THE DOG BAY LINE - A MAJOR SILURIAN TECTONIC BOUNDARY IN NORTHEAST NEWFOUNDLAND

The Dog Bay Line separates different Silurian rock groups in northeast Newfoundland. West of the line, terrestrial volcanic rocks and sandst ones (Botwood Group) overlie marine greywackes and conglomerates (Badg er Group). East of the line, red sandstones overlie shallow marine sha les and limestones (Indian Islands Group). Throughout Dog Bay, the lin e is marked by a disrupted zone of dark grey to black shales, volcanic rocks, and gabbros. Pervasive dextral, transpressive ductile deformat ion followed by successively more brittle extension with renewed dextr al movements mark the northwest side of the line on the coast. The Dog Bay Line is traceable for 100 km and it is open-ended. Dextral offset is deduced to be many tens of kilometres. The line trends northeast, parallel to outcrop belts, and both the line and outcrop belts are cur ved eastward at the coast. The Mount Peyton Batholith, dated at 420 +/ - 8 Ma, apparently cuts the line. The Dog Bay Line occurs within the D unnage Zone whose Cambrian-Ordovician rocks represent vestiges of the Iapetus Ocean. Northwest of the line, the Silurian rocks were deposite d on Ordovician rocks already accreted to Laurentia. Southeast of the line, the Silurian rocks were deposited on Ordovician rocks already am algamated with the continental Gander Zone. Timing of major movement a nd a Silurian marine to terrestrial depositional change recorded on bo th sides of the line agree within error with isotopic ages for the ons et of plutonism, regional deformation, and metamorphism in central New foundland. The Dog Bay Line may mark the terminal Iapetus Ocean.