Abstract

In the Humber Zone of the west Newfoundland Appalachians, the Middle Ordovician Taconian orogeny led to emplacement of the Humber Arm and other allochthons above a rift and shelf succession of late Precambrian to Early Ordovician age. Later deformation reversed this stacking in the Old Man's Pond area, where shelf sediments are thrust over the Old Man's Pond Group. East-vergent folds were subsequently developed in association with a regional, west-dipping cleavage that overprints the Taconian structures and intensifies eastward. Later, a major episode of shearing along normal-sense, northwest-dipping shear zones juxtaposed the Old Man's Pond Group against metamorphosed rift-related sediments and volcanics of the Hughes Lake Slice. This extensional episode may be related to intrusive events in adjacent central Newfoundland. The shear zones are overprinted by "cross folds," west-vergent folds, crenulation cleavage, and thrusts. These late structures are inferred to be related to westward transport of the rift–shelf succession and the Grenville-age basement rocks of the Long Range massif above a detachment surface, probably in Devonian time. The structural history is difficult to reconcile with a single, Devonian, "Acadian" orogeny, but is consistent with published isotopic data from central Newfoundland that suggest continued protracted tectonic activity in the Humber Zone in Silurian time.

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