TECTONIC EVOLUTION OF SALINIC AND ACADIAN HIGH GRADE METAMORPHIC TECTONITES OF THE MEELPAEG NAPPE NEAR PORT AUX BASQUES, SOUTHWESTERN NEWFOUNDLAND: SILURIAN SALINIC UNDERTHRUSTING OF THE GANDER MARGIN BENEATH COMPOSITE LAURENTIA AND LOCHKOVIAN EDUCTION, FOLLOWED BY BI-VERGENT NAPPE FORMATION AND DEXTRAL TRANSPRESSION DURING THE ACADIAN OROGENY

Upper amphibolite facies tectonites of the Meelpaeg nappe in SW Newfoundland were deformed during the Salinic and Acadian orogenies. D_1-2 form a progressive event that occurred between 451 and 417 Ma and led to south or southeasterly directed tectonic transport during protracted Salinic A-subduction of the Harbour Le Cou Group (HLCG), which forms part of the passive margin of the Ordovician Exploits backarc basin, beneath Laurentia. A-subduction transgressed into eduction during D₂ between 419 and 417 Ma following slab unbending, which in turn was preceded by slab breakoff. Eduction may have lasted until 413 Ma. Fragments of the Victoria arc (VA) isolated in the Exploits basin were structurally juxtaposed along the Grandys River shear zone (GRSZ) with the HLCG, which has a markedly different Ordovician magmatic evolution, and deeply buried together during and after D₁ but prior to peak metamorphism and syn-collision magmatism during the final stages of D₂. The different magmatic histories displayed across the GRSZ are typical of the Dog Bay Line. The GRSZ is outlined by a narrow band of the ca. 451 Ma Port aux Basques granite tectonite, which was folded into a complex F₂-F₃ interference structure. Unfolding suggests the GRSZ represents a D₁ thrust which emplaced the Port aux Basques Complex (VA) southwards over the HLCG. Salinic structures were overprinted by Acadian D₃ dextral transpression (≤ 412 Ma), which formed the bi-vergent Meelpaeg nappe emplacing amphibolite facies tectonites above greenschist facies rocks on all sides. The bounding fault zones accommodated dextral oblique reverse ductile shear prior to or early during F₃, which progressively steepened the faults and transformed the nappe into a positive flower structure. Amphibolite facies tectonites were exhumed to greenschist facies by 400 Ma. Deformation continued into the Middle Devonian but became progressively localised into dextral ductile-brittle fault zones. The Meelpaeg nappe dies out to the northeast and its anomalous degree of metamorphism is attributed to protracted underthrusting of the Cabot promontory. Salinic convergence was sinistral but became dextral during the diachronous Acadian orogeny. The switch from sinistral to dextral is proposed as a tool to separate Salinic from Acadian structures in this part of the orogen.