Geology and Mineral Resources - Vermont Geological Survey

Geology of Vermont

Structural Evolution of the Worcester Mountain Range, North-Central Vermont
by Marjorie Gale and Jonathan Kim
A poster presented at Geologic Association of Canada/MAC Meeting, May 2006

Click on individual images to enlarge or by Fig. # in box below.

Fig. 1 - Location; Fig. 2 - Lithotectonic Belts; Fig. 3 - (pdf) Regional Bedrock Map; Fig. 4 - Shaded Relief; Fig. 5 - Geology on Relief; Fig. 6 - Morrisville Quad (Springston et al., 1998); Fig. 7 - Southern Worcesters; Fig. 8 & 9 - Cross-section A-A and B-B'; Fig. 10 - Representative Structures: Row 1, Row 2, Row 3, Row 4; Fig. 11 - Equal Area Nets; Fig. 12 - Correlation with Quebec; Fig. 13 - Correlative Geology in Quebec.

ABSTRACT

The Worcester Mountain Range (WMR), located in the Rowe-Hawley Belt and east of the Baie Verte - Brompton Line, is composed of Pre-Silurian greenschist to amphibolite facies metasedimentary and meta-igneous rocks of the Hazens Notch, Ottauquechee, and Stowe formations. The rocks were deformed and metamorphosed during the Taconian and Acadian orogenies. The WMR is a fault-bounded, asymmetric, doubly-plunging anticlinorial window formed by superposition of four major deformational phases. Garnet-kyanite schist and amphibolite comprise the core of the WMR, whereas finer-grained chlorite-muscovite phyllites and schists compose the flanks and are in fault contact with albitic schists and mafic rocks of the Green Mountain slice. This presentation focuses on the 3-dimensional geometry generated by superposition of the ductile structural events.

The earliest deformation (D1) produced a penetrative schistosity parallel to compositional layering and a stack of east-dipping lithotectonic packages emplaced above the Prospect Rock Fault. The second deformational fabric (S2) is axial planar to isoclinal F2 folds and forms a composite S2/S1 fabric. The tectonic contact between amphibolites of the WMR core and the structurally underlying Hazens Notch schists is deformed by moderately-steeply north plunging F2 folds. S2/D2 kinematics are equivocal. Although most lithotectonic contacts predate S2, S2 is commonly the dominant fabric in the core schists of the WMR.

Acadian F3 folds are tight, shallowly plunging asymmetric folds with an axial planar spaced cleavage. S3 forms a composite anastamosing fabric with S2. D3 is responsible for the arching of the WM and the Green Mt. Anticlinorium. Garnet-kyanite schists of the WMR core plunge southward below Stowe Fm. phyllites at the southern closure of the antiform. The WMR antiform is truncated on the both sides by D3/reactivated D2 faults.

The fourth deformational phase (D4) affecting the WMR resulted in an irregular warping of the F3 antiformal axis by large-scale open folds that are visible along their ~30 km. length. S4 develops irregularly as a crenulation cleavage axial planar to F4 folds. F4 folds clearly deflect S3 to the east.

The architecture of the WMR reflects the juxtaposition of lithotectonic units during Taconian ductile thrusting followed by several phases of subordinate folding and faulting continuing through the Acadian Orogeny. The WMR terminates to the north via north-plunging F2 folds and to the south via south-plunging F3 folds. Acadian anticlinorial windows exposing coarse-grained garnet schists in the Becancour Dome and Carrineault Antiform in S. Quebec may be along-strike analogues for the WMR.