Towards a stratigraphic record of dynamic topography

The erosion of mountainous terrain and contemporaneous deposition of sediment within the adjacent basins modifies the applied load upon the surface of the Earth. A simple argument can then be made that an increase in rainfall would lead to increased erosion and deposition, and would therefore directly impact the dynamics and evolution of the linked mountain-basin system. Key to the exploration of this argument is an understanding of how efficient erosion is and the acceptable way to model long-term (> million years) Earth surface processes at relatively large spatial scales (100's of kilometres). Surface processes are complex and chaotic, and up-scaling from individual bed-load processes to the scale of a single river system has yet to be achieved. To overcome this we attempt to find the laws that describe sediment transport on a gross scale. We develop a general and simple length dependent diffusive sediment transport law to model both erosion and deposition that includes the concentrative effects of river systems. This allows us to collapse sediment transport onto a line and couple erosion and deposition with plate flexure. We use this model to interogate the impact of long-term (millions of years) change in rainfall and uplift rate on the stratigraphic archive of continental interior basins. Tilting of the interior of the North American continent due mantle flow, be it lithosphere instability or remnant subducting slabs, has been invoked to be the driver of change in rates and magnitude of river incision observed within the Great Plains. But, an increase in rainfall would also lead to an increase in river incision. Assuming that grains are deposited selectively by size we explore the signals recorded in the stratigraphic record by change in long-wavelength uplift and long-term rainfall. We find that under simple forcing conditions the modelled landscape is highly responsive to change in climate. Increase in rainfall causes incision of previously deposited materialand the pro-gradation of coarse grains down system. Increase in uplift within the catchment and subsidence within the basin, however, has a delayed effect as the landscape takes time to respond to the increase in slope. Furthermore, an increase in uplift causes pro-gradation of the depositional system, and subsequent incision if uplift rates reduce. The degree of incision is crucially dependent on the prescribed boundary condition at the depositional edge of the model domain. Fixed elevation increases the degree of incision, yet fixed slope reduces incision. The model predictions are supported by landscape development of the Rocky Mountain - Great Plains region from the late Miocene to Pleistocene, where: (1) incision was initiated in the Late Miocene due to tectonic tilting (without a significant change in grain-size); (2) deposition of a thick succession of conglomerate (coarser than underlying Miocene succession) ensued from ca. 3.5 to 2.6 Ma, associated with extreme sediment loads and a wetter mid-Pliocene climate; and (3) a period of incision from 2.6 Ma onwards that was climatically-driven, and coincides with the onset of Northern Hemisphere glaciation.