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 f or the western Transverse Ranges of California. The cross sections com pare the shortening that occurs along a transfer zone in which displac ement is transferred eastward from a surface reverse fault (Red Mounta in fault) to a blind thrust and from that to a combination of both a s urface 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 ami no-acid racemization of fossil mollusks, crustal shortening rates appe ar 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 o n 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 r ates determined by Global Positioning System surveys, taken over 4.6 y ears, 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.