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.

Location The Mascoma Dome is located in central western New Hampshire about 12 mi (20 km) east of the towns of Lebanonand Hanover (Fig. 1). Groups of outcrops at two localities aredescribed. Most of the major rock units can be seen by visitingeither locality separately, but some relationships between theunits are displayed only at one locality or the other. Both localitiescan be reached by passenger car or by charter bus. This section gives instructions for reaching each group separately. If both locality groups are to be visited on a single trip, the drive from one locality to the other can be shortened considerablyby navigating from the Mt. Cube and Mascoma 15-minutetopography maps. The starting point for reaching both localities is Exit 20 off I-89 about 1 mi (1.6 km) east of the junction of I-89with 1-91. The first locality is at the Dartmouth College Skiway at Holts Ledge in Lyme Center, New Hampshire (Mt. Cube 15-minute Quadrangle). Leave I-89 at Exit 20 and travel north on New Hampshire 12A. In West Lebanon continue northward on New Hampshire 10. On the north side of the village of Lyme, turn right on the unnumbered paved road leading eastwardthrough Lyme Center toward the Skiway. At 1.2 mi (1.9 km) eastof Lyme Center, follow the paved road that forks right to the Skiway. Park in one of the parking lots and walk to the base of the beginner's slope (the southmost trail on the west side of theroad). The trip continues on foot following

T he Errol quadrangle, New Hampshire–Maine (44° 45′–45° N. lat., 71°–71° 15′ W. long.) is underlain by approximately 27,000 feet of stratified metamorphic rocks that form the core of the Boundary Mountain anticlinorium. Approximately 22,000 feet of these rocks are probably Cambrian and Ordovician, overlain unconformably in the northwest corner of the area by a Siluro-Devonian sequence. The metamorphic rocks were intruded by a batholith of hornblende-biotite granodiorite (Taconic or Acadian) and several stocks of biotite-muscovite adamellite (Acadian). The pre-Silurian stratified rocks, in which the metamorphic grade ranges from the biotite to the sillimanite zone, are divided into three thick formations (from oldest to youngest): the Cambro-Ordovician(?) Aziscohos, the Ordovician (?) Albee, and the Ordovician(?) Dixville Formations. Most of these rocks are metasedimentary, but important metavolcanic units are present in the Dixville Formation and in the Siluro-Devonian sequence. The Aziscohos Formation is tentatively correlated with the Stowe and Ottauquechee Formations of Vermont and with the Mansonville of the Eastern Townships, Quebec. The Albee is traceable along strike to the type Albee in western New Hampshire and is correlated with the lower part of the Moretown Formation of Vermont and the lower part of the Beauceville Formation of the Eastern Townships. The Dixville is correlated with the Partridge and Ammonoosuc Formations of northwestern New Hampshire, the Cram Hill and upper Moretown Formations of Vermont, and the upper Beauceville of the Eastern Townships. The Siluro-Devonian rocks are correlated, at least in part, with the Seboomook Slate of northwestern Maine and the Eastern Townships. The stratified rocks in general trend northeast to north, but a major fold, the Diamond Peaks anticline, dominates the eastern and northeastern part of the area. It is believed to plunge south or southwest. Minor folds and other b lineations throughout most of the area plunge steeply with a strong vertical maximum and are believed to have been formed during a second stage of deformation. Because of the angular unconformity beneath the Siluro-Devonian rocks, it is inferred that broad folding occurred during the Taconic disturbance, although the major orogeny was Acadian.