Vincent Regard

Rocky shore platforms are critical coastal landforms whose morphology reflects the interplay between geological inheritance, sea-level history, and erosional processes. While Sunamura’s (1992) classification distinguishes three main platform types (A, B, C), the detailed morphology of these features—especially their submerged components—remains poorly understood due to limited access. Here, we investigate the geological and geomorphological controls on shore platform development along the Basque Coast (France), using high-resolution bathymetric data acquired during the SPLASHALIOT-1 cruise (R/V Haliotis, 2014). Our study focuses on two sites near Saint-Jean-de-Luz: Socoa (a long-term monitoring site of the Dynalit national observatory) and Kokotia, both underlain by Cretaceous–Paleogene turbidites deformed during the Pyrenean orogeny. By integrating bathymetric mapping, and geological observations, we reveal that both sites exhibit the characteristics of a Type A platform (sensu Sunamura, 1992): a planar, gently sloping surface (1–2°) extending 100–500 m offshore. The platform morphology is strongly influenced by lithological layering, with resistant turbidite beds shaping roughness and structural features. Notably, we observe no seaward cliff, challenging the hypothesis of ubiquity of low-tide cliffs in models of rocky coast evolution. Geological mapping extends onshore stratigraphy offshore, highlighting thrusts and normal faults ( e.g ., the Bidart–Pointe Sainte Anne Fault) that compartmentalize the platform. Our results suggest that platform morphology primarily records Holocene sea-level stabilization, with limited evidence of deeper terraces from the Last Interglacial (MIS5). The absence of prominent scarps or stepped features implies that cliff retreat rates (4–20 cm/a) have been insufficient to preserve older morphologies. We further discuss how structural roughness and lithological contrasts modulate wave energy dissipation, with implications for future cliff retreat under sea-level rise. This study underscores the need to account for geological heterogeneity in predictive models of rocky coast evolution.

Envelope surfaces are used in geomorphology to determine paleo‐topographic model surfaces. Several automatic methods have been proposed based on the search for high points in a digital elevation model (DEM) in sliding windows of variable size. This approach can be costly in terms of computing time when the DEMs are of high resolution and the valleys are wide, as the search window must be larger than the valleys in order to capture the high points. Here, we propose an alternative method that consists in determining the high points of the topography by inverting the topography, so that the crests become rivers, and selecting the points of the inverted topography for which the drainage area is greater than a threshold drainage area, Ac. These points correspond to the crests in the real topography. We test the ability of this method to fill valleys cutting into plateaus, while adjusting as closely as possible to the altitudes of the plateaus, in southern Peru and in France. We show that the quality of the envelope surface depends on Ac and that Ac can be estimated by multiplying the maximum incision of the topography by 8.2 in kilometres. The advantages of this method are (1) its speed of execution, in particular, because it benefits from fast algorithms for calculating drainage area and because it can be carried out on DEMs of degraded resolution without loss of quality, and (2) its applicability to all types of topography (canyons or mature landforms) because it is based on the structure of the hydrographic network of each topography rather than on a sliding window.

The position of the coastal waterline—the interface between land and sea—varies over time due to sedimentary and hydrodynamic processes influenced by climate variability. While previous studies have mainly focused on local morphodynamic or storm‐driven shoreline changes, the large‐scale climatic controls on global waterline variability remain poorly understood. This study addresses this gap by analyzing how climate‐driven environmental factors govern coastal waterline dynamics across seasonal to interannual timescales. Using a globally consistent, satellite‐derived data set of coastal waterlines from 1999 to 2019, combined with reanalysis‐based estimates of sea‐level anomalies, wave setup and freshwater runoff, we use an empirical mode decomposition framework to disentangle the contributions of these drivers. This approach isolates the dominant temporal components of waterline variability and identifies regional relationships with environmental forcing. Our results reveal two main regimes of climate‐driven waterline variability: (a) a seasonal pattern that intensifies with latitude and is primarily linked to storm‐driven wave energy flux and sea‐level oscillations; and (b) an interannual pattern that peaks in the tropics and is strongly modulated by large‐scale climate oscillations, such as ENSO, and by variability in fluvial discharge. We demonstrate that regional sea‐level and freshwater anomalies can alter the waterline by amounts comparable to those caused by waves. This challenges the conventional view that wave‐driven morphodynamics dominate waterline change. By framing the waterline as a climate‐sensitive indicator of coastal change, this study establishes the first global‐scale, data‐driven framework for diagnosing and predicting waterline variability. The results provide a basis for developing climate‐informed waterline forecasts and improving assessments of coastal vulnerability in the context of ongoing sea‐level rise.

In heterogeneous continental lithosphere, rifts propagate by growth and linkage of discrete segments. Linkage zone geometries reflect this process with different segment overlaps, kinematics, and mechanical properties. Recently, analogue and numerical models compared to natural examples (East African and West European rifts) have allowed significant progresses in understanding the localized transfer zones. Here, we focus instead on wide linkage zones in exceptionally hot crust settings, which is relevant for geothermal exploration.The Rio Grande Rift is a relatively narrow intra-mountainous system, active since the Miocene, contemporaneous with the Basin and Range extension. Despite substantial extension, rift basins remain at high elevations (>1000 m) with inherited rift shoulders reaching up to ~3700 m. These high elevations and Moho temperatures (800-900°C), indicate significant dynamic support.To investigate linkage kinematics and strain distribution, we compared analogue models inspired from and structural analysis from DEM-derived fault trajectories and published slip data of the Española basin, a 60-km-wide linkage zone connecting the San Luis and Albuquerque segments. We tested various model rheologies (sand-silicone ratios) and extension velocities to assess their impact on the rift architecture, strain partitioning and fault network. Our approach aims to constrain the 3D strain field evolution in the linkage zones and highlight the role of crustal rheology and inherited structures on the linkage zone geometry.The NE-SW trending Española basin comprises early-rift grabens and half-grabens preserved, beneath younger volcano-sedimentary deposits, as ‘embayments’ along the basin margins. Seismic data reveals a two-stage evolution. The early wide rift stage (30-15 Ma) produced distributed shallow basins above low-angle normal faults, consistent with the extension of a thermally weakened crust after the Oligocene magmatism. The late narrow-rift stage (15 Ma-present) showed higher extension rates and high-angle normal faults, with thicker and narrower basins. Española basin is bounded by the NE-SW trending Embudo and Tijeras left-lateral fault systems. Within the linkage zone, fault traces are both concave and convex, indicating a rotational strain component. Late-rift faulting forms multi-scale en-echelon patterns resulting from interaction between the N-S intra-basin faults and the oblique border faults.Xenolith studies documented Miocene crustal rheological changes: Oligocene crustal melting produced progressive granulitization and mechanical strengthening of the lower crust, which could have caused the localization of deformation during late rifting.Analogue sand-silicone models with a brittle-ductile transition at 5-10 km depth reproduced the Española basin architecture. The distributed deformation across then multiple N-S to NE-SW sub-basins, matching the observed alternation of narrow half-graben and graben tips forming ‘embayment’. Rotational strain in the linkage zone, produce convex-concave faults similar to those observed in the Española basin. Increasing extension velocity promotes strain localization, particularly along the NE-SW left-lateral fault, replicating the present-day strain pattern.These results demonstrate that the Española basin formed by rift segment linkage under simple orthogonal extension with increasing strain rate and progressive strengthening of the crust. Segment propagation drove a progressive tip rotation, oblique faulting, and localized strike-slip motion. The models reveal high fault connectivity within the linkage zones, with significant implications for geothermal exploration.

Quantifying sediment fluxes is an essential part of the Source‐to‐Sink approach in the understanding of sedimentary systems. However, the transfer of sediment from the source to the sink and the factors controlling it are still poorly understood. We focus on a small catchment coupled with its offshore deep‐sea fan: the Sithas system (Gulf of Corinth, Greece). We restore the volume of sediment eroded in the catchment using geomorphic constraints; quantify the volume of sediment deposited in the offshore basin, after revising the age model; and calculate erosional fluxes using the BQART model. This allows for the comparison of the reconstructed fluxes of sediment eroded and deposited since 800 ka across the entire source‐to‐sink system. For the Sithas coupled catchment‐deep‐sea fan system, we show an increase in sedimentary fluxes both in erosion and deposition since 800 ka and particularly since 400 ka, where cyclic variations of ~120 kyr are recorded in erosion and deposition compartments. We suggest that the overall increase in flux results from a change in the catchment size due to the tectonic evolution of the region. The record of cyclic variations from 400 kyr in fluxes matches with the maturity of the system and with the intensification of glacial cycles and tectonic constraints migration. We also suggest that the discrepancy between erosion and deposition reflects a temporary storage between source and sink areas, probably along the coast. This has changed since 30 ka, introducing the last phase of evolution characterised by phased source and sink dynamics, suggesting a lack of temporary storage and a connection between river outlet and submarine canyon head. This study shows that sediment fluxes are controlled by the catchment's size as well as by climatic and tectonic factors and that even a small sedimentary system can be affected by temporary sediment storage.

Cosmogenic isotope dating of rock samples taken from shore platforms is the only dating method for reconstructing the long-term evolution (centuries to millennia) of fully erosional rock coasts. Two techniques can be used: the first is based on the collection of samples on the platform and the distribution of their cosmogenic isotope concentrations over a transect perpendicular to the shoreline (e.g. Regard et al., 2012; Hurst et al., 2016; Swirad et al., 2020); the second, which is limited to low rates of recession, is implemented by measuring cosmogenic isotope concentrations in the colluvium at the foot of the cliff (Bossis et al., 2024). Here we focus on the first technique which involves fitting the distribution of observed concentrations to a model. This model allows the cliff retreat rate to be constant or variable over the Holocene, and shore platform either to erode in concert with the cliff retreat (steady state model) or to widen through time. The simple inversion model is as widely usable as possible and allows a quick, easy, robust and standardised calculation of exposure ages based on information such as cosmogenic isotope concentrations, present topography and relative sea level change. ReferencesBossis, R., Regard, V., Carretier, S., and Choy, S., 2024, Evidence of slow millennial cliff retreat rates using cosmogenic nuclides in coastal colluvium: EGUsphere, p. 1–15, doi:10.5194/egusphere-2023-3020.Hurst, M.D., Rood, D.H., Ellis, M.A., Anderson, R.S., and Dornbusch, U., 2016, Recent acceleration in coastal cliff retreat rates on the south coast of Great Britain: Proceedings of the National Academy of Sciences, v. 113, p. 13336–13341, doi:10.1073/pnas.1613044113.Regard, V., Dewez, T., Bourlès, D.L., Anderson, R.S., Duperret, A., Costa, S., Leanni, L., Lasseur, E., Pedoja, K., and Maillet, G.M., 2012, Late Holocene seacliff retreat recorded by 10Be profiles across a coastal platform: Theory and example from the English Channel: Quaternary Geochronology, v. 11, p. 87–97, doi:10.1016/j.quageo.2012.02.027.Swirad, Z.M., Rosser, N.J., Brain, M.J., Rood, D.H., Hurst, M.D., Wilcken, K.M., and Barlow, J., 2020, Cosmogenic exposure dating reveals limited long-term variability in erosion of a rocky coastline: Nature Communications, v. 11, p. 3804, doi:10.1038/s41467-020-17611-9.

Sequence stratigraphy has predominantly been used on seismic data to interpret basin stratigraphic architectures, especially offshore, but its application to onshore outcrops, and more specifically to the stratigraphic records of the last interglacial periods, remains comparatively rare. The preservation of marine terraces on the southern coast of the Gulf of Corinth (Greece) enables high-resolution study of exposed stratigraphic architectures within terrace deposits. Fourteen marine terraces corresponding to the last 400 ka are visible in outcrop. This study focuses on the detailed analysis of outcrop sequences in the marine terraces of the Gulf of Corinth, aiming to document multiple regression cycles recorded during past interglacials.The outcrops are roughly organised in km-large, including hundreds of meters-high Gilbert-type deltas. We identify clinoforms and facies associations thanks to panoramas and stratigraphic logs interpretations and then, we determine the different systems tracts: LST, TST, HST and FSST and the offlap break for the consecutive marine terraces. This enables us to illustrate the processes involved in the formation of marine terraces in relation to eustatic variations. The restored stratigraphic architecture captures an entire record of several cycles organised in an overall forced regression, from the most proximal facies to the most distal ones. This record encompasses the last 400 ka. On this record, we identify almost every HST and some considerable LST and narrow TST in discontinuity with the basement by interpreting topsets, foresets, bottomsets and discontinuities.We determine how coastline has migrated through time by establishing the trajectory path of the land-sea interface during the last 400 ka. We finally correlate the coastline migration rates with sediment flux dynamics to discuss the relationship between erosion processes and sea-level changes along the coastline. For example, we illustrate that deposition of sediments in the offshore is more important during low sea levels.The Gulf of Corinth outcrop sequences could serve as a natural laboratory for testing proxies and understanding the interplay between tectonics, climate, and sea-level changes for the past interglacials.

The estimation of the land-sea interface, or waterline, variability due to high energetic events - i.e. potentially inducing extreme coastal level and erosion- constitutes an important component for the comprehensive risk assessment of the global coastal zone. The large spatial scales and the requirement for real-time coastal risk assessment pose the need for timely forecasts of the key driving processes, in conjunction with the human stakes, exposed population, infrastructures and properties, at risk. Moreover, the interplay between the involved physical processes necessitates the inclusion of a large number of plausible scenarios, useful for the involved decision makers. We present a flexible and low computational-cost framework for the real-time estimation of the global waterline change and associated coastal risk. This framework utilizes satellite-derived probabilistic water level data for the global ocean, combined with state of the art wave and hydrological numerical data and up-to-date satellite observations of the waterline. This information is integrated with high spatial resolution exposure data, providing in real-time the assessment of the imminent risk at the global coastal zone. The outcome of the proposed approach may serve as an additional forecast tool for a first-pass risk assessment, facilitating both short-term and long-term risk mitigation studies.

The erosion of rocky coasts contributes to global cycles of elements over geological times and also constitutes a major hazard that may potentially increase in the future. Yet, it remains a challenge to quantify rocky coast retreat rates over millennia – a time span that encompasses the stochasticity of the processes involved. Specifically, there are no available methods that can be used to quantify slow coastal erosion (< 1 cm yr⁻¹) averaged over millennia. Here, we use the ¹⁰Be concentration in colluvium, corresponding to the by-product of aerial rocky coast erosion, to quantify the local coastal retreat rate averaged over millennia. We test this approach along the Mediterranean coast of the eastern Pyrenees (n=8) and the desert coast in southern Peru (n=3). We observe a consistent relationship between the inferred erosion rates and the geomorphic contexts. The retreat rates are similar, 0.3–0.6 mm yr⁻¹ for five samples taken on the Mediterranean coast, whereas two samples from vegetated colluvium have a lower rate of ∼ 0.1 mm yr⁻¹. The coastal retreat rate of the Peruvian site currently subject to wave action is similar to the Mediterranean coast (0.5 mm yr⁻¹), despite Peru's more arid climate. The other two Peruvian sites, which have not been subjected to wave action for tens of thousands of years, are eroding 20 times more slowly. The integration periods of the two slowest Mediterranean coast erosion rates may encompass pre-Holocene times, during which the sea level and thus the retreat rate were much lower. We explore here this bias and conclude that the associated bias on the inferred retreat rate is less than 80 %. These data show that rocky coasts are eroding 1 to 20 times faster than catchments in the same regions on average over the last few thousand years. We anticipate that this new method of quantifying slow rocky coastal erosion will fill a major gap in the coastal erosion database and improve our understanding of both coastal erosion factors and hazards.

We measured the long term physical denudation of the Ogooué River catchment using 10Be. These measurements are averaged over 25-200 ka (average 40 ka), depending on the physical denudation rate. The denudation rate of the Ogooué River catchment is slow (38 t/km2/a, 15 m/Ma), slightly higher than the Equatorial West Africa (from Senegal to Angola, 26 t/km2/a, 10 m/Ma). Physical denudation and chemical weathering fall within the same order of magnitude. Thus, although low, chemical weathering, is substantial compared to physical denudation, its contribution is likely over 30% of the total denudation.Denudation rates are spatially variable (from 10 to 60 t/km2/a) within this large Ogooué River catchment. Over the long term, this variability exhibits a fairly close match of physical denudation/chemical weathering, except in the Batéké Plateaux area, because they are made up of already weathered detrital material and their modern flux of solutes is therefore very low (~9.5 t/km2/a). The spatial distribution is similar to the one described in Moquet et al. (2021)'s work, i.e. the southern part of the catchment is denuding twice as fast as the northern part. We show here that the whole picture did not vary much since 100 ka, as shown by both methods giving consistent results. Faster denudation in the south of the catchment may be related to some more uplift than in the north caused by the south African superswell.