Crustal velocity structure from SAREX, the Southern Alberta Refraction Experiment

Abstract

Lithoprobe's Southern Alberta Refraction Experiment, SAREX, extends 800 km from east-central Alberta to central Montana. It was designed to investigate crustal velocity structure of the Archean domains underlying the Western Canada Sedimentary Basin. From north to south, SAREX crosses the Loverna domain of the Hearne Province, the Vulcan structure, the Medicine Hat block (previously considered part of the Hearne Province), the Great Falls tectonic zone, and the northern Wyoming Province. Ten shot points along the profile in Canada were recorded on 521 seismographs deployed at 1 km intervals. To extend the line, an additional 140 seismographs were deployed at intervals of 1.25–2.50 km in Montana. Data interpretation used an iterative application of damped least-squares inversion of traveltime picks and forward modeling. Results show different velocity structures for the major blocks (Loverna, Medicine Hat, and Wyoming), indicating that each is distinct. Wavy undulations in the velocity structure of the Loverna block may be associated with internal crustal deformation. The most prominent feature of the model is a thick (10–25 km) lower crustal layer with high velocities (7.5–7.9 km/s) underlying the Medicine Hat and Wyoming blocks. Based on data from lower crustal xenoliths in the region, this layer is interpreted to be the result of Paleoproterozoic magmatic underplating. Crustal thickness varies from 40 km in the north to almost 60 km in the south, where the high-velocity layer is thickest. Uppermost mantle velocities range from 8.05 to 8.2 km/s, with the higher values below the thicker crust. Results from SAREX and other recent studies are synthesized to develop a schematic representation of Archean to Paleoproterozoic tectonic development for the region encompassing the profile. Tectonic processes associated with this development include collisions of continental blocks, subduction, crustal thickening, and magmatic underplating.

PdS receiver function evidence for crustal scale thrusting, relic subduction, and mafic underplating in the Trans-Hudson Orogen and Yavapai province

“…We suggest that the deep positive event seen in Fig. 4 is analogous to a "Type III reflection pattern near the Moho" described by Cook (2002) as "reflections that can be traced from the lower crust to beneath the reflection Moho". A similar mantle signal can be seen in numerous studies, event that may be evidence of relic subduction and structural underplating of oceanic crust.…”

“…Recent and more extensive investigations of this region, however, do not exist. There have been, however, numerous studies to the west of the U.S. Trans-Hudson Orogen including the COCORP, Deep Probe, and CD-ROM experiments, which reveal visible subduction zone scars across the WY-Proterozoic boundary (Cheyenne Belt) (Yuan and Dueker, 2005), an indistinct Moho west of the Trans-Hudson Orogen (Allmendinger, 1982;Brown et al, 1983;Braile, 1989;Nelson et al, 1993;Baird et al, 1996), possibly resulting from a gradational phase change (Smithson, 1989), and a high velocity lower crustal layer within the Wyoming province, which may be the result of Proterozoic underplating and metamorphism of the Archean crust (Braile, 1989;Clowes et al, 2002;Gorman et al, 2002;Snelson et al, 2005;Karlstrom et al, 2005).…”

3-D geomechanical–numerical model of the contemporary crustal stress state in the Alberta Basin (Canada)

“…5) by directional kriging. These are data from seismic refraction studies (Bouzidi et al, 2002;Burianyk et al, 1997;Clowes et al, 2002;Fernández-Viejo and Clowes, 2003;Halchuk and Mereu, 1990;Németh et al, 1996Németh et al, , 2005Spence and McLean, 1998;Welford et al, 2001;Zelt and White, 1995) and from teleseismic studies (Gu et al, 2011;Shragge et al, 2002).…”

Sharp mantle transition from cratons to Cordillera in southwestern Canada

“…While global discontinuity maps [e.g., Flanagan and Shearer, 1998;Gu et al, 2003;Lawrence and Shearer, 2008] suggest a relatively flat MTZ, diminished discontinuity topography could be partially attributable to the averaging effects of large Fresnel zones [Neele et al, 1997]. The use of higher-resolution approaches such as receiver functions was previously limited to sparsely populated national network stations [Cassidy, 1995;Bostock, 1996], linear temporary arrays [Shragge et al, 2002;Courtier et al, 2006], or crustal-scale investigations [Clowes et al, 2002;Bostock et al, 2010;Cassidy, 1995].…”

“…The resulting broadband data improved the understanding of craton-Cordillera transition [Mercier et al, 2008;Dalton et al, 2011] and the depth/integrity of lithospheric roots [Shragge et al, 2002]. These deep-probing experiments are complemented by a series of shallow refraction and reflection lines, e.g., the Alberta Basement Transect and the Southern Alberta Refraction Experiment Clowes et al, 2002], which primarily targeted the basement and crustal structures in the WCSB using active-source techniques Clowes et al, 2002]. A key outcome from these deployments is a significant crustal/mantle seismic gradient from cratons to terranes near the Cordilleran Deformation Front [e.g., Mercier et al, 2008;Dalton et al, 2011], which is generally supported by the shear and compressional speeds from global [Grand and Helmberger, 1984;Grand, 1994;Bijward et al, 1998;Grand, 2002;Ritsema et al, 2004;Montelli et al, 2004;Obayashi et al, 2006;Simmons et al, 2010] and regional [Grand, 1994;Grand et al, 1997;Frederiksen et al, 2001;Shragge et al, 2002;van der Lee and Nolet, 1997;Nettles and Dziewonski, 2008] tomographic inversions.…”