TECTONIC EVOLUTION OF THE EASTERN BORDER FAULT FOOTWALL, DURHAM QUADRANGLE, CT

The Eastern Border fault (EBf) of the Hartford Basin is known for syn-depositional Mesozoic normal motion, but new fieldwork shows evidence of Paleozoic thrust motion. The Durham quadrangle is halved by the NE-SW trending EBf. The western hanging wall contains Mesozoic non-marine sedimentary rocks and flow basalts. The eastern footwall contains metamorphic rocks of the Killingworth Dome, at the southern end of the Bronson Hill Terrane. The footwall units of the Rodgers (1985) Durham quadrangle, based on prior mapping for an unpublished open-file map by Jelle de Boer, identify the most proximal unit to the EBf as the Ordovician Collins Hill formation, composed of rocks interpreted to have sedimentary (Och) and volcanic (Ochv) protoliths.

We report here on new mapping designed to build on de Boer’s (1964) mapping. Our work has begun in Miller’s Pond State Park, offering a NW-SE transect across-strike within 1 km of the EBf. The lithology within the park includes several rock types. Plagioclase gneisses and granofels dominate locally, with interwoven schists containing variable muscovite/biotite ratios. Pegmatite is pervasive. The dominant foliation is ~210 35NW. Our interpretation proposes that this foliation is not preserved bedding as formerly interpreted but is more likely transposed foliation developed within a meta-igneous protolith. Fieldwork around Miller’s Pond documents evidence supporting upper amphibolite facies Alleghanian ductile thrusting. This includes amphibole lineations plunging ~300 and boudin long-axes plunging ~310, suggesting high strain ~100 m.y. earlier than Triassic exhumation. Fold vergence in the park corroborates a top-to-the-southeast shear sense, consistent with Alleghanian thrust motion within the EBf zone. Previously unrecognized lower amphibolite facies mylonites, possibly developed from recrystallized magmatic rocks part way through exhumation, provide further evidence that these rocks are not the water-laid pyroclastic rocks they were once-thought. Late quartz veins and brittle faults and joints finally record lower greenschist facies Mesozoic deformation. Collectively these structures record a progressive reversal of displacement from thrust to normal motion, from deep to shallow conditions, over a 100 million year history of movement.