Publication: Large extensional structures developed during emplacement of a
crystalline thrust sheet: the Mondoñedo nappe (NW Spain)
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Publication Date
2003
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Elsevier Ltd.
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Abstract
The Mondoñedo nappe is a crystalline thrust sheet characterized by large recumbent folds, regional intermediate-pressure metamorphism,
synkinematic intrusion of granitoids during nappe emplacement, and an extensional ductile shear zone developed within the nappe during
thrusting. A large tectonic window permits the study of the footwall unit, revealing another extensional shear zone contemporaneous with
thrusting and a low-pressure metamorphic evolution, in contrast to that of the hanging wall unit. The two main extensional shear zones
produced E–W extension parallel to the direction of orogenic shortening and normal to the orogenic structural trend. Furthermore,
subordinate N–S longitudinal extension was accommodated by normal faults in the footwall, and some of these faults were used as lateral
ramps in late stages of thrusting.
The role of the extensional shear zones and faults described is discussed in the context of an evolving orogenic wedge dominated by plate
convergence but characterized by large-scale rheological heterogeneities within it. Deep-seated viscous flow, triggered by heat accumulation,
seems to account for the horizontal stretching and probable tapering of the orogenic wedge, which was induced by gravitational instabilities
due to partial melting and underplating by buoyant continental crust.