Abstract

A model is presented for what may be the first recognized ancient analogue of migration of a Mendocino-style triple junction, exposed at mid-crustal levels in the eastern New England Appalachians (northeastern United States). In the model, the mid-Paleozoic subvertical crustal-scale dextral transpressive Norumbega fault system formed as a result of subduction of an oceanic ridge and associated transform fault, similar to the evolution of the modern San Andreas fault system in western North America. Ridge subduction in eastern New England started in the latest Silurian at the latitude of the Coastal Volcanic belt in eastern Maine, causing bimodal plutonism and volcanism in the area. Subduction of an associated transform fault, and the transition from convergence between the northern Rheic Ocean plate and Laurentia to dextral transpression between the southern Rheic Ocean plate and Laurentia, resulted in development of the Norumbega fault system starting in the Middle to Late Devonian. The Norumbega triple junction gradually moved ∼200 km southwestward throughout the Devonian, similar to northward migration of the Mendocino triple junction at the northern end of the San Andreas fault system over the past ∼30 m.y. The ridge subduction model explains why convergent tectonics was ongoing until the latest Devonian or earliest Carboniferous in eastern Massachusetts, analogous to motion along the modern Cascadia subduction zone (western North America), while to the north, a progressive southwestward transition from predominantly convergent tectonics to predominantly strike-slip movement along an active Norumbega fault system occurred, reflecting migration of a triple junction similar to the Mendocino triple junction.

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