In mountain rivers, sediment from landslides or debris flows can alluviate portions or even full reaches of bedrock channel beds, influencing bedrock river incision rates. Various landscape evolution models have been developed to account for the coevolution of alluvial cover and sediment-flux-dependent bedrock incision. Despite the commonality of their aims, one major difference between these models is the way they account for and conserve sediment. We combine two of the most widely used sediment conservation schemes, an Exner-type scheme and an erosion-deposition scheme, with the saltation-abrasion model for bedrock incision to simulate the coevolution of sediment transport and bedrock incision in a mixed bedrock-alluvial river. We compare models incorporating each of these schemes and perform numerical simulations to explore the transient evolution of bedrock incision rates in response to changes in sediment input. Our results show that the time required for bedrock incision rates to reach a time-invariant value in response to changes in sediment supply is over an order of magnitude faster using the Exner-type scheme than the erosion-deposition scheme. These different response times lead to significantly different time-averaged bedrock incision rates, particularly when the sediment supply is periodic. We explore the implications of different model predictions for modeling mixed bedrock-alluvial rivers where sediment is inevitably delivered to rivers episodically during specific tectonic and climatic events.

Seismic instruments placed outside of spatially extensive hazard zones can be used to rapidly sense a range of mass movements. However, it remains challenging to automatically detect specific events of interest. Benford's law, which states that first non-zero-digit of given datasets follow a specific probability distribution, can provide a computationally cheap approach to identifying anomalies in large datasets and potentially be used for event detection. Here, we select raw seismic signals to derive the first-digit distribution. The seismic signals generated by debris flows, landslides, lahars, and glacier-lake-outburst floods follow Benford's law, while those generated by ambient noise, rockfalls, and bedload transports do not. Focusing on debris flows, our Benford's-law-based detector is comparable to an existing random forest method for the Illgraben, Switzerland, but requires only single station data and three non-dimensional parameters. We suggest this computationally cheap, novel technique offers an alternative for event recognition and potentially for real-time warnings.

We present an O(n) complexity and implicit algorithm for the two-dimensional solution of the Stream Power Incision Model (SPIM) enriched by a discharge threshold term and taking into account variability in rainfall and thus discharge. The algorithm is based on the formulation developed by Deal et al (2018) and the generalization of the FastScape algorithm \cite{BraunWillett2013}where the slope is approximated by first-order accurate finite difference. We consider a variety of discharge thresholds that vary in their dependence on channel slope. The algorithm requires finding the root of a non-linear equation using a Newton-Raphson iterative scheme. We show that the convergence of this scheme is unconditional, except for a narrow range of model parameters where the threshold increases with the slope and for low discharge variability. We also show that the rate of convergence of the iterative scheme is directly proportional to the slope exponent n in the SPIM. We compare the algorithm to analytical solutions and to numerical solutions obtained using a higher-order finite difference scheme. We show that the accuracy of the FastScape algorithm and its generalization presented here is comparable to other schemes for values of n>1. We also confirm that the FastScape algorithm and its generalization to variable discharge+threshold conditions does not need to satisfy the CFL condition and provides an accurate solution for both small and very long time steps. We finally use the new algorithm to quantify how the existence of an erosional threshold strongly affects the length of the post-orogenic decay of mountain belts.

Deep incised glacial valleys surrounded by high peaks form the modern topography of the Southern Patagonian Andes. Two Miocene plutonic complexes in the Andean retroarc, the cores of the Fitz Roy (49°S) and Torres del Paine (51°S) massifs, were emplaced at 16.7±0.3 Ma and 12.5±0.1 Ma, respectively. Subduction of ocean ridge segments initiated at 54°S, generating northward opening of an asthenospheric window with associated mantle upwelling and orogenic shortening since 16 Ma. Subsequently, the onset of major glaciations at 7 Ma caused drastic changes in the regional topographic evolution. To constrain the respective contributions of tectonic convergence, mantle upwelling and fluvio-glacial erosion to rock exhumation, we present inverse thermal modeling of a new dataset of zircon and apatite (U-Th)/He from the two massifs, complemented by apatite 4He/3He data for Torres del Paine. Our results show rapid rock exhumation recorded in the Fitz Roy massif between 10.5 and 9 Ma, which we ascribe to mantle upwelling and/or crustal shortening due to ridge subduction at 49°S. Both massifs record a pulse of rock exhumation between 6.5 and 4.5 Ma, which we interpret as the result of the onset of Patagonian glaciations. After a period of erosional quiescence during the Miocene/Pliocene transition, increased rock exhumation since 3-2 Ma to present day is interpreted as the result of alpine glacial valley carving promoted by reinforced glacial-interglacial cycles. This study demonstrates that along-strike thermochronological studies provide us with the means to assess the spatio-temporal variations in tectonic, mantle, and surface processes forcing on rock exhumation.

The uplift of the southern African Plateau is often attributed to mantle processes, but there are conflicting theories for the specific timing and drivers of topographic development. Evidence for most proposed plateau development histories is derived from continental erosion histories, marine stratigraphic architecture, or landscape morphology. Here we use a landscape evolution model to integrate these three types of data for southern Africa, including a large dataset of low temperature thermochronology, sediment flux rates to surrounding marine basins. We explore three main hypotheses for surface uplift: 1) southern Africa was already elevated at the time of Gondwana breakup, 2) uplift and continental tilting occurred in the mid-Cretaceous, or 3) uplift occurred in the mid to late Cenozoic. We test which of these three intervals of plateau development are plausible by using an inversion method to constrain the range in erosional and uplift model parameters that can best reproduce the observed data. Results indicate two families of uplift histories are most compatible with the data. Both have limited initial topography with some topographic uplift and continental tilting starting in the east at ~95 Ma. In one acceptable scenario, nearly all of the topography, ~1400 m, is created at this time with little Cenozoic uplift. In the other acceptable scenario, only ~500 m of uplift occurs in the mid-Cretaceous with another ~850 m of uplift in the mid-Cenozoic. The two model scenarios have different geodynamic implications, which in the future could be evaluated by direct comparison between geodynamic and landscape model predictions.

We use a Landscape Evolution Model (FastScape S2S) to explore the impact of inherited topography in the foreland domain of a rising mountain range on its stratigraphic architecture and sediment accumulation history, inspired by the northern Pyrenean foreland. We simulate an uplifting half mountain range, its foreland basin and forebulge, and beyond, an open marine domain. We ran models with 4 different initial reliefs in the foreland domain: an initially flat foreland domain at sea-level, an elevated flat continental foreland (+300 m), a pre-existing 1 km-deep and 100 km-wide bathymetry at the location of the future foreland basin associated with a forebulge domain either at sea-level or elevated at +300m. All models show a prograding mega-sequence associated with building of mountain topography and development of the flexural foreland basin and forebulge, coalescence of alluvial fans at the foot of the range, progressive continentalization of the foreland domain, and burial of the forebulge. An initially elevated foreland domain ultimately produces a thinner foreland basin while an initially deep foreland basin produces a thicker one. After 10-13 Myr, the initial relief of foreland domain is smoothed out and the landscape does not exhibit a record of pre-existing relief. In contrast, the stratigraphic architecture of the foreland basin allows to trace inherited relief with deep marine sediments in the initially deep foreland basin, marine sediments onlapping and then burying the forebulge initially at sea-level, and continental sediments onlapping and burying the initially elevated foreland domain. We compare these interpretations to the Pyrenean retro-foreland.