Diachronous supracrustal extension in an intraplate setting and the origin of the Connecticut Valley–Gaspé and Merrimack troughs, northern Appalachians

Abstract

In the Appalachians of mainland Canada and New England, Silurian/Early Devonian rocks are preserved in the Connecticut Valley-Gaspé and Merrimack troughs, and rest unconformably or in fault contact with older rocks belonging to Laurentia and to Gander/Avalon, respectively. The Silurian/Early Devonian rocks consist of marine clastic deposits with subordinate carbonates, lava flows and terrestrial deposits. The origin of these sedimentary basins is still poorly understood. Metamorphic ages and structures in the Laurentian margin, major unconformities and syn-sedimentary normal faulting in both troughs argue for significant crustal extension during deposition. The Connecticut Valley-Gaspé and Merrimack troughs are separated by inliers of pre-Ordovician to Ordovician rocks which we interpret as Silurian basement highs that would have been buried in Devonian times to form a composite sedimentary basin. Volcanic rocks are widely distributed in time and space in both basins, and are mostly subalkaline within-plate tholeiites, which is consistent with a tectonic setting involving crustal extension rather than with subduction. Granitic plutons are abundant in the Merrimack trough and attest to high temperatures at mid-crustal levels. It is suggested that crustal extension was responsible for the formation of both basins and that heating of the lower crust in the Merrimack trough during the Silurian was the result of synorogenic collapse likely triggered by delamination at the Laurentia-Medial New England boundary. Delamination of the subducted slab and the upwelling of the asthenosphere would have caused (1) isostatic uplift and formation of basement highs, (2) magmatism in the lower crust and regional-scale contact metamorphism in the upper crust, and (3) collapse of metamorphic terranes and the formation of subsiding sedimentary basins.

Silurian extension in the Upper Connecticut Valley, United States and the origin of middle Paleozoic basins in the Québec embayment

“…Numerous workers have argued for a period of post-Taconian crustal extension leading to the development of the CVGT and CMT (Osberg and others, 1989;Keppie and Dostal, 1994;van Staal and de Roo, 1995;Rankin, 1996a;Karabinos, 1998;Bourque and others, 2000;Moench and Aleinikoff, 2002;Castonguay and Tremblay, 2003;Tremblay and Pinet, 2005). The locus of late Silurian lithospheric attenuation that we have documented for the BHA is spatially between the two basins and we seek a common cause for the extension.…”

“…Support for Silurian extension in the CMT and Gaspé comes from the geochemical signature of magmatic suites in Maine, Gaspé, and New Brunswick (Bédard, 1986;others, 1989, 1993;Hon and others, 1992;Fitzgerald and Hon, 1994). Recent papers, (for example, others, 1989, 1993;Hon and others, 1992;Keppie and Dostal, 1994;Tremblay and Pinet, 2005), considered the Late Silurian and Early Devonian igneous rocks a cogenetic group and proposed a common extensional environment for the group. We suggest that this may be ill advised and urge that, where age control is adequate, Silurian and Devonian suites be considered separately.…”

Guidebook for Field Trips in Western Maine and Northern New Hampshire

“…Third, an emergent landmass might have shed a buoyant freshwater lid across the narrowing Central Maine Trough to the northwest, thus further amplifying eutrophication. An alternative mechanism for the onset of black shale deposition entails the formation of isolated basins within an overall extensional upper plate setting (Tremblay and Pinet, 2005;). …”

Smalls Falls Revisted: A Journey Through a Paleozoic Sedimentary Basin

“…Third, an emergent landmass might have shed a buoyant freshwater lid across the narrowing Central Maine Trough to the northwest, thus further amplifying eutrophication. An alternative mechanism for the onset of black shale deposition entails the formation of isolated basins within an overall extensional upper plate setting (Tremblay and Pinet, 2005;Reusch and van Staal, 2012).…”