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Chapter 16: Geophysical framework of the Appalachians and adjacent Grenville Province

By
Steven R. Taylor
Steven R. Taylor
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Published:
January 01, 1989

A number of geophysical studies, including nonseismic (gravity, magnetics, resistivity, and heat flow) and seismic (refraction, teleseismic P-wave residuals, Pn time terms, reflection, inversion of teleseismic receiver functions, and surface-wave dispersion) results, are combined with geological observations to obtain a picture of the subsurface structure in the Appalachians. Based on differences in geologic history and geophysical structure, the Appalachians can be divided across strike into a northern and southern segment separated by a line marking the approximate northward extension of Alleghanian deformation. This boundary is located approximately in the vicinity of southern New England. Along strike, the mountain chain can be divided into belts of either continental or oceanic affinity. This along-strike boundary exists in the vicinity of the Inner Piedmont/Charlotte belt (or possibly into the slate belt) in the southern Appalachians, and in the Connecticut Valley synclinorium in the northern Appalachians.

In the northern Appalachians, the Grenville Province (exposed in the Adirondacks) is characterized by a relatively uniform crust on the basis of refraction models. However, detailed analysis of reflection profiles and structure derived from inversion of crustal receiver functions illustrates a more complex picture. A zone of prominent reflectors is seen to exist at depths of approximately 18 to 26 km, which correlates well with a high-velocity anomaly inferred from the receiver functions. Separate studies of crustal receiver functions also suggest the existence of relatively low shear velocities and possibly a high Poisson’s ratio in the lower crust. These structures correlate with a highly conductive lower crust inferred to exist from electromagnetic sounding. The lack of Moho reflections and Ps converted phases suggests that the crust-mantle boundary in the region is gradational.

Farther east, the crust appears to be thicker beneath the Taconic thrusts where the thrust (at ~2-km depth) and underlying shelf sequence have an aggregate thickness of about 4.5 km. The reflector at a depth of 4.5 km continues beneath the Green Mountains, suggesting that they are allochthonous. Gravity, refraction, and reflection data suggest a fundamental change in crustal character east of the Precambrian outliers and serpentinite belt in the Connecticut Valley synclinorium. A regional gravity high, found to the east of the Precambrian outliers, indicates that there is a deep-seated increase in crustal density. This is further supported by the existence of high velocities in the lower crust in the central orogenic belt, observed from refraction data. In this transitional zone, there is a thick series of east-dipping reflectors observed on reflection profiles extending through much of the crust and flattening beneath the Bronson Hill anticlinorium and Merrimac synclinorium. Moho reflections are observed on the Consortium for Continental Reflection Profiling (COCORP) profiles, and magnetotelluric data suggests that the lower crust is fairly resistive. Data from Pn time terms and teleseismic P-wave residuals suggest that the crust is slightly thicker in this central belt. Farther east, refraction models and traveltime residuals indicate that the crust thins beneath the Avalon block, and the high-velocity lower crust is absent.

In the southern Appalachians, the Grenville crust shows many similarities to that in the north, including the gradational crust-mantle boundary and absence of high velocities in the lower crust. However, the crust appears to be much thicker on the average and approaches thicknesses of 50 km in some localities beneath the Valley and Ridge and western Blue Ridge Province.

COCORP reflection profiling illustrates that the Valley and Ridge, Blue Ridge, and Inner Piedmont are allochthonous and have been thrust over an early Paleozoic shelf sequence. The magnitude of thrusting may be at least 175 km and points out the importance of thin-skinned tectonics in continental collision episodes. The root-zone for the southern Appalachian décollement has not yet been identified and is a topic of considerable controversy. Gravity, magnetic, reflection, and refraction data all suggest that a fundamental change in crustal character occurs in the vicinity of the Inner Piedmont-Charlotte belt boundary. However, it remains unclear whether this region represents a suture zone separating North America from accreted island-arc sequences or a décollement extending farther east beneath the slate belt and into the Coastal Plain. Similar to the western part of the Connecticut Valley synclinorium in the northern Appalachians, a zone of east-dipping reflectors is observed near the Inner Piedmont-Charlotte belt boundary. However, in the northeast Georgia profile the reflectors do not appear to penetrate the crust, and a shallow-dipping reflector can be traced farther east beneath the Charlotte belt/slate belt. The southeast-dipping reflectors penetrate much of the crust on the southernmost COCORP line in southwestern Georgia where North American rocks are juxtaposed with rocks that were probably once part of the African continent. A well-defined Moho reflection is observed on reflection profiles to the east and southeast of these dipping reflectors.

The gravity and magnetic data all suggest that the crust to the east of the Inner Piedmont-Charlotte belt boundary is of oceanic affinity. In contrast to the northern Appalachians, the southern refraction data indicates that the crust thins dramatically beneath the Charlotte belt-Carolina slate belt, and no evidence of high velocities is observed in the lower crust. Interestingly, the Charlotte belt and slate belt rocks are thought to be of Avalon affinity, and the crustal velocity structure shows many similarities to the Avalon block in the northern Appalachians.

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Contents

GSA Memoirs

Geophysical Framework of the Continental United States

L. C. Pakiser
L. C. Pakiser
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Walter D. Mooney
Walter D. Mooney
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Geological Society of America
Volume
172
ISBN print:
9780813711720
Publication date:
January 01, 1989

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