Changes in rock uplift rates and fault slip rates over the past 125 ky in the western Transverse Ranges

Nate W. Onderdonk, Clay Kelty, Ian McGregor, & Eric Tutterow

Published August 16, 2021, SCEC Contribution #11663, 2021 SCEC Annual Meeting Poster #083

A sequence of Quaternary fluvial and marine terraces in the Western Transverse Ranges (WTR) of California record the pattern and rates of rock uplift, folding, and faulting over the past 125 ky and show that deformation is not steady through time.

We mapped, surveyed, and dated (with pIR-IRSL luminescence dating) terraces along the major drainages within the Santa Maria Basin and along the western coastline between Point Conception and Point Sal. Three prominent levels of fluvial terraces are present throughout the Santa Maria Basin and grade into marine terraces formed during sea-level high stands.

The Orcutt Formation is the oldest terrace level and was deposited across a broad low-relief alluvial plain between 125 and 85 ka during a period of tectonic quiescence and planation of older structures. By 85 ka, faults and folds within the Santa Maria Basin began to lift and deform the Orcutt Formation. A subsequent fluvial terrace level developed in drainages incised into the growing topography between 80 and 60 ka and was also lifted, folded, and faulted by continued tectonic activity. The youngest terrace level was deposited between 30 and 45 ka, and is not deformed by most structures, indicating that tectonic activity within the Santa Maria Basin had largely ceased by this time.

Differences in uplift rates from two levels of marine terraces in the Point Conception area to the south, however, show an increase in the rate of deformation since ~50 ka. Previous studies of marine terraces along the Santa Barbara coast also show an increase in uplift rates over the same time period. This suggests a southward shift in deformation during the past 50 ky.

We propose that the episodic nature of deformation in the Santa Maria Basin is due to temporal changes in how slip along the basal detachment, that has been proposed to underlie the WTR, is transferred to the shallower structures observed at the surface. The Santa Maria Basin structures may be a back-thrust complex that alternates with the thrust front farther south under the Santa Barbara Channel.

Key Words
western Transverse Ranges, santa maria basin, episodic deformation

Citation
Onderdonk, N. W., Kelty, C., McGregor, I., & Tutterow, E. (2021, 08). Changes in rock uplift rates and fault slip rates over the past 125 ky in the western Transverse Ranges. Poster Presentation at 2021 SCEC Annual Meeting.

Related Projects & Working Groups
Earthquake Geology