A SEISMIC INVESTIGATION OF THE CRUST AND MOHO UNDERLYING THE PEACE RIVER ARCH, CANADA

The Peace River Arch is a structural uplift in northwestern Alberta which has experienced an anomalous history of movement with respect to the surrounding Western Canada Sedimentary Basin. The supracrustal geology of the region provides a detailed record of the vertical motion of the Arch in the Phanerozoic era. In an attempt to gain information on the underlying crystalline crust and upper mantle in the region, the Peace River Arch Seismic Experiment (PRASE) was carried out by the Geological Survey of Canada and several Canadian universities in the summer of 1985. Four reversed seismic refraction lines, each 300-350 km in length, were shot. The two-dimensional, laterally homogeneous models developed from the in-line data suggest that there is little lateral variation in velocity in the region. The upper 2-4 km of each section was modelled by sedimentary layers. The upper 20 km of the models are composed of horizontal, laterally homogeneous layers with velocities increasing from 6.0 to 6.5 km/s. The data from three of the lines indicate an intracrustal discontinuity at a depth of 16-24 km. The data from a line running along the axis of the Arch, suggests a second discontinuity at 30 km. The lower half of the crust increases in velocity to 6.6-7.2 km/s. The upper mantle has an average velocity of 8.13 +/- 0.08 km/s and has a much lower velocity gradient than the overlying crust. There is some variation in the nature of the Moho, which is generally well defined, nearly horizontal, and occurs at a depth between 36 and 43 km. There is little correlation of the arch uplift structure with the structure in the underlying crust. There appears to be some local thickening of the crust immediately below the axis of the Arch. Upward warping of the velocity contours below the axis of the Arch in the lower half of the crust may also be an indication of the uplift at depth. No definite driving mechanism can be established for the origin of the Peace River Arch.