Conceptual model of a two plate collision
We reconstruct seven sequential positions of the Acadian deformation front and foreland basin to illustrate the northwestward migration of the orogenic belt across Maine and adjacent areas from Late Silurian to Middle Devonian time. The reconstructions are based on (1) U-Pb and 40Ar/39Ar ages of pretectonic, syntectonic, and posttectonic plutons; (2) conodont and palynomorph ages of key strata that predate or postdate the local age of deformation; and (3) interpretations of the depositional settings of these strata: far foreland, foreland basin, inner trench slope, and orogenic hinterland. Tight correlations between isotopically and paleontologically dated events are made possible by recent improvements in the Silurian-Devonian time scale.
During early Ludlow time (~423 Ma), the deformation front must have lain near the present midline of the Fredericton Basin, between the posttectonic Pocomoonshine pluton (423 Ma) to the southeast and the coeval graptolitic Smalls Falls Formation, 70 km across strike to the northwest. During early Lochkovian time (~417 Ma), the deformation front was near the present midline of the Central Maine Basin, as recorded by the northwestward advance of slope deposits of the Carrabassett Formation across axially transported turbidites of the Madrid Formation. The early Emsian (407–406 Ma) deformation front lay along or near the Lobster anticlinorium; its position is bracketed between the posttectonic Russell Mountain pluton (406 Ma) to the southeast and the coeval brachiopod-bearing Tomhegan molasse, 50 km across strike to the northwest.
At the Emsian-Eifelian boundary (~394 Ma), the deformation front was located along the Pennington-Munsungun anticlinorium, northwest of the posttectonic Mapleton and Trout Valley Formations, but southeast of the youngest nonmarine clastic rocks in the pretectonic Fish River Lake Formation. At the Eifelian-Givetian boundary (~387.5 Ma), the deformation front was probably somewhere near the midline of the Connecticut Valley-Gaspé Basin, northwest of a belt of posttectonic plutons in Québec, including the 384-Ma Scotstown pluton, but southeast of three occurrences of deformed Eifelian carbonates along the basinÍs northwestern margin. At the Givetian-Frasnian boundary (~382.5 Ma), the deformation front was somewhere to the northwest of these Eifelian carbonate outcrops. The orogen thus migrated northwestward about 240 km across strike (present distance) in about 40.5 Ma. Meanwhile, on the outboard (southeasterly) side of the orogen, a boundary between deformed and undeformed rocks advanced southeastward at least 50 km, probably during the Early and (or) Middle Devonian.
The migration pattern of the orogen and foreland basin suggests that during collision, a southeasterly plate that included the Acadian orogenic wedge and its Avalonian backstop overrode the Taconic-modified margin of North America. The implied minimum plate-convergence rate is about 6 mm/yr. The actual rate must have been considerably faster because this calculation was done on a nonpalinspastic base map; a more accurate estimate will have to await a careful assessment of Acadian shortening. If shortening reduced Maine to half its pre-Acadian width (a conservative estimate in light of the regional-scale tight to isoclinal folding), this situation would imply a plate-convergence rate of 12 mm/yr, which is at the slow end of the normal range of modern plate motions. The reconstructed positions of the orogen and foreland basin also constrain the setting of several Silurian and Devonian episodes of volcanism and plutonism, certain post-Acadian deformations in the Acadian orogenic hinterland, and certain pre-Acadian deformations in what was then the Acadian foreland.
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Michael Diggles
Created: December 12, 2000 (cad)
Updated: February 15, 2007 (bwr, mfd)