Abstract

A zone of fine-grained laminated mylonitic rocks is described that is related to the formation of a tectonic slide produced during the refolding of an earlier fold. These mylonitic rocks are characterized by the presence of mesoscopic isoclinal folds whose axial surfaces are parallel with the overall orientation of the lamination and are genetically related to the early folds. Originally equidimensional garnets are tectonically flattened in the axial planes of these early folds as a result of the later folding and it is suggested that the mylonitic lamination is produced directly by flattening perpendicular to the lamination resulting in extensionsal flow parallel with the zone of mylonitic rocks.Thin sections of the laminated mylonitic rocks show the presence of strained augen and ribbon quartz separated by zones of small strain-free polygonal grains which have originated by syntectonic recrystallization. The two former structures have a geometric relation with the early isoclinal structures, whereas the polygonal grains are related to late orthorhombic mesoscopic folds found only within the mylonitic rocks. This latter fabric is interpreted as resulting from restricted extensional flow within the mylonitic zone. The geometry of the fabric is the same within and without the mylonitic zone, the only variation being the intense development of very small strain-free polygonal grains, which from experimental evidence require a high strain rate for their formation. Hence, these mylonitic rocks are interpreted as a zone of high strain rate produced by flattening perpendicular to their plane of flow but otherwise they have a deformational history similar to that of the enveloping rocks.

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