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Unformatted text preview: q 2006 Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or firstname.lastname@example.org. Geology; September 2006; v. 34; no. 9; p. 733–736; doi: 10.1130/G22793.1; 3 figures; Data Repository item 2006154. 733 The orogenic superstructure-infrastructure concept: Revisited, quantified, and revived N.G. Culshaw Department of Earth Sciences, Dalhousie University, Halifax, Nova Scotia B3H 3J5, Canada C. Beaumont Department of Oceanography, Dalhousie University, Halifax, Nova Scotia B3H 4J1, Canada R.A. Jamieson Department of Earth Sciences, Dalhousie University, Halifax, Nova Scotia B3H 3J5, Canada ABSTRACT The historical superstructure-infrastructure concept (S-I) ex- pressed contrasts in structural style and metamorphic grade be- tween shallow and deep orogenic levels. Two-dimensional thermal- mechanical models provide a quantitative explanation in terms of progressive crustal shortening and thickening (phase 1), thermal relaxation and rheological weakening (phase 2), and ductile flow at depth (phase 3). Results predict an upper-crustal superstructure, dominated by early steep structures, separated across a subhori- zontal high-strain zone from a ductile infrastructure with late gent- ly dipping structures; this is consistent with observations from the western Superior Province. These models can account for contrasts in structural style, metamorphic grade, seismic reflectivity, and age between upper- and lower-crustal levels. In contrast to convention- al thrust-tectonics models, the revived S-I model shows how young structures can form beneath older ones during progressive con- vergence, thereby encouraging reassessment of standard seismic reflection interpretations. Keywords: tectonics, crustal structure, superstructure, infrastructure, seismic reflectivity, Superior Province. INTRODUCTION The historical terms ‘‘superstructure’’ and ‘‘infrastructure’’ denot- ed marked contrasts in structural style and metamorphic grade between upper and lower tectonic levels in orogenic core zones (e.g., Wegmann, 1935; Haller, 1956). Superstructure represents a low-grade upper level with early, typically upright structures, whereas infrastructure consti- tutes a high-grade, migmatitic lower level with late, gently inclined structures overprinting early structures. A detachment or shear zone (Abschwerungszone) may commonly be observed between tectonic levels. Since the development of quantitative thrust-belt models (e.g., Boyer and Elliott, 1982; Davis et al., 1983), orogenic forelands and core zones have generally both been interpreted in terms of thrust tec- tonics (e.g., Butler, 1986; Meissner et al., 1991; Cook and Varsek, 1994). The superstructure-infrastructure (S-I) concept has therefore been largely abandoned, although comparable ideas have persisted to explain similar observations (e.g., ‘‘orogenic lid’’; Laubscher, 1983)....
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This note was uploaded on 03/19/2012 for the course GEOLOGY 6 taught by Professor Rm during the Spring '12 term at UNAM MX.
- Spring '12