Abstract

Intragranular strain has been measured by the twinned calcite strain-gage technique from the hinges and limbs of three single-layer minor folds with limb dips of 15°, 48°, and 67°. The folds are in unmetamorphosed, Silurian-age limestone beds enclosed in shale, in the Appalachian Valley and Ridge province of central Pennsylvania. In all three folds, the maximum compressive strain axes are subparallel to bedding and tend to plunge toward the inner arcs of the hinges. The principal deviatoric compressive strains in the fold cross sections range from −0.48 ± 0.77 to −4.75 ± 0.78 percent; the largest compressive strains are in the gentlest fold and the smallest ones are in the tightest fold. Syntectonic stylolites are abundant in the folds and approximate a fanning cleavage. Filled extension fractures occur normal to bedding on the outer arc of the hinges of the two tightest folds. Filled fractures on the limbs of the same folds began as extension fractures subparallel to bedding and evolved into throughgoing thrust faults. A significant amount of the folding deformation is evidently accomplished by pressure solution and by displacement on fractures.

The folds are interpreted as buckle folds and distinguished from transverse bends and passive folds on the basis of the mechanical contrast between the limestone and enclosing shale beds and the orientation and distribution of the principal strain axes, stylolites, and fractures. The orientations of the principal strain axes are best fit by buckle-fold models. Strain models of pure bending, layer-parallel shear, and shear parallel to the hinge plane are shown to be inadequate by themselves even as first-order approximations.

A simple rheological model including intragranular strain (twin and translation gliding, grain boundary adjustments) and pressure solution fits the inferred relationships between these features. The model presumes that intragranular strain has a yield stress, whereas pressure solution does not, and that at high stresses pressure solution is more important than intragranular strain. The model predicts that when conditions are suitable for pressure solution, more of the strain will occur by this mechanism than by twin gliding and related mechanisms. This can explain the overall low values of measured intragranular strain.

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