Quaternary rate of folding of the Ventura Avenue anticline, western Transverse Ranges, southern California
Upper Quaternary terraces of the Ventura River, California, are uplifted, tilted, and folded over the Ventura Avenue anticline. Rates of uplift and tilting have decreased since inception of the structure over the past 200 ka. Assuming that the chronology, based on amino-acid racemization, 14C dates, and soils correlation, is approximately correct, then the minimum possible average rate of uplift in the axial region of the fold has decreased from ∼14 mm/yr to 2 mm/yr during the past 200 ka. Interval rates of uplift for the periods 200 ka to 80 or 105 ka, 80 or 105 ka to 30 ka, and 30 ka to present are, respectively, about 20 mm/yr, 9 mm/yr, and 5 mm/yr. The rate of tilting shows a similar trend, decreasing from ∼5.8 urad/yr, 2.5 urad/yr, and 1.2 urad/yr for the same time intervals, respectively. Based on the mechanics of flexural slip folds in stratified sedimentary rocks, these data suggest that the rootless Ventura Avenue anticline is a fold that has been shortening at a relatively constant rate of about 9 mm/yr since its inception.