Structural history of the Honey Hill fault and adjacent terranes in southeastern Connecticut, USA

Fleischer, Noah

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Structural history of the Honey Hill fault and adjacent terranes in southeastern Connecticut, USA

Fleischer, Noah

Advisor

Kuiper, Yvette D.

Date Issued

2022

Abstract

The Honey Hill fault is a northerly-dipping ductile mylonite zone located in southeastern Connecticut. The fault separates rocks of the Ganderian Putnam terrane in the NE and overlying metasedimentary rocks of the Merrimack belt in the NW from the Avalon terrane to the S, where Ganderia and the Avalon terrane are Gondwanan-derived terranes. Bedrock geologic mapping at a 1:50,000 scale across the WNW-striking segment of the Honey Hill fault and adjacent terranes, structural analysis of the map area and in situ U-Pb laser-ablation inductively coupled plasma- mass spectrometry (LA-ICPMS) monazite analysis of samples from the fault were carried out as part of a USGS EDMAP-funded project to investigate the structural history, motion sequences and timing of metamorphism along the Honey Hill fault and in adjacent domains. The goal of this research was to understand the tectonic history of the fault and to interpret its role in the formation and breakup of Pangaea. Within the study area, the Putnam terrane is composed of Cambrian to earliest Devonian gabbro, gneiss, schist and diorite, and Permian mylonite. The Merrimack belt is composed of Late Ordovician to Early Devonian gneiss and schist. The Avalon terrane is composed of Neoproterozoic to Permian granite, gneiss, amphibolite, schist, quartzite, and pegmatite, and Triassic silicified rock/quartz vein. The Honey Hill fault shows mylonitic and cataclastic rocks formed by significant deformation of units adjacent to the fault. These rocks, and the various structures and shear sense indicators within them, reveal at least three regional deformation events along the Honey Hill fault and in the adjacent terranes. The structures observed in the study area were formed during these events. The Putnam terrane (and Merrimack belt?) were significantly deformed (pre-D1 deformation) prior to the main deformation along the Honey Hill fault. D1 deformation occurred primarily along the Honey Hill fault and in the Avalon terrane with reverse, reverse-sinistral or sinistral-reverse motion, because of ~NE-directed motion of the Avalon terrane. D2 deformation occurred along the Honey Hill fault and in the adjacent terranes, and involved normal, normal-sinistral, or sinistral-normal movement, due to ~SE-directed extension between the Avalon terrane and the rocks to the N and W. Normal movement began at temperatures at or above the greenschist facies zone (3300- 500°C), as indicated by mineral assemblages along shear fabrics, and ended with faults and joints in the brittle zone (£300°C). D3 deformation involved ~N-trending steeply-dipping faults, veins and joints and occurred primarily in the Avalon and Putnam terranes, as a result of of ~E- directed extension and was exclusively brittle deformation. U-Pb LA-ICPMS analysis of in situ monazite of samples along the Honey Hill fault reveal that metamorphism occurred during various events between 411±34 and 242±10 Ma. Two possible growth clusters with weighted means of Concordia ages at 410±21 Ma (MSWD = 0.0021) and 314.6±3.1 Ma (MSWD = 1.04) are consistent with the early Acadian orogeny and the beginning of the Alleghanian orogeny, respectively. Periods of spread-out monazite dates at ~379-338 Ma and ~299-261 Ma may reflect continuous late orogenic growth or mixing of age domains. Two monazite grains with Concordia ages of 379±15 Ma and 361±14 Ma provide the first evidence of Acadian metamorphism along the Honey Hill fault and in the southern Avalon terrane. Pre-D1 deformation is attributed to the Acadian orogeny and earlier events, based on published work. D1 deformation may also be attributed to the Acadian orogeny based on Devonian ages in the Avalon terrane along the Honey Hill fault from this study, but more likely to the Alleghanian orogeny, based on abundant Pennsylvanian-Permian ages from this study and other published work. D2 ductile/brittle and D3 brittle deformation are attributed to post- Alleghanian extension and breakup of Pangaea, based on published ages.

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Structural history of the Honey Hill fault and adjacent terranes in southeastern Connecticut, USA

Fleischer, Noah

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