From transient landscape to transient stratigraphy: Characterising the response of sediment routing systems to tectonic and climatic perturbations

The sedimentary record is a unique archive of past erosional responses to tectonic or climatic change. However, the inversion of stratigraphy for these forcing mechanisms remains non-trivial. A problem to date is that geomorphic studies of landscape response to tectonic or climatic perturbation are rarely linked to concomitant changes in basin deposition. Conversely, stratigraphers often focus on the role of accommodation in generating stratal architecture, and neglect the geomorphic controls on the volume, caliber and locus of sediment supplied from eroding landscapes to neighbouring basins. Here, we bridge this key research problem by integrating field observations of catchments responding to a well-constrained increase in fault slip rate in the Central Apennines, Italy, with a simple landscape-to-basin model that describes a footwall to hanging wall sediment routing system crossing a normal fault. Erosion within the modelled catchment is driven by the concentrative effects of the flux of rainfall through the catchment and by hillslope diffusion, acting on a tectonic displacement field associated with cumulative co-seismic slip on a single fault. We solve for erosion using a continuum approach, and we predict the volume and grain-size of sediment released from the catchment at steady-state and during a transient perturbation, using pdfs of grain-size based on observations from catchments in the Apennines. Sediment is deposited in the hanging wall basin by filling tectonically generated accommodation and we predict the down-system variation in grain-size using a self-similarity-based model that drives grain-size fining by selective deposition. Using this simple approach we explore the effects of long-term changes in fault slip rate and rainfall on the catchment and within the basin. We find that variations in both rainfall and tectonic uplift produce diagnostic, but complex, responses in sediment supply, downstream grain-size trends and stratigraphic geometry. In particular, the grain-size distribution of sediment eroded from the catchment is critical to the resultant granulometric texture and stratigraphic geometries of basin sedimentary rocks. Both increases in slip rate and rainfall can generate an increase of coarse gravel into the sedimentary basin system. For a large slip rate increase, the median grain-size entering the system increases faster than the sediment supply, leading to a transient, short and coarse basin deposit. Conversely, for increased rainfall, sediment supply and median grain size increase, leading to progradation of the fan toe and lengthening of the depositional system, before catchment slopes decline and sediment supply falls. These findings are compared with the known hanging wall evolution of tectonically perturbed catchments in Italy. Our results show that the way in which climatic and tectonic changes are recorded in the sedimentary archive is strongly influenced by the dynamics of sediment release from upland catchments, and we demonstrate that transient landscapes necessitate transient stratigraphy