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Earth and Environmental Sciences pres.

Smith Lecture: Origin and Evolution of the Nashoba Terrane in Eastern Massachusetts, and Connections with the Norumbega Fault Zone in Maine: Potential for a Late Devonian Triple Junction

Yvette D. Kuiper, Department of Geology and Geological Engineering, Colorado School of Mines

The Nashoba terrane in eastern Massachusetts is located between the Avalon terrane to the SE and Ganderian terranes to the NW. It consists of Cambrian-Ordovician mafic to felsic metavolcanic and interlayered metasedimentary rocks, intruded by a series of Silurian to earliest Carboniferous dioritic to granitic plutons. Trace element geochemistry and Sm/Nd results of the oldest volcanic rocks indicate an early Paleozoic arc/back-arc origin of the terrane. The mafic volcanic rocks had a primitive source. The oldest intermediate and felsic rocks incorporated older, isotopically evolved crustal material, indicative of a Ganderian basement below the terrane. Based on negative εNd values, model ages and detrital zircon ages, metasedimentary rocks were derived from mixed, generally evolved, sources with Archean though Ordovician input, indicating a Ganderian source.
The Nashoba terrane is interpreted as a Ganderian arc-backarc complex similar to the Penobscot and/or Popelogan-Victoria arc systems and the Tetagouche-Exploits backarc basin on the leading (NW) margin of Ganderia in the northern Appalachians. The Nashoba terrane, however, is now located on the trailing (SE) margin of Ganderia and must have been there since ~430 Ma, when subduction-related plutons intruded the terrane and deposition of the ~430-407 Ma Merrimack terrane sediments to the NW took place

Major and trace element geochemical and Sm/Nd data of ~430-350 Ma dioritic and granitic plutons in the Nashoba terrane indicate subduction-related convergence. Significantly younger model ages in the youngest granitic rocks may reflect Avalonian input after 350 Ma. Acadian and Neo- or post-Acadian metamorphism occurred between ~425 Ma and ~360 Ma, related to convergence between the Nashoba, Merrimack and Avalon terranes and possibly subsequent exhumation of the Nashoba terrane.
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, but no obvious equivalent fault zone has been found in eastern Massachusetts. 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, and the Norumbega fault zone may have formed as result of subduction of a mid ocean ridge, similar to the San Andreas Fault.

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.

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