Abstract The pre-accretionary shapes of cratonic margins form successions of promontories and re-entrants inherited from the rifting of supercontinents. In accretionary orogens, the extent of deformation related to a collision with a continent characterized by an irregular margin is obfuscated through the partitioning of deformation along pre-existing structures. In the Northern Appalachians, the extent of the deformation related to the oblique collision of the Meguma terrane with the composite Laurentian margin is disputed. Herein, we use a framework based on modern collisional settings to investigate the Late Devonian to Mississippian deformation inboard of the Avalonia–Meguma boundary and evaluate the regional tectonic setting. We combine published shear zone kinematic interpretations, deformation ages and regional 40 Ar/ 39 Ar cooling ages with structural interpretation of aeromagnetic and gravimetric depth slices covering the Northern Appalachians. We find that the deformation related to the collision of the Meguma terrane, attributed to the Neoacadian orogeny, has a larger structural footprint than previously documented. While this deformation is partitioned in multiple structures in the Canadian Appalachians, northern New England is characterized by rapid crustal deformation, high palaeoelevation and fast erosional exhumation, similar to modern syntaxis structures.

Abstract Oblique accretion zones often display transpressive, strike-slip and transtensive structures of different orientations with respect to the convergence axis. The Late Devonian oblique collision of Meguma within the Canadian Appalachian orogen is often characterized as transpressive. However, the simultaneous opening of the Maritimes Basin indicates that the orogenesis also partitioned into an extensional component. In this context, few of the shear zones reactivated during this time period have been characterized while accounting for the possibility of strain partitioning. To do so, we characterize the kinematics and timing of deformation of the Eastern Highlands Shear Zone (EHSZ) and the Coinneach Brook Shear Zone (CBSZ) on Cape Breton Island, Nova Scotia, using a combination of U–Pb geochronology (zircon, monazite, xenotime and apatite) and 40 Ar/ 39 Ar geochronology in situ and step-heating experiments (amphibole, muscovite and biotite). Results show that the Silurian EHSZ was reactivated at c. 385–367 Ma and the CBSZ was formed at c. 395–369 Ma, both yielding oblique kinematics. Together, these structures accommodated the rapid exhumation of the Cape Breton Highlands during the docking of the Meguma terrane. This study thus highlights the heterogeneous distribution of transtensive and transpressive deformation during the Neoacadian Orogeny.

Abstract This study re-examines a reported mylonitic ‘metabasic granulite’ block in a megabreccia that is the most outboard igneous rock outcrop in the Avalon terrane, near the Meguma–Avalon terrane boundary in the northern Appalachians. The block of foliated gabbro is one of several igneous rock blocks in a largely dissolved salt wall and in style of deformation, mineralogy, lithogeochemistry and Sm/Nd isotopes resembles foliated and locally mylonitized late Devonian–early Carboniferous gabbro plutons along the Cobequid Shear Zone to the north. Garnet porphyroclasts in the foliated gabbro are exceptional, with a distinctive composition of Alm 55 Pyr 25 Grs 13 And 4 Sps 3 . Inclusions of pyroxene, andesine and ilmenite, lack of zoning and corroded rims suggest the garnets are antecrysts. Elsewhere in the world, garnets of similar composition in arc-related andesites are interpreted to be from disintegration of comagmatic cumulate material at 0.8–1.0 GPa under hydrous conditions. The Clarke Head foliated gabbro has two mafic components, one resembling arc-related hydrous magma and the other with tholeiitic back-arc character, similar to coeval rocks in the Cobequid Highlands. The gabbro is a product of complex mixing in crustal magma chambers and rapid rise of magma containing lower crustal antecrysts along strike-slip faults.