Mineral compositions of small peridotite bodies in an Ordovician collision zone of the Vermont Appalachians provide important clues to their tectonic environment of origin. The bodies have been deformed and partially serpentinized under greenschist- to lower amphibolite-facies conditions during the Ordovician and Devonian. Before serpentinization, the peridotite bodies were dunite as shown by their mineral assemblage and by their high MgO, and low Ti and Al whole-rock contents. Despite deformation and metamorphism, remnant olivine and spinel grains occur; their compositions are taken to represent conditions prior to regional metamorphic events. High Mg/(Mg+Fe) in olivine and very high Cr/(Cr+Al) in spinel indicate that the peridotites formed as highly-depleted mantle residues. The compositions are similar to those in harzburgite and dunite from some ophiolites and from fore-arc regions of subduction zones. Accordingly, the southern Vermont peridotites probably formed in a forearc, supra-subduction zone during the Early Palaeozoic. They were subsequently emplaced by obduction of the upper plate of an east-facing subduction complex.

New magnetic and seismic datasets offer a unique opportunity to document the geometry of the Appalachian front in a poorly studied segment of the orogen. Interpretation of high-resolution magnetic data allows, for the first time, the documentation of the contact between the autochthonous St Lawrence Platform and the Appalachians and highlights the regional significance of previously poorly documented ENE faults that experienced post-Ordovician strike-slip motion. Seismic data reveal tectonic slices in the foreland domain underlying the Appalachians and show that the depth of the décollement at the base of the Appalachian tectonic wedge varies significantly. Taken together, geological, magnetic and seismic data suggest that the geometry of the Appalachian front exhibits significant variations in map and cross-section views and recorded a polyphased structural history.

The anisotropy of magnetic susceptibility (AMS) is widely and routinely used to measure the preferred orientations of Fe-rich minerals in undeformed and weakly deformed granite plutons. The interpretation of the mapped AMS fabrics depends on rock-textural observations, on the map patterns of the fabrics in plutons, and on comparisons of the pluton fabrics to tectonic structures in the country rocks. The AMS may document emplacement-flow related fabrics, but the emplacement fabrics may be reworked or completely overprinted by rather weak tectonic strains of the magma mush or the cooling pluton, especially in syntectonic intrusions. The Late Devonian Canso granite pluton is an excellent example of overprinting of emplacement fabrics by weak tectonic strains. The Canso pluton was emplaced ca. 370 Ma along the boundary between the Meguma and Avalon tectonic terranes, in the northern Appalachian orogen. The AMS was mapped along two traverses that cross the pluton and that are perpendicular to the terrane boundary. Textural evidence suggests the rocks underwent very modest post-full crystallisation strains. The AMS records the dextral transcurrent shearing that occurred on the terrane boundary during emplacement and cooling of the Canso pluton, supporting interpretations that weakly deformed syntectonic granites can be used as indicators of regional bulk kinematics. AMS fabrics in Late Devonian granites of the Meguma Terrane suggest partitioning of the non-coaxial shearing into the terrane bounding fault, with pure-shear dominated deformation further from the fault. Numerical simulations suggest that the kinematics recorded by the fabrics in the Canso pluton was simple-shear, or transpression or transpression with small components of pure shear oriented perpendicular to the bounding shear zone.

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.

Ordovician metamorphic rocks of the Casco Bay Group are exposed in an approximately 170 km long NE-trending belt (Liberty-Orrington belt) in southern and south-central Maine. Geochemical analysis of rocks within the Spring Point Formation (469±3 Ma) of the Casco Bay Group indicate that it is an assemblage of metamorphosed bimodal volcanic rocks. The mafic rocks (originally basalts) have trace element and Nd isotopic characteristics consistent with derivation from a mantle source enriched by a crustal and/or subduction component. The felsic rocks (originally rhyolites and dacites) were likely generated through partial melting of continental crust in response to intrusion of the mafic magma. Relatively low initial εNd values for both the mafic (−1.3 to +0.6) and felsic (−4.1 to −3.8) rocks suggest interactions with Gander zone continental crust and support a correlation between the Casco Bay Group and the Bathurst Supergroup in the Miramichi belt of New Brunswick. This correlation suggests that elements of the Early to Middle Ordovician Tetagouche-Exploits back-arc basin can be traced well into southern Maine. A possible tectonic model for the evolution of the Casco Bay Group involves the initiation of arc volcanism in Early Ordovician time along the Gander continental margin on the eastern side of the Iapetus Ocean basin. Slab rollback and trenchward migration of arc magmatism initiated crustal thinning and rifting of the volcanic arc around 470 Ma and resulted in the eruption of the Spring Point volcanic rocks in a back-arc tectonic setting.

The Topsails igneous terrane of western Newfoundland contains several intrusive and volcanic suites underlain and separated by screens of older intrusive rocks. The heterogeneous Hungry Mountain complex yielded U-Pb zircon upper and lower intercept ages of 2090 ± 75 Ma and 467 ± 8 Ma, demonstrating a significant inherited component of Aphebian age, while an adjacent suite of relatively massive granodioritic to granitic rocks yielded a slightly discordant U-Pb zircon age of 460 ± 10 Ma. The 438 ± 8 Ma age of the Rainy Lake complex, a suite of island arc type intrusive rocks, suggests it forms part of a Silurian magmatic episode, which also included Springdale Group bimodal volcanics (429 ± 4 Ma), and peralkaline granite and subvolcanic porphyries which intrude the Springdale Group (429 ± 3 Ma and 427 ±3 Ma, respectively). Most igneous units contain a slight component of inherited zircon, but initial 87Sr/86Sr ratios (average 0·704) are similar to calculated ‘Bulk Earth’ values at this time.

Available data suggest that the Topsails terrane formed an oceanic tract with active volcanic island arcs when obduction commenced in early Ordovician time. The subsequent magmatic history, including the major but short-lived early Silurian magmatism, can be directly or indirectly related to obduction processes, including over-riding of the Topsails terrane by ophiolitic allochthons. There is no evidence of any Acadian (Devonian) igneous activity in the Topsails terrane.