The seismic velocity structure of northern Appalachian crust around western Newfoundland

Abstract

The crustal velocity structure beneath western Newfoundland is constrained by a reanalysis of older, regional refraction profiles together with an analysis of one new profile. Initial interpretation of the older data gave inconsistent and discontinuous structures that are difficult to reconcile with more recent deep reflection profiles. We also show that traveltimes predicted by the earlier models often do not yield acceptable fits to the original observations. Our reinterpretation reveals a simpler pattern, in which the crust is characterized by a persistent, high-velocity, lower crustal (HVLC) layer. This layer has velocities of 7.0–7.9 kmls and thicknesses of 5–23 km. It is thickest beneath the Grenville crustal block, east of the Appalachian structural front, and thins or is possibly absent within the Central block. Analysis of the new, much higher resolution profile off western Newfoundland confirms the existence of the HVLC layer with a velocity of 7.2 kmls and thicknesses of 11–19 km, increasing to the northeast. The upper crust has well-defined velocities of 6.2–6.4 kmls and is overlain by a complex sandwich of sediment layers with principal velocities of 3.9, 4.95, and 5.58 kmls and maximum total thicknesses of 8.5 km in the south to 5.5 km in the north. Total crustal thickness varies from 39 to 43 km from south to north. Comparison of the velocity–depth models with the pattern of deep crustal reflectivity revealed by deep multichannel profiles shows that the HVLC layer is coincident with a zone of flat-lying reflectors that terminate to the west at the base of the crust beneath the Appalachian structural front. The HVLC may continue eastward to cover a broad region of central Newfoundland as suggested by the older data, but its association with the reflectivity is not clear.