Crustal structure of the western New England Appalachians and the Adirondack Mountains

Abstract

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 ∼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. Pois son'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.

Stratigraphy, geochronology, and accretionary terrane settings of two Bronson Hill arc sequences, northern New England

“…According to geophysical studies, the southeastern Grenville margin now lies near the Silurian tectonic hinge in western Maine and southwestern New Hampshire, but much farther west in northeastern Vermont (see arrows 1, 3, and 2, respectively, in Fig. 1, plotted from Stewart et al (1993), Ando et al (1984) and Hughes and Luetgert (1991)). …”

“…! 1 Southeast edge of Grenville basement: 1, Stewart et al (1993); 2, Hughes and Luetgert (1991); 3, Ando et al (1984). X h Vms deposits having strongly radiogenic lead: X h , Hampshire Hills; X m , Milan; X g , Gardner Mountain.…”

Global geoscience transect 8: Quebec-Maine-Gulf of Maine transect, southeastern Canada, northeastern United States of America

“…Because the seismic velocity in shi,iJlow Grenvillian crust is higher (-6.6 krn/s), the top of the low~r crust cannot be clearly identified therein (Hughes and Luetgert, 1991 ). The interpretation that the lower crust attained its present configuration during late Paleozoic and early Mesozoic extension is supported by several other lines of evidence.…”

“…to show the same phenomenon. Seismic refraction data from northern New Hampshire (Hughes and Luetgert, 1991) suggest that the top of the ·lower crust is at about 18 km depth. This corresponds closely to the depth on COCORP line 6 at which east-dipping reflectors, interpreted as decollements, die out, and where laminated subhorizontal reflectors appear in the lower crust in conjunction with a distinct Moho reflection.…”

Imaging beneath an opaque basaltic layer using densely sampled wide‐angle OBS data1

“…Owen and Barton 1990, experiment described by Hughes and Luetgert 1991). In marine work, OBSs allow the recording of signals from high-volume airgun arrays up to a source-receiver offset of 100 or more kilometres (e.g.…”