HIGH STRAIN AND DECOUPLED DEFORMATION IN THE MANTLE OF GNEISS DOMES

Two N-S trending sets of mantled gneiss domes are first-order features of the northern Appalachians. The western belt contains thirteen domes that expose either 1 Ga Laurentian basement rocks or approximately 475 Ma rocks of the Shelburne Falls arc in the cores. The eastern belt contains twenty-one gneiss domes cored by either 600 Ma crust of Ganderian affinity or approximately 450 Ma rocks of the Bronson Hill arc. The domes in both belts are surrounded by rocks of Silurian and Devonian age, which were deposited or erupted in two N-S trending basins, and deformed during the Devonian Acadian orogeny. The Chester dome in southeastern Vermont is 60 km long, elongated parallel to N-S striking Acadian structures, and only 15 km wide. Core gneisses in the dome preserve E-W oriented Grenvillian fabrics and structures. In contrast, Neoproterozoic to Ordovician units that mantle the Chester dome contain strong N-S striking Acadian fabrics that are parallel to foliation in the surrounding Silurian and Devonian rocks. The mantling units are dramatically thinner than elsewhere in southern Vermont, and they are commonly absent. A strong spatial correlation between the attenuated and excised mantling units and highly strained mylonitic rocks suggests the presence of a ductile, normal-sense shear zone, which commonly extends into the uppermost part of the Mesoproterozoic gneisses. Garnet-bearing rocks from the core and mantle of the dome record 2 to 3 kb of decompression during metamorphism, whereas rocks above the high-strain zone were metamorphosed during nearly isobaric conditions. Strain markers, kinematic indicators, reconstructed P-T paths, and stratigraphic omission suggest that extension occurred during northward and upward extrusion of rigid lower to middle crustal wedges of Proterozoic and Ordovician quartz-feldspar-rich gneisses relative to a thick tectonic cover of the surrounding Silurian and Devonian rocks, which were transported westward in large-scale nappes prior to and synchronous with doming at approximately 380 Ma. High strain focused in the mantling units served to decouple the westward transport of nappes from structurally lower northward extrusion of rigid core gneisses in the domes and to accommodate the dramatic contrast in strain between rocks in the dome cores and surrounding units.