Abstract Active faults accommodate tectonic plate motion through different slip modes, some stable and aseismic, others characterized by the occurrence of large earthquakes after long periods of inactivity. Although the slip mode estimation is of primary importance to improve seismic hazard assessment, this parameter inferred today from geodetic observations needs to be better constrained over many seismic cycles. From an analytical formulation developed for analyzing fault scarp formation and degradation, we show that the final topographic shape generated by one earthquake rupture or by creep (i.e., continuous slip) deviates by as much as 10–20%. This result opens up the theoretical possibility of inverting the number of earthquakes and their sizes from scarp morphologies. Our approach highlights the importance of trade-offs between fault slip history and diffusive processes. An identical topographic profile can be obtained either with stable fault creep together with rapid erosion, or with a single earthquake rupture followed by slow erosion. Our findings reveal that the previously noticed variation of the diffusion coefficient with the age of the scarp may be an artifact related to the history and mechanism of fault slip. These inferences, derived from the simplest possible diffusion model, are likely to be even more pronounced in nature.
Multiple uplift events, either by discrete earthquakes or creep, will steepen and thus apparently rejuvenate fault scarps, raising the possibility that fault slip history leaves a hidden morphological signature. Here we explore this idea by proposing a new analytical formulation to simulate the scarp degradation generated by faulting at regular intervals. Our formulation fills the gap between the single rupture and the creeping fault proposed solutions. We show that the morphology of degrading fault scarps generated by one major or multiple minor earthquakes with the same final total uplift deviates by as much as 10-20%. Our inversion approach highlights the importance of trade-offs between fault slip history and erosion intensity. An identical topographic profile can be obtained either with a stable creep and an intense erosion or with a single seismic event and a weak erosion. Finally, our findings reveal that the previously noticed variation of the diffusion coefficient with time may be an artifact related to the kinematics of faulting. These inferences, derived from the simplest possible diffusion model, are likely to be even more pronounced in nature.
Worldwide rivers annually export about 19 Gigatons of sediments to the ocean that mostly accumulate in the coastal
zones and on the continental shelves. This sediment discharge testifies of the intensity of continental erosion
and records changes in climate, tectonics and human activity. However, natural and instrumental uncertainties
inherent to the in-situ measurements of sediment discharge prevent from conclusive estimates to better understand
these linkages. Here we develop a new method, using the Gravity Recovery and Climate Experiment (GRACE)
satellite data, to infer mass-integrative estimates of sediment discharge of large rivers to the ocean. GRACE satellite
provides global gravity time series that have proven useful for quantifying mass transport, including continental
water redistribution at the Earth surface (ice sheets and glaciers melting, groundwater storage variations) but has
been seldom used for monitoring sediment mass transfers so far. Here we pair the analysis of regularized GRACE
solutions at high spatial resolution corrected from all known contributions (hydrology, ocean, atmosphere) to a
particle tracking model that predicts the location of the sediment sinks for 13 rivers with the highest sediments
loads in the world. We find that the resulting GRACE-derived sediment discharges off the mouth of the Amazon,
Ganges-Brahmaputra, Changjiang (Yangtze), Indus, Magdalena, Godavari and Mekong rivers are consistent with
in-situ measurements. Our results suggest that the lack of time continuity and of global coverage in terrestrial
sediment discharge measurements could be reduced by using GRACE, which provides global and continuous data
since 2002. GRACE solutions are regularly improved and new satellite gravity missions are being prepared hence
making our approach even more relevant in a near future. The accumulation of sediments over time will keep
increasing the signal to noise ratio of the gravity time series, which will improve the precision of the GRACE-
derived sediment discharges values.
The Pyrenean Range results from the collision between Eurasia and Iberia plates that started during the Santonian. The Pyrenean orogeny is considered to end in Early Miocene times, since from that period onwards, Iberia shows no more differential motion with respect to the rest of Europe and becomes part of the Eurasian plate. This coincided with the slowing down of African-Eurasian convergence. Nevertheless, the Pyrenean relief still displays an unexpected strong relief and a combination of various geological methods highlights the occurrence of local Upper Miocene to present-day uplifts in the Pyrenees (low temperature thermochronological dating, sedimentology, karstology, palynology, palaeoaltimetry). While it has already been studied extensively in the Eastern Pyrenees, fewer works refer to the Western Pyrenean uplift. Depending on the method considered, the timing and the vertical amplitude of this uplift differs.
We are initiating a geomorphological study of the incisions in the Western Pyrenees in order to constrain this post-shortening uplift. We focused on the Aspe Valley, where Miocene highly elevated – low relief erosion paleosurfaces observed as relicts on top of the interfluves have been strongly dissected. This dissection was achieved in several steps, including a low erosion episode, followed by an acceleration of erosion which could be attributed to the Late Miocene to actual Western Pyrenean uplift event. The sharp incisions are highlighted by erosion triangular facets which cannot be attributed to glacial erosion as they occur beyond the last glacial deposits. We attempt to reconstitute the landscape prior to this late incision in order to quantify the subsequent erosion and to try to link it with foreland deposits.
Evidence of a Late Miocene to present-day uplift is well described in the Eastern Pyrenees and at a larger scale, in the Central Massif, the peri-alpine area and in northern and central Iberia thus suggesting an uplift at a lithospheric scale. However, discrepancies in the uplift rates indicated by already published data are found at a kilometric scale in the Western Pyrenees, which leads us to consider a possible contribution of both local and regional processes to explain the recent incisions in the Aspe Valley.
This work is funded and carried out in the framework of the BRGM-TOTAL project Source-to- Sink.
Abstract. We develop a Matlab program named LAPS (Lagrangian Advection of Particles at Sea) to simulate the advection of suspended particles in the global ocean with a minimal user effort to install, set and run the simulations. LAPS uses the 3D sea current velocity fields provided by ECCO2 to track the fate of suspended particles injected in the ocean, at specific places and times, during a period of time. LAPS runs with a short configuration file set by the user and returns the distribution of the particles at the end of the advection. A continuous tracking option is also available to record the complete trajectory of the particles throughout the entire period of advection. The effect of water waves, or Stokes drift, which alter sea surface current velocities, can also be taken into account. The principle and usage of the program is detailed and then applied to three case studies. The first two cases studies are applied to suspended sediment transport. We show how LAPS simulations can be used to investigate the spatio-temporal distribution of fine particles observed by satellites in the upper ocean. We also estimated sediment deposit areas on the seafloor as a function of sediment grain sizes. The third case study simulates the dispersion of microplastic particles during a tropical cyclone, and shows how the Stokes drift, which is significant during storm events, alters the particles trajectories compared to the case where the Stokes drift is neglected.
The long-term erosion of steep landscapes is punctuated by dramatic erosional events that can remove significant amount of sediments within a time-scale shorter than a seismic cycle. However, the role of such large erosional events on seismicity is poorly understood. We use QDYN, a quasi-dynamic numerical model of earthquake cycles to investigate the effect of a large erosional event on seismicity. The progressive evacuation of landslide sediments is modelled by a transient normal stress decrease. We show that erosional events with a shorter duration compared with the duration of a seismic cycle can significantly increase the seismicity rate, even for small stress changes. Moreover, large erosional events with a shorter period compared with the earthquake nucleation time-scale can change earthquake size distribution by triggering more small events. Those results suggest that large erosional events can significantly affect seismicity, illustrating in turn the short-term impact of surface processes on tectonics.
Abstract. The impact of glaciers on the Quaternary evolution of mountainous landscapes remains controversial. Although in situ low-temperature thermochronology offers insights on past rock exhumation and landscape erosion, the methods also suffer from biases due to the difficulty of sampling bedrock buried under glaciers. Detrital thermochronology attempts to bypass this issue by sampling sediments, at e.g. the catchment outlet, that may originate from beneath the ice. However, the age distributions resulting from detrital thermochronology do not only reflect the catchment exhumation, but also the patterns and rates of surface erosion and sediment transport. In this study, we use a new version of a glacial landscape evolution model, iSOSIA, to address the effect of erosion and sediment transport by ice on the form of synthetic detrital age distributions. Sediments are tracked as Lagrangian particles which can be formed by bedrock erosion, transported by ice or hillslope processes and deposited. We apply our model to the Tiedemann glacier (British Columbia, Canada), which has simple morphological characteristics, such as a linear form and no connectivity with large tributary glaciers. Synthetic detrital age distributions are generated by specifying an erosion history, then sampling sediment particles at the frontal moraine of the modelled glacier. An assessment of sediment transport shows that 1500 years are required to reach an equilibrium for detrital particle age distributions, due to the large range of particle transport times from their sources to the frontal moraine. Next, varying sampling locations and strategies at the glacier front leads to varying detrital SPDFs, even at equilibrium. These discrepancies are related to (i) the selective storage of a large proportion of sediments in small tributary glaciers and in lateral moraines, (ii) the large range of particle transport times, due to varying transport lengths and to a strong variability of glacier ice velocity, (iii) the heterogeneous pattern of erosion, (iv) the advective nature of glacier sediment transport, along ice streamlines, that leads to a poor lateral mixing of particle detrital signatures inside the frontal moraine. Finally, systematic comparisons between (U-Th)/He and fission track detrital ages, with different age-elevation profiles and relative age uncertainties, show that (i) the nature of the age-elevation relationship largely controls the ability to track sediment sources, and (ii) qualitative first-order information may still be extracted from thermochronological system with high uncertainties (> 30 %) depending on erosion pattern. Overall, our results demonstrate that detrital age distributions in glaciated catchments are strongly impacted not only by erosion and exhumation but also by sediment transport processes and their spatial variability. Combined with bedrock age distributions, detrital thermochronology offers a means to constrain the transport pattern and time of sediment particles. However, our results also suggest that detrital age distributions of glacial features like frontal moraines, are likely to reflect a transient case as the time required to reach detrital thermochronological equilibrium is of the order of the short-timescale glacier dynamics variability, as little ice ages or recent glaciers recessions.
