CONVERGENCE-RATES ACROSS A DISPLACEMENT TRANSFER ZONE IN THE WESTERN TRANSVERSE RANGES, VENTURA BASIN, CALIFORNIA

Three cross sections are balanced and retrodeformed to the top of the Pleistocene Saugus Formation and to a horizon close to the base of the Jaramillo subchron to yield crustal shortening and shortening rates for the western Transverse Ranges of California. The cross sections compare the shortening that occurs along a transfer zone in which displacement is transferred eastward from a surface reverse fault (Red Mountain fault) to a blind thrust and from that to a combination of both a surface reverse fault (San Cayetano fault) and the blind thrust. Using an age of 250+/-50 ka for the top of the Saugus Formation based on amino-acid racemization of fossil mollusks, crustal shortening rates appear to have abruptly accelerated through time from 3(+4)/(-3) to 5+/-4 mm/yr between 250 and 975 ka to 20+/-6 to 28+/-1 mm/yr since 250+/-50 ka. Using an age of 500 ka for the top of the Saugus Formation based on paleomagnetic stratigraphy, there is little increase in deformation rates through time, ranging from 5+/-5 to 8+/-6 mm/yr between 975-500 ka and from 10+/-3 to 14+/-1 mm/yr since 500 ka. Crustal convergence rates determined by Global Positioning System surveys, taken over 4.6 years, indicate a shortening rate of 7-10 mm/yr across the basin. This is consistent with the slower rates of deformation calculated using an age of 500 ka for the top of the Saugus Formation.