Determining The Impact Of Faulting On The Rate Of Erosion In A Low-relief Landscape: A Case Study Using In Situ Produced ~(21)ne On Active Normal Faults In The Bishoptuff, California

摘要

Normal faults exposed in the Volcanic Tableland in the southeastern part of the Bishop Tuff provide a unique opportunity to investigate whether tectonic faulting and associated fracturing control the rate of erosion in the vicinity of actively growing faults. If fracturing increases with fault displacement, erosion close to the faults should be more rapid for faults with higher displacement. Here we use in situ produced cosmogenic ~(21)Ne to quantify the erosion rate in the flat footwall blocks of three normal faults that have maximum vertical displacements between ～12 and ～160 m. ~(21)Ne concentrations in surface samples from volcanic bedrock and desert pavements along the three faults are nearly uniform and demonstrate that erosion is spatially invariant, both along individual faults and between faults with different tectonic offsets. Hence, faulting and fracturing of rocks adjacent to the active normal faults in the Bishop Tuff do not have a significant impact on the rate of erosion in the sub-horizontal footwall blocks next to the faults. This is further supported by samples from sites which are unaffected by faulting but record a similar erosion rate as samples located near faults. Using the density of the Bishop Tuff and a published ~(40)Ar/~(39)Ar eruption age for the tuff (760+2 ka), the absolute amount of erosion since eruption has been quantified. At most sample sites the time-integrated amount of erosion ranges between ～0.9 and ～1.9 m, which is equivalent to average erosion rates between ～1.2 and ～2.5 m/Ma. Samples of bedrock-derived clasts from desert pavements have slightly higher ~(21)Ne concentrations than adjacent bedrock outcrops and thus yield higher apparent exposure ages (calculated assuming no erosion). Thus, desert pavements are more suitable for determining the minimum eruption ages of yet-undated welded tuffs than bedrock exposures. Desert pavements from the Bishop Tuff yield exposure ages that are ～60% of the tuffs ~(40)Ar/~(39)Ar eruption age.