Abstract

Analysis of three-dimensional morphological properties of 24 coastal drainage basins that have evolved in areas of low (<1 m/1,000 yr), intermediate (1-3 m/1,000 yr), and high (>3 m/1,000 yr) rates of uplift near the Mendocino triple junction (MTJ), coastal northern California, identified channel gradients as the best indicator of tectonism in the landscape. Lower-order tributaries best reflect tectonically controlled differences. 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 rates. Variations in steepness of first-order channel gradients indicate that (1) high uplift rates in the wake of the MTJ have existed for at least 73,000 yr and (2) differential tilt of the region to the north and south of the current locus of highest uplift rate is occurring in association with regional uplift. Although first-order streams are excellent indicators of areas of both high uplift rates and regional differential tilting, they are less useful in distinguishing between areas of low and intermediate uplift rate. Analysis of the longitudinal profile of the main trunk stream of ten of the 24 drainage basins, using the stream-gradient index, was more useful to categorize broadly the uplift rates and to distinguish between streams in low- and intermediate-uplift-rate areas. Although the hypotheses tested herein do not identify the exact mechanism off uplift in the MTJ region, they do indicate that the nature of deformation is most likely regional tilt rather than crustal shortening with localized compressional folding and thrust faulting. They also indicate that geomorphic responses trail in the wake of the uplift-rate response caused by development of a slab window beneath the North American plate; (1) maximum uplift rates occur ∼9 km south of the northern boundary of the slab window, (2) maximum mean first-order channel gradient occurs 16 km south of the northern edge of the slab window, and (3) maximum drainage-basin relief occurs 20 km south of the northern edge of the slab window. Associated lag times between passage of the southern edge of the subducted Gorda plate slab and geomorphic responses are ∼160,000 yr for maximum uplift rates, ∼290,000 yr for maximum channel gradients, and ∼370,000 yr for maximum relief.

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