A SURVEY OF MICROSTRUCTURES ACROSS A MAJOR LITHOTECTONIC BOUNDARY IN THE TOWN OF CRAFTSBURY, NORTHERN VERMONT

The Town of Craftsbury straddles a major lithotectonic boundary (Richardson Memorial Contact (RMC)) that separates Pre-Silurian (PS) metamorphic rocks of the Green Mountain Belt (GMB) on the west side from those of Silurian-Devonian (SD) age in the Connecticut Valley Belt (CVB) on the east side. Phyllitic quartzites (Moretown Fm) and black phyllites (Cram Hill Fm) comprise the GMB whereas SD black phyllites and quartzites (Northfield Fm) and siliceous marbles interlayered with black phyllites (Waits River Fm) comprise the CVB. Although originally defined as a Silurian unconformity, the RMC and its equivalents in Vermont and S. Quebec locally are coincident with a high angle Devonian fault.

The Ordovician Taconian Orogeny only deformed and metamorphosed PS rocks whereas the Devonian Acadian Orogeny affected rocks on both sides of the RMC. Bedrock mapping in Craftsbury defined two dominant foliations (Taconian =S1, S2; Acadian = S3, S4) for each orogeny. At the mesoscale, the S1/S2 composite in the Moretown Fm is “pinstriped” and has alternating Qtz and Mica domains. Flattened quartz grains in S1/S2 display a strong shape-preferred orientation. Stretching leading to the formation of quartz rods that parallel fold axes involved the dynamic recrystallization of quartz aggregrates. In the Cram Hill Fm, quartz and mica pressure shadows (S1/S2) surround albite grains parallel to lineation. Late S4 crenulation cleavage planes in the Moretown Fm are pressure solution enhanced, spaced, smooth, parallel, and discrete.

In SD rocks, S3 varies from a spaced to a crenulation cleavage and is associated with the overall structure of the CVB whereas S4 is always a superposed crenulation cleavage and is related to the D4 Brownington Syncline. In the marbles, flattened grains of albite and calcite, graphite bands, and alternating Qtz and Mica domains define S3. The S4 crenulation cleavage is only weakly expressed in thin section. S4 cleavage planes are widely spaced, smooth, parallel, and gradational. Quartz displays only weak lattice-preferred orientations.

Our distribution of samples is ideal for evaluating the significance and evolution of strain gradients across the RMC boundary. Ongoing microstructural analysis is focused on evaluating the kinematics of superposed deformations.