Correlation and dating of Quaternary alluvial-fan surfaces using scarp diffusion Leslie Hsu * , Jon D. Pelletier Department of Geosciences, University of Arizona, 1040 E. Fourth St., Tucson, AZ 85721, USA Received 7 June 2002; received in revised form 18 August 2003; accepted 22 August 2003 Available online 20 November 2003 Abstract Great interest has recently been focused on dating and interpreting alluvial-fan surfaces. As a complement to the radiometric methods often used for surface-exposure dating, this paper illustrates a rapid method for correlating and dating fan surfaces using the cross-sectional shape of gullies incised into fan surfaces. The method applies a linear hillslope-diffusion model to invert for the diffusivity age, jt (m 2 ), using an elevation profile or gradient (slope) profile. Gullies near the distal end of fan surfaces are assumed to form quickly following fan entrenchment. Scarps adjacent to these gullies provide a measure of age. The method is illustrated on fan surfaces with ages of approximately 10 ka to 1.2 Ma in the arid southwestern United States. Two areas of focus are Death Valley, California, and the Ajo Mountains piedmont, Arizona. Gully-profile morphology is measured in two ways: by photometrically derived gradient (slope) profiles and by ground-surveyed elevation profiles. The jt values determined using ground-surveyed profiles are more consistent than those determined using photo-derived jt values. However, the mean jt values of both methods are comparable. The photometric method provides an efficient way to quantitatively and objectively correlate and relatively-date alluvial-fan surfaces. The jt values for each surface are determined to approximately 30–50% accuracy. D 2003 Elsevier B.V. All rights reserved. Keywords: Alluvial fans; Diffusion; Geochronology; Quaternary 1. Introduction Alluvial fans are common in many arid and semi- arid regions such as the southwestern United States. In these areas, alluvial-fan deposits provide an important record of Quaternary tectonics and climate change. On a single fan, these deposits are often expressed geo- morphically as multiple surfaces that can be distin- guished from one another by their relief above the active channel, soil and varnish development, dip of the surface, and degree of dissection and degradation (e.g. McFadden et al., 1989; Bull, 1991; Hooke and Dorn, 1992). Accurate ages for these fan surfaces are essential for developing a regional chronology of Quaternary deposits and for determining the relative roles of tectonism and climate in Quaternary fan evolution (Ritter et al., 1995). For example, since local tectonic events are not likely to be regionally synchronous, similar fan-surface ages over a broad region would support climatic triggering of episodes of alluvial-fan deposition. In semiarid areas around the world, many studies have argued for linkages 0169-555X/$ - see front matter D 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.geomorph.2003.08.007 * Corresponding author. Present address: University of Cali- fornia, Berkeley, USA. Fax: +1-510-643-9980. E-mail address: lhsu@eps.berkeley.edu (L. Hsu). www.elsevier.com/locate/geomorph Geomorphology 60 (2004) 319 – 335