were developed to estimate the seismic-signal-penetration limit, which varies between 100 and 220 km d epth. Amplitude-decay analysis and Q estimation show that a seismically unr eflective zone within the Vulcan structure is not caused by inadequate sign al penetration. This blank zone is interpreted as part of an intrusive comp lex that has overprinted the preexisting structural fabric. Unlike most oth er parts of Alberta, the reflection Moho is indistinct and the uppermost ma ntle (45-60 km depth) is reflective, particularly for source-receiver offse ts > 10 km. South-dipping reflectivity in the lower crust and upper mantle beneath the Loverna block and northern Vulcan structure gives way to subhor izontal reflectivity beneath the Medicine Hat block. We interpret this refl ectivity as compositional layering and (or) zones of ductile deformation th at were previously part of the mafic lower crust, but that have now undergo ne metamorphic transformation to eclogite. The deepest observed reflection is an isolated, gently north-dipping event at similar to 120 km depth.