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).
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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
Until a few decades ago, structural and regional geology were traditionally the preserve of field geologists. They usually mapped areas of outcropping deformed rocks and supplemented their work by laboratory studies of rock deformation and by theoretical work. Structural geology became tied to the geology of uplifts, folded belts, and underground mines, all of which were accessible to direct observation. Since World War II we have witnessed a tremendous development of geophysics in oceanography and in petroleum geology. Academic geophysicists in oceanography led their geological colleagues into modern plate tectonics and industry geophysicists developed reflection seismology into a superb structural mapping tool that penetrated the subsurface.
Today we are facing a situation where instruction and textbooks in structural geology are almost entirely dedicated to rock deformation, analytical techniques in detailed field geology and summaries of plate tectonics. Illustrations based on reflection seismic profiles are virtually absent in textbooks of structural geology. These texts illustrate only the parts of the proverbial elephant, together with some conjecture, but without ever offering a glimpse of the whole elephant.
Some of the reason cited for the relative scarcity of published reflection profiles are: 1) the confidentiality of exploration data; 2) difficulties in the photographic reduction and reproduction of seismic profiles for a book format; 3) the two-dimensional nature of vertical reflection profiles; and 4) the obvious distortions in reflection profiles that are typically recorded in time.
The AAPG leadership felt that it was time to attempt to correct the situation and to produce this picture and work atlas. The first volumes, of what may become a series of volumes, are addressing an audience that includes: petroleum geologists concerned with structural interpretations; exploration companies that provide in-house training; the AAPG continuing education program; and academic colleagues interested in updating their curricula in structural geology by inclusion of reflection profiles from the “real world” in their teaching.
The atlas is not meant to be a textbook in reflection seismology (instead we listed some at the end of this introduction) nor a text in structural and/or regional geology. Our intent is simply to provide a teaching tool.