GSA Annual Meeting in Denver, Colorado, USA - 2016

Paper No. 347-8
Presentation Time: 9:00 AM-6:30 PM


PRICE, Nancy A. and LAHART, Eleanor A., Department of Geology, Portland State University, PO Box 751, Portland, OR 97207-0751,

Recent research has brought new focus to the metasedimentary rocks on the western edge of the Goshen Dome, Appalachian Orogen, Western Massachusetts, USA. The Goshen Dome is a tectonic window through the metasedimentary rocks of the Connecticut Valley Trough (CVT). West-vergent isoclinal folds consistent with thrust emplacement of the CVT, with a décollement inferred at its base. Snoeyenbos and Peterman (2015) and Peterman and Snoeyenbos (2015) show that garnet-kyanite schist-hosted phosphatic garnet at the edge of the dome display the same chemical signatures and mineral textures seen in the ultra-high pressure rocks of Rhodope, Greece, suggesting a more complex tectonic history may be preserved in the rocks of the Goshen Dome.

In this study, we collected an across-strike sample transect in the southwest of the Goshen Dome. Structurally below the location of the décollement (locally-inferred, unobserved in this study) are a layered amphibolite and a garnet-kyanite schist unit (host to the phosphatic garnet). These units are different from the other metasedimentary units on the edge of the dome in that they contain distinct metamorphic mineral assemblages and display outcrop-scale isoclinal-folds and complex boudin structures not observed elsewhere in the transect. S-C microstructures in fine-grained layers near the base of the garnet-kyanite schist unit suggest some degree of localized shear. Structurally below the garnet-kyanite schist unit are an amphibole gneiss unit, an amphibole-quartz-garnet gneiss unit, and a garnet schist (+/- staurolite) unit that are continuous along strike (mean strike and dip: 159°, 35º). The amphibole-rich units are foliation-dominated with a weak mineral lineation (e.g. 6° -> 338º), consistent with flattening strain. The schist units display shear boudins consistent with south-directed shear (intersection lineation- e.g. 19º-> 275º). The isoclinal folds of the garnet-kyanite schist unit are also overprinted by shear folds, suggesting that if the rocks at the edge of the dome were assembled into a package, they were structurally juxtaposed prior to the onset of south-directed shearing. The mechanism of dome formation is currently undetermined for the Goshen Dome; we have yet to find evidence for west-directed shear as might be expected on the western edge of a gneiss dome.