ANOMALOUS MANTLE STRUCTURE AT THE PROTEROZOIC PALEOZOIC BOUNDARY IN NORTHEASTERN US

Using teleseismic P wave tomography we detect very large (similar to 5 %) variation in the compressional velocity of the upper mantle beneath the Proterozoic-Paleozoic boundary in the Northeastern US. In this re gion, the upper mantle is heterogeneous on the 100 km scale to a depth of 300 km. Below this depth, the mantle appears to be laterally homog enous, at least on the scale of 200-250 km. Given the tectonic history of the region, the large amplitude, short wavelength heterogeneities can not be caused by temperature variations, but may be due to chemica l heterogeneity or variations in the amount and orientation of flow-in duced seismic anisotropy.