Crustal Structure in the Adirondacks
S. Klemperer, L. Brown, J. Oliver, C. Ando, S. Kaufman, 1983. "Crustal Structure in the Adirondacks", Seismic Expression of Structural Styles: A Picture and Work Atlas. Volume 1–The Layered Earth, Volume 2–Tectonics Of Extensional Provinces, & Volume 3–Tectonics Of Compressional Provinces, A. W. Bally
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The Adirondack dome of northeastern New York State is located near the eastern edge of the North American craton. As the southeastern extension of the Grenville Province, it constitutes the largest well-exposed Proterozoic crystalline terrain in the United States. COCORP has recorded a series of lines across the dome and the onlapping Paleozoic cover sequence (Brown et al, 1982; see location map Figure 1). In this article, line drawings are presented for lines 1, 7 and 10 (Figure 2) and data from line 7 is reproduced (Figure 3). Despite the extreme degree of deformation and metamorphism of the surface rocks, reflections from throughout the crust were recorded. The most striking result of this survey is a set of layered, gently northwest-dipping reflectors observed to span a depth range of about 17 to 25 km (10.5 to 15.5 mi) beneath the Marcy anorthosite massif. Such an observation is thusfar unique, but the geological interpretation of these data are still ambiguous.
The rocks of the Adirondack mountains comprise a multiply-folded stratigraphic sequence of highgrade metasedimentary and metavolcanic rocks which are intruded by syntectonic bodies of metanorthosite and metagabbro (Isachsen and Fisher, 1970). Recent studies (McLelland and Isachsen, 1980) suggest that at least four phases of folding have occurred, and that the complex surface outcrop pattern is produced by the interference of large nappe structures with wavelengths of many kilometers. The Highlands region, which includes most of the Adirondack dome (Figure 1), consists principally of granulite facies rocks which were buried to a depth of 20 to 25 km (12.4 to 15.5 mi) during peak metamorphism (Bohlen, Essene, and Hoffman, 1980), The Highlands are bounded on the northwest by the Carthage-Colton mylonite zone, beyond which are the Lowlands, dominated by metasedimentary rocks equilibrated at somewhat lower pressures (equivalent to 16 to 21 km, or 10 to 13 mi, depth) (Brown, Essene, and Kelly, 1978).
The regional exposure of rocks metamorphosed to such high pressures raises the question, how were they brought to the surface? Since the Adirondack crust is now of normal thickness, was it formerly much thicker? If so, was this greater thickness caused by continental collision and subduction, or by some kind of igneous underplating? Several models have been proposed (Baer, 1981; Dewey and Burke, 1973; Seyfert, 1980; Wynne-Edwards, 1976) which represent the Grenville orogeny variously as the product of large-scale continental obduction, crustal thickening by basement reactivation to absorb continental convergence, or drift of a continent over a hot zone or chain of hot spots. A possibly related question concerns the current domical structure of the Adirondack basement. The present surface topography is apparently post-Devonian (Crough, 1981) but the age of uplift is uncertain. Isachsen (1975) has argued from geodetic data that the Adirondack dome is still rising, but this theory has been questioned (Brown and Reilinger, 1980).