DETERMINING UPLIFT RATES AND PATTERNS FROM QUATERNARY MARINE TERRACES OF THE POINT REYES PENINSULA, CALIFORNIA

We are using uplifted marine terraces to evaluate the contractional component of deformation associated with the Point Reyes segment of the San Andreas fault (SAF). The terraces consist of platforms that are cut by waves and subsequently blanketed by beach and fluvial deposits. The platforms now exposed on land were cut during sea-level highstands and can be correlated to eustatic events whose ages and elevations are known. We used a Global Positioning System (GPS) to measure terrace elevations along five coast-perpendicular transects. We created terrace profiles for each transect and correlated the terrace surfaces between transects.

We are using the terrace data to evaluate the style of deformation and to estimate uplift rates. The terraces have been uplifted during the formation of a broad syncline that makes up the peninsula. The terrace flights we measured are on the eastern limb of the fold. We did not measure terraces on the western limb of the fold because they are poorly developed there. The lowest terrace increases in elevation southward along the coast, because the southern end of the terrace is on the limb of a fold, whereas the northern end of the terrace is closer to the fold axis where the uplift rate has been less. The fold does not appear to be plunging because we can correlate terraces at the same elevation parallel to the fold. To estimate the uplift rate we are obtaining ages for the lowest terrace, where sediments are preserved. Preliminary ages suggest that the terrace sediments may be as young as 30 ka, but our results are not yet complete. We will present our estimate of uplift rates based on a larger data set.