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Crustal structure of the superior and Grenville provinces of the Northeastern Canadian Shield

DIVISION OF SEISMOLOGY EARTH PHYSICS BRANCH DEPARTMENT OF ENERGY, MINES AND RESOURCES, OTTAWA, Canada
UNIVERSITÄT ERIDERICIANA KARLSRUHE, GERMANY, KARLSRUHE

Abstract

A large scale seismic refraction experiment was mounted in the Superior and Grenville provinces of the NE Canadian Shield during the summer of 1968. The purpose was to investigate possible differences in their crustal structure and to study the nature of the large gravity anomaly which is centered along the boundary or front between them.

We tentatively interpret the confusing P_n data as indicating a weak anisotropic velocity behavior in the material at the top of the upper mantle, coupled with considerable topographic relief on the Mohorovii transition and a complex crustal structure above. The preferred upper mantle velocity is 8.06±0.06 km/sec with a maximum deviation caused by the anisotropy of 0.17 km/sec. We are not able to discern a marked difference in the upper mantle velocities beneath the two provinces, but we cannot be definite on this point.

The average velocity of the crust is 6.57±0.03 km/sec and appears to be the same over the whole region. Crustal thickness varies from 48 to 29 km over the survey area with the average thickness in the Superior being 34 km compared to 39 km in the Grenville to the south. The crust is thickest along the Grenville Front and has an average value of 45 km along our profile.

We have made extensive use of synthetic seismograms to interpret the later events on our three seismic profiles. Our preferred models each show a family resemblance. The upper crust has a sharp velocity gradient at the surface with velocities increasing from 5.0 to approximately 6.2 km/sec. From a depth of 1 or 2 km down to the Conrad discontinuity the upper crust is essentially homogeneous except for a low-velocity channel, the depth of whose bottom varies from 6 to 15 km. The channel is thickest, about 5 km, and deepest at the east end of the central profile along the Grenville Front and the center of the gravity anomaly. The channel appears to rise and to become thinner to the north and south.

The Conrad discontinuity lies at a depth of approximately 14 km in the Superior, 21 km in the Grenville and 24 km along the Front. The material below the Conrad has a velocity from 6.5 to 6.8 km/sec and a small velocity gradient of roughly 0.02 sec^–1. Approximately 12 km above the Mohorovii boundary, some of the profiles indicate another low-velocity channel, but in any case, at this depth or below the channel, all of the profiles require a model with a strong velocity gradient down to the crust-upper mantle transition. Here, the velocity rises sharply but more or less continuously and, if there is a discontinuity at all, it is probably a jump of less than 0.02 km/sec.

The thickening of the low-velocity channel and the thickening of the upper crust along the Front together are able to explain much of the strong local negative Bouguer anomaly there. The strong regional trend across the area is not simply explained by our models.

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