INFERRING POST-PLIOCENE SURFACE AND BEDROCK UPLIFT AND EROSION IN THE WESTERN KLAMATH MOUNTAINS, U.S, FROM DIGITAL TOPOGRAPHY

The Klamath Mountains Province (KMP), northern California/Southern Oregon, is situated at the juncture of the Mendocino Triple Junction, the southern boundary of the Cascades Volcanic Arc and the Juan de Fuca-North America convergent margin, and the western boundary of the Basin and Range province. KMP topography extends from sea level to over 2 kilometers of elevation. The mean elevation of the central KMP is greater than one kilometer, defining a regional dome of elevated topography. Surface uplift and rock exhumation have been ongoing since the Late Pliocene. A quasi-planar, regionally-extensive erosional surface termed the Klamath Peneplain (KP) is exposed in coastal regions at/near sea level and at elevations in excess of 2 kilometers more than 100 kilometers inland. Pleistocene marine and non-marine deposits have aggraded on the KP, thus preserving the surface. The geometry of the peneplain and the average amount of uplift and erosion may be calculated by interpolating a westward-dipping surface through the basal peneplain exposures. This assumes the peneplain was continuous over the entire western KMP. Time-averaged long-term uplift rates reach a maximum of 0.4mm/yr in the east. The total amount and rate of erosion since formation of the KP may be calculated by subtracting this interpolated surface from modern topography. These time-averaged results indicate that approximately 3850 km3 of material has been removed since the Pliocene at a rate of 0.00077 km3/yr. The maximum long-term erosion rate is 0.29 mm/yr (Salmon River). Our maps suggest that less than sixteen percent of western KMP topography existed before the Pliocene. Paleotopography with elevations greater than a kilometer must have existed in the eastern KMP at the time of maximum peneplanation. On-going analyses of longitudinal profiles of major rivers throughout the KMP indicate that knickpoint elevations and abnormal profile convexities are consistent with Pliocene to recent uplift within the KMP. We speculate that modern surface uplift and rock exhumation in the KMP may be attributed to one or more of the following; 1) northward migration of the Blanco F.Z., 2) recent duplexing of the Franciscan Complex or other accreted terranes beneath the KMP, and 3) development of a serpentine wedge beneath the KMP.