VARIABLE AMOUNTS AND RATES OF FLUVIAL AND MARINE TERRACE DEFORMATION IN THE WESTERN TRANSVERSE RANGES OF CALIFORNIA SUGGEST THAT SLIP RATES ON SOME OF THE FAULTS HAVE CHANGED SIGNIFICANTLY OVER THE PAST 100 KYR

The western Transverse Ranges of California is an active fold and thrust belt with high rates of shortening (7 to 12 mm/yr) and rock uplift (up to 7 mm/yr). We used late Quaternary fluvial and marine terraces as markers to measure deformation rates across the major faults in the inverted Santa Maria Basin that makes up the northwestern part of the mountain belt. Mapping, surveying, and luminescence dating of multiple terrace levels show that rock uplift and fault slip rates have changed along some of the major faults over the past 100 kyr.

Luminescence (pIR-IRSL) dates from 22 sites within the Santa Ynez River watershed show that there are three regional strath terrace levels that formed during warm and dry climate intervals at 95 - 85 ka (Qt3), 75 - 55 ka (Qt2), and 45 - 30 ka (Qt1). The oldest terrace levels have been lifted, folded, and faulted across the Baseline and Santa Ynez River faults, and rock uplift rates were 0.9 to 3.4 mm/yr in the hanging walls of the faults between Qt3 and Qt2 time. However, the deformation and rock uplift rates decreased after Qt2 time, and the Qt1 terraces are undeformed across these two faults. Conversely, the Santa Ynez fault displaces Qt1 terraces with a dip-slip rate of 0.48 mm/yr since ~45 ka. Differences in rock uplift measured in the hanging wall of the fault where it intersects the coastline suggest that the slip rate of the Santa Ynez fault may have increased over the past ~85 kyr while rock uplift rates decreased along the Baseline and Santa Ynez River faults to the north.

These apparent variations in rock uplift and fault slip rates over periods of 10,000 to 100,000 years are too long to be attributed to normal variations in fault rupture recurrence intervals in an active tectonic plate boundary. We therefore interpret that these variations reflect changes in how and where regional shortening across the mountain belt is accommodated. This variable style of deformation may have also occurred in the past based on the presence of multiple angular unconformities in the Miocene to Pleistocene sedimentary deposits preserved in the hanging walls of these faults. Thus, the changes in deformation rates recorded by the late Quaternary terraces may be the most recent manifestation of a longer-term cycle of faults turning off and on at the western end of the Transverse Ranges.