THE EAST DERBY FAULT: EVIDENCE FOR LATE PALEOZOIC DEXTRAL MOTION WITHIN THE CONNECTICUT VALLEY SYNCLINORIUM

Preliminary work shows that the East Derby fault zone (EDFZ) is broader than indicated by Rodgers (1985), and has a protracted late Palezoic history. This northeast trending fault cuts kyanite grade rocks and juxtaposes the Orange-Milford Belt, on the east, against Connecticut Valley Synclinorium (CVS) rocks, on the west. The core of the fault zone is defined by greenschist facies phyllites and phyllonites, but our investigations show the zone to be much wider. In fact, stratigraphic units mapped between Rodgers' fault zone and the porphyritic Harrison Gneiss are orthoschists derived from, and cutting, this gneiss. Zoned, 1-5cm long, K-feldspars with Carlsbad twinning show development of marginal microcline twinning, significant grain size reduction and local to near complete replacement by myrmekite. Magmatic biotite recrystallizes into biotite-rich folia separated by quartz and plagioclase domains and ribbons. Albite twinned plagioclase recrystallizes to finer grained, zoned and untwinned metamorphic plagioclase. Closer to the mapped EDFZ, K-feldspar grain-size reduction becomes more complete and the developing biotite schistocity is joined by equal proportions of muscovite, forming centimeter long folia; muscovite fish and kinked grains become common. Finally biotite nearly disappears and is replaced by muscovite with minor chlorite. These textures show continued grain size reduction and transition from biotite schist to chlorite phyllite. Thus deformation occurred during waning metamorphic conditions from middle amphibolite to lower greenschist facies.

Regional geochronology (Lanzirotti, 1996; Moecher et. al., 1997) defines the metamorphic cooling history. Metamorphism peaked in the middle Devonian Acadian Orogeny. Cooling through the Argon closure temperature of amphibole (~500°C) and muscovite (~350°C) occurred ~ 340 and 300 Ma respectively, showing that the margin of the Harrison Gneiss was being strongly deformed throughout the Carboniferous. Kinematic indicators in biotite-rich orthoschists and greenschist facies phyllites suggest right lateral motion persisted during most of this retrogression and very possibly into the Permian. Thus the EDFZ may appreciably translate the Orange-Milford Belt southwestward relative to other CVS rocks to the west.