EVIDENCE FOR TECTONIC WEDGING OF AVALON TERRANE ROCKS INTO THE GANDER ZONE, SOUTHERN NEW ENGLAND

Similar ages of Neoproterozoic rocks and similar sequences of early Paleozoic sedimentary rocks strongly support the correlation of the rocks SE of the Lake Char-Bloody Bluff fault system in southern New England to the Avalon terrane of Newfoundland. In Connecticut, new limited Nd and Pb whole-rock isotopic data generally support this correlation. However, rocks in the core of the Lyme dome correlate with the Gander zone of central Newfoundland. Specifically, ~590 Ma igneous rocks of the Selden Neck block in the Avalon terrane immediately south of the Honey Hill fault show positive epsilon Nd values of ~5 and 207Pb/204Pb ratios of ~15.59. These values suggest a primitive, depleted source for these granodioritic rocks, and overlap with similar values in the Avalon Peninsula of Newfoundland. In contrast, the isotopic values in 620 Ma old rocks from within the Lyme dome are near 0 and 15.65 respectively, reflecting an older crustal source for the granitic and granodioritic rocks analyzed so far. These isotopic compositions are indistinguishable from those of igneous rocks of the Gander zone in central Newfoundland, to which we tentatively correlate the rocks of the Lyme and Clinton domes. The reassignment of Lyme dome rocks to the Gander zone is tectonically significant. In the northern and maritime Appalachians equivalents of the Putnam-Nashoba, Merrimack, and Central Maine blocks are cover terranes to Gander basement. However, in southern New England north and west of the Honey Hill-Lake Char-Bloody Bluff fault zone they are underlain by the Avalon terrane. The new identification of Gander zone rocks in the Lyme dome, structurally below the Avalon zone thus places the Avalon terrane in a Gander sandwich, where the Avalon terrane was thrust between Gander cover and basement. Thus the final docking of the Avalon terrane to this part of North America seems to have involved tectonic wedging of Avalon into the Gander zone. Increasingly abundant geochronological data show that this wedging began in the Carboniferous and continued with shortening and heating in the Permian. These latter processes culminated in the latest Paleozoic with the folding of the Gander-Avalon terrane boundary forming the Lyme dome, and finally tilting to the north that allowed this unusually deep level of exposure.