Sébastien Castelltort

How plausible are high-frequency sediment supply-driven cycles in the stratigraphic record?

Abstract This paper is an attempt to quantify the plausible time scales of clastic sediment supply variations at the entrance of sedimentary basins. Our approach is based on the sedimentary system concept, which simplifies natural systems by dividing them into three zones of dominant processes: the erosion, the transfer, and the sedimentation subsystems. We examine recent results from geomorphology, which show that frequent climate changes can induce high-frequency sediment flux variations at the outlet of the source area. We put forward the crucial role of the transfer subsystem, which conveys sediment from the erosion zone to the basin. By applying a diffusive model to a number of worldwide rivers, we extend from large (>1000 km) to intermediate (>300 km) rivers the previous finding that the transfer subsystem acts as a buffer for short periods sediment pulses (tens to hundreds of kiloyears). This implies that high-frequency stratigraphic cycles in clastic accumulations fed by large drainage systems are unlikely to reflect sediment supply cycles of tens to hundreds of thousands of years of periodicities.

Coupled model of surface water flow, sediment transport and morphological evolution

This paper presents a mathematical model coupling water flow and sediment transport dynamics that enables calculating the changing surface morphology through time and space. The model is based on the shallow water equations for flow, conservation of sediment concentration, and empirical functions for bed friction, substrate erosion and deposition. The sediment transport model is a non-capacity formulation whereby erosion and deposition are treated independently and influence the sediment flux by exchanging mass across the bottom boundary of the flow. The resulting hyperbolic system is solved using a finite volume, Godunov-type method with a first-order approximate Riemann solver. The model can be applied both to short time scales, where the flow, sediment transport and morphological evolution are strongly coupled and the rate of bed evolution is comparable to the rate of flow evolution, or to relatively long time scales, where the time scale of bed evolution associated with erosion and/or deposition is slow relative to the response of the flow to the changing surface and, therefore, the classical quasi-steady approximation can be invoked. The model is verified by comparing computed results with documented solutions. The developed model can be used to investigate a variety of problems involving coupled flow and sediment transport including channel initiation and drainage basin evolution associated with overland flow and morphological changes induced by extreme events such as tsunami.

Placing limits to shortening evolution in the Pyrenees: Role of margin architecture and implications for the Iberia/Europe convergence

Estimating shortening in collision belts is critical to reconstruct past plate motions. Balanced cross-section techniques are efficient in external domains but lack resolution in the hinterland. The role and the original extent of the continental margins during the earliest stages of continental convergence are debated. Here we combine existing and new sequentially restored cross sections in the central Pyrenees, with Iberia/Europe (IB/EU) plate kinematic reconstructions and new apatite fission track, zircon (U-Th)/He, and U/Pb ages to discuss higher and lower bounds of crustal shortening and determine the amount of distal margin sutured during collision. We show that after extension in the Albian (~110 Ma), a 50 km wide extremely thinned crustal domain underwent subduction at 83 Ma. Low-temperature data and thermal modeling show that synorogenic cooling started at 75–70 Ma. This date marks the transition from suturing of the highly extended margin to collision of the more proximal margin and orogenic growth. We infer a relatively low crustal shortening of 90 km (30%) that reflects the dominant thick-skinned tectonic style of shortening in the Pyrenees, as expected for young (Mesozoic) and weak lithospheres. Our proposed reconstruction agrees with IB/EU kinematic models that consider initially rapid convergence of Iberia, reducing from circa 70 Ma onward. This study suggests that plate reconstructions are consistent with balanced cross sections if shortening predicted by age-dependent properties of the continental lithosphere is taken into account.

Model shows that rivers transmit high-frequency climate cycles to the sedimentary record

Rivers are a major component of sediment routing systems that control the transfer of terrigenous sediments from source to sink. Although it is widely accepted that rivers are perturbed by millennial-scale climatic variability, the extent to which these signals are buffered or transferred down river systems to be recorded in sediments at or beyond the river mouth remains debated. Here, we employ a physically based numerical model to address this outstanding issue. Our model shows that river transport strongly amplifies high-frequency sediment flux variations arising from changing water discharge, due to positive feedback between discharge and the channel gradient. This behavior is distinctly different from short-period sediment flux signals (with constant water discharge) where the output sediment flux is strongly dampened within the river, due to negative feedback between the channel gradient and sediment concentration. We conclude that marine sedimentary basins may record sediment flux cycles resulting from discharge (and ultimately climate) variability, whereas they may be relatively insensitive to pure sediment flux perturbations (such as for example those induced by tectonics).

Dynamic constraints on the crustal-scale rheology of the Zagros fold belt, Iran

Thin-skinned fold-and-thrust belts are generally considered as the result of contractional deformation of a sedimentary succession over a weak decollement layer. The resulting surface expression frequently consists of anticlines and synclines spaced in a fairly regular manner. It is thus tempting to use this spacing along with other geological constraints to obtain insights into the dynamics and rheology of the crust on geological time scales. Here we use the Zagros Mountains of Iran as a case study, as it is one of the most spectacular, well-studied thin-skinned fold-and- thrust belts in the world. Both analytical and numerical models are employed to study what con- trols fold spacing and under what conditions folding dominates over thrusting. The models show that if only a single basal decollement layer is present underneath a brittle sedimentary cover, deformation is dominated by thrusting, which is inconsistent with the data of the Zagros fold belt. If we instead take into account additional decollement layers that have been documented in the fi eld, a switch in deformation mode occurs and crustal-scale folding is obtained with the correct spacing and time scales. We show that fold spacing can be used to constrain the friction angle of the crust, which is ~5° the Zagros fold belt. This implies that on geological time scales, the upper crust is signifi cantly weaker than previously thought, possibly due to the effect of fl

Modes and rates of horizontal deformation from rotated river basins: Application to the Dead Sea fault system in Lebanon

Partitioning of horizontal deformation between localized and distributed modes in regions of oblique tectonic convergence is, in many cases, hard to quantify. Here we use the geometry of river basins and numerical modeling to evaluate modes and rates of horizontal deformation associated with the Arabia-Sinai relative plate motion in Lebanon. We focus on river basins that drain Mount Lebanon to the west and are bounded by the Yammouneh fault, a segment of the Dead Sea fault system that transfers left-lateral deformation across the Lebanese restraining bend. We quantify a systematic counterclockwise rotation of these basins and evaluate drainage area disequilibrium using the χ metric. The analysis indicates a systematic spatial pattern whereby tributaries of the rotated basins appear to experience drainage area loss or gain with respect to channel length. A kinematic model reveals that since the late Miocene, 23%–31% of the relative plate motion parallel to the plate boundary has been distributed along a wide band of deformation to the west of the Yammouneh fault. Taken together with previous, shorter-term estimates, the model indicates little variation of slip rate along the Yammouneh fault since the late Miocene. Kinematic model results are compatible with late Miocene paleomagnetic rotations in western Mount Lebanon. A numerical landscape evolution experiment demonstrates the emergence of a similar pattern of drainage area disequilibrium in response to progressive distributed shear deformation of river basins with relatively minor drainage network reorganization.

Deglaciation and glacial erosion: a joint control on magma productivity by continental unloading

Glacial-interglacial cycles affect the processes through which water and rocks are redistributed across the Earths surface, thereby linking the solid Earth and climate dynamics. Regional and global scale studies suggest that continental lithospheric unloading due to ice melting during the transition to interglacials leads to increased continental magmatic, volcanic, and degassing activity. Such a climatic forcing on the melting of the Earths interior, however, has always been evaluated regardless of continental unloading by glacial erosion, albeit the density of rock exceeds that of ice by approximately 3 times. Here we present and discuss numerical results involving synthetic but realistic topographies, ice caps, and glacial erosion rates suggesting that erosion may be as important as deglaciation in affecting continental unloading. Our study represents an additional step toward a more general understanding of the links between a changing climate, glacial processes, and the melting of the solid Earth.

Magmatic pulse driven by sea-level changes associated with the Messinian salinity crisis

Between 5 and 6 million years ago, during the so-called Messinian salinity crisis, the Mediterranean basin became a giant salt repository. The possibility of abrupt and kilometre-scale sea-level changes during this extreme event is debated. Messinian evaporites could signify either deep- or shallow-marine deposits, and ubiquitous erosional surfaces could indicate either subaerial or submarine features. Significant and fast reductions in sea level unload the lithosphere, which can increase the production and eruption of magma. Here we calculate variations in surface load associated with the Messinian salinity crisis and compile the available time constraints for pan-Mediterranean magmatism. We show that scenarios involving a kilometre-scale drawdown of sea level imply a phase of net overall lithospheric unloading at a time that appears synchronous with a magmatic pulse from the pan-Mediterranean igneous provinces. We verify the viability of a mechanistic link between unloading and magmatism using numerical modelling of decompression partial mantle melting and dike formation in response to surface load variations. We conclude that the Mediterranean magmatic record provides an independent validation of the controversial kilometre-scale evaporative drawdown and sheds new light on the sensitivity of magmatic systems to the surface forcing.

Detecting eustatic and tectonic signals with carbon isotopes in deep-marine strata, Eocene Ainsa Basin, Spanish Pyrenees

We explore the potential of high-resolution carbon isotope stratigraphy to provide an independent record of global sea-level changes in a 1600-m-thick succession representing ∼5 m.y. of slope sedimentation in the Eocene Ainsa Basin of Spain. The restricted physiographic setting of the basin results in a bulk δ 13 C carb signal that accurately correlates with the coeval eustatic curve from the New Jersey (USA) passive margin. We show that much of the deep-water sediment gravity flow (SGF) deposits are emplaced during eustatic lowstands and fine-grained marly intervals between SGFs correlate with rising and highstand sea levels. However, we also detect a substantial interval of SGF deposition during a sea-level highstand, confirming the nonuniqueness of the controls on clastic deposition. This approach provides a new way to assess the origin of depositional sequences and improve stratigraphic predictions in basins with limited chronostratigraphic control.

Immediate and delayed signal of slab breakoff in Oligo/Miocene Molasse deposits from the European Alps

High-resolution 32–20 Ma-old stratigraphic records from the Molasse foreland basin situated north of the Alps, and Gonfolite Lombarda conglomerates deposited on the southern Alpine margin, document two consecutive sedimentary responses - an immediate and delayed response - to slab breakoff beneath the central Alps c. 32–30 Ma ago. The first signal, which occurred due to rebound and surface uplift in the Alps, was a regional and simultaneous switch from basin underfill to overfill at 30 Ma paired with shifts to coarse-grained depositional environments in the foreland basin. The second signal, however, arrived several million years after slab breakoff and was marked by larger contributions of crystalline clasts in the conglomerates, larger clast sizes, larger sediment fluxes and shifts to more proximal facies. We propose that this secondary pulse reflects a delayed whiplash-type erosional response to surface uplift, where erosion and sediment flux became amplified through positive feedbacks once larger erosional thresholds of crystalline bedrock were exceeded.