Seismic Crustal Structure Northwest of Thunder Bay, Ontario
- Muawia Barazangi and
- Larry Brown
Published Online: 15 MAR 2013
Copyright 1986 by the American Geophysical Union.
Reflection Seismology: The Continental Crust
How to Cite
Young, R. A., Wright, J. and West, G. F. (1986) Seismic Crustal Structure Northwest of Thunder Bay, Ontario, in Reflection Seismology: The Continental Crust (eds M. Barazangi and L. Brown), American Geophysical Union, Washington, D. C.. doi: 10.1029/GD014p0143
- Published Online: 15 MAR 2013
- Published Print: 1 JAN 1986
Print ISBN: 9780875905143
Online ISBN: 9781118670118
- Seismic reflection method—Congresses
A seismic refraction-wide angle reflection survey of limited scope was carried out in the Shebandowan-Atikokan-Savant Lake region of western Ontario using open pit mine blasts as the principal energy sources. Conclusions drawn from the data are: 1) The seismic crustal structure varies laterally from one part of the area to another, but the variations are not substantial, nor is the crustal structure anywhere very unusual. The interpreted depth of the M discontinuity lies everywhere in the range 37–44 km. P wave velocity everywhere rises in the lower crust, reaching at least 7 km/s near the M discontinuity, whereas the upper crust displays a relatively more uniform velocity of less than 6.3 km/s. In three of the four interpreted sections, velocity jumps rapidly at an intermediate depth to define a distinct lower crustal layer. The depth to the top of this lower crustal layer, however, is quite variable (13–21 km). 2) An 8 km thick, near-surface capping of slightly higher velocity is interpreted for the more northerly profile which passes through the Sturgeon Lake-Savant Lake greenstone belt. It appears to be a manifestation of the more frequent occurrence of mafic metavolcanic material in crust which otherwise exhibits granitoid velocities (6.0 km/s). The result lends support to the view that greenstone belts of the Superior Province are generally of limited depth extent and are underlain by granitoids. 3) The M discontinuity at least beneath the northerly profile is a complex zone about 5 km thick, seemingly with a lamellar structure. Upper mantle velocity is not well defined by the surveys, but is consistent with previous regional estimates of about 8.1 km/s. On the northerly profile, there is evidence for a rise in mantle velocity to 8.3 km/s at about 50 km depth.