Department or Program

Geology

Abstract

New detailed structural mapping and zircon geochronology in the southern half of the Mt. Crescent 7.5’ quadrangle in Randolph, NH was completed for the USGS/NHGS StateMap program in an effort to better understand the geometry and deformation history of the Jefferson Dome, a mantled gneiss dome that is part of the Bronson Hill Anticlinorium. Overall structures support a Latest Silurian to Devonian, Acadian or Neoacadian timing of deformation for the Dome with subsequent Alleghanian shearing, followed by later Early Mesozoic brittle faulting. Structural data in the form of foliations in the Jefferson Dome were collected for this study. Three different types of foliations were found: 1) no foliation; 2) single foliations that are likely dome related and 3) mylonitic S-C foliations defining cross-cutting shear zones. The zones of no foliation are somewhat randomly distributed throughout the Dome. Similarly the dips of the singly foliated rocks are variably distributed across the Dome. Equal area projections of the Dome foliations indicate a classic anticlinal dome shape with a trend and plunge of 62º, 8º. However, the spatial distribution of the foliation data does not support the traditional antiformal shape. Instead a dome-like structure with two limbs with either many parasitic folds or later variable dip realignment due to faulting. Broad zones with abundant mylonitic foliations that present S-C fabrics and sigma porphyroclasts striking NE-SW show primarily reverse dip-slip shear and cut the single and no foliation zones. These represent a complex array of shear zones. Lenses of Ammonoosuc Volcanics are present within the shear zones showing that there must be some Ammonoosuc Volcanics located at depth. Newly mapped silicified pods, up to 10 m in width, are indicative of late brittle faults striking NE-SW and NW-SE. Kinematic indicators for the faults are absent. New detrital and crystallization zircon geochronology constrains the timing of deformational events in the area. A newly mapped foliated metasedimentary xenolith within an unfoliated granite yielded a maximum depositional age of 429.3 +/- 7.0 Ma constraining the Dome deformation to after the middle Silurian. A fine grained granite with no foliation except for zones of S-C fabrics yielded a crystallization age of 334.0 +/- 2.2 Ma constraining the Dome deformation between the Latest Silurian to Devonian, likely attributed to diapiric rise and compression from the Acadian and Neoacadian orogenies. The timing of the shear zones is constrained to the Late Carboniferous, likely a result of movement during the Alleghanian orogeny. The timing of the brittle faults is constrained to the Early Mesozoic since they are not present in the 178.4 +/- 1.1 Ma granite porphyry unit and are likely related to the rifting of Pangea.

Level of Access

Open Access

First Advisor

Eusden, Jr., Dykstra

Date of Graduation

5-2018

Degree Name

Bachelor of Science

Number of Pages

94

Components of Thesis

2 pdf files

Download

Open Access

Available to all.

Share

COinS