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The Late Ordovician-Early Silurian Bruns wick subduction complex in northern New Brunswick preserves a nearly intact southeast-facing forearc terrane with clear links among Silurian subduction, underplating, exhumation, and forearc sedimentation. The forearc terrane grew over time due to successive accretion and underplating of seamounts and isolated ribbons made up of continental and transitional crust concomitant with foreland migration of the trench. A sliver of strongly deformed epidote-blueschists derived from an accreted seamount was stacked together with slightly lower-pressure winchite-bearing tectonites and medium-to high-pressure greenschists of the Fournier, California Lake, and Tetagouche blocks into a series of D 1 metamorphic nappes during Salinic closure of the Tetagouche-Exploits backarc basin. The blueschists (~375 °C, 7.2 kbar) locally preserve mineralogical (zoning from actinolite or barroisite core to a rim of glaucophane) and microstructural evidence for a counterclockwise pressure-temperature (P-T) path ascribed to underthrusting beneath the young oceanic lithosphere of the Fournier block shortly after the inception of subduction at ca. 450 Ma. The next phase of accretion is represented by underplating of the California Lake block at ca. 442 Ma. This may have caused emplacement of the blueschists above the slightly lower metamorphic grade Canoe Landing Lake winchite-bearing nappe (350 °C, 6.5 kbar). Underplating of the Tetagouche block at ca. 435 Ma extruded the blueschist sliver and underlying nappes of the California Lake block immediately above greenschistfacies nappes (~5.8 kbar) and beneath lowerpressure (4-5 kbar) greenschist-facies basalt and gabbro of the ophiolitic Fournier block, which was deformed and imbricated earlier, probably during accretion of the California Lake block. Shear sense indicators confi rm that the hanging-wall shear zone of the blueschist sliver accommodated Early Silurian normal motion. D 1 is generally time-transgressive due to down-stepping of the underplating-related deformation. Metamorphic relicts, as well as zoning in amphibole and epidote, indicate that the Canoe Landing Lake and other nappes underwent clockwise P-T paths related to subduction of the cool oceanic lithosphere and/or accretion of the California Lake and Tetagouche blocks.

The Dog Bay Line separates different Silurian rock groups in northeast Newfoundland. West of the line, terrestrial volcanic rocks and sandstones (Botwood Group) overlie marine greywackes and conglomerates (Badger Group). East of the line, red sandstones overlie shallow marine shales and limestones (Indian Islands Group). Throughout Dog Bay, the line is marked by a disrupted zone of dark grey to black shales, volcanic rocks, and gabbros. Pervasive dextral, transpressive ductile deformation followed by successively more brittle extension with renewed dextral 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 curved eastward at the coast. The Mount Peyton Batholith, dated at 420 ± 8 Ma, apparently cuts the line.The Dog Bay Line occurs within the Dunnage Zone whose Cambrian–Ordovician rocks represent vestiges of the Iapetus Ocean. Northwest of the line, the Silurian rocks were deposited on Ordovician rocks already accreted to Laurentia. Southeast of the line, the Silurian rocks were deposited on Ordovician rocks already amalgamated with the continental Gander Zone. Timing of major movement and a Silurian marine to terrestrial depositional change recorded on both sides of the line agree within error with isotopic ages for the onset of plutonism, regional deformation, and metamorphism in central Newfoundland. The Dog Bay Line may mark the terminal Iapetus Ocean.

The Topsails igneous terrane of western Newfoundland contains several intrusive and volcanic suites underlain and separated by screens of older intrusive rocks. The heterogeneous Hungry Mountain complex yielded U-Pb zircon upper and lower intercept ages of 2090 ± 75 Ma and 467 ± 8 Ma, demonstrating a significant inherited component of Aphebian age, while an adjacent suite of relatively massive granodioritic to granitic rocks yielded a slightly discordant U-Pb zircon age of 460 ± 10 Ma. The 438 ± 8 Ma age of the Rainy Lake complex, a suite of island arc type intrusive rocks, suggests it forms part of a Silurian magmatic episode, which also included Springdale Group bimodal volcanics (429 ± 4 Ma), and peralkaline granite and subvolcanic porphyries which intrude the Springdale Group (429 ± 3 Ma and 427 ±3 Ma, respectively). Most igneous units contain a slight component of inherited zircon, but initial 87Sr/86Sr ratios (average 0·704) are similar to calculated ‘Bulk Earth’ values at this time.Available data suggest that the Topsails terrane formed an oceanic tract with active volcanic island arcs when obduction commenced in early Ordovician time. The subsequent magmatic history, including the major but short-lived early Silurian magmatism, can be directly or indirectly related to obduction processes, including over-riding of the Topsails terrane by ophiolitic allochthons. There is no evidence of any Acadian (Devonian) igneous activity in the Topsails terrane.