Chronology and rates of faulting of Ventura River terraces, California

Abstract

New evidence concerning the chronology of four late Pleistocene terraces of the Ventura River near Oak View, California, that are vertically offset and tilted by reverse and reverse flexural-slip faults provides a means of estimating rates of fault movement and downcutting by the Ventura River. Radiocarbon ages of charcoal contained within terrace deposits date two of the terraces. Correlation of soils that have developed on terrace deposits and extrapolation of the rate of vertical displacement of the Arroyo Parida-Santa Ana fault (not a flexural-slip fault) date the others. Resulting age estimates for the four main late Pleistocene terraces are Qt5b = 30,000 Qt6a = 38,000, Qt6b = 54,000, and Qt6c = 92,000 yr B.P. Vertical-slip rates on flexural-slip faults range from <0.3 to 1.1 mm/yr and apparently are related to the rate, form, and mechanics of folding of the north limb of the Ayers Creek–Canada Larga syncline. Average rates of downcutting of the Ventura River for several intervals during the late Pleistocene, estimated from the chronology and relative elevation of river terraces north of the Arroyo Panda–Santa Ana fault upstream from the zone of faulting, vary from ∼0.5 to 1.3 mm/yr. The range in rates probably reflects variations in local uplift as well as adjustments to changing eustatic sea level, climatic conditions, and/or regional deformation of the western Transverse Ranges. The average rate of downcutting of the Ventura River north of the zone of flexural-slip faulting is ∼0.8 mm/yr, compared with 1.2 to 2.2 mm/yr in the deformation zone immediately to the south. This apparently indicates that during tectonic deformation there is an approximate balance between the rate of uplift due to faulting and folding and the rate of downcutting by the fluvial system.