Evolution of the Appalachian laurentian margin: Lithoprobe results in western Newfoundland

The Humber Zone of the western Newfoundland Appalachians represents the ear ly Paleozoic Laurentian margin established by Neoproterozoic rifting. After a period of passive margin thermal subsidence, Taconian deformation began in the Early Ordovician with westward thrusting. Subsequently, an extensive foreland basin developed beneath the Gulf of St. Lawrence. It records rapi d Late Ordovician to Early Silurian subsidence; mid-Silurian erosion; and r enewed Late Silurian to Devonian subsidence. The Humber Zone was traversed by Lithoprobe seismic reflection profiles. Within the external part of the orogen, seismic reflections in the upper crustal section appear more cohere nt where seismic lines are parallel to fold hinges. Some subhorizontal refl ectors are interpreted as thrust sheets of shelf limestone, but others prob ably represent intrabasement structures. A group of moderately northwest-di pping reflections probably represents late extensional shear zones. On the Bale Verte Peninsula, low-angle reflections passing beneath the Bale Verte Line are probably also late extensional shears, possibly reactivating earli er thrusts. Tectonism in the Humber Zone probably began with attempted east ward subduction of the Laurentian margin. Deep burial of the margin, accomp anied by eclogite-facies metamorphism, probably coincided with rapid subsid ence in the foreland basin. Later Barrovian metamorphism was associated wit h cleavage development and east-directed shear, and with dextral oblique sl ip, in Pale Verte Peninsula. Later Silurian sinistral transpression with th rusting east of the Bale Verte Line was followed by dextral transpression t o transtension. "Acadian" thrusting dominated the western margin of the oro gen in the Devonian and possibly earliest Carboniferous.