APPLYING GIS-BASED RESIDUAL CONTOURING TO LANDSCAPE EVOLUTION STUDIES IN THE MENDOCINO TRIPLE JUNCTION AND SOUTHERN CASCADIA FOREARC REGIONS, NORTHERN CALIFORNIA

Drainage basins exhibit two end-members for assessing fluvial response to baselevel change - interfluves where erosion is minimal and channel networks that limit local baselevel lowering. We developed a GIS approach to residual contouring that differences envelopes(E) constructed from interfluve altitudes and subenvelopes(S) constructed from stream channel altitudes. We constructed two envelope types: one depicts a surface of minimal lowering tangent to ridge crests, the other depicts a surface of ridge-capping late Pleistocene deposits (Hookton Formation). Subenvelopes were constructed from DEM drainage networks of a specified order and larger. Envelope and subenvelope grids were used to calculate three residual types: (1) envelope residuals (R_E=E-S) representing minimum incision; (2) subenvelope residuals (R_S=S₁-S₂) representing relief in a portion of the drainage network; and (3) topographic residuals (R_Z=E-DEM) representing erosional lowering. We used these techniques to estimate latitudinal gradients in uplift across the southern edge of the Gorda plate inland of the Mendocino triple junction (Mtj) and to identify structural controls on incision in the forearc of the southern Cascadia subduction zone. Latitudinal trends in R_S values (inferred uplift) across the region of Mtj migration indicate that the highest uplift rates occur north of the highest elevations, and that both coincide with inferred plate margin locations. From these trends, we infer northward-migrating uplift and a pattern of landscape evolution inferred by Koehler (1999) and modeled by Furlong and Govers (1999). In the region of active forearc contraction north of the Mtj, axial trends in high R_S values correlate with axes of mapped fault-related folds. Conversely, high R_E values occur on the dissected flanks of folds and along major river valleys oriented transverse to the prevailing NNW structural trend. We believe R_S patterns indicate tectonic forcing of channel gradient in low-order basins, and R_E patterns indicate rapid incision in high-order channels intersecting fold and thrust structures. Based on correlating residual trends with mapped structures, we infer a fault-related fold that links the offshore North Spit anticline with warped, late Pleistocene strath terraces in the onshore North Fork Elk River valley.