Acadian deformation in the shallow crust: an example from the Siluro-Devonian Arisaig Group, Avalon terrane, mainland Nova Scotia

摘要

The Silurian - Early Devonian Arisaig Group of the Avalon terrane in northern mainland Nova Scotia consists mainly of thinly bedded sandstones, siltstones, and shales deposited in a near shore environment. These strata were deformed in the middle Devonian to form regional northeast- to NNE-trending folds and record deformation processes in the shallow crust during the Acadian orogeny, one of the most regionally extensive orogenic events in the Canadian Appalachians. Structural features in the Arisaig Group are consistent with fold propagation associated with thrust fault geometry and coeval local extension recorded by a set of conjugate normal faults. Many outcrop-scale folds have sheared limbs and show evidence of a complex progressive deformation. Folding was predominantly accomplished by bulk rotation and flattening above thrust fault tips. Early structures (D_1-D_2) produced regional cylindrical folds, whereas later (D_(3a), D_(3b), D_(3c)) structures produced conical folds. D_1-D_3 fold orientations show high variability, but are consistent with progressive deformation related to reactivation and coeval dextral strike-slip movement along the Hollow Fault. The style of deformation is compatible with models in which strain is partitioned into preexisting shear zones in the basement, with folds in the overlying Arisaig Group initiated above the tips of upward-propagating thrusts as second-order structures related to movement along those shear zones. Taken together, these data indicate that fold mechanisms and geometry in the shallow crust during the Acadian orogeny in mainland Nova Scotia may be related to dextral strike-slip along major faults in the basement and co-genetic upward-propagating thrusts that rotated and flattened overlying strata.