Changxing Long

The Britt Domain in the southwestern Grenville Province of the Canadian Shield is believed to be a deep (ca. 20km) exposure of high grade (upper amphibolite facies) mid-crustal rocks of the predominantly granitic and granodioritic composition. Vertical reflection data across the Britt Domain show a series of subhorizontal, discontinous reflections and wide-angle records reveal a complex `shingling pattern' of crustal arrivals. This work addresses the overall velocity structure of the middle crust exposed in the Britt domain and the origin of these reflections based on laboratory velocity measurements, in-situ seismic data and surface geology.

Laboratory studies of eighty velocity samples from the Britt Domain show that average (area-weighted mean) P- and S- wave velocities at 600 MPa are 6.36 km/s and 3.67 km/s respectively, and the average velocity anisotropy is weak. Strong reflection coefficients occur between mafic rocks and granitic rocks and intermediate coefficients occur between diorite and other lithologies. Shear zones do not generate significant reflection coefficients because they occur within granitic gneisses and impedance contrasts are difficult to develop in this lithology.

In-situ refraction data (LITHOPROBE Abitibi-Grenville line AB) demonstrate a uniform velocity structure to a depth of 15 km in the central Britt Domain with a P-wave velocity of 6.15 km/s, an S-wave velocity of 3.55 km/s and linear vertical gradients at depth of 0.02 km/s/km and 0/01 km/s/km, respectively. Comparison of laboratory and refraction data suggests that the upper crust of the Britt Domain is granodioritic in composition. The lower crust beneath the Grenville Front Tectonic Zone in the central Britt Domain appears to be composed of diorite at the top but becomes increasingly mafic toward the Moho.

One and two-D seismic modelling shows that scattered bodies of mafic-intermediate gneiss within the host gneisses of granitic composition are the most likely causes of reflectivity in this area. Large scale folded structures are probably responsible for the `shingles' revealed by wide-angle reflection. These structures are not revealed on near vertical reflection records probably because rugosity (second order folds) of the boundaries and velocity heterogeneity above them break the reflectors into segments on the seismic section, and they could not be recovered by conventional interpretation routines. While strong lower crustal reflectivity in extensional terranes may be due to lithologic lamination, this study also suggests that such reflections in compressional settings may be produced by merged reflections from the peaks and troughs of deep seated folds.

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Supervisor: Matt Salisbury