The stratigraphy and structure of the southern part of the Hare Bay allochthon, N.W. Newfoundland

Smyth, W. R. (Walter Ronald) (1973) The stratigraphy and structure of the southern part of the Hare Bay allochthon, N.W. Newfoundland. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

The allochthonous sequence south of Hare Bay comprises four distinct tectonic slices which from the structurally lowest to the highest are: the Maiden Point Slice, the Croque Head Slice, the St. Julien Island Slice and the White Hills Slice. The allochthonous sequence was emplaced westwards in Middle Ordovician time over an autochthonous, mainly carbonate, sequence. The top of the autochthon consists of a northeasterly derived flysch that contains detritus derived from the allochthon. This flysch records a decrease in grain size, bed thickness and sand content, from north to south across the area. -- The study has shown that the slices of the allochthon display different stratigraphy, different internal deformation and different metamorphic history. The slice contacts are in most places marked by a few tens of meters of black shaley melange. -- The Maiden Point Slice consists mainly of coarse quartzo-feldspathic greywackes and minor mafic volcanic rocks. The Croque Head Slice consists of finer grained greywackes than those in the Maiden Point Slice. The St. Julien Island Slice consists of sandy limestone in fault contact with a polymictic conglomerate. The conglomerate contains volcano-plutonic detritus which was probably derived from an island arc to the east. The White Hills Slice contains a partial ophiolite, structurally underlain by metavolcanic rocks and minor amounts of metasedimentary rocks that record pre-emplacement polyphase deformation and metamorphism. These metamorphic rocks consist of pyroxene-bearing amphibolites, amphibolites, and greenschists that display decreasing metamorphic grade and intensity of deformation downward and away from the overlying ultramafic rocks. They are interpreted as a dynamothermal aureole related to obduction and earliest transport of oceanic crust and mantle over previously undeformed and unmetamorphosed supracrustal rocks. -- Emplacement of the allochthon in Middle Ordovician time produced a slaty cleavage, rare recumbent folds and low greenschist metamorphism in the lower tectonic slices and in the melange zones. The autochthonous rocks are in most places unaffected by the emplacement deformation (Taconic orogeny), except at Canada Bay where melange is absent and an imbricate structure and associated northwest-facing recumbent folds are developed. -- After Middle Ordovician emplacement the whole area was affected by the Acadian orogeny. The pre- and syn-emplacement structures are refolded about open, upright to moderately inclined folds and an associated crenulation cleavage is widely developed. -- The evolution of the allochthon and autochthon is described in terms of a plate-tectonic model related to the birth and destruction of a continental margin and a marginal ocean basin from late Precambrian to upper Paleozoic times. Comparisons with the other "Taconic" allochthons found along the western margin of the northern Appalachians suggests that similar tectonic processes controlled their evolution.

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