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Penobscottian-Grampian-Finnmarkian orogenies as indicators of terrane linkages

By
Robert B. Neuman
Robert B. Neuman
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Michael D. Max
Michael D. Max
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Published:
January 01, 1989

The Penobscot orogeny in the Appalachians, the Grampian orogeny in the British Isles, and the Finnmarkian orogeny in the Scandinavian Caledonides are Late Cambrian–Early Ordovician events that characterize terranes having similar pre- and post-deformation features. Together they record a major orogen-wide closing of the Cambrian Iapetus Ocean basin.

The Penobscottian is the principal pre-Acadian deformation event in the Gander and related terranes of the Appalachians. The upper Proterozoic–Cambrian rocks deformed by it include subduction-related mélanges. In the northern Appalachians its timing is best constrained in northern Penobscot County, Maine, where polydeformed slate and sandstone (Grand Pitch Formation), dated Cambrian(?) by Oldhamia, are overlain by the singly deformed Arenig-age Shin Brook Formation. Basal conglomerate of the Shin Brook contains Grand Pitch clasts, and higher tuff contains Early Ordovician (late Arenig) Celtic province shelly fossils that indicate deposition around a volcanic island in cool waters of moderate to high latitude, remotely distant from contemporaneous warm equatorial waters of the North American (Laurentian) continental margin.

The Grampian, in the Scottish–Irish orthotectonic Caledonides, deformed and metamorphosed miogeoclinal, upper Precambrian to lower Middle Cambrian Dalradian rocks that were largely derived from the Laurentian craton. An early tectonothermal phase, probably associated with subduction that produced blueschists, was followed by the main Barrovian metamorphism (510 to 480 Ma). Fossils in the Arenigian post-tectonic rocks of the South Mayo trough have strong North American affinities. Dalradian equivalents in the paratectonic Caledonides (Howth Peninsula, Ireland; Anglesey, Wales) suggest rifting of the Cambrian miogeocline, confirmed by the cool-water Celtic province Arenig shelly fauna of Anglesey.

The Finnmarkian event deformed and metamorphosed rocks of the Late Proterozoic–Cambrian(?) Baltic continental-rise prism of Baltic provenance and obducted slabs of ophiolite. The minimum of its isotopic age range, 540 to 480 Ma, is confirmed by the Llanvirnian Otta serpentinite conglomerate that unconformably overlies an ophiolite remnant. Fossils from the conglomerate suggest cool-water Baltic and ocean-island affinities.

The sequences deformed by these penecontemporaneous events record a complex history of the Late Proterozoic–Cambrian Iapetus Ocean, including miogeoclinal sedimentation on the margins of Laurentian, Baltic, and Armorican cratons, and the rifting and closing of intervening oceanic tracts. At the time of their deformation, these sedimentary sequences occupied the margins of a very large Early Ordovician Iapetus Ocean.

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Contents

GSA Special Papers

Terranes in the Circum-Atlantic Paleozoic Orogens

R. D. Dallmeyer
R. D. Dallmeyer
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Geological Society of America
Volume
230
ISBN print:
9780813722306
Publication date:
January 01, 1989

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