DUCTILE TERRANE ASSEMBLY EXPOSED IN THE DEEP RIVER AND ESSEX QUADRANGLES, MIDDLESEX COUNTY, CONNECTICUT

Exposed in the 7.5’ by 15’ area of the Deep River and Essex quadrangles in eastern Connecticut are rocks that record the assembly of six fault-bound slivers of peri-Gondwanan crust under anatectic conditions. These include Ediacaran orthogneisses of the Avalon and Gander terranes and four terranes built on Ganderian basement: the Ordovician Bronson Hill volcanic arc (BH) and Ordovician to Devonian metasediments of the Merrimack, Putnam-Nashoba, and Central Maine terranes (MT, PNT, and CMT, respectively). New 1:24,000-scale mapping reveals that foliations within each terrane (D1) are transposed into ductile shear zones (D2) defined by mylonitic and blastomylonitic fabrics. Major terrane boundaries include the Mt. Parnassus shear zone, which divides the MT and CMT, the Bonemill Brook fault, which juxtaposes the BH against the MT and CMT, and the Honey Hill fault, across which the MT and PNT wrap the wedge of Avalon known as the Selden Neck block. Fabrics and faulted contacts are crumpled into a series of overturned folds with NE-plunging fold hinges (D3) including the 3 x 3.5 km Vincent Pond synformal sheath fold. Further shortening coincided with regional tilting to the north and produced a stack of N-dipping thrust faults (D3’). Structurally higher faults commonly include pseudotachylites, whereas contemporaneous structures in the south are marked by lineated granitic gneisses. All phases of ductile deformation produced granitic pegmatites. Older pegmatites are completely dismembered and comminuted into the dominant foliation. Younger pegmatites are typically transposed top to the south into boudins. Planar, massive, undeformed pegmatites are also prevalent. Geochronological data constrains D2 and D3 to Alleghanian orogenesis. Brittle deformation in the area is Mesozoic or younger and is subdivided into an earlier phase of NE-trending subvertical vein networks of epidote, quartz, chlorite, or zeolite and joints with bleached walls (D4) followed by a final event involving N- and NW-trending normal faults (D4’). Collectively, the deformation of these rocks documents the clockwise rotation of Laurentia and Gondwana during the assembly and breakup of Pangaea. The high-grade structures provide exceptional insight of the behavior of the lower crust under high strain and, locally, high strain rate conditions.