EDMAPS/NEWMAPS IN SOUTHEASTERN NEW HAMPSHIRE

EDMAP has supported UNH Earth Sciences graduate and undergraduate geologists mapping bedrock underlying nearly the equivalent of seven 7.5-minute quadrangles. Emphasis has concentrated on the metasedimentary, migmatitic, and igneous rocks within the three tectonostratigraphic zones that comprise southeastern New Hampshire: The coastal Rye Complex (RC), the Merrimack Trough (MT), and the northwest-bounding Massabesic Gneiss Complex (MGC). Several regionally significant faults have been refined and extended, and two new faults are defined, all recording at least one early ductile followed by later brittle deformational events.

The RC is an 8km by 20km variably mylonitized band of metasedimentary and meta-igneous rocks of probable early Paleozoic age. Ultrahigh strain zones subdivide and bound this belt. East of the Great Commons fault zone (GCFZ), and prior to brittle movement on this fault, low pressure facies series metamorphism reached sillimanite + muscovite grade. Between the GCFZ and the west-bounding Portsmouth fault zone (PFZ) felsic and mafic mylonites dominate. Latest movement is Alleghanian.

West of the RC and in sharp tectonic contact lies the generally lower grade, multiply deformed Siluro-Ordovician metasedimentary rocks of the Merrimack Trough crosscut by Devonian and younger plutons. Regional metamorphism is greenschist facies adjacent to the PFZ and increases westward into amphibolite facies (reaching sillimanite + muscovite grade). Late brittle movement on the dextral NE-trending phyllonitic Nannie Island and Calef fault zones, and on the Flint Hill and related faults, variably offset metamorphic isograds from Massachusetts to Maine.

Past interpretations involving a gradational contact between metasedimentary rocks of the MT and the enigmatic, multiply migmatized (?) Massabesic Gneiss Complex are much revised with the recognition of lower Silurian to Devonian (?) metasedimentary rocks of the Central Maine Terrane on both sides of the MGC anticlinorium. We view an extended history of accretion, metamorphism(s), folding and faulting, refolding and intrusion, continued shear involving early ductile and later brittle faulting as a punctuated continuum from late Silurian through early Permian, followed by Mesozoic extension and related magmatic activity.