THE TECTONIC STRUCTURE OF THE CRUST AND UPPER MANTLE OF THE NORTHEASTERN UNITED STATES AND SOUTHEASTERN CANADA FROM SEISMOLOGICAL STUDIES

Over the past several decades there have been a number of different seismological studies 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 Norumbega, Clinton Newbury and Bloody Bluff faults are imaged 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. The Chain Lakes Massif is a distinctive crustal block that extends down into the lower crust. The Moho is a surface with many regional variations in depth and character. Along coastal Maine is it a sharp, flat boundary at a depth of about 30 km. From the Maine coast to Quebec the Moho has some sharp downdrops with intervening flatter areas, although there are some unexplained significant variations in Moho depth, such as along strike beneath the Central Maine Basin. At the eastern edge of the Grenville terrane the Moho appears to be a gradational boundary. 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.