Seismic refraction data in the Gulf of Saint Lawrence: implications for the lower-crustal blocks.
A refined model for the wide-angle reflection-refraction profile 88-3 that crosses the foreland basin of the Appalachian orogen is presented. The two major layers of the cratonic crust have velocities of 6.2 and 6.7 km·s -1 with low gradients. Near the exposed North American craton, the 6.7 km·s -1 velocity appears at the shallow depth of 13 km and the M discontinuity at greater than 40 km depth. Towards the orogen, a southeastward-dipping ramp is developed at mid-crustal levels at the top of the 6.7 km·s -1 layer. Near the Appalachian Front lower-crustal velocities typical of the Grenville are replaced with a velocity of 7.2 km·s -1 . The collinear reflection profile confirms the southward-dipping ramp, the shallowing M discontinuity, and the lateral changes in the lower crust. Refraction profiles combined with reflection and gravity data are used to distinguish the Grenville lower-crustal block. The Grenville lower-crustal block is defined as a pattern of lower-crustal and upper-mantle reflectivity, with associated velocities, and a more negative gravity anomaly than lower-crustal blocks of the Appalachian terranes. The Grenville block terminates northwest of the coast of Newfoundland near the Appalachian Front. Thus, this lower-crustal province does not underlie the Humber tectono-stratigraphic zone. This is different than the relationship of the edge of the Grenville lower-crustal block to surface zones in New England. A thick-skinned tectonic style with basement-involved deformation appears to be typical of the Newfoundland Appalachians; in contrast, a thin-skinned structural style is observed in New England. The change in structural styles occurs in the bend of the Appalachian orogen across the Gulf of St. Lawrence.