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The orogenic superstructure-infrastructure concept: Revisited, quantified, and revived

¹ Department of Earth Sciences, Dalhousie University, Halifax, Nova Scotia B3H 3J5, Canada
² Department of Oceanography, Dalhousie University, Halifax, Nova Scotia B3H 4J1, Canada
³ Department of Earth Sciences, Dalhousie University, Halifax, Nova Scotia B3H 3J5, Canada

The historical superstructure-infrastructure concept (S-I) expressed contrasts in structural style and metamorphic grade between 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 subhorizontal high-strain zone from a ductile infrastructure with late gently 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 conventional thrust-tectonics models, the revived S-I model shows how young structures can form beneath older ones during progressive convergence, thereby encouraging reassessment of standard seismic reflection interpretations.

Key Words: tectonics • crustal structure • superstructure • infrastructure • seismic reflectivity • Superior Province

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