LATERAL VARIATIONS OF STRAIN IN EXPERIMENTAL FORCED FOLDS

Packages of rock layers, deformed under confining pressure by the upli ft and rotation of a steel-block forcing assembly, translate both towa rds, and away from, the margin of the principal uplifted block. The re sulting asymmetric forced folds, and especially their long, planar lim bs, exhibit along-layer variations in strain. Alternations of layer el ongation and contraction occur along profiles extending away from the antiform/synform couplet. Layer-normal strains are mostly nil, so the longitudinal strains largely equate to volume strains in these plane-s train models. Spaced anomalies in outcrops, indicating either increase d cementation, or erosional weakness, may suggest that similar process es operate in nature to produce variations in damage caused during the flexural-slip process. Two, non-exclusive explanations are offered to account for the patterns of strain observed in the experiments: (1) t hey may be caused by decaying wavetrains of small-scale flexural defle ctions (and their local strain patterns) related to the bending of the major forced folds; or (2) they may be caused by a 'patchy' developme nt of layer-parallel slip, and the consequent spatial variability in d isplacements. (C) 1998 Elsevier Science B.V. All rights reserved.