RATES OF SURFICIAL AND TECTONIC PROCESSES IN THE AFAR RIFT (ETHIOPIA) BASED ON REBOUND TEST HAMMER MEASUREMENTS AND ROCK WEATHERING

The Afar rift in Ethiopia is a subaerial exposure of the late stage of continental rifting. Landscape evolution and tectonism are tightly linked, and understanding rates of landscape processes provides important constraints on assessing rates of tectonic activity. Rebound test hammer measurements can serve as a proxy for degree of weathering, consequently, may provide an inexpensive method for estimating the ages of rock surfaces. This method may be particularly suited for the Afar region due to the volcanic bedrock of varying age and the slow weathering rates in this arid environment. In this study, an N-type Schmidt rebound test hammer was used to measure weathering of rock surfaces using a double-strike method to estimate a weathering index. This method measures the difference between subsequent strikes at the same location on a surface, and the difference reflects the thickness of a weathered rind. In this study, a minimum of 50 double-strikes were taken at each sample location to ensure large sample sizes. Samples were collected from recent lava flows, alluvial fan clasts, and stream valleys incised into Early Quaternary Afar Stratoid basalt. Rebound values taken from surfaces of known age (lava flows and Holocene lake stands) were used to calibrate the Weathering Index as a function of time. Preliminary results are consistent with increasing degrees of weathering down the stream valleys, consistent with headward incision. Alluvial fans show increasing weathering with inferred age consistent with the estimations of Weathering Index as a function of time. Once calibrated, the weathering index allows an inexpensive means of estimating the approximate ages of landforms and rates of tectonic and surficial processes.