QUANTIFYING UPLIFT RATES ALONG THE PACIFIC/NORTH AMERICAN PLATE BOUNDARY IN THE WAKE OF THE PASSAGE OF THE MENDOCINO TRIPLE JUNCTION USING SOIL DEVELOPMENT ON FLIGHTS OF FLUVIAL TERRACES

For the past six years, we have studied the soil development on flights of fluvial terraces in order to better understand the timing of valley development in the wake of the northward migration of the Mendocino Triple Junction in northern California. We present soils data from two areas, 1) the Cache Creek Natural Area (CCNA), east of Clear Lake in Lake County, approximately 220km SSE of the triple junction region, and 2) the Confusion Hill area along Highway 101 in Mendocino County, approximately 60km SE of the triple junction region.

In the CCNA, we find from the soils data and valley geomorphology, that river incision has occurred for at least the past 200,000 +/-100,000 years. The style of incision appears to be episodic, with incision events or episodes separated by intervals of stability and valley widening. Flights of 4-9 river terraces are preserved over a time span of perhaps 0.1 million years in the CCNA. The average rate of uplift estimated by the relative ages of river terraces based upon soil development is approximately 1 meter/ka, varying between about 0.26 m /ka to more than 1m/ka for the oldest surfaces.

In the Confusion Hill area, we see the same episodic incision over approximately the same time span, but the rate of incision is significantly higher and appears to have recently accelerated. Since about 40,000 years ago, the uplift rate is as high as 4m/ka. This high rate of incision is undoubtedly due to the bulge of uplift surrounding the triple junction, a tectonic/geomorphic signature corroborated by previous researchers. In both locations we believe incision has occurred over a much longer time interval (perhaps as much as a million years), but here we only report on the time interval spanned by our soils studies.

In both regions, there are older surfaces (estimated to be 10⁵ years in age) with soils that have well developed Bt horizons (A/Bw/Bt/Cox/Cn profiles), whereas the younger surfaces have soils with Bw horizons (A/Bw/Cox/Cn profiles. The clay percentages of the older soils are substantially greater (e.g. 46% vs 14%), and all profile development indices yield numerical values suggesting an order of magnitude difference in soil ages. In lieu of numerical ages we find the relative age estimates provided by soil chronosequences to be an excellent proxy for an understanding of geologic rates in this tectonic setting.