THE ORDOVICIAN HISTORY OF THE HUMBER MARGIN IN THE NORTHERN APPALACHIANS

Conflict between paleomagnetic and geologic data concerning the opening of the Iapetus Ocean can be resolved by invoking a multistage rifting process. Initial separation between Laurentia and Gondwana may have taken place at c. 570 Ma, while another block rifted-off at c. 540-530 Ma. The latter block, referred to herein as the Dashwoods-Chain lakes block (DCLB), remained within the realm of Laurentian faunas and hence was only separated from the Humber margin by a narrow seaway (Humber seaway), which was partially floored by oceanic crust. Paleomagnetic data is consistent with such a scenario. The existence of the DCLB also allows resolving the conflict between the loading history of the Humber margin, the first evidence of which is in the Arenig (c. 475 Ma), and evidence for Late Cambrian- Tremadoc (c. 500-485 Ma) interaction between Laurentian crust and Iapetan oceanic crust (Baie Verte oceanic tract). The Baie Verte oceanic tract (BVOT) is represented by Late Cambrian-Tremadoc suprasubduction zone oceanic crust, which typically includes boninitic rocks. These rocks are thought to have been generated during subduction initiation and formation of an infant extensional arc during rapid hinge retreat of the downgoing plate. Partial subduction of the buoyant DCLB resulted in crustal thickening and intense metamorphism, westward emplacement of the BVOT and finally in failure of the subduction zone, which stepped-back towards the west into the Humber seaway at the end of the Tremadoc. Closure of the Humber seaway led to emplacement of the DCLB and the BVOT onto the Humber margin. Cessation of west-directed subduction at the end of the Middle Ordovician led to slab break-off, which generated numerous granites and high temperature metamorphism in the DCLB. A well developed southeast-facing Middle Ordovician-Early Silurian accretionary complex in Newfoundland, the Annieopsquotch accretionary tract, indicates that slab break-off was rapidly followed by a subduction polarity reversal. The remnants of this accretionary complex are largely hidden beneath the Connecticut-Gaspe synclinorium in the New England Appalachians.