AN ACADIAN-ALLEGHANIAN-MESOZOIC STRETCHING FAULT, SOUTHERN NEW ENGLAND,LOCALIZED BY RODINIAN STRUCTURAL INHERITANCE

The retrograde phyllonitic East Derby shear zone (EDsz) in southern Conn. is recognized as a part of a stretching fault that was locally active during Acadian and Alleghanian orogenesis, with brittle reactivation in the Jurassic. Stretching faults develop where shortening occurs in pure shear against rock units of contrasting strengths causing differential stretch and thus displacement along their contact (Means, 1989). The EDsz developed between feldspar-rich orthogneisses and pelitic schists along a NNE trend from Bridgeport, Conn. to the Hartford Basin. Strongly foliated and lineated orthogneiss bodies >50 km long have aspect ratios of > 10:1 in the EDsz. To the SE the flanking pelitic Wepawaug Schist is also stretched to the NNE. During the Acadian orogeny both schist and orthogneiss bodies were at amphibolite facies conditions, giving them similar strengths and both record a NNE stretch. Ages of syntectonic monazite of ~380 Ma associated with high-grade fabrics document the Devonian age of this deformation. Continued stretching under waning Alleghanian greenschist facies conditions was concentrated in the schist because the ductility contrast between the two units was stronger at these cooler conditions. The telescoping of the Wepawaug’s Acadian isograds documents thinning by >50%. Deformation was localized along the orthogneiss - schist boundary to create the EDsz, and ⁴⁰Ar/³⁹Ar ages of muscovite in multiple overprinting foliations document recrystallization between ~365 and 265 Ma. Alleghanian phyllonites are traced to the NE where they cut the Bronson Hill terrane in the Rockville area. Overlapping P-T-time paths between phyllonites on either side of the Hartford Basin confirm simultaneous development of the two phyllonite zones and the lack of measurable horizontal displacement shows little lateral translation. Rather the vertically stretched older isograds argue for vertical extrusion. Finally, the NNE trending brittle fault system through Meriden, Connecticut cuts Early Jurassic basalt ridges and produced breccias in both shear zones. The mapped deformation tracks the inferred location of the New York promontory against the former Rodinian crust showing structural inheritance along this boundary from lower to upper crustal conditions through ~ 200 m.y. of activity.