Thinning of the upper mantle during late Paleozoic appalachian orogenesis

Across the Appalachian orogen of New England, the splitting of core-refracted shear waves from a wide range of arrival directions indicates the presence of two nearly uniform horizontal layers of anisotropic upper mantle. The anisotropy in the lower layer has a fast axis nearly parallel to the absolute motion of the North American plate and thus is attributed to basal shear as the plate plows through asthenospheric mantle. The anisotropy of the upper layer is inferred to be a fossil fabric, residing in lithospheric mantle. The finite extension direction of the upper fabric is subhorizontal and oriented normal to the local trend of the Appalachian orogen. The upper fabric 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 synconvergent or postconvergent extension, developed for Tibet, predict rapid surface uplift and increased heat flow due to lithospheric thinning, consistent with coeval late orogenic mantlederived magmatism in both the northern Appalachians and Morocco.