THE NASHOBA TERRANE: A NEW TECTONOSTRATIGRAPHY AND SHARED STRUCTURAL STYLES WITH THE MERRIMACK BELT IN MASSACHUSETTS

Recent STATEMAP-sponsored geologic mapping of several 7.5' quadrangles in east-central Massachusetts describe a new tectonostratigraphy and structural history for migmatitic gneisses of the Cambro-Ordivician arc complex represented, in part, by the Nashoba Formation. While preserved sedimentary structures are absent, four discrete subunits can be mapped based on distinct lithologies. From structurally lowest to highest, they are: A felsic magnetite-bearing bt gneiss, a magnetite and silliminite rich mu-bearing bt paragneiss, gt-bearing sulfidic bt-gneisses interlayered with sulfidic schist, amphibolite, and marble, and a calc-silicate rich bt-gneiss.

The Nashoba terrane exhibits near-identical early and late styles of deformation to those in the adjacent Nashua sub-belt of the Merrimack terrane: Early amphibolite-facies isoclinal folding (D1) overprinted by tight upright folding (D2) and associated thrust faults define the map-scale geometry of tectonostratigraphic units between the two terranes. D2 occurred syn-peak upper amphibolite facies metamorphism in the Nashoba terrane while defined by retrograde greenschist facies fabrics in the Merrimack terrane. These are overprinted by outcrop-scale sinistral strike-slip motion (D3) in the Nashoba terrane progressively transitioning to late chlorite-grade NW-side down oblique extension (D4) exhibited in both terranes.

Correlation of the above sequence with existing geochronology, structural petrology, and tectonic studies by other workers in the area suggest that D1 in both terranes occurred prior to and/or during ~370 Ma Acadian orogenesis. Existing geochronology shows D2 fold and fabric development to be diachronous both within and across terranes: occurring both before and after ~363 Ma, with progressive development possibly as late as ~325 Ma, in the Nashoba terrane. D2 in the Nashua sub-belt of the Merrimack terrane occurs after ~330 Ma in the Carboniferous, as late at ~293 Ma in the Permian, during Alleghenian orogenesis. NW-side-down extension and associated rotation of M1 metamorphic isograds in the Merrimack Terrane, and exhumation of the Nashoba terrane, are generally accepted to have occurred during later stages of Alleghenian orogenesis in the Permian.