FORMATION OF NEW ENGLAND GNEISS DOMES

One of James W. Skehan's (1961) early contributions to New England geology was his study of the Green Mountain Anticlinorium in the Wilmington-Woodford area, Vermont. Part of his study was an analysis of the Sadawga Pond and Lake Rayponda domes, and he identified the formation of gneiss domes as one of the important problems in the northern Appalachians and in orogens worldwide. At the time of his research, models of gneiss dome formation were commonly guided by Eskola's (1949) classic work, which invoked the vertical remobilization of older plutons into nonconformably overlying sediments. Ramberg's (1967) influential experimental studies on diapirs explained domes as the buoyant rise of core gneisses into denser metasediments. Later, plate tectonic theory inspired new interpretations of gneiss domes in southeastern Vermont. Stanley and Ratcliffe (1985) argued that contacts between Mesoproterozoic core rocks and younger Neoproterozoic to Paleozoic cover rocks around the domes are thrust faults. This interpretation, however, fails to account for the dramatic attenuation and omission of Paleozoic cover units around the domes. I suggested that the Vermont domes formed by Acadian extension during gravitational collapse (Karabinos, 2001), but this model is at odds with the extremely narrow width of the orogenic belt at this latitude. Northward extrusion of quartz-feldspar-rich core gneisses beneath mica-rich Silurian units during Acadian shortening explains many of the critical observations. The domes are surrounded by high-strain zones (HSZ), which decoupled deformation in the core gneisses from that in the overlying nappes of Silurian metasediments. Units were dramatically thinned or omitted in the HSZ. Sense of shear indicators suggest that rocks above the HSZ were displaced southwest relative to rocks below it. P-T paths of rocks from below the HSZ in the Chester dome indicate decompression of several kbars during metamorphism, whereas rocks above the HSZ record nearly isobaric conditions. These observations are consistent with normal-sense displacement between the core of the domes and the mantling sequence during Acadian deformation. During extrusion, the core gneisses cut upsection into the nappes of Silurian rocks and the thickness of the intervening Lower Paleozoic section was dramatically reduced.