GSA Connects 2021 in Portland, Oregon

Paper No. 195-14
Presentation Time: 2:30 PM-6:30 PM


FLEISCHER, Noah, Geology and Geological Engineering, Colorado School of Mines, 1516 Illinois Street, Golden, CO 80401 and KUIPER, Yvette D., Department of Geology and Geological Engineering, Colorado School of Mines, 1516 Illinois Street, Golden, CO 80401

The Honey Hill Fault (HHF) is a north-dipping ductile mylonite zone located in southeastern Connecticut. The fault separates rocks of the Ganderian Putnam terrane in the northeast and overlying metasedimentary rocks of the Merrimack belt in the northwest from the Avalon terrane to the south. Geologic mapping was carried out for three months during summer of 2021 as part of a USGS EDMAP project in order to understand the structural history of the HHF and adjacent domains. Additional funding for accommodations was provided by GSA as part of the Graduate Student Research Grant program. Within the field area, the Putnam terrane is composed of Cambrian-Silurian rocks that include gabbro, gneiss, schist, amphibolite, granofels, pegmatite and marble. The Merrimack belt is composed of Silurian to Early Devonian rocks that include gneiss, schist, granofels and metamorphosed calcareous turbidite. The Avalon terrane is composed of Neoproterozoic to Devonian rocks that include granite, gneiss, schist, pegmatite and quartzite. The HHF zone shows mylonitic and cataclastic rocks. Along the HHF, S-C fabrics and north-verging folds indicate north-side-down movement. The foliation in the Putnam terrane generally dips shallowly to moderately to the west-northwest and in the Avalon terrane moderately to the north. Fold hinge lines and mineral lineations in both the Avalon and Putnam terranes generally plunge to the northwest, except in the southeast portion of the field area in the Avalon terrane, where they plunge to the north or northeast. Structures in the Putnam terrane are primarily a result of late Silurian accretion of the Avalon terrane during the Acadian orogeny, while those in the Avalon terrane are primarily a result of the Alleghanian orogeny (formation of Pangea), based on prior work by others. The difference between northwest-plunging lineations and fold hinge lines in the Avalon terrane in the western part of the field area and north-northeast plunging ones in the southeast may be related to local dome-shaped structures within the Avalon terrane to the south. The normal movement along the HHF most likely occurred when Pangaea rifted apart during the Mesozoic, as this is the latest known movement that is consistent with normal movement.