SEISMIC VELOCITIES IN THE SHALLOW CRUST OF WESTERN NEW-ENGLAND AND NORTHERN NEW-YORK

The Ontario-New York-New England refraction study (O-NYNEX) provides a set of high-quality data for a tomographic study of an intraplate con tinental region with low seismic activity. This study focuses on the r egion crossed by the United States part of the O-NYNEX profile. Tomogr aphic inversion of the travel-time data is used to construct a three-d imensional model of the P- and S-wave seismic velocity distribution wi thin the shallow crust of this region. Velocity structure is resolved in the top 15 km of the crust, with the most detailed picture obtained for the depth range between 1 and 5 km. The upper crust of the Proter ozoic Grenville Province is found to have a P-wave velocity 0.2 to 0.3 km/sec higher than the Paleozoic Appalachians. The area of anorthosit ic intrusions within the Adirondack Mountains is characterized by P-wa ve velocities 0.3 km/sec higher and S-wave velocities 0.1 km/sec highe r than the average values for the Grenville Province. This high-veloci ty anomaly extends to the depth over 5 km, possibly as deep as 16 km. The velocity structure of central Vermont is found to be closer to tha t of the Grenville Province than to the rest of the Appalachians. Cent ral New Hampshire shows relatively small lateral variation of seismic velocity. No strong correlation is seen between velocity anomalies and intrusive bodies of the White Mountains magma series.