Folds with axes parallel to the extension direction: an experimental study

Abstract

Experimental studies of single- and multilayer folding have generally considered shortening of layers oriented perpendicular to the maximum extension direction X (i.e. layers parallel to YZ), or in a more limited number of cases, oblique layers still containing the intermediate Y axis. Few experimental studies have considered the case where the extension direction X lies within the layer itself, although in nature folds with axes parallel to X are quite common. These folds have often been ascribed to passive rotation of fold axes during continued shear, but it has been shown both theoretically and experimentally that active buckle folds can also develop with axes parallel to X. Single- and multilayer analogue model experiments were performed on planar layers oriented initially perpendicular to the intermediate Y axis, and with the extension direction X lying within the layer itself. All experiments were conducted in plane strain - either in pure shear or simple shear. Paraffin waxes of different melting ranges were used as analogues for rocks with a power-law rheology (stress exponent around 2-3). With a viscosity ratio of ca. 30:1, no measurable fold amplification was discernible for shortening of 36% or shear strain of 3.6. Neither domed initial perturbations with circular sections parallel to the layer nor cylindrical perturbations elongate parallel to the initial stretching direction were significantly amplified. Only at much higher viscosity ratios (ca. 600:1) did active buckle folding develop. This folding at high viscosity ratio was associated with flow of the matrix in the X direction around the layer, developing a strong linear fabric parallel to X in the matrix immediately adjacent to the layer. The development of thisflow discontinuity between matrix and layer may be characteristic of active buckling of layers parallel to XZ, with fold axes parallel to X.