Abstract

Near the exposed Grenville craton, a two-layer crust with velocities of 6.2 km s–1 and 6.65 km s–1 and low gradient is measured. The 6.65 km s–1 velocity is observed at the shallow depth of 15 km and the Moho at greater than 40 km depth. Towards the Appalachian orogen, a southwards dipping ramp is developed in the mid crust on the top of the 6.65 km s–1 layer. A collinear reflection profile confirms the southwards dipping ramp, the shallowing Moho and the lateral changes in the lower crust. A published refraction profile that crosses this reflection profile near the southern end confirms the termination of the 6.65 km s–1 layer and its replacement by a slower velocity. With the publishing of wide angle reflection–refraction (WAR) profile 88-2, two pairs of lines are now available in the gulf that cross the craton into the orogen with both WAR and deep seismic reflection coverage. The combined data sets are used to distinguish the Grenville lower crustal block based on patterns of reflectivity and associated velocities. Near the Gaspé, the Grenville lower crustal block extends southwestward and, by extrapolation of the tectonostratigraphic zones offshore, underlies the Humber zone. In contrast, profiles in the gulf to the west of Newfoundland imply the continuous Grenville lower crustal block terminates seaward of the Humber surface terrane. Along-strike variations in the extent of Grenville lower crustal block beneath the northern Appalachians are suggested as an important factor in determining the style of crustal deformation.

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