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Uwe Ring and Osama M. K. Kassem
The nappe rule: why does it work?
Journal of the Geological Society of London (December 2007), 164(6):1109-1112
Abstract: Index Terms/Descriptors: Latitude & Longitude:
GeoRef, Copyright 2007, American Geological Institute. Reference includes data from GeoScienceWorld, Alexandria, VA, United States. Reference includes data from The Geological Society, London, London, United Kingdom
The nappe rule stresses that imbrication emplaces nappes with higher metamorphic grade above those with lower grade. This means that the overriding nappes must be exhuming during nappe stacking; otherwise metamorphic grade would increase structurally downwards. The high-grade metamorphic internides of many orogens are characterized by a pervasive nappe-emplacement-related subhorizontal foliation indicative of vertical ductile shortening (thinning). By quantifying finite strain in the Gran Paradiso Massif, Italian Alps, we show that vertical ductile thinning associated with a nappe-emplacement-related subhorizontal foliation caused sufficient exhumation of the overriding nappes and can thus explain the occurrence of higher-grade rocks above lower-grade ones.
Alps; crustal shortening; crustal thinning; deformation; depth; ductile deformation; Erfaulet Unit; Europe; exhumation; finite strain analysis; foliation; grade; Gran Paradiso Massif; Gran Paradiso Unit; high pressure; imbricate tectonics; Italy; metagranite; metaigneous rocks; metamorphic rocks; metasedimentary rocks; nappes; orogeny; overburden; overprinting; P-T conditions; pressure; Southern Europe; strain; tectonics
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