Geochemistry of post-Acadian, Carboniferous continental intraplate basalts from the Maritimes Basin, Magdalen Islands, Québec, Canada

Major late Paleozoic faults, many with documented strike-slip motion, have dissected the Ordovician-Devonian Appalachian orogen in the Maritime Provinces of Atlantic Canada. Activity alternated between east-west faults (Minas trend) and NE-SW faults (Appalachian trend). NW-SE faults (Canso trend) were probably conjugate to Minas-trend faults. Major dextral movement, on faults with Appalachian trend, in total between 200 and 300 km, began in the Late Devonian. This movement initiated the Maritimes Basin in a transtensional environment at a releasing bend formed around a promontory in the Laurentian margin and thinned the crust, accounting for the major subsidence of the basin. Appalachian-trend strike slip continued in the Mississippian but was accompanied by major movement on E-W Minas-trend faults culminating around the Mississippian-Pennsylvanian boundary, juxtaposing the Meguma and Avalon terranes of the Appalachians close to their present-day configuration. However, strike slip continued during the Pennsylvanian-Permian interval resulting in transpressional deformation that reactivated and inverted earlier extensional faults. A final major episode of transtension, mainly sinistral, occurred during the Mesozoic opening of the Atlantic Ocean. Restoration of movements on these faults, amounting to several hundred kilometers of slip, explains anomalies in the present-day distribution of terranes amalgamated during early Paleozoic Appalachian tectonism. In the restored geometry, the Nashoba and Ellsworth terranes of Ganderia are adjacent to one another, and the Meguma terrane lies clearly outboard of Avalonia. A restored post-Acadian paleogeography, not the present-day geometry of the orogen, should be used as a basis for reconstructions of its earlier Paleozoic history.

https://era.library.ualberta.ca/items/331c261f-4469-426b-9403-138fecee58b8/view/28b0ffc9-02f3-4a3b-b8e6-5e7db4455ce6/Waldron&al2015FaultsTectonics.pdf

Late Paleozoic strike‐slip faults in Maritime Canada and their role in the reconfiguration of the northern Appalachian orogen

Fifteen UPb (zircon) radiometric age determinations have been made on igneous rocks of Middle Devonian to Early Carboniferous age from the southern margin of the Magdalen basin in Cape Breton Island and northern mainland Nova Scotia. Volcanic rocks interbed with early rift-basin sedimentary rocks with some palynological biostratigraphy; dated intrusive rocks cut these sedimentary units. Our biostratigraphically constrained ages are in close agreement with the current Devonian time scale. Combined with previously published data, the age determinations show that igneous activity occurred in four pulses: Middle Devonian (390385 Ma), early Late Devonian (375370 Ma), latest Devonian to early Tournaisian (365354 Ma), and late Tournaisian to early Visean (ca. 339 Ma). Middle Devonian (385389 Ma) volcanic rocks are confined to the Guysborough Group. The Fisset Brook Formation (basalt and minor rhyolite) in the type area and elsewhere in Cape Breton Island and northern mainland Nova Scotia is Late Devonian (c...

Chronology of Devonian to early Carboniferous rifting and igneous activity in southern Magdalen Basin based on U–Pb (zircon) dating

The Acadian orogeny, involving deposition of clastic wedges, deformation, metamorphism, magmatism, and exhumation, is limited in time to the Devonian, and in space to the northern mainland Appalachians. Conventional interpretations attribute it to collision between Laurentia and Avalon or Meguma terranes. However, advances in paleogeography indicate that the Avalon and Meguma terranes were accreted to Laurentia in the Late Ordovician and Early Silurian, which was synchronous with closure of the Iapetus Ocean. On the other hand, the Rheic Ocean remained open to the south. In this context, we propose that the Acadian orogeny developed on an Andean-type margin, and attribute it to flattening of the subduction zone as a consequence of collision of an oceanic plateau surrounding a plume. This model explains: (1) SE to NW diachronism in the onset of deformation throughout the Devonian; (2) the development of a ~400-380 Ma magmatic arc gap in Maritime Canada that was abruptly terminated in the Meguma terrane at ...

The Acadian Orogeny in the Northern Appalachians

Three seismically defined lower crustal blocks (LCBs) have been recognized to underlie the familiar tectonic-stratigraphic zones of the Canadian Appalachians. Tectonic development of the orogen is inferred to have been strongly influenced by collision of the two outboard LCBs (Central and Avalon LCBs) against the sharply irregular North American craton (Grenville LCB), as evidenced by an abrupt offset in the trend of the orogen localized at this irregular edge. The LCBs are restored to a precollisional configuration by (1) removing Carboniferous strike-slip motion on major faults linking through the Magdalen pull-apart basin; and (2) removing Devonian(?) offset along a hypothesized fault through the present-day Strait of Canso (Canso fault) to realign the Central and Avalon LCBs, presumed to have been continuous prior to accretion to North America. The restoration leads to (1) a qualitative explanation for deep seismic reflection observations across the Magdalen basin which suggest that portions of the underlying crust do not correspond to any of the three principal LCBs; and (2) a plate tectonic model for the orogen which suggests the backarc basin separating the Taconian (Ordovician) arc from the eastern margin of the early Paleozoic Iapetus ocean was at least as wide (>450 km) as the offset in the irregular Grenville LCB, inherited from rifting of Iapetus. The present-day Southwest Newfoundland Transform Margin may be a segment of the Canso fault reactivated during Mesozoic opening of the Atlantic ocean.

Deep seismic structure and plate tectonic evolution of the Canadian Appalachians

We have determined crustal thickness in the Gulf of St. Lawrence, an area that corresponds to an offset of the main northern Appalachians units. A "complete" Bouguer anomaly was calculated from recent depth-to-basement compilations and sediment densities from well data. The Moho surface was obtained by inverting the Bouguer anomaly, assuming a single density contrast at depth, and using an average depth provided by deep reflection seismic data. The resulting crustal model shows a Moho depth of 42–44 km beneath the Grenville Craton, north of the Appalachian deformation front. South of this front, the depth to Moho displays a pronounced thinning of the crust beneath the Carboniferous Magdalen Basin. This is in striking contrast to the deep seismic data, which give a Moho depth of about 43 km. The modelling of the Bouguer anomaly in the Magdalen Basin, taking into account the seismic reflection and refraction data, reconciles these different results and suggests that a 43 km deep Moho beneath the basin is as...

Crustal thickness under the Gulf of St. Lawrence, northern Appalachians, from gravity and deep seismic data

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Crustal thickness under the Gulf of St. Lawrence, northern Appalachians, from gravity and deep seismic data