GROWTH OF THE ANNIEOPSQUOTCH ACCRETIONARY TRACT BY UNDERPLATING OF ARC AND BACKARC CRUST BENEATH COMPOSITE LAURENTIAN MARGIN, NEWFOUNDLAND APPALACHIANS: IMPLICATIONS FOR ACCRETIONARY MECHANISMS AT CONVERGENT MARGINS

The Annieopsquotch accretionary tract comprises a thrust stack of Lower to Middle Ordovician arc and backarc terranes that were accreted to the composite Laurentian margin of Iapetus during Middle to Upper Ordovician. Geological relationships suggest that the constituent terranes of the Annieopsquotch accretionary tract initially formed outboard of a peri-Laurentian Dashwoods microcontinent in an extensional arc, but occupied a lower plate setting with respect to Dashwoods during accretion. Metamorphic mineral assemblages indicate that the terranes were underplated beneath the composite Laurentian margin at depths ranging from ~ 3 km up to >18 km. We infer the accretion of the terranes to be controlled by the brittle-ductile transition in the hydrated crust. The decoupling of brittle from ductile crust resulted in very high aspect ratios of the terranes, which comprise thin (<5 km) but very large (up to 25 x 250 km) slabs of supracrustal arc rocks and ophiolite crust. Arc basement and ophiolitic mantle are not preserved and were either underplated at a greater depth or subducted and recycled back in the mantle. The accreted crust forms a reasonable approximation to bulk continental crust requiring little post-accretionary modification; hence, the accretion of arc-backarc complexes which occupy a lower plate setting can form an important mechanism for creation of new continental crust required to balance crustal loss at convergent margins.