Three-Dimensional Size-Analysis of Folds of Quartz Veins in the Psammitic Schist of the Oboke District, Shikoku

Three-dimensional size-analysis of folds of quartz veins in the psammitic schist of the Oboke district, Shikoku, has been performed. Many of fold-forms observed on the facies of quartz veins show lens-like form elongated along the fold axis, associating bifurcation of folds. A linear relationship is found, between layer-thickness (T) and arc-length (La) as measured on a given joint surface normal to the fold axis and La increases with increase of T. The mode value of La/T ratios is 8-8.5. The La values on individual folds are not always constant as measured on several sections normal to the fold axis, because the folds show commonly lens-like form elongated along the fold axis. The ratios between axial length of fold (Lfa) and T (Lfa/T) appear to show a bimodal distribution, through Lfa tends to increase with increase of T. One of the mode values of Lfa/T ratios is 35 and the other is 65. The small values of Lfa/T ratio tend to be more frequently found on the folds with larger interlimb angles rather than on those with smaller interlimb angles. During the process of growing of amplitude of fold appears to occur frequently the phenomenon that adjacent folds approximately oriented on a straight line are combined with each other into one fold, associating change of La value.