DUCTILE TO BRITTLE FAULTING AT THE WESTERN END OF THE HONEY HILL FAULT ZONE, GILLETTE CASTLE STATE PARK, CT

In 2005 Jelle De Boer and I led a field trip to the western end of the Honey Hill fault for the New England Intercollegiate Geologic Conference (NEIGC). The stops described in this guide were favorites of Jelle’s, which he visited on may field trips with Wesleyan students and community members. Over the 15 years since this trip these stops have become favorites of mine as well. In this presentation I will provide a virtual field trip to Gillette Castle State Park discussing spectacular ductile to brittle fault structures exposed there and emphasizing their structural significance and educational value. The work includes new mapping by Wesleyan students including construction of 3D virtual outcrop models and large-scale mapping of the park as part of a 2020 geologic mapping course.

The Honey Hill fault has been recognized as a major north-dipping regional structure since it was first mapped by Lundgren (1950’s to 60’s). It is presently understood as a terrane bounding fault that juxtaposes Gander cover in its hanging wall (exposed to the north) against Avalon basement in its footwall (exposed to the south). In the vicinity of Gillette Castle, the fault contact is covered by Quaternary deposits, but hanging wall rocks are spectacularly exposed in a series of southwest-facing cliff bands rising above the Connecticut River. The fault contact itself was intercepted by a borehole 221 m below the modern river floodplain. Bedrock within the park consists of Late Silurian Hebron Formation intruded by Permian(?) pegmatites. The predominant foliation has a northeast strike (~200°) and northwest dip (~15°). Conspicuous 1-10 cm thickness foliation-parallel compositional layering in the Hebron Fm. is defined by purple-brown biotite and greenish amphibole rich layers. Multiple generations of pegmatites cut the foliation and are variably transposed into near parallelism with it. Just above the floodplain is a pseudotachylyte-bearing fault zone. Melt-generating slip surfaces parallel the regional foliation while linking structures and injection veins cut it. The geometry of linking structures indicates that this latest brittle slip was top to the north or normal-sense. This new mapping confirms previous regional work indicating a protracted slip history including normal and reverse sense motion within the Honey Hill fault zone.