Crustal structure and surface zonation of the Canadian Appalachians: implications of deep seismic reflection data

Abstract

In 1986, 1181 km of marine seismic reflection data was collected to 18–20 s of two-way traveltime in the Gulf of St. Lawrence area. The seismic profiles sample all major surface tectono-stratigraphic zones of the Canadian Appalachians. They complement the 1984 deep reflection survey northeast of Newfoundland. Together, the seismic profiles reveal the regional three-dimensional geometry of the orogen.Three lower crustal blocks are distinguished on the seismic data. They are referred to as the Grenville, Central, and Avalon blocks, from west to east. The Grenville block is wedge shaped in section, and its subsurface edge follows the form of the Appalachian structural front. The Grenville block abuts the Central block at mid-crustal to mantle depths. The Avalon block meets the Central block at a steep junction that penetrates the entire crust.Consistent differences in the seismic character of the Moho help identify boundaries of the deep crustal blocks. The Moho signature varies from uniform over extended distances to irregular with abrupt depth changes. In places the Moho is offset by steep reflections that cut the lower crust and upper mantle. In other places, the change in Moho elevation is gradual, with lower crustal reflections following its form. In all three blocks the crust is generally highly reflective, with no distinction between a transparent upper crust and reflective lower crust.In general, Carboniferous and Mesozoic basins crossed by the seismic profiles overlie thinner crust. However, a deep Moho is found at some places beneath the Carboniferous Magdalen Basin.The Grenville block belongs to the Grenville Craton; the Humber Zone is thrust over its dipping southwestern edge. The Dunnage Zone is allochthonous above the opposing Grenville and Central blocks. The Gander Zone may be the surface expression of the Central block or may be allochthonous itself. There is a spatial analogy between the Avalon block and the Avalon Zone. Our profile across the Meguma Zone is too short to seismically distinguish this zone from the Avalon Zone.

Provenance of tectonic history of the Gander Zone in the Caledonian/Appalachian Orogen: Implications for the origin and assembly of Avalon

“…The clastic rocks of the Gander Zone are thus autochthonous to parautochthonous with respect to their basement. The seismically defined lower crustal block underneath the Gander Zone (Marillier et al, 1989;Stockmal et al, 1990) thus probably represents its original basement. Quinlan et al (1992) recently argued on the basis of continuity of seismic reflectors that the lower crustal block is similar to the basement of the Hermitage Flexure (Bras d'Or-Brookville belt) in southwestern Newfoundland (Fig.…”

Influence of deformation and fluids on Ar retention in white mica: Dating the Dover Fault, Newfoundland Appalachians

“…1; Blackwood and Kennedy, 1975;Dallmeyer et al, 1981;Holdsworth, 1994) and is correlated at depth with a geophysically-defined crustal-scale structure that offsets the Moho (Keen et al, 1986;Marillier et al, 1989;Stockmal et al, 1990). Sinistral shear is recorded in a ~20 km wide NE-striking zone of ductile shear that parallels the Dover Fault, the Wing Pond shear zone ( Fig.…”

Magmatic and tectonic evolution of the Cambrian-Ordovician Laurentian margin of Iapetus: Geochemical and isotopic constraints from the Notre Dame Subzone, Newfoundland

“…1) and the Baie Verte line (Williams and St. Julien, 1978), respectively. Seismic reflection studies show that the entire Notre Dame subzone is allochthonous, underlain by the eastward-dipping extension of the ancient Laurentian continental margin (Marillier et al, 1989;Quinlan et al, 1992). Geologic data provide no evidence of the spatial relationships between the oceanic terranes and continental lithosphere during the Cambrian and Early Ordovician.…”