External Basement Massifs in the Northern Appalachians: The Link Between Rodinia and Pangea

External basement massifs in the Northern Appalachians expose Mesoproterozoic rocks and mark the eastern edge of Laurentia. They are important because they record the Neoproterozoic rifting of Rodinia and the Paleozoic collisions of arcs and continents that eventually formed Pangea. Some distinctive units, such as 1.35 to 1.31 Ga arc-related tonalites in the Green Mountain massif of Vermont, and 1.17 Ga granites in the Berkshire massif in Massachusetts establish correlation of the massifs with coeval rocks in the Adirondacks. However, some younger units, such as a suite of 1.00 Ga syn-tectonic anatectic sills in the Berkshire massif, and 960 Ma post-orogenic granites in the Green Mountain massif, have no counterparts in the Adirondacks, and suggest that the Grenville orogenic cycle, and relaxation of perturbed isotherms, persisted somewhat longer in the external massifs.

The external massifs also mark the boundary between the Neoproterozoic-Early Paleozoic shelf sequence to the west and slope-rise sequence to the east. Locally, basal clastics that record the rift-drift transition unconformably overlie basement rocks in the massifs. The massifs probably formed as rifted blocks above low-angle normal faults, and the normal faults were later reactivated as thrusts during Paleozoic collisions. More outboard rifted blocks formed ribbon continents separated from Laurentia by narrow seaways.

The Berkshire massif was emplaced during the Silurian Salinic or Devonian Acadian orogeny as a rigid intracrustal wedge. The leading edge of the massif-wedge is commonly an overturned fold, and it closely followed the boundary between the Taconic thrust sheets and the carbonate shelf rocks. Wedging was favored by the mechanical contrast between the rigid basement gneisses and clastic rocks of the massif compared to mica-rich schists of the Taconic and the carbonate rocks of the shelf sequence.