EVALUATING PERMANENT STRAIN AND SURFACE UPLIFT IN THE SOUTHERN CASCADIA FOREARC

Vertical motions of the crust along convergent margins are driven primarily by elastic strain accumulation during interseismic periods, but may also record a component of permanent, non-elastic deformation of the overriding plate. Separating these components of modern strain fields is challenging, as rates of permanent strain accumulation that drive mountain building are difficult to directly measure. In the southern Cascadia forearc, preservation of a relict landscape in the Klamath and Siskiyou Mountains allows assessment of rates and patterns of deformation during Plio-Quaternary mountain building. This relict landscape, the “Klamath Peneplain” of Diller (1902), is preserved as low-relief interfluves and ridgetops that exhibit extensively weathered bedrock and regolith and, in some places, are capped with terrestrial and marine deposits. Near Crescent City, CA, these surfaces are inferred to have once been continuous with, and graded to, late Pliocene marine deposits.

To reconstruct the former extent of the low-relief landscape represented by these remnant surfaces, we conducted a GIS-based analysis of both mapped surface remnants (Irwin, 1997) and topographically continuous ridgelines. Observations of the surface remnants indicate the flat-topped ridges of the Peneplain relics grade downwards from the eastern extent of the Klamath mountains to the coast. We confirmed the presence of fluvial sand and gravel deposits on several of these surfaces; gravel deposits range from a few meters to tens of meters in thickness and are composed of cobble to boulder-size clasts. Clear imbrication and bedding suggest these were deposited in ancient fluvial channels atop the relict landscape. Deposits range from 400 – 900 meters above modern canyons and provide direct constraints on the magnitude of incision in the past few Myr. By differencing the present-day landscape and the reconstructed surface we estimated the minimum mass removal by erosion. We are currently using this result to model the flexural-isostatic response to quantify the magnitude and spatial pattern of rock uplift due to isostatic rebound during growth of the Coast Ranges. When tied to the Pliocene sea level, our results will constrain the relative contribution of permanent strain to modern-day vertical motion in this region of the Cascadia forearc.