Geomorphic Indicators of Active Fold Growth: South Mountain - Oak Ridge Anticline, Ventura Basin, Southern California
Antonio Azor Pérez, Edward A. Keller, & Robert S. YeatsPublished June 2002, SCEC Contribution #650
South Mountain–Oak Ridge, near Ventura, California, is an asymmetric anticlinal uplift forming at the present time above the active, buried Oak Ridge reverse fault. Shortening along the Oak Ridge fault accumulated largely in Quaternary time and is responsible for the growth and present topography of the westernmost 15 km of the ridge during the past 0.5 m.y. Tectonic geomorphic analysis using several indices of active tectonics provides information concerning fold growth. Stream-gradient indices are relatively high in the northern, eroded fold scarp of the ridge, a pattern consistent with the existence of active, rapid slip on the Oak Ridge fault. Mountain-front sinuosity along the northern slope of the anticlinal ridge roughly decreases from ∼2 to 1 toward the westernmost 10 km of observed surface folding. Valley floor width to valley height ratios along the northern flank of the ridge generally decrease westward from ∼1.5 to 0.5. Values of the hypsometric integral along the northern flank increase significantly from ∼0.35 to 0.4 (maximum ∼0.55) from east to west. Drainage density varies from ∼4 to 6 km/km2 along both flanks of the South Mountain–Oak Ridge anticline. Entrenchment of streams into the (southern) backlimb of the fold along the westernmost 9 km of the structure decreases from ∼20 m to <1 m from east to west. Apparent backlimb rotation, as measured by dip of strata along the westernmost 7 km of the fold, decreases from east to west, from ∼35° to 20°.
Citation
Azor Pérez, A., Keller, E. A., & Yeats, R. S. (2002). Geomorphic Indicators of Active Fold Growth: South Mountain - Oak Ridge Anticline, Ventura Basin, Southern California. Geological Society of America Bulletin, 114(6), 745-753. doi: 10.1130/0016-7606(2002)114<0745:GIOAFG>2.0.CO;2.
