Strucural geology of the southwestern shore of Conception Bay, eastern Avalon zone, Newfoundland Appalachians

Riveros, Claudia Paz (1998) Strucural geology of the southwestern shore of Conception Bay, eastern Avalon zone, Newfoundland Appalachians. Masters thesis, Memorial University of Newfoundland.

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Abstract

The Newfoundland Avalon Zone of the eastern Appalachians has, for many years, been affected by two major tectonic events: the late Proterozoic Avalonian Orogeny. which occurred prior to the final collision of the Gondwanan and Laurentian continents, and the mid-Paleozoic Acadian Orogeny. associated with the final accretion of the Gondwanan to Laurentian continents. Recent geochronological data show, however, that the Avalon Zone was affected by multiple late Proterozoic thermal events (ca.760 Ma. ca. 680 Ma. ca. 630-600 Ma and ca. 575-550 Ma) and by the Paleozoic Salinic and Alleghenian-Hercynian orogenies. Systematic constraints on the structural characteristics of these tectonic events are lacking in the Avalon Zone. The present study addresses this problem via a detailed investigation of the structural and stratigraphic relationships observed along the southwestern shore of the Conception Bay, eastern Avalon Zone. The cross-cutting relationships between a pervasive regional (S₂) cleavage and structures truncated by the sub-Cambrian unconformity are used to distinguish between D₁ Proterozoic and D₂ post-Cambrian structures. Distinctive structural characteristics observed through the study site are used, further, to identify four distinctive styles and relative ages of deformation: D₁ₐ, D₁, D₂ₐ and D₂. D,. and D,b reflect two phases of deformation associated with the Avalonian Orogeny. Both appear to have occurred in a late period of this prolonged and diachronous orogenic event. D,. is a block faulting extensional event that produced north-trending block faults. These have produced the north-trending Brigus and Holyrood horsts. This .. brittle" deformational event produced few identifiable mesoscale features and no cleavage. D,b was a north-south shortc:ning event. D,b structures include northwest-trending dextral kink band sets and northeast-trending sinistral kink band sets, northwest- and northeast-trending open to isoclinal F, folds, and regional dome and basin and periclinal structures. Some of the faults that underwent normal displacement during D,. were reactivated as strike-slip faults during D,b. North- to north-northeast-trending faults (e.g .• Brigus, Marysvale and Topsail faults) underwent apparent sinistral strike slip and north-northwesttrending faults (e.g., Bacon Cove, Duffs and Holyrood faults) underwent apparent dextral strike slip during D,b. A cleavage (S1) is commonly developed in discrete zones in tight to isoclinal F, folds. but is regionally non-penetrative. This restricted development of S, reflects the semi-brittle deformational regime of 0 11, (moderate to low temperature and mean stress conditions). D2a represents a mid-Paleozoic contractional event related to the Acadian and/or Salinic orogenies. D2a is characterized by a pervasive. generally upright cleavage (SJ that indicates east-west contraction. Upright. open. north- to north-nonheast-trending folds (FJ overlie east- and west-directed thrust and reverse faults. Some of these D1a faults are inverted 0 1 structures. Cambrian beds preserved in the core of F2 synclines and in inverted D,. horsts provide the only direct age constraint (post early Cambrian) on the D1a structures. S2 can transect the F2 axial traces by up to 15" in plan view. It is inferred that the F1 fold trends are strongly influenced by the orientation of the associated faults (formed during earlier deformational events). whereas the S2 cleavage more directly reflects the D1a east-west shortening. The bedding of Cambrian rocks and S1 display similar rotations adjacent to some of the north-trending faults. Cambrian rocks are preserved in D1b down-faulted blocks. These features are interpreted to indicate post-S2 normal fault separation on reactivated D,. and/or D1a faults. There is also post-S2 apparent sinistral and dextral strike-slip along minor northwest-trending faults in Cambrian rocks. Collectively. these fault movements are interpreted to indicate northwest-southeast oriented extension during D2a. These structures could be associated with the mid-Devonian to early Permian Alleghenian-Hercynian orogeny and/or opening of the North Atlantic during the Mesozoic.

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