IDENTIFICATION OF MULTIPLE STAGES OF DEFORMATION IN THE DEEP RIVER QUADRANGLE, CONNECTICUT USA

New, STATEMAP-funded bedrock mapping in the Deep River quadrangle, Connecticut, has led to the identification of new ductile faults within and among the four distinct tectonic terranes exposed in the study area. Our results corroborate existing interpretations regarding the occurrence of the Putnam-Nashoba and Avalon terranes. However, we recognize a more complex deformational history in the upper amphibolite facies and locally anatectic rocks of the Bronson Hill and Merrimack terranes than previously reported.

The Hebron Complex (Shc) of the Merrimack terrane contains gritty biotite schist with interbedded calcareous granofels. Early deformation of the Shc produced a penetrative, subhorizontal, axial planar foliation and transposed the calcareous layers into isoclinal, NE-verging folds with subhorizontal, NW-trending fold axes. The base of the Shc is truncated by the shallowly N-dipping Honey Hill fault zone, below which lie tectonic slivers of the Tatnic Hill Fm and Avalon basement. The structural top of the Shc is separated from the overlying Brimfield Schist (also of the Merrimack terrane) at its northeastern margin by a previously unrecognized ~350m thick zone of massive to mylonitic pegmatites and foliated cataclasites. To the west, the Shc is divided from the Bronson Hill arc by the subvertical, N-trending Cremation Hill fault zone (CHFZ). Within ~1.5km of the CHFZ, the early foliation in the Shc is deformed by asymmetrically overturned folds with gently E-dipping long limbs, finer-grained, subvertical short limbs, and subhorizontal N-trending fold axes. These structures suggest top to the west displacement. The transposition of the fold axes of folded beds into the shear zone additionally suggest a strong dextral shear component.

West of the CHFZ, the gneisses of the Bronson Hill arc contain a NE-dipping foliation. Included metabasaltic dikes are folded with NE-dipping axial planes and NE-plunging fold axes. In contrast to the rocks of the Shc, neither the old foliation of the Bronson Hill arc rocks nor the fold axes of folded amphibolite dikes are transposed against the CHSZ. These observations suggest the Bronson Hill arc served as a tectonic buttress against which the weaker Shc was first crumpled and then deflected during Alleghanian orogenesis.