Thinning of the upper mantle during late Paleozoic Appalachian orogenesis

Across the Appalachian orogen of New England, the splitting of core-refract ed shear waves from a wide range of arrival directions indicates the presen ce of two nearly uniform horizontal layers of anisotropic upper mantle. The anisotropy in the lower layer has a fast axis nearly parallel to the absol ute motion of the North American plate and thus is attributed to basal shea r as the plate plows through asthenospheric mantle. The anisotropy of the u pper layer is inferred to be a fossil fabric, residing in lithospheric mant le, The finite extension direction of the upper fabric is subhorizontal and oriented normal to the local trend of the Appalachian orogen. The upper fa bric is consistent over a broad region beneath and west of the New England Appalachians, which indicates that it formed after Devonian closure of the Iapetus ocean, probably during or after the Paleozoic Acadian and Alleghany orogenies. Tectonic scenarios for syn-convergent or postconvergent extensi on, developed for Tibet, predict rapid surface uplift and increased heat fl ow due to lithospheric thinning, consistent with coeval late orogenic mantl e-derived magmatism in both the northern Appalachians and Morocco.