Abstract

The Laramide orogeny of the western United States is proposed as a modern analogue for the Silurian-Devonian Acadian orogeny and subsequent diachronous, voluminous, short-lived magmatism and basin formation in the northern Appalachians. Shallowing of the Benioff zone accounts for several enigmatic features associated with plate convergence in the northern Appalachians, including (1) Wenlockian-Ludlovian termination of arc-related magmatism in the Avalon terrane followed by a period of relative magmatic quiescence from 395 to 380 Ma, and (2) diachronous migration of the Acadian deformation front from ca. 415 Ma in the southeast to ca. 370 Ma in the northwest, extending more than 600 km into the continental interior. The flattening of the subduction zone is attributed to overriding of a plume by the convergent margin, which may explain (1) the abrupt termination of magmatic quiescence by 380–370 Ma, and voluminous felsic magmatism and production of plume-related lamprophyres in the southeast (Meguma terrane) as the plume thermally eroded the oceanic lithosphere, causing melting of the lower crust; (2) Late Devonian regional high-temperature, low-pressure metamorphism in the Meguma terrane related to the thermal anomalies above a plume; (3) synchronous Devonian emplacement of Meguma gold deposits and associated siderophile elements, possibly derived from fluid circulation above an ascending plume; (4) rapid Late Devonian uplift and erosion of as much as 10 km due to dynamic uplift over a plume; (5) migration of magmatism to the north (Avalon terrane, Cobequid highlands) so that plume-related Carboniferous magmatism occurred in and around the Carboniferous-Permian Maritimes basin; (6) the high-density lens at the base of the crust beneath the Maritimes basin, the product of plume-derived underplated mafic rocks; and (7) a subsidence mechanism for formation of the Maritimes basin by cooling of a decapitated plume head.

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