THE SEISMIC STRUCTURE OF THE CRUST AND UPPER MANTLE OF THE NORTHEASTERN UNITED STATES

Over the past several decades there have been a number of different seismological studies by Boston College geophysicists of the structure of the crust and upper mantle beneath the northeastern U.S. and southeastern Canada. The techniques used in these studies have included seismic reflection surveys, seismic refraction surveys, apparent velocity measurements, time-term tomography inversions and receiver function measurements. The results from these studies can be used to provide an overview of the tectonic structure in this region today as it is preserved in the crust and upper mantle. The Clinton Newbury and Bloody Bluff faults are imaged using receiver functions as major features that cut the entire crust and offset the Moho. The Adirondack Mountains dip eastward beneath Vermont to approximately the New Hampshire border. Variations in the Poisson’s ratio across the region reveal distinctive crustal blocks and features, such as the Grenville beneath Vermont, the distinctive Chain Lakes Massif, and the Norumbega Fault, all of which extend through the entire crust. Seismic studies in central Maine reveal a number of distinctive belts of rocks that trend NE-SW. The Moho is a surface with many variations in depth both perpendicular to the continental margin as well as parallel to the margin. Along coastal Maine the Moho is a sharp, flat boundary at a depth of about 30 km, whereas at the eastern edge of the Grenville terrane the Moho appears to be a gradational boundary at a depth of more than 40 km. The uppermost mantle beneath the accreted terranes of coastal New England has a P velocity of about 8.05 km/s with no observed increase with depth down to 60-70 km. On the other hand, the uppermost mantle beneath the Grenville terrane has a P velocity of about 8.23 km/s immediately beneath the Moho with a velocity increase at about 50-60 km depth. These crustal images can be used to provide constraints on the tectonic assembly of the terranes in this region.