CONTRASTS IN CRUSTAL CHARACTER BETWEEN MEGUMA AND AVALONIA BASED ON WIDE-ANGLE SEISMIC REFLECTION/REFRACTION PROFILES ALONG THE SCOTIAN MARGIN AND ACROSS NOVA SCOTIA, CANADA

Two wide-angle seismic reflection/refraction profiles provide new information about the velocity characteristics of the Meguma terrane of southern Nova Scotia in comparison to adjacent Avalonia. Line 99-1 extends ENE along the Scotian shelf from southwest Nova Scotia to offshore Avalonian southeastern Cape Breton Island; line 99-2 extends NNE across the Scotian shelf, mainland Nova Scotia, and the Avalonian Antigonish Highlands, and into the Gulf of St Lawrence. Velocity models for both lines derived by forward modelling of travel times show a P-wave velocity of 5.5-6.0 km/s in the uppermost crust under the Meguma terrane, interpreted to characterize metasedimentary rocks of the Cambrian Goldenville Group. P-wave velocity gradually increases through the underlying upper, middle, and lower Meguma crust. The Mohorovičić Discontinuity (Moho) at a depth of about 37-38 km separates lower crust with a P-wave velocity of 6.6-6.7 km/s from upper mantle with a velocity of 8.0 km/s. The crust thins to ~30 km near the edge of the shelf, as well as under the Meguma terrane in the onshore. In contrast, Avalonian crust displays a different velocity profile, with a thick high-velocity (6.8-7.1 km/s) lower crust and a deeper Moho (~40 km). Line 99-2 shows that the boundary (Chedabucto Fault at surface) between Meguma and Avalonia crust is steep and appears to offset the Moho. On offshore line 99-1, the velocity model confirms that thin Meguma crust underlies the Orpheus graben and that the fault between Meguma-Avalonia crust is at the northern margin of the graben. The bounding fault dips to the south beneath the graben, and the crust of Avalonia extends only ~30 km under the Meguma terrane south of the graben. Poisson's ratios calculated from velocities of P- and S-waves in the crust under the Meguma terrane are 0.19 to 0.22, lower than average for continental crust. In contrast, a Poisson's ratio of 0.25 for the crust under Avalonia shows that it differs from that under Meguma.