Journal of the Geological Society; December 2007; v. 164; no. 6;
p. 1109-1112; DOI: 10.1144/0016-76492007-020
© 2007 Geological Society of London
The nappe rule: why does it work?
Uwe Ring1 and
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.
Copyright © 2008 by Geological Society of London