DEFORMATION OF A CONTINENTAL MARGIN DURING SUBDUCTION POLARITY REVERSAL: APPALACHIANS OF WESTERN NEWFOUNDLAND

The Humber Zone of western Newfoundland preserves well known structures associated with Taconian westward emplacement of allochthons above a carbonate platform and foreland basin, but these were overprinted by Salinian structures during subduction polarity reversal and subsequent accretion. Further reactivation of older structures occurred during Acadian shortening.

Distinct stratigraphic successions – including deep-water, shelf, and shelf-edge units derived from the Laurentian passive margin – are contained within in-sequence thin-skinned (D1a) and platform-cutting thrusts (D1b), dated between 477 Ma and 459 Ma. Disrupted units and mélange, with scaly S1 cleavage, are found along D1a thrust contacts. The younger and structurally lower D1b thrusts are shear zones, the lowest of which involved crystalline basement of the Long Range Inlier. D1b thrust stacks have been further uplifted and deformed during Salinian D2 episodes. F2 east-vergent map-scale folds with regional northwest-dipping axial planar S2 cleavage deform earlier thrust sheets and related structures. Across map-scale F2 folds, outcrop-scale F1 folds change from west-vergent recumbent to upright to east-vergent. Subduction polarity reversal and therefore the onset of a proto-Salinian orogenic episode is constrained by syn-tectonic mineral grains that formed along cleavage planes around 451 Ma. The main Salinian orogeny is recorded in a regional northwest-dipping cleavage associated with subduction direction during Salinian D2 episodes, constrained at ~443 Ma. Most D2 contacts are characterized by mylonitic foliation and stretching lineation as shear zones. Thrusts associated with D3 episodes are mainly unfolded, cut up-section to the west and offset older thrust stacks leading to a complex compartmentalization of three generations of faults. S3 cleavage planes are only preserved locally recording ages ~419 Ma.

Isotopic age dates from oppositely dipping fabrics demonstrate that subduction polarity reversal occurred over only a few million years during the Ordovician, consistent with other bi-vergent orogens in the past and present.