The Bronson Hill Arc is a bimodal volcanic arc and associated sediments that formed over a west (?) dipping

subduction zone Subduction is a geological process in which the oceanic lithosphere is recycled into the Earth's mantle at convergent boundaries. Where the oceanic lithosphere of a tectonic plate converges with the less dense lithosphere of a second plate, the ...

during the Ordovician period (c. 475 - 450 million years ago (Ma)) as part of the

Taconic Orogeny The Taconic orogeny was a mountain building period that ended 440 million years ago and affected most of modern-day New England. A great mountain chain formed from eastern Canada down through what is now the Piedmont of the East coast of the Unit ...

. These rocks are presently well exposed along the Connecticut River Valley of

Vermont Vermont () is a state in the northeast New England region of the United States. Vermont is bordered by the states of Massachusetts to the south, New Hampshire to the east, and New York to the west, and the Canadian province of Quebec to ...

and

New Hampshire New Hampshire is a U.S. state, state in the New England region of the northeastern United States. It is bordered by Massachusetts to the south, Vermont to the west, Maine and the Gulf of Maine to the east, and the Canadian province of Quebec t ...

. The arc is evidenced by

plutonism Plutonism is the geologic theory that the igneous rocks forming the Earth originated from intrusive magmatic activity, with a continuing gradual process of weathering and erosion wearing away rocks, which were then deposited on the sea bed, re- ...

and extrusive volcanicsm, including the

Ammonoosuc Volcanics The Ammonoosuc Volcanics is a rock unit in parts of New Hampshire and Vermont in the United States. This unit is named for the Ammonoosuc River that runs through the portion of New Hampshire that houses the Ammonoosuc Volcanics. Setting The Am ...

(c. 461 Ma from U/Pb

zircon Zircon () is a mineral belonging to the group of nesosilicates and is a source of the metal zirconium. Its chemical name is zirconium(IV) silicate, and its corresponding chemical formula is Zr SiO4. An empirical formula showing some of the r ...

dates) and the overlying Partridge Formation (c. 457 Ma from graptolites in the formation). It is related to the slightly older Shelburne Falls arc that sits to the west. These rocks were metamorphosed and deformed during the Acadian Orogeny and the Alleghenian Orogeny.

References

*Robert H. Moench and John N. Aleinikoff, "Stratigraphy, Geochronology, and Accretionary Terrane Settings of Two Bronson Hill Arc Sequences, Northern New England", ''Physics and Chemistry of the Earth, Parts A/B/C'', Volume 27, Issues 1–3, January 2002, pages 47–95. Geology of New Hampshire Geology of Vermont Terranes {{US-geology-stub