ACADIAN EXTENSION IN WESTERN NEW ENGLAND

A dramatic feature of the geology of western New England is the thinness of some Paleozoic units east of the Green Mountain and Berkshire massifs in southeastern Vt. and western Mass. compared to equivalent units north, west, and south of the massifs. These structurally thinned units are widely interpreted as preserving a Taconian accretionary wedge and arc complex. The attenuation of these units is most compelling around the Chester dome where rocks lithologically equivalent to the Pinney Hollow, Ottauquechee, Stowe, and Missisquoi Formations are locally one to two orders of magnitude thinner than they are elsewhere. P-T paths for rocks structurally below the attenuated zone in the Chester dome suggest 2.5 kbar of decompression during Acadian metamorphism (Vance and Holland, 1993), equivalent to approx. 10 km of denudation. Boudinaged layers and stretched porphyroblasts within the attenuated zone also indicate extension. Rosenfeld (1968) recognized west-southwest directed layer-parallel extension and layer-perpendicular shortening in stretched pebble conglomerates of the Silurian Shaw Mountain Formation showing that extension is not restricted to pre-Silurian rocks. Thus, extension and crustal thinning were accomplished by internal strain as well as displacement on multiple faults distributed throughout a significant crustal section. Assuming that decompression resulted entirely from tectonic denudation and that average fault dips were 10 to 30 degrees, the relative displacement of rocks above and below the attenuated zone during extension was between 60 to 20 km. Early Acadian crustal thickening led to metamorphism at pressures up to approx. 10 kbar around the Chester dome (Kohn and Spear, 1990). Below the attenuated zone, garnet cores record this higher-pressure metamorphism and contain abundant rutile inclusions. Garnet growth was interupted by a temperature decrease accompanying extension and decompression. A second lower-pressure prograde metamorphism and garnet growth stage, lacking rutile inclusions, produced unconformity textures and reflects a temperature increase caused by enhanced heat flow. Peak metamorphic temperatures were reached after extension but prior to later, dome-stage Acadian folding.