An Explanation for the Development of Oblique Normal Faults on Tops of Anticlines
A series of experiments with paper sheets were performed to observe the rotation of fold axis and development of normal faults during transpressional deformation, a condition exists in central and southern California. Under right-lateral shear, it was observed that axes of newly created folds rotated anticlockwise when the direction of the applied transpressional force didn’t change, or when the direction of the applied force changed from more compressional to less compressional but more shearing. Clockwise rotation was observed when the direction of the applied force changed from more shearing to less shearing but more compressional. The observed maximum anticlockwise rotation of a fold axis was 35°, achieved when applied force changed from pure compression to simple shear. The maximum clockwise rotation observed was 22°, achieved when applied force changed from transpression to compression during the experiment. The experimental results can be used to explain the origin of some of the fold-axis-oblique normal faults on some anticlines in central and southern California. In case of Belridge anticline in central California on which normal faults developed less than 30° clockwise from the fold axis, the normal faults may initially developed parallel to the fold hinge. As the right-lateral transpressional deformation continued and the fold axis rotated anticlockwise, the normal faults, as discontinuous material “lines”, remained in the same orientation, creating angles between the fold axis and the fault trend. In case of Wilmington anticline in southern California on which most normal faults are at high angles with the fold axis, the normal faults might have developed late in the folding process. The dragging force created by the nearby Newport-Inglewood fault is the main drive for the creation of the high-angle normal faults. In spite of the satisfaction in explaining these field cases, the experimental observations are in contradiction with some previously reported conclusions that folds in Southern California rotated clockwise rather than anticlockwise since Pliocene.
AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California