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 discontinuit y 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) la yer. Near the Appalachian Front lower-crustal velocities typical of the Gre nville are replaced with a velocity of 7.2 km . s(-1). The collinear reflec tion profile confirms the southward-dipping ramp; the shallowing M disconti nuity, and the lateral changes in the: lower crust. Refraction profiles com bined with reflection and gravity data are used to distinguish the Grenvill e lower-crustal block. The Grenville lower-crustal block is defined as a pa ttern of lower-crustal and upper-mantle reflectivity, with associated veloc ities, and a more negative gravity anomaly than lower-crustal blocks of the Appalachian terranes. The Grenville block terminates northwest of the coas t of Newfoundland near the Appalachian Front. Thus, this lower-crustal prov ince does not underlie the Humber tectono-stratigraphic zone. This is diffe rent than the relationship of the edge of the Grenville lower-crust al bloc k to surface zones in New England. A thick-skinned tectonic style with base ment-involved deformation appears to be typical of the Newfoundland Appalac hians; in contrast, a thin-skinned structural style is observed in New Engl and. The change in structural styles occurs in the bend of the Appalachian orogen across the Gulf of St. Lawrence.