THE ANNIEOPSQUOTCH ACCRETIONARY TRACT, CENTRAL NEWFOUNDLAND: FROM INITIATION OF WEST-DIRECTED SUBDUCTION TO ARC-ARC COLLISION

The Annieopsquotch Accretionary Tract (AAT) in Central Newfoundland comprises a relatively narrow SE-directed thrust duplex that is sandwiched between the ensialic Notre Dame arc and the Red Indian Line.. The Red Indian line is the fundamental Iapetan suture in the Newfoundland Appalachians, along which at least 3000 km of oceanic lithosphere was destroyed during Early to Late Ordovician convergence between Laurentia and the peri-Gondwanan Victoria arc, which was constructed on the leading edge of Ganderia. The AAT comprises a series of structural panels containing Lower to Middle Ordovician suprasubduction zone ophiolites (480-473 Ma) and arc/backarc complexes (473-460 Ma) that formed above a west-dipping subduction zone. The structural panels become younger eastwards, suggesting successive accretion to Dashwoods. Initiation of west-dipping subduction at c. 481 Ma is inferred to have taken place as the result of a plate reorganization induced by the start of the Laurentia-Dashwoods collision along promontories in the Humber margin and is evidenced by formation of the c. 200 km long Annieopsquotch ophiolite belt (AOB). Approximately 10 Ma after its formation, the AOB together with the c. 473 Ma Buchans arc, which was built on a crustal flake that had moved into the arc-trench gap through sinistral forearc slivering, were accreted and underthrust beneath the still hot Notre Dame arc, producing an inverted metamorphic sequence. Accretion was possibly induced by the transfer of convergence from the now closed Humber Seaway to the Iapetus Ocean immediately east of Dashwoods. After accretion of the AOB and the Buchans arc, eastward retreat of the subduction zone generated the 465-460 Ma Red Indian Lake arc and Skidder backarc. Collision of the east-facing Red Indian lake arc with the west-facing Victoria arc between 455 and 450 Ma terminated accretion in the AAT and subduction stepped eastwards in the Exploits backarc basin