Evolution of passive margin escarpments: what can we learn from low-temperature thermochronology?
Résumé
Recent studies integrating geomorphology, thermochronology, cosmogenic erosion rate estimates, and numerical modeling suggest that escarpment evolution may take place following two dramatically different modes: (1) parallel retreat from the escarpment's original position at the continent-ocean boundary to its present-day inland position and (2) formation-in-place by progressive downwearing of a plateau initially located between the coast and a preexisting inland drainage divide. Using a three-dimensional finite element model to solve the heat transfer equation, we show that the mode of migration of a passive margin escarpment can be constrained by low-temperature (apatite (U-Th)/He) thermochronology. We first couple the heat equation solver to a surface processes model that predicts the two different escarpment evolution modes from only slightly different initial conditions. We predict (U-Th)/He age distributions that are markedly different for the two scenarios. We perform a thorough investigation of the model behavior to determine under which circumstances thermochronological data can be used to constrain passive margin escarpment dynamics. These conditions include (a) a tall escarpment, (b) a high geothermal gradient, and/or (c) a low flexural rigidity of the lithosphere. We demonstrate that to determine the rate and mode of escarpment migration from low-temperature thermochronology, one needs to collect samples along transects perpendicular as well as parallel to the escarpment. Tightest constraints on escarpment development are provided by (in ascending order) the minimum (U-Th)/He age encountered seaward of the escarpment, the location of where the minimum age is found, the slope of the age-distance relationship (in a direction perpendicular to the coast), and the slope of the age-elevation relationship (from a transect parallel to the escarpment). We finally demonstrate that there are situations where thermochronological data sets do not provide constraints on the mode of escarpment migration, such as along the escarpment of southeastern Australia, where migration has possibly been very rapid. Using the Neighborhood Algorithm method, we are, however, able to extract from an existing apatite (U-Th)/He data set very useful constraints on the evolution of the southeastern Australian escarpment, including the duration of the migration event (<15 Myr), the local geothermal gradient (32°–40°C km−1), and the effective elastic thickness of the underlying lithosphere (6–8 km).
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