INSIGHTS INTO MID-CRUSTAL LEVELS OF A LATE DEVONIAN TRIPLE JUNCTION: INTERACTION BETWEEN THE DEXTRAL NORUMBEGA FAULT AND THE PARTIALLY MELTED NASHOBA TERRANE IN SOUTHEASTERN NEW ENGLAND

The Norumbega fault system in New England is a Paleozoic northeast-trending orogen-parallel, subvertical, predominantly dextral transpressive fault system. It was active at least as early as in the Middle Devonian, during the Acadian orogeny. It consists of one major shear zone and numerous associated localized mylonite zones. It extends from southwestern New Brunswick to southern Maine and New Hampshire, and possibly farther southeast. However, its exact location in eastern Massachusetts not fully understood. While potential connections with various shear and fault zones have been made, no steep predominantly dextral transpressive fault or fault zone has been recognized in eastern Massachusetts. Instead, at the location of the projected Norumbega fault, the northwesterly-dipping Nashoba terrane exists, which was partially melted in the Middle to Late Devonian as a result of the Acadian orogeny. Northeastern Massachusetts thus represents a Middle to Late Devonian transition zone between transcurrent movement, similar to the San Andreas Fault, to the northeast, and a northwesterly directed subduction zone, similar to the Cascadia subduction zone, resulting in the accretion of Avalon, to the south. If the analogue is correct, northeastern Massachusetts may represent a mid-crustal exposure of a Late Devonian triple junction.

In our model, Late Devonian southeastern New England is an equivalent to the San Andreas Fault-Cascades system at mid-crustal levels, but rotated by ~180 degrees, and ~360 My earlier. The Laurentian/Ganderian, Rheic and ‘Avalonian’ plates are equivalents of the North American, Pacific and San Juan plates. In eastern Massachusetts, part of Avalon remained behind on a microplate attached to the spreading Rheic ocean, while to the north the spreading ridge was subducted and the Norumbega transform fault created. The Norumbega fault zone may have been largely deflected into the ocean in or north of northeastern Massachusetts, similar to the deflection of the San Andreas Fault towards the north into the Mendocino fracture zone in the ocean. Some evidence for minor dextral offset exists across the Nashoba terrane, suggesting that splays of the Norumbega fault may have continued across the terrane at mid-crustal levels, perhaps into Avalon, along the eastern side of the Narragansett basin.