The geomorphic record of deep lithospheric deformation

Where deformation of the upper crust is minimal or absent, a young history of topographic change can be related to active or recent processes occurring in the lower crust and mantle lithosphere. Geomorphic systems, fluvial systems in particular, are sensitive indicators of topographic change manifested by the erosional/depositional record, drainage pattern perturbations and landscape form. Evolution of landscape form over geologically relevant timescales (Myr) is commonly derived from low-temperature (<125 °C) histories of rocks in the shallow crust (1 to 4 km) using apatite (U-Th)/He and fission-track dating methods. Quantitative and qualitative measurements of landscape change through topographic analysis, field observations and thermochronometry can be used to understand rates and processes of deformation in the deep lithosphere usually through simple (or sometimes complex) geodynamic models. Utilizing the geomorphic record in this manner is particularly constructive because we cannot otherwise directly observe or quantify deformation in the deep lithosphere in active orogens. The success of this approach depends on our ability to understand relationships between topographic change and lithospheric deformation, and to quantify the response of surface processes to evolving landscape form. Examples from the eastern Tibetan Plateau and the Sierra Nevada, California illustrate how geomorphic evolution of mountain ranges and plateaus may be related to lower crustal flow and mantle lithosphere removal. In these cases, a combination of multi-disciplinary data is used to delineate a holistic view of lithospheric evolution from the surface to the upper mantle that can be used to quantify rates of deformation and improve our understanding of deformation mechanisms. Future directions in linking surface and deep lithospheric processes will be aided by new thermochronometric techniques and further development of quantitative laws that govern surface processes, which will allow us to better understand landscape response to tectonic forcing. In turn, these advances may be applied for a more complete understanding of lithospheric dynamics.