STRUCTURAL MAP ACROSS THE NASHOBA TERRANE, EASTERN MASSACHUSETTS, SCALE 1:300,000

The Nashoba terrane (NT) is a Ganderian early Paleozoic arc/back-arc complex in the southeastern New England Appalachians. It was multiply deformed and metamorphosed under upper amphibolite facies conditions during the latest Silurian to Late Devonian Acadian orogeny, associated with the accretion of the Avalon terrane to the SE. Mapping of structures across the terrane was completed to better understand the deformation history of the terrane, and the enigmatic juxtaposition of the NT against greenschist facies rocks of the Merrimack belt to the NW and Avalon terrane to the SE. The NW side of the NT preserves sillimanite-bearing metapelitic, granitic and gneissic rocks with localized partial melt, and local amphibolitic and calc-silicate gneissic rocks. The SE side preserves mafic metavolcanic, granitic, gneissic and locally sillimanite-bearing partially melted metapelitic and calc-silicate gneissic rocks.

Five deformation events affected the NT. Moderately NW- and SE-dipping to subvertical foliation is present throughout the NT and preserves isoclinal folds. These are refolded by ~410-360 Ma NW-side-down asymmetric folds across the terrane. SE-side-down folds are present along the SE boundary of the NT. Localized top-to-the-SE asymmetric folds are preserved along shallowly to moderately NW-dipping foliation within the NT. Sinistral ~405-340 Ma shear zones within and along the boundaries of the NT are syn- to post- NW-side-down movement. Subvertical, NW- and SE-dipping mylonitic and brittle-ductile shear zones are also preserved throughout the NT, and post-date asymmetric folds. En echelon veins are locally present along the SE NT, are variable in orientation, and crosscut all deformation events.

Asymmetric folds that preserve mainly NW-side-down movement occurred during the Acadian orogeny. Sinistral movement occurred contemporaneous to NW-side-down movement, but continued after conclusion of the Acadian orogeny. These events may have caused the exhumation and juxtaposition of the NT against the greenschist facies rocks of the adjacent terranes. The ages of the younger deformation events are unknown, but may be related to deformation associated with the end of the Acadian orogeny, or deformation during or after the Alleghanian orogeny.