FLUVIAL INCISION ACROSS ACTIVELY UPLIFTING FOREARCS: LESSONS LEARNED FROM THE OLYMPIC MOUNTAINS, WASHINGTON, AND NORTHERN APENNINES, ITALY: KIRK BRYAN AWARD

Terrace straths record the incisional history, both vertical and horizontal, of a river carving and lowering its valley bottom. In the tectonically active setting, the carving of straths is affected by local tectonics as well as rock-type, base level, and watershed hydrology. Wide straths are preferentially cut along graded stream reaches where (1) the rock-type is soft, or particularly susceptible to erosion during frequent wetting and drying, (2) the channel is at capacity, and (3) the channel has a meandering pattern with stable discharges consistent with cooler, moister climates. Once created, the separation of the strath from the valley long profile records the movement of rocks through an orogenic wedge. Perhaps the two most revealing observations to emerge from detailed mapping of strath separations is that (1) the uplift rate across the orogenic wedge is not constant and (2) the horizontal movement of rocks is both significant and can be greater than the vertical rate of rock uplift, especially in distal parts of the wedge. For the Olympic Mountains, we prefer an interpretation of greater rock uplift in the central massif rather than climatically-induced profile gradient lowering to explain upstream divergence of terraces because of the constant incision rate at a point for terraces of different age and the long-term record of steady exhumation from thermochronologic data. In the central massif, the observed divergence actually underestimates the true rate of rock uplift because the straths are moving horizontally at twice the rate of vertical rock uplift. These observations are even better appreciated in the northern Apennines where straths begin their life as unconformities in the Apennine foredeep or in the lower reaches of river valleys, are shortened, uplifted, and advected horizontally through the topographic wedge where they pass through the range crest, extend, and collapse back down to sea level. From the material reference frame, these straths record the passage of an orogenic wave. Continued research into strath genesis and their relationships with valley profiles will undoubtedly enhance our understanding of the geodynamic and surficial process interactions of active orogens.