Bedrock Geology, Geochronology, and Tectonic History of the Bronson Hill Anticlinorium in Northern New Hampshire

Liza A. Folsom, Bates College


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 and the greater Appalachians. The mapping straddles the boundary between the Bronson Hill Anticlinorium and the Central Maine Belt and was supported by complementary detrital and igneous zircon geochronology to determine the sequence of events in the study area. In the Bronson Hill Anticlinorium section, the mapping revealed three plutonic rock types within the Ordovician Oliverian Jefferson Batholith, a pink biotite monzogranite, a gray monzogranite, and a gray monzogranite with Ammonoosuc Volcanic intrusions as well as several different variations of the Ordovician Ammonoosuc Volcanics including amphibolite (Oam), 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. Across the Mahoosuc Fault separating the Bronson Hill Anticlinorium and Central Maine Belt is the Silurian Perry Mountain Formation (Spm). Additionally, small lenses of Devonian to Carboniferous two mica granite (CDtmg) were found in both the Bronson Hill Anticlinorium and Central Maine Belt. The first phase of deformation began with the normal Mahoosuc Fault juxtaposing the Bronson Hill Anticlinorium and Central Maine Belt as the Acadian orogenic front migrated northwest. This was followed by D1 folding in the Early Acadian orogeny that consisted of northeasterly trending isoclinal folds of S0 bedding and layering with strong S1 foliations and more uncommon L1 lineations found in both the Bronson Hill Anticlinorium and Central Maine Belt. D2 folding followed in the Mid-Devonian Late Acadian orogeny, characterized by open, again northeast-trending folds with a weak or nonexistent S1 axial planar cleavage that deformed the S1 and S0 fabrics in both the Bronson Hill Anticlinorium and Central Maine Belt. Doming occurred in the Late Devonian Neoacadian orogeny and is visible as a change in S1 and S0 regional dip from northwest to southeast effectively 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 on two Bronson Hill Anticlinorium volcaniclastic samples, Oamq and Oamdq, while igneous zircon geochronology was done on one Bronson Hill Anticlinorium felsic meta-tuff, Oamf. The Oamq detrital zircon sample yielded a maximum depositional age of 465 ± 6 Ma and was characterized by Ordovician aged arc sources. A maximum depositional age was unable to be identified for sample Oamdq. However, the analysis provided background on the terrane fingerprints of the sample suggesting it is dominated by older basement sources. The igneous sample, Oamf, yielded an age of 452 ± 8 Ma.