The ages of fluvial and volcanic landforms using beryllium-10, aluminum-26, and chlorine-36 exposure age dating

To determine the ages of bedrock river terraces (straths) and a relatively young volcanic feature, I collected samples in northern Pakistan along the Indus River and from Kilbourne Hole, a maar in New Mexico, for cosmogenic exposure age dating. The samples were subjected to a variety of chemical processing techniques to extract Be, Al, and Cl for analysis using accelerator mass spectrometry. With the assistance of personnel at Lawrence Livermore National Laboratory, measurements of 10Be/9Be, 26Al/ 27Al, and 36Cl/Cl in over 50 samples were obtained. Interpretation of these data was aided by whole rock Cl measurements performed at the National Institute for Standards and Technology.; Field tests and evidence show that 10Be and 26 Al exposure ages of bedrock straths along the Indus River appear to closely approximate the abandonment age of the strath. Patterns of strath age and height reveal fast bedrock incision rates along the Indus River that increase with decreasing age and that increase in proximity to an active reverse fault that cuts across the river. These patterns suggest that within the last 60 ka, stream power has increased and that the Indus River has maintained a relatively steep longitudinal profile despite downstream uplift. The Indus River appears to be maintaining a dynamic equilibrium profile form balanced between stream power and differential bedrock uplift along its course.; At Kilbourne Hole, in New Mexico, exposure ages were modeled from 10Be and 26Al measurements in olivine and from 36Cl measurements in basalt. Modeled 10Be and 26Al exposure ages in olivine are in surprising agreement with modeled 36Cl exposure ages in whole rock basalt and yield a minimum eruption age of 30 ka. 40Ar/39Ar dating of lower crustal sanidine erupted during maar formation yield spuriously old ages in excess of 700 ka. Given the thermal history of the samples, the excess Ar may have resulted from a high ambient P_Ar in the lower crust or be related to uptake of atmospheric Ar during rapid cooling following eruption.