FOLDS WITH AXES PARALLEL TO THE EXTENSION DIRECTION - AN EXPERIMENTAL-STUDY

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 limit ed number of cases, oblique layers still containing the intermediate Y axis. Few experimental studies have considered the case where the ext ension direction X lies within the layer itself, although in nature fo lds with axes parallel to X are quite commonly seen. These folds have often been ascribed to passive rotation of fold axes during continued shear, but it has been shown both theoretically and experimentally tha t active buckle folds can also develop with axes parallel to X. Single - and multilayer analogue model experiments were performed on planar l ayers oriented initially perpendicular to the intermediate Y axis, and with the extension direction X lying within the layer itself. All exp eriments were conducted in plane strain-either in pure shear or simple shear. Paraffin waxes of different melting ranges were used as analog ues for rocks with a power-law theology (stress exponent around 2-3). With a viscosity ratio of ca. 30:1, no measurable fold amplification w as discernible for shortening of 36% or shear strain of 3.6. Neither d omed initial perturbations with circular sections parallel to the laye r nor cylindrical perturbations elongate parallel to the initial stret ching direction were significantly amplified. Only at much higher visc osity ratios (ca. 600:1) did active buckle folding develop. This foldi ng at high viscosity ratio was associated with flow of the matrix in t he X direction around the layer, developing a strong linear fabric par allel to X in the matrix immediately adjacent to the layer. The develo pment of this flow discontinuity between matrix and layer may be chara cteristic of active buckling of layers parallel to XZ, with fold axes parallel to X.