USING INHERITED OSL SIGNALS TO QUANTIFY LANDSCAPE DYNAMICS: EXAMPLES FROM THE JALISCO BLOCK TO THE FAULTS OF SIERRA DE JUáREZ (MEXICO)

Cosmogenic radionuclides (CRN) and optically stimulated luminescence (OSL) are two recent geochronological techniques that have expanded the capabilities of quantifying geomorphic processes and the age of landforms. Traditionally, OSL has been used to date the age of deposits (< 0.5 Ma) and in some cases such ages complement the results obtained from CRNs. Here we extend the usage of OSL signals in order to: (1) determine the presence of erosional pulses in low energy depositional settings and (2) explore if OSL signals can provide additional information about the erosion rates in the landscape as ¹⁰Be-based catchment-averaged denudation rates do.

For the first approach we extracted and analyzed two cores from Sayula and San Marcos lakes located in the triple junction of the Jalisco Block (west-central Mexico). Even though OSL signals exhibit the expected pattern of increasing luminescence age with depth, our analysis of cores using a Pulsed Photon Stimulated Luminescence unit indicate that there has been different rates of deposition and changes in the precedence of sediment. These results are also supported with geochemical analysis of major elements. For the second approach we analyzed the faults of Oaxaca and Donají which form the front of Sierra de Juarez (southern Mexico). From the active channels incising across thefaults we extracted sediment samples to determine the ¹⁰Be catchment-averaged erosion rates and to measure the OSL signals. At Donají fault, OSL signals closely scale with ¹⁰Be catchment-averaged denudation rates (R² = 0.78 for IRSL and R² = 0.71 for BLSL) and the last correlates with the mean basin rainfall (R² = 0.86). At Oaxaca fault, OSL signals are negatively correlated with ¹⁰Be catchment-averaged denudation rates. Based on our OSL and ¹⁰Be data, as well as on topographic analyses we propose that the landscape along the Oaxaca fault is in disequilibrium due to the fault offset, whereas in Donají, which is known to be a transfer fault, the landscape seems to have reach a state close to equilibrium. Here we demonstrate that OSL signals, supported with ¹⁰Be-derived denudation rates, can yield information on the rates that prevail in the landscape.