The case for crustal wedging in the assembly of the southern New England Appalachians
|Location||International Geological Congress,oslo 2008|
|Author||Wintsch, Robert۱; Aleinikoff, John۲; Dorais, Michael۳; Walsh, Gregory۲; Kunk, Michael۲|
|Holding Date||21 September 2008|
New petrologic and isotopic investigations of Neoproterozoic rocks in southern New England identify orthogneisses that were derived from either primitive or evolved sources. Pb and Nd isotopic compositions show that some orthogneisses have negative εNd and high 207Pb/206Pb ratios (derived from evolved sources), whereas other orthogneisses have εNd of zero to positive values and low 207Pb/206Pb (derived from primitive sources). The former isotopic ratios are found in orthogneisses exposed in the Lyme dome (Walsh et al., 2007) and other domes in coastal Connecticut while the latter isotopic ratios are found in rocks folded around the Lyme dome, and traditionally assigned to the Avalon terrane. By comparison with Neoproterozoic rocks in the type sections of Newfoundland, these suites of rocks are assigned to the Gander and Avalon zones, respectively. When the tilted fold of the Lyme dome is restored, the interpretive cross section reveals a tectonic wedge similar to that identified in the subsurface in Newfoundland (van der Velden et al., 2004).
The identification of Gander zone rocks in southern New England permits a reinterpretation of the structure of southern New England, both onshore and offshore along seismic line to the south. Onshore, the Avalon-Gander boundary is characterized by a distinctive aeromagnetic high caused by magnetite-bearing orthogneisses that can be traced offshore to the south. The aeromagnetic pattern defines an omega-shape and shows that the Gander zone widens to ~150 km south of Long Island near 40°N latitude, where it is crossed by the USGS seismic line # 36 (Hutchinson et al., 1986; Phinney, 1986). These data suggest that the Gander zone is bounded by strong reflectors, one on the east against the Avalon terrane ~ 120 km south of Block Island, and one on the west against Laurentia ~80 km east of the New Jersey coast. With this revised understanding of the locations of terrane boundaries, the assignment of the tectonic belts revealed in the cross-strike seismic line (Phinney, 1986) are re-evaluated. The Avalon-Gander boundary dips west for 50 km to the Moho, where Gander crust was delaminated from the mantle. Regional geochronology and thermochronology suggest that delamination occurred in the late Paleozoic (Hercynian). The Gander-Laurentia boundary is less well defined, but may dip west to mid-crustal levels and then east, in a wedge-shape that implies intra-crustal wedging of Gander crust from mantle. Ages of magmas in the hanging wall onshore suggest that the wedge was emplaced in the Silurian (Salinic?). We conclude that crustal wedging as interpreted by van der Velden et al. (2004) in Newfoundland appears also to have occurred in southern New England, as evident in both surface outcrops in southern New England, and offshore seismic reflection and aeromagnetic data. These data imply that crustal-scale wedging was a major mechanism of continental assembly and crustal thickening during Paleozoic orogenesis.