Abstract

Mesozoic pelagic limestones in Umbria (northern Italy) were deformed by Miocene-Pliocene flexural slip folding under 1 to 2 km of cover; there are no signs of metamorphism. Under these conditions the deformation was brittle, as indicated by extension veins and by foraminifera that show no measurable distortional strain. Nevertheless, the limestone is often tightly folded; the folding mechanism combined dissolution of limestone along stylolites subparallel to fold axes with deposition of sparry calcite in extension veins normal to the stylolites. Thus, at the scale of a hand specimen, deformation occurred through rigid-body displacements, but e-twinning in the vein calcite shows a minor component of distributed strain. Spaced cleavage on the limbs of major folds also formed by dissolution. This is shown by (1) the presence of insoluble residues on the cleavage surfaces, (2) imbrication of insoluble chert nodules that indicates tens of percent shortening parallel to bedding, (3) deflection of cleavage away from strain shadows protected by chert nodules and toward places where the nodules have been telescoped, and (4) offsets of bedding at bedding-cleavage intersection that resulted from removal of limestone by dissolution along the cleavage surfaces.

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