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Geology; October 1992; v. 20; no. 10; p. 923-926; DOI: 10.1130/0091-7613(1992)020<0923:FORFDS>2.3.CO;2
© 1992 Geological Society of America
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Formation of recumbent folds during synorogenic crustal extension (Austroalpine nappes, Switzerland)

Nikolaus Froitzheim1

1 Geologisch-Paläontologisches Institut der Universität, Bernoullistrasse 32, CH-4056 Basel, Switzerland)

In sedimentary rocks of the Austroalpine nappes in eastern Switzerland, several generations of Alpine (Cretaceous-Tertiary) folds can be distinguished by overprinting relations. The second generation (D2), probably Late Cretaceous in age, includes spectacular, kilometre-scale folds refolding the earlier formed nappe pile. D2 folds are interpreted here as resulting from east-southeast-directed crustal extension rather than, as previously assumed, from north-south crustal shortening, for the following reasons: (1) D2 folds are restricted to a predominantly ductile layer underlying the brittlely extended "orogenic lid," and (2) D2 folds are associated with structures indicating extension in a direction parallel to the extension direction of the brittle layer. Before the east-southeast extension started, the layering of the rocks was already steeply inclined owing to earlier D1 folding. The deformation producing the second-phase folds probably had a noncoaxial component with a top-to-the-east-southeast shear sense and a component of coaxial deformation leading to shortening in the vertical direction and stretching in an east-southeast, horizontal direction. The folds developed with axes oblique to parallel to the direction of bulk stretching. I suggest that folding of initially nonhorizontal layers by ductile crustal extension is an important deformation mechanism in the Austroalpine and possibly also in other parts of the Alps. Initially steep orientations of layers resulted from transpressive deformation in the obliquely convergent Alpine belt, and crustal extension parallel to the chain resulted from gravitational spreading. I propose that this process can lead to the often-encountered phenomenon of stretching parallel to the axes of folds.




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