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.