Margaret M.Burke

The Grenville Front Tectonic Zone (GFTZ) is a pronounced geological and geophysical discontinuity which extends from northern Labrador to the southern United States. A deep seismic reflection profile (GLIMPCE Profile J) across the GFTZ in Lake Huron shows a series of high amplitude, multicyclic, dipping reflections. This work addresses the origin of these reflections based on laboratory and in situ measurements of compressional wave velocities in GFTZ rocks.

Forty-one samples of three main lithologic groups were used for laboratory velocity studies. Orthogneiss samples, including highly deformed mylonitic rocks from the boundary fault of the GFTZ, have an average velocity of 6.29 km/s at 600 MPa and seismic anisotropy ranging from 1-6%. Paragneiss samples have an average velocity of 6.31 km/s at 600 MPa and seismic anisotropy ranging from 2-15%. Mafic gneisses exhibit anisotropy from 4-16% and an average velocity of 6.88 km/s at 600 MPa.

In situ profiles show very shallow (ca. 300 m) level alteration (fracturing, weathering, etc.) indicated by very low compressional wave velocities for both orthogneiss and paragneiss assemblages. However, in situ measurements of compressional wave velocities at slightly deeper levels show good agreement with low pressure laboratory velocities. Data from orthogonal profiles across the boundary fault rocks also indicate minimal seismic anisotropy for the mylonitic rocks exposed.

Detailed one-dimensional models, based on mapped dikes along Collins Inlet, show that very thin layers (ca. 30 cm thick) can produce reflections, if clustered in some fashion so as to resemble units with intermediate velocity that have thicknesses on the order of seismic wavelengths. A more general, two-dimensional model shows that contacts between mafic gneisses and other lithologies are the most likely contributors to reflectivity in this area of the GFTZ.

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Supervisor: Matt Salisbury