STRAIN INDUCED RETROGRADE METAMORPHISM IN AN ALLEGHANY FAULT IN SOUTH CENTRAL CONNECTICUT

New field studies of the East Derby Shear Zone (EDSZ) of Rodgers (1985) confirm the presence of a major zone of retrogression and overprinting fabrics at the western margin of the Orange-Milford Belt (OMB) in south-central Connecticut. Mapping along three transects across the fault zone shows a ~500 meter wide belt of chlorite/muscovite (lower greenschist facies) phyllites interior to amphibolite facies metamorphic rocks. Fabrics trending 015 in the eastern staurolite/kyanite bearing Wepawaug Schist are transposed and retrogressed to ~030 in the EDSZ. Progressive partial and then complete replacement of porphyroblasts of biotite, garnet and staurolite by chlorite and muscovite follows this transposition. The west side of the EDSZ is marked by greenschist facies mylonitic Wepawaug schist against the stronger Silurian Pumpkin Ground granodioritic gneiss, except along intervening slivers of high grade Wepawaug Schist where the eastern fabric transposition is mirrored. Existing geochronology (Lanzirotti and Hanson, 1996; Growdon et al. 2013) shows that the Middle Devonian metamorphism of the Wepawaug Schist with muscovite cooling ages >360 Ma are replaced by greenschist facies muscovite with crystallization ages < 300 Ma.

Kinematic indicators along the EDSZ such as S-C fabrics, asymmetric quartz and chlorite beards, and transposed quartz veins show dextral displacement. The relatively stronger Pumpkin Ground orthogneiss, which was on the edge of the Laurentian plate, acted as a buttress during lower green-schist facies Permian faulting. The EDSZ trends northeast beneath Triassic sediments of the Hartford basin, and may reemerge as the Westminster West Fault (WWF) (McWilliams et al. 2010; Walsh et al. 2012) in western New Hampshire. Together these results support the escape tectonics model proposed by Growdon et al. (2013).