Our ability to understand the importance and implications of contemporary erosion rates and fault slip rates is partially hindered by our inability to put them into a natural, historic context. Recently, a method for estimating erosion rates that integrate over the past 10,000 to 100,000 years has developed.  This method for estimating long-term, basin-wide erosion rates takes advantage of the fact that Beryllium-10 (10Be) is produced by interactions of cosmic radiation with quartz within the top meter below Earth's surface.  This method has developed rapidly over the past decade (Bierman 2004) and fills a critical gap of understanding erosion and landscape evolution on 10,000-100,000 year timescales, for which we have no other rigorous method. Despite widespread use of the method, a few anomalies have been observed and are yet to be addressed, which are likely have important implications for application of the method. Foremost among these anomalies is the grain size dependency that has been observed in some studies (Brown 1995, 1998; Matmon 2003), but not in others (Clapp 2000, 2001, 2002; Granger 1996). Furthermore, in studies that have reported a grain size dependency (ie. the erosion rate estimated from the 10Be inventory in sand differs significantly from that estimated from cobble-sized alluvium), the proposed mechanisms differ and have never been rigorously tested (Belmont 2005a). This study will directly test one of the hypothesized mechanisms (H1) and directly test the hypothesis (H2) that uniform footwall relief, despite an along-strike gradient in tectonic displacement along the Lemhi fault, eastern Idaho, reflects the efficacy of surface processes to remove material.

The Lemhi Range is the footwall block of a ~150 km multi-segment fault in the northeastern margin of the Basin and Range Province, western USA. A variety of rock types are present including a Precambrian quartzite, Ordovician quartzite, Permian to Lower Pennsylvanian sandstone and Eocene mixed silicic and basaltic volcanic ejecta. Relief and catchment slope are uniform over the middle 100 km of the range and decreases to zero at both ends despite the fact that tectonic displacement increases from both ends towards strike center (Densmore 2004).

Brown (1995) hypothesized (H1) that the grain size dependency of 10Be, such that larger grains are 10Be deficient, which infers faster erosion rates, can be attributed to a hillslope process. Landslides contribute large cobbles, which are derived from deeper in the soil/rock profile and have therefore been shielded from cosmic radiation, which quickly attenuates with depth below the surface. Finer materials are generated where sediments slowly creep down hillslopes due to biological and physical processes. Thus, only sampling the sand fraction, as is typically done, leaves one susceptible to missing the 10Be signal carried in larger grains and also makes the measurement contingent on the amount of 'dilution' of the 10Be signal as larger grains break down into sand sized particles.

To test H1, we have identified several catchments in the Lemhi Range to sample. These catchments offer replication of sampling similar rock types across the displacement field as well as different rock types experiencing similar tectonic displacement. Our approach is to perform nested sampling of four basins, including three sites in each basin and two grain sizes (sand and cobble sized grains) at each location. Surficial mapping of geology and hillslope processes is necessary in each basin to understand how each of the experimental basins are eroding. Extensive granulometry of river sediments at each site will allow us to tightly constrain grain size distributions and grain size reduction during fluvial transport. The nested sampling approach also allows us to investigate whether or not progressive breakdown of larger grains dilutes the 10Be signal in alluvial sediments, thereby changing the erosion rate estimate, and if that process is strongly coupled to rock type. Well constrained basin-wide erosion rates for these catchments will allow us to rigorously test the hypothesis of Densmore (2004) that along-strike consistency in catchment slope and relief is an artifact of the efficacy of erosional process in the Lemhi Range (H2), which is a crucial question in the effort to develop predictive models of the topographic response to tectonic activity and earthquake prediction.		  
 

Belmont (2005) In situ terrestrial cosmogenic nuclides in alluvial sediment: grain size matters. GSA Prog w/Abs Annual Meeting Paper No. 190-20
Bierman (2004) Rock to sediment- slope to sea with 10Be-Rates of landscape change. An Rev EPSL 32: 215-255.
Brown (1995) Denudation rates determined from the accumulation of in-situ-produced 10Be in the Luquillo Experimental Forest, Puerto Rico. Earth and Pl Sci Let. 129: 193-202.
Brown (1998) Determination of predevelopment denudation rates of an agricultural watershed (Cayaguas River, Puerto Rico) using in-situ-produced 10Be in river-borne quartz. Earth and Pl Sci Let. 160: 723-728.
Clapp (2002) Using 10Be and 26Al to determine sediment generation rates and identify sediment source areas in an arid region drainage basin. Geomorphology. 45: 89-104.
Clapp (2001) Rates of sediment supply to arroyos from upland erosion determined using in situ produced cosmogenic 10Be and 26Al. Quat Research. 55: 235-245.
Clapp (2000) Sediment yield exceeds sediment production in arid region drainage basins. Geology. 28: no.11, 995-998.
Densmore (2004) Footwall topographic development during continental extension. J Geophys Res. 109, F03001.
Granger (1996) Spatially averaged long-term erosion rates measured from in situ-produced cosmogenic nuclides in alluvial sediment. Journal of Geology. 104: no.3, 249-257.
Matmon (2003) Erosion of an ancient mountain range, the Great Smoky Mountains, North Carolina and Tennessee. Am J of Sci. 303: no.9, 817-855.