Evolution of the Bronson Hill arc and Central Maine basin, northern New Hampshire to western Maine: U-Pb zircon constraints on the timing of magmatism, sedimentation, and tectonism

Abstract

ABSTRACT The Ordovician Bronson Hill arc and Silurian–Devonian Central Maine basin are integral tectonic elements of the northern Appalachian Mountains (USA). However, understanding the evolution of, and the relationship between, these two domains has been challenging due to complex field relationships, overprinting associated with multiple phases of Paleozoic orogenesis, and a paucity of geochronologic dates. To constrain the nature of this boundary, and the tectonic evolution of the northern Appalachians, we present U-Pb zircon dates from 24 samples in the context of detailed mapping in northern New Hampshire and western Maine. Collectively, the new geochronology and mapping results constrain the timing of magmatism, sedimentation, metamorphism, and deformation. The Bronson Hill arc formed on Gondwana-derived basement and experienced prolonged magmatic activity before and after a ca. 460 Ma reversal in subduction polarity following its accretion to Laurentia in the Middle Ordovician Taconic orogeny. Local Silurian deformation between ca. 441 and 434 Ma may have been related to the last stages of the Taconic orogeny or the Late Ordovician to early Silurian Salinic orogeny. Silurian Central Maine basin units are dominated by local, arc-derived zircon grains, suggestive of a convergent margin setting. Devonian Central Maine basin units contain progressively larger proportions of older, outboard, and basement-derived zircon, associated with the onset of the collisional Early Devonian Acadian orogeny at ca. 410 Ma. Both the Early Devonian Acadian and Middle Devonian to early Carboniferous Neoacadian orogenies were associated with protracted amphibolite-facies metamorphism and magmatism, the latter potentially compatible with the hypothesized Acadian altiplano orogenic plateau. The final configuration of the Jefferson dome formed during the Carboniferous via normal faulting, possibly related to diapirism and/or ductile thinning and extrusion. We interpret the boundary between the Bronson Hill arc and the Central Maine basin to be a pre-Acadian normal fault on which dip was later reversed by dome-stage tectonism. This implies that the classic mantled gneiss domes of the Bronson Hill anticlinorium formed relatively late, during or after the Neoacadian orogeny, and that this process may have separated the once-contiguous Central Maine and Connecticut Valley basins.