Valerio Olivetti

Quantifying rock uplift rates using channel steepness and cosmogenic nuclide–determined erosion rates: Examples from northern and southern Italy

Rock uplift rates can be difficult to measure over 10 3 –10 5 yr time scales. If, however, a landscape approaches steady state, where hillslope erosion and rock uplift rates are steady and locally similar, then it should be possible to quantify rock uplift rates from hillslope erosion rates. Here, we test this prediction by comparing channel steepness index values and 10 Be catchment-averaged erosion rates to well-constrained rock uplift rates in two landscapes in Italy. The first field area is the Romagna Apennines, northern Italy, where rock uplift rates are relatively uniform, between 0.2 and 0.5 mm/yr (regional mean 0.40 ± 0.15 [SE] mm/yr), and have been steady since 0.9 Ma. The second area is the region around northeastern Sicily and the southernmost Italian peninsula, where rock uplift rates are higher and exhibit a strong spatial gradient, from ∼0.7 to ∼1.6 mm/yr (regional mean 1.09 ± 0.13 [SE] mm/yr). In both regions, channel steepness indices and 10 Be erosion rates vary directly with rock uplift rates. Although there is considerable variability in erosion rates, regionally averaged rates in both the northern (0.46 ± 0.04 [SE] mm/yr) and southern (1.21 ± 0.24 [SE] mm/yr) areas accurately measure rock uplift rates. Although channel steepness indices do not quantify rock uplift rates, they are useful for (1) identifying regional patterns of rock uplift, (2) identifying areas where uplift rates might be expected to be uniform, and (3) informing 10 Be sampling strategies. This study demonstrates that, together, channel steepness and hillslope erosion rates can provide a powerful tool for determining rock uplift rates.

Spreading pulses of the Tyrrhenian Sea during the narrowing of the Calabrian slab

The opening of the Tyrrhenian Sea has been punctuated by short-lived episodes of oceanic accretion on separate small backarc basins during early Pliocene (Vavilov basin) and early Pleistocene (Marsili basin) time. These spreading pulses are related to slab rollback and are synchronous with the reduction of the subduction zone width during the formation of the narrow Calabrian arc. Using laboratory models, we investigated the long-term and transient effects of the reduction of slab width on the subduction kinematics. We found that the abrupt reduction in slab width results in a pulse of acceleration of the trench retreat velocity, as the balance between driving and resisting forces acting on the slab is temporarily modified. Our findings also show that the time scale and amplitude of spreading observed in the Tyrrhenian Sea can be experimentally fitted if the scaled viscosity of the uppermost part of the mantle ranges between 10 19 and 10 20 Pa s.

Spatio-temporal evolution of intraplate strike-slip faulting: The Neogene–Quaternary Kuh-e-Faghan Fault, central Iran

Central Iran provides an ideal region in which to study the long-term morphotectonic response to the nucleation and propagation of intraplate faulting. In this study, a multidisciplinary approach that integrates structural and stratigraphic field investigations with apatite (U + Th)/He (AHe) thermochronometry is used to reconstruct the spatio-temporal evolution of the Kuh-e-Faghan Fault in northeastern central Iran. The Kuh-e-Faghan Fault is a narrow, ~80-km-long, deformation zone that consists of three main broadly left-stepping, E-W–trending, dextral fault strands that cut through the Mesozoic–Paleozoic substratum and the Neogene–Quaternary sedimentary cover. The AHe thermochronometry results indicate that the intrafault blocks along the Kuh-e-Faghan Fault experienced two major episodes of fault-related exhumation at ca. 18 Ma and ca. 4 Ma. The ca. 18 Ma faulting/exhumation episode is chiefly recorded by the structure and depositional architecture of the Neogene deposits along the Kuh-e-Faghan Fault. A source-to-sink scenario can be reconstructed for this time frame, where topographic growth caused the synchronous erosion/exhumation of the pre-Neogene units and deposition of the eroded material in the surrounding fault-bounded continental depocenters. Successively, the Kuh-e-Faghan Fault gradually entered a period of relative tectonic quiescence and, probably, of regional subsidence, during which a thick pile of fine-grained onlapping sediments was deposited. This may have caused resetting of the He ages of apatite in the pre-Neogene and the basal Neogene successions. The ca. 4 Ma faulting episode caused the final exhumation of the fault system, resulting in the current fault zone and topography. The two fault-related exhumation episodes fit with regional early Miocene collision-enhanced uplift/exhumation, and the late Miocene–early Pliocene widespread tectonic reorganization of the Iranian Plateau. The reconstructed long-term, spatially and temporally punctuated fault system evolution in intraplate central Iran during Neogene–Quaternary times may reflect states of far-field stress changes at the collisional boundaries.

Differential Exhumation Across the Longriba Fault System: Implications for the Eastern Tibetan Plateau

The deformation processes at work across the eastern margin of the Tibetan Plateau remain controversial. The interpretation of its tectonic history is often polarized between two deformation models: ductile flow in the lower crust and shortening and crustal thickening accommodated by brittle structures in the upper crust. Many geological investigations on this plateau margin focused on the Longmen Shan, at the western edge of the Sichuan Basin. However, the Longriba fault system (LFS) located 200 km northwest and parallel to the Longmen Shan structures provides an opportunity to understand the role of hinterland faults in eastern Tibet geodynamics. For this reason, we investigate the exhumation history of rocks across the LFS using (U-Th)/He and fission track ages from apatite and zircon. Results show a significant contrast in cooling histories across the Maoergai fault, the southernmost fault of the LFS. South of the Maoergai fault, the bedrock records a rapid increase in exhumation rate since ~10-15 Ma. In contrast, the area north of the fault has experienced steady cooling since ~25-35 Ma. We attribute this cooling contrast to ~2 km of differential rock uplift across the Maoergai fault, providing the first evidence of activity of the LFS in the Late Cenozoic. Our results indicate that deformation of the eastern Tibetan margin has been partitioned into the LFS and the Longmen Shan over an ~200 km wide block, which should be incorporated in future studies on the regions deformation, and in both above-mentioned deformation models.

Dating the topography through thermochronology: Application of Pecube code to inverted vertical profile in the eastern Sila Massif, southern Italy

The Sila Massif is a small part of an orogenic wedge that sits on top of the narrow and active Calabrian subduction zone. The topography of the Sila Massif is characterized by a plateau region whose age and origin has been long debated. Here we integrate new apatite (U-Th)/He data from the eastern flank of the massif with existing apatite fission-track (AFT) data, to constrain the topographic evolution of the massif. The new AHe ages range from 9.7 Ma to 49.8 Ma and overlap the AFT ages indicating that a phase of rapid Cenozoic exhumation was followed by an abrupt decrease of the exhumation rate. A steep/inverse AFT age-elevation relationship from a vertical profile on top of the summit area of the north-eastern Sila may records post-exhumation relief degradation, which is consistent with the low-relief upland topography. To test this hypothesis we performed inverse numerical modeling using Pecube code. Integrating the new AHe ages and the numerical modelling results with the geological constraints we propose a new model for the regional topographic evolution from 30 Ma to the present.