Velocity structure along the Western Trans-Hudson Oregen as determined from seismic refraction data
Creators: name: Long, Denise Therese
Publisher: name: University of British Columbia
Year: 2009

description: During the summer of 1993, a seismic refraction /wide-angle reflection experiment was conducted over the exposed and sediment-covered part of the Paleoproterozoic Trans-Hudson Orogen in Saskatchewan and Manitoba. Three seismic lines, up to 750 km in length, were surveyed. One extended E-W across the orogen, one ran N-S along the eastern central region, and one extended N-S along the western part of the orogen. The latter, Line R3, is the focus of this study. From north to south it crosses exposed Archean rocks of the deformed and reworked Wollaston and Peter Lake domains, the 1855 Ma magmatic arc Wathaman batholith, the Paleoproterozoic metasedimentary Rottenstone domain, the arc-volcanic La Ronge belt and the gneissic-plutonic Glennie domain, and continues south beneath Phanerozoic cover. Based on Archean basement windows and reflection data, the Glennie and adjacent Paleoproterozoic domains are underlain by an Archean microcontinental block, the Sask craton. Principal objectives of the study are to determine the velocity structure of the different domains, crustal thickness and the northward and southward extent of the Sask craton. The data show very good Pg, Pn and PmP phases which are, respectively, waves turning in the upper crust, below the Moho and reflecting off the Moho. Many sections also show reflections from intracrustal structures. The phases are interpreted through iterative travel time inversion and synthetic seismogram forward modelling for amplitude comparisons to provide velocity structural models. Results show significant lateral variation in velocities and crustal thickness. The crust is thickest ( ≈ 50 km) below the southern part of the line; it thins rapidly for 200 km to the north until it is less than 40 km thick. Below the exposed Glennie domain, crustal thickness increases to about 45 km; below the domains to the north, however, it remains near 40 km. Uppermost mantle velocities are generally high, 8.1-8.2 km/s, but in one region below the Phanerozoic cover where crust is thinnest, well constrained values of 8.5 km/s are determined. This region lies below Prince Albert in the area where diamondiferous kimberlites have been found. Similar results, high uppermost mantle velocities associated with regions of diamondiferous kimberlites, have been documented in Russia. The velocity structure sections are compared to nearly coincident reflection data collected in 1994. Generally, there is good agreement between characteristics of the velocity models and the reflections sections. Somewhat higher lower crust velocities for the Sask craton tend to end below the La Ronge-Rottenstone domains, indicating the northern limit of the Sask craton (and consistent with interpretations from the reflection data). Three relatively higher lower crustal velocities extend southward to the end of the line, indicating the southward continuation of the Sask craton. Variations of the crustal thickness and upper mantle velocities are explained by the process of eclogitization in which lower-density, lower-velocity materials in the lower-most crust are metamorphosed to high-density, high-velocity materials. Exactly why this occurs in this particular region is uncertain. Perhaps the thrusting of the thick (?) Archean craton alters the state of the lower crust making it more susceptible to eclogitization. Much further study is needed to justify this hypothesis.