Collaborative Research: Tropical cyclone imprint on late Quaternary alluvial fans of Baja California: Key for understanding arid regions landscape evolution

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Alluvial fan formation in desert environments is still a debated subject, and application of conceptual models developed for fan formation in specific regions to areas with different climate settings generally lacks robust quantitative support. This international collaborative project aims to input data into these conceptual models by testing alternate hypotheses on the formation of Late Quaternary alluvial fans in the southern Baja California peninsula in Mexico. The central hypothesis is that tropical cyclones have performed most of the geomorphic work during this period and that seasonal high intensity precipitation has been coupled to rapid weathering of bedrock to generate cyclic alluvial fan aggradation in response to millennial-scale climate variability. Combined cosmogenic nuclide depth profile age determination and optically stimulated luminescence depth profile dating will be supported with detailed analyses of sedimentology and soil development of the sedimentary units in the Late Pleistocene-Holocene. These techniques are aimed to determine simultaneously sediment production on the hillslopes and delivery and accumulation rates in the alluvial units. The results will yield a clear picture of when the alluvial units were built, when the alluvial surfaces stabilized, how fast sediment was deposited, and how long the sediment was stored in the hillslopes before transport and deposition. This project (supported by NSF's programs in Geomorphology and Land Use Dynamics, P2C2, and EPSCoR, and Office of International Science and Engineering) will produce datasets and knowledge that are of interest to a larger scientific audience and to the broader public. First, we will test a widely used model of alluvial fan aggradation for the deserts of southwestern North America. Second, we will test how the relevance of tropical cyclones has evolved over millennial timescales in this area and help to analyze tropical cyclone effects in other arid and semi-arid regions of the world (in particular, we will link with studies that assess activity of Eastern Pacific tropical cyclones over the last centuries and their effects on ecosystems and human population in the rapidly growing areas of southwestern U.S. and northwestern Mexico to improve hazard characterization of these storms). Third, our more precise correlation of alluvial fans surfaces normally used as paleoseismological markers will improve determination of earthquake recurrence intervals and of local and regional fault slip rates, increasing our understanding of fault kinematics not only in neighboring areas in Baja California but also in all arid southwestern North America and other regions where alluvial surfaces are used to infer earthquake recurrence rates.