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Journal of the Geological Society; 2007; v. 164; issue.6; p. 1109-1112;
DOI: 10.1144/0016-76492007-020
© 2007 Geological Society of London

Short Communication

The nappe rule: why does it work?

Uwe Ring1 & Osama M.K. Kassem2

1 1Department of Geological Sciences, Canterbury University, Christchurch, New Zealand (e-mail: uwe.ring{at}canterbury.ac.nz)
2 2Geology Department, National Research Center, 12622 Dokki, Giza, Egypt

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.