The Nature of the Bronson Hill Anticlinorium - Central Maine Terrane Contact in the North Berlin 7.5’ Quadrangle, New Hampshire

Department or Program

Geology

Abstract

The Berlin 7.5’ Quadrangle is situated along the boundary between the Ordovician metavolcanic and igneous rocks of the Bronson Hill Anticlinorium (“BHA”) and the Silurian metasedimentary units of the Central Maine Terrane (“CMT”) along a boundary locally known as the Mahoosuc Fault (“MF”). First mapped in detail by (Billings and Fowler-Billings, 1975), as part of the Gorham 15’ quadrangle, the location of this boundary was largely preserved by (Lyons et al., 1997; Moench et al., 1995) which they mapped as a normal fault with an eastward dip. However, completion of the bedrock mapping work at 1:24,000 scale in this quadrangle newly suggests that the boundary is expressed by multiple generations of Acadian Orogeny folding as well as dip reversal from doming of the upwelling of Oliverian Jefferson Pluton of the BHA. A 1:15,000 scale bedrock map showing the fault was created via ArcMap software in addition to an interpretive A-A’ cross section for a string of data from two traverses on 7/16/2020 and 7/17/2020 (Plate 1). Additionally, equal area stereonet projections for data from each generation of folding and corresponding annotated outcrop photographs were created to highlight the nature of folding along the boundary transect. The map and cross section feature several D1, nappe-stage, N-plunging isoclinal folds along the transect. Four stages of deformation in the study area are as follows: D0) Mahoosuc Faulting, juxtaposing the Ordovician BHA units against the Silurian CMT units. It was proposed to have originated as a normal fault dipping west that was later reconfigured to have an eastward dip, resembling a thrust fault, via D3 Oliverian doming (Eusden et al., 2006); D1) Isoclinal nappe-stage recumbent folds at macroscopic scale some time before the peak metamorphism in the Acadian Orogeny. Multiple field photographs have been annotated showing the distinctive S0 bedding planes not oriented parallel with S1 schistosity, in addition to early fold data and F1 hinge lines from four stations graphically represented on stereonet projections; D2) Late stage open folding, refolding D1 folds at mesoscopic scale; and D3) Regional-scale dip reversal from Oliverian doming within the Bronson Hill Anticlinorium. Because of this, a fault-fold-fold- domed model is proposed for the boundary in the northeastern area of the Berlin 7.5’ quadrangle. Additional outcrop data from North and East of the transect would be helpful to constrain the extent of the fault. Assuming the various models are accurate, this complex folding along the Mahoosuc Fault in this area as compared to differing interpretations of the BHA/CMT boundary in Southern New Hampshire (Thompson, 2018) point to the fact that the boundary must be varied and complex along its length from Connecticut to Maine, and just one tectonic model cannot be applied for its entire length.

Level of Access

Open Access

First Advisor

Dyk Eusden

Date of Graduation

5-2021

Degree Name

Bachelor of Arts

Number of Pages

48

Components of Thesis

1 Writing document PDF, 1 Map document PDF.

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Open Access

Available to all.

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