Dating gully incision into an abandoned alluvial surface along the eastern termination of the Altyn Tagh Fault Dr. Nimrod Wieler Geological Survey of Israel

Headcut migration and gully incision are amongst the most important geomorphological processes driving soil erosion and land degradation in arid and semi-arid environments. Previous studies that investigated gully incision often constrained the timing of incision using the age of the surface incised and/or the age of post-incision sediment, because of difficulties in directly measuring the age of erosional features. As channels are inherently younger than the landform they incise and are necessarily older than the sediments they accumulate, this common practice may cause large uncertainties in soil erosional rates. To address this problem, we focused on a unique site along the Altyn Tagh Fault (ATF) at its eastern termination near 97°E, where three adjacent gullies incise into a 52 ka abandoned alluvial desert surface and are laterally offset by the ATF by a different magnitude. To estimate the age of each offset gully separately, we employed a locally calibrated version of the stream-power-law, a commonly used model for channel incision. We used high-resolution topography together with the age and reconstructed initial topography of the surface to empirically constrain the model parameters for adjacent gullies that incise the same fan surface but are not offset by the fault. Our computed ages for the offset gullies are all younger than the surface they incise and proportional to the magnitude of offset of the gullies, thus supporting the validity of our approach. Additional field work and geomorphologic analyses are needed to further test this approach in different settings.