Geomorphic response to late Quaternary tectonism: Coastal northern California, Mendocino triple junction region.

Abstract

Different patterns of uniform-uplift periods associated with passage of the Mendocino triple junction (MTJ) and a change in tectonic regime are based on altudinal spacing analyses of nine flights of marine terraces. Rates of uplift appear to increase from <1 m/ka to 4-5 m/ka, with periods of most rapid uplift that are progressively younger toward the triple junction. The MTJ was about 55 km to the south at about 1 ma ago, where uplift has been uniform at about 1.3 m/ka during the last 300 ka. Maximum uplift of about 4 m/ka is occurring 20-40 km south of the MTJ; prior to about 100-150 ka uplift was slow, being only 0.2-0.5 m/ka. Ten km to the north of the MTJ, accelerated uplift has occurred only during the last 60 ka. Relative strength of soil development in sandy marine deposits on five Pleistocene and Holocene (1.7 to 120 ka) marine terraces near the MTJ is one basis for relative age estimates and correlation of terrace soils. Organic carbon content in the upper 70 cm of the soil profile increases exponentially, and pH decreases exponentially; for both properties steady state is attained by 40 ka. Whole profile content of clay, total free iron oxyhydroxides (Fe(d)), and total free and para-crystalline aluminum oxyhydroxides increase nearly linearly until at least 120 ka. Comparison of these soils with four marine terrace soils (103-405 ka) 100-120 km to the south indicates that maximum percent values of clay and Fe(d) increase exponentially, with highest values reached at about 120 ka. Analysis of three-dimensional morphological properties of 25 coastal drainage basins that have evolved in areas of low (<1 m/ka), intermediate (1-3 m/ka), and high (>3 m/ka) rates of uplift near the MTJ identified channel slopes as the best indicator of tectonism in the landscape. Lower order tributaries reflect tectonically-controlled differences best. The largest streams examined, of third order, are able to adjust to most base-level change and maintain their profile form, whereas lower order streams farther upstream tend to accumulate the effects of net base-level fall, and have steepest profiles in the areas of highest uplift. Although first order streams are excellent indicators of highest uplift rate areas and regional differential tilting, they are less useful in distinguishing between low and intermediate uplift rate areas. Analysis of the longitudinal profile of the main trunk stream of 10 of the 25 drainage basins with the stream-gradient index (Hack, 1957) was useful to broadly categorize uplift rates, and to distinguish between low and intermediate uplift rate streams.