ACADIAN DEXTRAL TRANSPRESSION AND STRAIN PARTITIONING ALONG THE SOUTHEASTERN MARGIN OF THE PUMPKIN GROUND GNEISS, CONNECTICUT, USA

A major sheared boundary between kyanite-sillimanite grade gneisses on the west and staurolite and lower-grade rocks on the east at the southeastern margin of the Acadian high in SW Conn. formed during Acadian dextral transpression. Reaction softening localized strain along the contact between relatively strong orthogneisses and relatively weak schistose metasedimentary rocks just west of the East Derby fault zone (Rodgers, 1985). Concomitant reaction softening within the Pumpkin Ground orthogneiss produced discrete zones of high strain now marked by mylonitic orthogneiss. Dextral shearing first deformed randomly oriented, euhedral, zoned 1-7 cm long plagioclase phenocrysts present in low strain domains of the Pumpkin Ground gneiss into subhedral grains with asymmetric tails parallel to a developing foliation, then progressively into mm-diameter porphyroclasts in a schistose matrix. The resulting foliation dips ~ 85° toward 300. Microscopic features supporting dextral motion include S-C fabrics, asymmetric mineral fish, and mantled and domino-type fragmented porphyroclasts. Mineral and other lineations (mica streaks; quartz-feldspar rods, intrafolial fold axes) plunge ~30° toward 220.

To the east, in the Ordovician Trap Falls formation (Rodgers, 1985), prograde staurolite and garnet have sigma and delta type tails and asymmetric mineral fish indicative of dextral shear at amphibolite facies temperatures (~600°C). Dextrally offset pegmatite dikes at the gneiss-schist contact confirm this high-grade dextral displacement. New exposures of the contact show that the Silurian (Sevigny and Hanson, 1993) Pumpkin Ground gneiss intrudes previously schistose metasediments to the east. This suggests that rocks now defining the western New England Acadian metamorphic high may have already experienced Early Silurian metamorphism, a timing consistent with the Salinic orogeny. The crystallization of Late Devonian syntectonic metamorphic sphene (~362 Ma, Sevigny and Hanson, 1993) indicates that dextral transpression persisted into the waning stages of the Acadian. Thus this SE boundary of the Acadian metamorphic high appears to record oblique rather than orthogonal plate convergence during much of the Acadian.