A TIME-TRANSGRESSIVE RELATIONSHIP BETWEEN SILURIAN SALINIC OROGENESIS, MAGMATISM AND THE OLD RED SANDSTONE SEDIMENTATION IN THE NEWFOUNDLAND APPALACHIANS

An intimate relationship between Silurian terrestrial red bed sedimentation (Old Red sandstone), slab breakoff-related magmatism and deformation is proposed in the Newfoundland Appalachians. Red bed sedimentation started during the Early Silurian (earlier than in the British Caledonides), and records the progressive rise of the Salinic mountains in the tectonic hinterland of the Appalachian orogen in Newfoundland follwing accretion of the Gander margin to composite Laurentia. The red beds were mainly deposited in molasse-style foreland basins in front of an east-propagating terminal Salinic deformation front. New U-Pb zircon dating of volcanic rocks interlayered with the Silurian red beds in key structural locations combined with the existing geochronological database suggest that the sedimentary rocks become progressively younger from west to east and overstep the Gondwanan-derived terranes. We propose that deposition of these red beds is a proxy for the timing of cratonization of the accreted terranes. Eastward migration of the terminal Salinic deformation front was accompanied by an eastward shift of the locus of slab-breakoff magmatism above a progressively widening asthenospheric window. The asthenospheric window is interpreted to have formed following entrance of the leading edge of the Gander margin into the Salinic subduction zone due to a combination of progressive steepening of the down going plate, slab-breakoff and eduction of the Gander margin. Eduction occured during the Salinic collision and may have been driven by the trench migration of the Acadian coastal arc that was built upon the trailing edge of the Gander margin. Formation of the coastal arc was due to closure of the Acadian seaway that separated Avalonia from the trailing edge of Ganderia. Cosure had started while the Salinic collision was ongoing along Ganderia's leading edge. The resultant thinning of the lithosphere beneath the Salinic orogen immediately prior to the onset of the Early Devonian Acadian orogeny, the latter a result of the collision between of Avalonia with composite Laurentia, set the stage for generation of the widespread bloom of syn-collision Acadian magmatism in the overriding plate (composite Laurentia).