Collaborative Research: Processes Driving Rock Uplift and Flexural Deformation following Convergent Tectonics: The Fluvial Terrace Record, Ebro Basin, Spain

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Lewis 0088714McDonald 0088601Establishing dynamic links between rock uplift and long-term patterns and rates of fluvial incision away from plate margins is a major contemporary challenge in tectonic geomorphology. We are engaged in a multifaceted project that addresses fundamental questions about how mountainous topography and fluvial systems evolve following active crustal shortening. The Spanish Pyrenees and adjacent Ebro Basin comprise an outstanding locality for isolating processes that drive rock-uplift (or subsidence) in orogens where active compression and crustal thickening have ceased. A marked feature of the Ebro Basin is the striking variation in stream incision into ~25 million year old sedimentary rocks. Incision is minimal along the axis of the basin, but reaches more than 1000 m in the south-central Pyrenees. Our data suggest that long-term patterns and rates of fluvial incision in ranges flanking the Ebro Basin are driven by intraplate phenomena, which form the three competing hypotheses we are testing: Streams in this region incise in response to one, or more, of the following mechanisms: (1) rock uplift by isostatic response to removal of mass from the orogen and adjacent foreland basin; (2) rock uplift driven by upwelling of hot, buoyant asthenosphere perhaps related to adjacent, active crustal extension and volcanism; (3) regional base level lowering, perhaps related to the Messinian salinity crisis, Quaternary eustatic sea level changes, or both. We are developing a precise chronostratigraphy (based on radiocarbon, luminescence, and cosmogenic nuclide dating and soils) of selected stream terraces in the Ebro Basin for which spatial and temporal rates of fluvial incision can be reconstructed. Results will be used to generate a conceptual model for the processes that drive rock-uplift (or subsidence) in orogens where active crustal thickening has ceased. Our multidisciplinary approach requires expertise in structural geology, soil stratigraphy, geochronology, tectonic geomorphology, geodynamic modeling, and regional geology. We focus on the central Pyrenees for this work, but our methodology has direct application to both modern and ancient mountain ranges throughout the world.