CRUSTAL STRUCTURE OF THE WESTERN NEW-ENGLAND APPALACHIANS AND THE ADIRONDACK MOUNTAINS

We present an interpretation of the crustal velocity structure of the New England Appalachians and the Adirondack Mountains based on a seismic refraction/wide-angle reflection experiment in eastern North America extending from the Adirondacks in New York State through the northern Appalachians in Vermont and New Hampshire to central Maine. Modeling of the eastern portion of the profile within the New England Appalachians shows a subhorizontal layered crust with upper crustal velocities ranging from 5.5 to 6.2 km/s, a midcrustal velocity of 6.4 km/s, and a lower crustal velocity of approximately 6.8 km/s. Crustal thickness increases from 36 km beneath Maine to 40 km in Vermont. Little evidence is seen for structures at depth directly related to the White Mountains or the Green Mountains. A major lateral velocity change in the upper and mid crust occurs between the Appalachians and the Adirondacks. This boundary, projecting to the surface beneath the Champlain Valley, dips to the east beneath the Green Mountains and extends to a depth of approximately 25 km below the eastern edge of the Connecticut Valley Synclinorium in Vermont. The Tahawus Complex, a series of strong horizontal reflections at 18-24 km depth beneath the Adirondack Highlands, is seen to dip eastward beneath Vermont. Upper crustal rocks in the Adirondack Mountains have Poisson's ratios of 0.28 +/- 0.01 that can be correlated with the Marcy Anorthosite. Poisson's ratios of 0.24 +/- 0.01 calculated for rocks of the Connecticut Valley Synclinorium indicate a siliceous upper crust in Vermont. The lower crust is considered to be best represented by intermediate to mafic granulites; a high Poisson's ratio (0.26-0.27) tends to support a mafic lower crust in the New England Appalachians. This seismic refraction/wide-angle reflection experiment provides further evidence for the obduction of the allochthonous western Appalachian units onto Grenvillian crust above a zone of detachment that penetrates at least to midcrustal depths and was the locus of successive Paleozoic thrusting.