Characterising landscape response, erosional supply and sediment flux along active normal faults

Abstract

The erosional fluxes produced from tectonically-active areas determine landscape evolution in upland areas, the mass balance of orogens, and set the initial boundary conditions for depositional stratigraphy. However, deciphering the connections between tectonic forcing, landscape response, erosion and the resulting sediment fluxes, and the rates and timescales at which they operate, remains a first-order challenge in the Earth Sciences. In this thesis, I address this challenge using normal faults in Southern Italy as an excellent template to study the links between tectonics, geomorphology and erosion, taking an empirical approach that combines DEM-based data, structural constraints on active faulting, field data, and cosmogenic radionuclide analysis (CRN). The first part of this thesis derives new constraints about the tectonic activity and evolution of eight major normal faults in Southern Italy: the Vallo di Diano, East Agri and Monti della Maddalena faults, in the Southern Apennines; and the East Crati, West Crati, Serre, Cittanova and Armo faults, in Calabria. I build detailed throw and throw rate profiles for these faults, and I demonstrate that they have throws between ∼ 640-1430 m, and time-averaged throw rates between ∼ 0.6-1.4 mm/yr. For the Calabrian faults, I estimate that regional uplift has uplifted both their hangingwall and footwall blocks by up to ∼ 200-1300 m, depending on location, and that their footwall ranges have variable proportions of inherited pre-faulting relief of up to ∼ 300-800 m. For all the studied faults, channel steepness varies along fault strike in a similar way that fault activity does, with their steepness indices becoming ∼ 5-10m^0.9 steeper for each 0.1 mm/yr increment in throw rates. The long profiles of all the channels draining the footwall ranges have one to two sets of knickpoints, revealing past base level changes. I decipher the magnitudes and origin of these changes, and for the Southern Apennines faults, their timing. In the second part of this thesis, using these faults as well-constrained settings, I investigate hillslope erosional supply and catchment sediment fluxes as a function of tectonic rate, associated landscape response, and lithology. First, I focus on quantifying how lithology controls the delivery processes and the grain size distributions supplied from hillslopes. For this, I have compiled a new landslide inventory of nearly 3000 landslides along the studied faults, based on previously published maps and newly recognized landslides, which I combine with field estimates of rock-mass strength and a field-derived area-volume scaling relationship. This data set reveals that landsliding in the area is not volumetrically dominated by large landslides (𝛽 = 1.98), with 83% of the landslides being < 0.1 km^2 and shallower than 3 m. It also shows that weaker rocks are considerably more prone to landsliding, and that landslides generate fluxes of material that are significant for the catchments sediment export, of the order of 10^2-10^3 m3/yr. Combining these data with field-measured grain size distributions from bedrock weathering products and landslides, I demonstrate that stronger rocks weather into coarser fragmentation products, and that landslides supply systematically coarser material than weathering, with lithology influencing the degree of coarsening. By comparing these grain sizes with those measured at the outlets of footwall channels, I show that sediment export to the hangingwall basins becomes coarser with increasing depths of incision in the catchments, due to the combination of enhanced landsliding and transport capacity in more incised catchments. Finally, I quantify catchment-averaged 10Be erosion rates on twelve representative footwall catchments, which range between 0.10-0.81 mm/yr, and I demonstrate that these are controlled by fault throw rates and the degree of transient incision in the catchments. Overall, erosion removes ca. 70% of the rock being uplifted by the faults, which adds new insights into the mass balance of the study area. The data also reveals that those areas above knickpoints, which have not responded yet to tectonics, are eroding at consistent background rates of ∼ 0.12 mm/yr. I show that, from this background value, erosion rates increase with the degree of transient catchment incision and associated landsliding. These transient erosional dynamics are also reflected by the grain sizes exported from these catchments, with those with greater sediment fluxes and incision delivering significantly coarser grain sizes, likely driven by enhanced coarse supply from landslides. Remarkably, the high degree of correlation of the CRN erosion rates with fault throw rates implies that despite the landsliding, CRN samples from the studied catchments provide reliable estimates. I hypothesize that this is because landslides are frequent, generally small and shallow, and are stored on the hillslopes for up to > 10^3 yrs, and I validate this hypothesis using a published numerical model of landsliding and CRN dynamics. Consequently, this thesis demonstrates that tectonic activity and its associated landscape response control erosion rates and sediment fluxes along active normal faults; that landslides are an integral part of these sediment fluxes; and that lithology modulates landslide occurrence and the grain sizes supplied from hillslopes. The results from this thesis provide new insights into the extensional tectonics, landscape evolution, and seismic hazard in Southern Italy; and the controls of hillslope supply and erosional dynamics in transiently responding settings. Some of these results can be readily incorporated into coupled models of tectonics and landscape response, sediment-flux dependent models of fluvial incision and basin stratigraphy; and models of the cosmogenic nuclide dynamics of landslide-prone catchments.