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The rotation of garnet porphyroblasts around a single fold, Lukmanier Pass, Central Alps
Authors:Peter Visser  Neil S Mancktelow
Abstract:The orientation of the straight internal foliation Si within large ( 5 mm) garnet porphyroblasts has been measured relative to the orientation of the external foliation Se around a single antiform of 0.5 m wavelength, which folds the dominant regional foliation. The internal foliation is not constant in orientation, but varies consistently both with position around the fold and with the porphyroblast ellipticity. The dip of Si (hinge dip taken as zero) is consistently less than the dip of Se; it increases with increasing dip of Se and with increasing ellipticity of the porphyroblasts. Si effectively defines a fold with an opening angle greater than that in the external foliation. The opening angle of this fold in Si decreases with increasing porphyroblast ellipticity. The observed variation in the orientation of Si can be explained qualitatively by a flattened flexural flow model for fold development, as could be expected for folding of a pre-existing, strongly anisotropic foliation. The measurements clearly demonstrate that rotation of porphyroblasts relative to geographical co-ordinates did occur during the development of this fold and that a model based on the classical theories of rotation of stiff inclusions in a weaker viscous matrix is most appropriate.
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