Geological evolution of middle to late Paleozoic rocks in the Avalon Terrane of northern mainland Nova Scotia, Canadian Appalachians; a record of tectonothermal activity along the northern margin of the Rheic Ocean in the Appalachian-Caledonide orogen

The Early Silurian-Early Devonian Arisaig Group, in the Avalon terrane of Nova Scotia, consists of a thick ( approximately 1900 m) sequence of unmetamorphosed fossiliferous siliciclastic strata that unconformably overlies the 460 Ma bimodal Dunn Point Formation volcanic rocks and is unconformably overlain by basalts and red clastic rocks of the McArras Brook Formation. The Dunn Point volcanic rocks were deposited when Avalonia was a microcontinent, in a New Zealand-type arc setting approximately 1800 km north of Gondwana and 1700-2000 km south of Laurentia. Geochemical, Sm-Nd, and U-Pb (detrital zircon) isotopic data of all Arisaig Group strata show fundamental differences from the underlying Avalonian rocks, indicating that they were not derived from Avalonian basement. These data are instead compatible with derivation from Baltica, implying that Avalonia had accreted to Baltica by the earliest Silurian and that the Arisaig Group is part of a clastic sequence that has overstepped Appalachian-Caledonide terrane boundaries. The lack of penetrative deformation and the approximately concordant nature of the contact between the Dunn Point Formation and the Arisaig Group suggest that this portion of Avalonia was located on the trailing edge of the Avalonia plate during the collision. Regional syntheses suggest that the basin was initiated by local transtension during oblique sinistral collision between Avalonia and Baltica. An overall trend toward increasingly negative epsilon (sub Nd) values in the clastic rocks toward the top of the Arisaig Group is thought to reflect increasing input from Laurentia by the time of deposition of the Early Devonian strata. The basin also preserves evidence of loading in the Late Silurian, which is thought to reflect the development of a foreland basin and the ongoing shortening across the orogen associated with the onset of the Acadian orogeny. The unconformity between the Arisaig Group and the overlying McArras Brook Formation is the local expression of the deformation associated with Acadian orogeny in the Antigonish Highlands. The orientation and style of Acadian deformation preserved in the Arisaig Group is compatible with dextral movement along major northeast-trending faults, consistent with evidence of regional dextral shear along the northern margin of the Rheic Ocean in the Middle Devonian. Late Devonian-early Carboniferous deposition of the predominantly continental clastic rocks of the Horton Group occurred around the Antigonish Highlands in a series of grabens and half-grabens, most notably represented by the St. Mary's basin, which originated by dextral shear along the boundary between the Meguma and Avalon terranes. Continued episodes of dextral shear in the late Carboniferous resulted in localized regions of transtension and basin development, and also in episodes of transpression, manifested by intense deformation, thrusting, and S-C fabric development. Taken together, Middle Devonian-late Carboniferous episodes of dextral shear reflect the local accommodation of oblique convergence and eventual collision between Gondwana and Laurussia.