BEDROCK GEOLOGY, GEOCHRONOLOGY, AND TECTONIC HISTORY OF THE BRONSON HILL BELT IN NORTHERN NEW HAMPSHIRE

Bedrock mapping of the northern half of the Berlin 7.5’ quadrangle in northern New Hampshire was completed as part of the USGS/NHGS StateMap program to understand the spatial distribution, contact relations, ages of the rocks, and their deformational history in the Bronson Hill Anticlinorium (BHA) and the adjacent Central Maine Belt (CMB). The mapping straddled the boundary between the BHA and the CMB and was supported by zircon geochronology. In the BHA, mapping revealed two plutonic units within the Ordovician Oliverian Jefferson Dome, a pink biotite monzogranite and a gray monzogranite, as well as several different variations of the Ordovician Ammonoosuc Volcanics (Oam) including a volcanic conglomerate (Oamc), felsic metatuff (Oamf), rusty granofels and gneiss (Oamr), micaceous quartzite (Oamq), and a dark gray quartz/magnetite granofels (Oamdq). The Ammonoosuc Volcanics were intruded by, then deformed with the Oliverian units. Mapping in the CMB revealed the Silurian Perry Mountain Formation (Spm). Lenses of Devonian to Carboniferous two mica granite were found in both the BHA and CMB. The first phase of deformation began with the normal Mahoosuc Fault juxtaposing the BHA and CMB prior to the Acadian and at the end of CMB basin extension. This was followed by D₁ folding in the early Acadian resulting in northeasterly trending isoclinal folds. D₂ folding followed in the mid-Devonian with open, northeast trending folds. Late Devonian doming is visible as a change in regional dip from northwest to southeast; reversing the dip of the Mahoosuc Fault. Northeast striking, near-vertical silicified zones then resulted from late-Mesozoic brittle normal (?) faults.

U-Th-Pb detrital zircon geochronology was performed yielding maximum depositional ages of 440.3 ± 7.5 Ma for the Spm (CMB) and 465.6 ± 5.3 Ma for the Oamq (BHA). The Oamf (BHA) sample yielded an igneous age of 451.1 ± 2.8 Ma. Some zircons also record evidence of Acadian and Neoacadian metamorphism.

Though the BHA was formed as an island arc in the late Taconic there is little evidence of Taconic-aged deformation or metamorphism in BHA rocks. Rather, most of the ductile deformation and metamorphism is attributed to the Acadian and Neoacadian orogenies, with minor brittle deformation during the breakup of Pangea.