ALLEGHANIAN METAMORPHIC OVERPRINT OF ACADIAN ROCKS IN SOUTHERN NEW ENGLAND: EVIDENCE FROM STRUCTURE, PETROLOGY AND THERMOCHRONOLOGY

New structural and lithologic data support interpretations from 40Ar/39Ar thermochronology that the Alleghanian overprint of southern New England was caused by thrusting of structurally higher nappes to the S and E. Mapping around the Rockville quadrangle (Connecticut) shows that mylonitic fault zones separate domains in the Bronson Hill terrane with lineations gently plunging either toward 330° or 290°. New U-Pb dating of sphene associated with these penetrative fabrics demonstrates Alleghanian crystallization, while the vergence of these structures show a top to the S and E motion sense. This reflects a SE transport of Bronson Hill rocks over Central Maine and Merrimack rocks northwest of the Willimantic dome. New mapping in the Hopyard basin near Colchester, CT supports Lundgren's (1962) correlation of these rocks with the Bronson Hill terrane rocks. Structural analysis shows that these schists and gneisses have a mylonitic fabric with a penetrative lineation gently plunging 330°, verging SSE. Discordance of foliation attitudes across the Bronson Hill-Central Maine contact confirm a fault contact, define theses rocks as a klippe, and show Bronson Hill rocks here also moved toward ~150° over Central Maine terrane rocks.

The identification of these rocks and fabrics in these widely spaced areas, as well as kinematic indicators in the Merrimack terrane foot wall that confirm S and E vergence of hanging wall rocks leaves little doubt that the Alleghanian event involved thrusting of nappes of structurally higher western terranes to the SE over lower terranes.