TECTONIC EVOLUTION OF THE CONNECTICUT VALLEY REGION IN SOUTH-CENTRAL VERMONT AND WEST-CENTRAL NEW HAMPSHIRE

New 1:24,000-scale geologic mapping and supporting research in the Connecticut Valley from Lebanon, NH to Bellows Falls, VT provides new evidence for the tectonic evolution of the valley and demonstrates that it is the locus of complex faulting throughout its Phanerozoic history.

Metamorphosed Mesoproterozoic through Devonian rocks, locally intruded by Cretaceous rocks, underlie much of the valley. The Cambro-Ordovician Moretown slice overlies 1.3 Ga gneiss of the Chester dome along the Keyes Mountain fault, which represents the Ordovician suture (Red Indian Line) between Laurentia and Ganderia. Silurian and Devonian rocks of the Connecticut Valley trough (CVT) overlie the Moretown slice. The deposition of sediments in the CVT occurred above the Silurian unconformity from ~424-407 Ma.

In the Bronson Hill arc (BHa), the lower part of the Ordovician Ammonoosuc Volcanics (Oa) is intruded by the 473 ± 5 Ma Plainfield tonalite. Volcanics in the upper part of the Oa yield ages of 460 ± 2 and 457 ± 5 Ma. The age of the Partridge Fm. is constrained by a maximum depositional age (MDA) from detrital zircons at 452 Ma and a volcanic rock dated at 450 ± 5 Ma. Eleven samples from the Oliverian Plutonic Suite range in age from 473-440 Ma. Silurian zircons (~422 Ma) in the Rangeley Fm. and a population of zircons (~408 Ma) from the Littleton Fm. constrain their MDA. Five new SHRIMP U-Pb zircon ages from the New Hampshire Plutonic Suite range in age from 400-360 Ma and constrain the age of D1 and D2.

Multiple generations of faults truncate Devonian Acadian F1 fold nappes. The BHa structurally overlies the CVT along the Acadian Monroe fault with preserved tectonic mélange in the footwall. Upright dome-stage F2 folds post-date amphibolite facies metamorphism and locally developed into sheath folds in high-strain zones. F3 folds exhibit sinistral rotation associated with lower greenschist facies faults. Major Late Paleozoic Alleghanian to Mesozoic shear zones transpose stratigraphy, early structures, and peak metamorphic isograds. ⁴⁰Ar/³⁹Ar white mica growth ages (300-250 Ma) indicate that retrograde deformation continued into the latest Paleozoic and earliest Mesozoic. Apatite fission track data show that brittle faults were active prior to about 100 Ma, and experienced Late Cretaceous and even Paleocene re-activation.