Stéphane Bonnet

Climatic forcing of erosion, landscape, and tectonics in the Bhutan Himalayas

A fundamental objective in studies of climate-erosion-tectonics coupling is to document convincing correlation between observable indicators of these processes on the scale of a mountain range. The eastern Himalayas are a unique range to quantify the contribution of tectonics and climate to long-term erosion rates, because uniform and steady tectonics have persisted for several million years, while monsoonal precipitation patterns have varied in space and time. Specifically, the rise of the Shillong plateau, the only orographic barrier in the Himalayan foreland, has reduced the mean annual precipitation downwind in the eastern Bhutan Himalaya at the Miocene-Pliocene transition. Apatite fission-track (AFT) analyses of 45 bedrock samples from an E-W transect along Bhutan indicate faster long-term erosion rates outside of the rain shadow in the west (1.0‐1.8 mm/yr) than inside of it in the east (0.55‐0.85 mm/yr). Furthermore, an AFT vertical profile in the latter segment reveals a deceleration in erosion rates sometime after 5.9 Ma. In this drier segment of Bhutan, there are remnants of a relict landscape formed under a wetter climate that has not yet equilibrated to the present climatic conditions. Uplift and preservation of the paleolandscape are a result of a climate-induced decrease in erosion rates, rather than of an increase in rock uplift rate. This study documents not only a compelling spatial correlation between long-term erosion and precipitation rates, but also a climatically driven erosion-rate change on the scale of the eastern Himalayas, a change that, in turn, likely influences that region’s recent tectonic evolution.

Landscape response to climate change: Insights from experimental modeling and implications for tectonic versus climatic uplift of topography

We present the results of an experimental investigation of the concurrent action of tectonic uplift and climate variation on relief evolution. We designed an experimental apparatus that allows the study of erosion of laboratory-scale topographies that evolve under given uplift and rainfall rates. For constant uplift and rainfall rates, the experimental topography evolves toward a statistical steady state defined by a mean elevation constant with time. Starting from such a steady state and keeping the input uplift rate constant, a subsequent change in the rainfall rate yields a change in the mean elevation of the landscape to a new equilibrium elevation. An increase in precipitation yields a lower mean steady-state elevation, whereas for a decrease in precipitation the surface is uplifted. We define this phenomenon as a climatically induced surface uplift, as opposed to a tectonically induced surface uplift. The climatically and tectonically induced surface uplifts correspond to different dynamics of denudation so that it is theoretically possible to differentiate between the climatic or tectonic causes of surface uplift from records of output sediment fluxes.

Large‐scale relief development related to Quaternary tectonic uplift of a Proterozoic‐Paleozoic basement: The Armorican Massif, NW France

The topography of basements located in intraplate domains has often been interpreted in terms of planation theory by examining the remnants of paleosurfaces. In this paper, we address the significance of the present-day drainage networks that erode such topographies by looking at the relief development on the Armorican Massif basement (northwestern France). Using digital elevation model (DEM) analysis, the topography of this massif is characterized by large-scale (∼250 km) relief variations that define high elevation domains (HEDs). These domains control the pattern of drainage networks by setting the location of the main drainage divides and the polarity of the regional slopes. Large-scale relief is strongly associated with the existence of scarps along inherited fault zones, suggesting that relief development is mainly controlled by tectonics through fault reactivation. A surface evolution study shows that drainage networks developed during the Quaternary, which corresponds to the timing of the tectonic activity which also led to relief development. The study of Quaternary fluvial erosion through a new method for measuring incision shows that the drainage basins of the HEDs record greater amounts of base level fall during Quaternary times. This implies that differential uplift is responsible for the large-scale relief development of the Armorican Massif during the Quaternary. Our study suggests that as seen in active settings, the topography of basements corresponds to a dynamic signal between tectonics and erosional processes which can be used, for example, in the study of intraplate deformation.

Influence of piedmont sedimentation on erosion dynamics of an uplifting landscape: An experimental approach

Models of relief development generally assume that eroded products are evacuated far from the landscape, whereas in nature they are often deposited at the foot of mountain belts, within continental environments. Because piedmont aggradation can modify the base level for erosion, we investigate the influence of piedmont sedimentation on the dynamics of an upstream relief. We developed an experimental study of relief dynamics using laboratory-scale models submitted to uplift under runoff-driven erosion. We compare the dynamics of topographies surrounded, or not, by a depositional belt made of eroded products coming from upstream. Piedmont aggradation acts on the dynamics of the upstream relief by modifying the relative uplift rate (applied uplift rate minus aggradation rate) that denudation tends to balance. Relief denudes at a lower rate than the applied uplift rate, so the mean elevation of the uplifting topography rises. When the time scale of aggradation is higher than the time scale of relief development, the topography cannot reach a steady state between denudation and the applied uplift rate as long as aggradation occurs. However, in this case denudation balances a continuously varying relative uplift rate during a dynamic equilibrium phase of the topography.

Relative uplift measured using river incisions: the case of the armorican basement (France)

River incision is a potential tool to quantify uplift of continental areas. However, river incision measurements using valley depths are difficult because of their variability at the catchment scale. We present a new method to quantify incision, by measuring valley depth according to their width, taking into account all the valleys in each catchment. Systematic differences in valley depths between catchments are quantified, related to relative uplift between them. An application on Brittany makes it possible to quantify a relative uplift of 30 m between two catchments during Quaternary times.

Les vallees fossiles de la baie de la Vilaine; nature et evolution du prisme sedimentaire cotier du Pleistocene armoricain

The study of a dense network of high resolution seismic profiles in the bay of Vilaine, INSU-CNRS cruise Geovill, have led to the characterization of the architecture of the sediment wedge preserved between the coast and the 50 m isobath. This wedge lies on a substratum composed of three seismic units, U1, U2 and U3 respectively attributed to metamorphic and magmatic rocks, Lutetian and Ypresian sandy carbonates and post-Eocene sediments. The coastal sediment wedge comprises three major units. A basal unit (U4), dated around 600 to 300 ky BP, interpreted as braided river sandy conglomerates. A median unit (U5) corresponding to estuarine and fluvial sandstones and clays that give way to the west to mouth bar sandstones. A sommital unit (U6) attributed to marine argillites and barrier island sandstones dated from 8110+ or -200 years at the base. These three units are bounded by two major surfaces: an unconformity between U4 and U5 and a marine (wave and tidal) ravinement surface between U5 and U6. The unconformity is interpreted as a sequence boundary between two depositional sequences: a lower one with U4 seismic unit and a topmost one with U5 and U6 seismic units. Based on the available datations, the lower sequence is attributed to the Saalian and/or Elsterian glacial cycles and, the upper sequence to the Weichselian (lowstand systems tract) and to the Holocene marine transgression (transgressive systems tract). The passage from one sequence to the other corresponds however to a drastic shift in the paleoflow directions (60 degrees ) in the Bay of Vilaine closely related to the main faults orientations. The tectonic activity in Brittany during the Pleistocene, linked to intraplate stress, seems to exert a control on sediment architecture in the coastal wedge. Indeed, the tilt of the Armorican Massif during that period has caused a complete rejuvenation of the fluvial profiles in land and the separation of the paleo-Vilaine from the Paleo-Loire river courses.

Mio–Pliocene to Pleistocene paleotopographic evolution of Brittany (France) from a sequence stratigraphic analysis: relative influence of tectonics and climate

The Mio–Pliocene in Western Europe is a period of major climatic and tectonic change with important topographic consequences. The aim of this paper is to reconstruct these topographic changes (based on sedimentological analysis and sequence stratigraphy) for the Armorican Massif (western France) and to discuss their significance. The Mio–Pliocene sands of the Armorican Massif (Red Sands) are mainly preserved in paleovalleys and are characterized by extensive fluvial sheetflood deposits with low-preservation and by-pass facies. This sedimentological study shows that the Red Sands correspond to three main sedimentary environments: fluvial (alluvial fan, low-sinuosity rivers and braided rivers), estuarine and some rare open marine deposits (marine bioclastic sands: ‘‘faluns’’ of French authors). Two orders of sequences have been correlated across Brittany with one or two minor A/S cycles comprised within the retrogradational trend of a major cycle. The unconformity at the base of the lower cycle is more marked than the unconformity observed at the top, which corresponds to a re-incision of the paleovalley network. A comparison of the results of the sequence stratigraphy analysis with eustatic variations and tectonic events during the Mio–Pliocene allows (1) to discuss their influence on the evolution of the Armorican Massif and (2) to compare the stratigraphic record with other west-European basins. The unconformity observed at the base of the first minor cycle may be attributed to Serravallian–Tortonian tectonic activity and/or eustatic fall, and the unconformity of the second minor cycle may be attributed to Late Tortonian–Early Messinian tectonic activity. The earlier unconformity is coeval with the development of a ‘‘smooth’’ paleovalley network compared to the jagged present-day relief. A single episode of Mio–Pliocene deformation recorded in Brittany may be dated as Zanclean, thus explaining the lack of the maximum flooding surface except in isolated areas. From this study, five paleogeographic maps were drawn up also indicating paleocurrent directions: three maps for the lower cycle (Tortonian retrogradational trend, Late Tortonian to Early Messinian maximum flooding surface and Messinian progradational trend) and two for the upper cycle (Pliocene retrogradational trend and Piacenzian maximum flooding surface). These maps show (1) the variations of paleocurrent directions during the Mio–Pliocene, (2) the extent of estuarine environments during the maximum flooding intervals and (3) a paleodrainage watershed oriented NNW–SSE following the regional Quessoy/Nort-sur-Erdre Fault during the retrogradational trend of the upper cycle and possibly during the progradational trend of the lower cycle. The present-day morphology of the Armorican Massif is characterized by (1) incised

Giant quartz vein formation and high-elevation meteoric fluid infiltration into the South Armorican Shear Zone: geological, fluid inclusion and stable isotope evidence

Giant quartz veins associated with the South Armorican Shear Zone record important fluid circulation during the Hercynian period. Regional-scale mapping of veins allows two groups of veins to be identified, on the basis of their geometric relationship with the South Armorican Shear Zone. Veins in the first group are parallel to the shear zone, whereas those in the second group developed in a direction oblique to it. The former probably record infiltration of fluids along permeable pathways in highly deformed zones; the latter may represent crustal-scale tension gashes in the regional context. Most quartz veins have δ18O values between 10 and 16‰ indicating a mid-crustal origin for the fluids. Microthermometry on fluid inclusions from euhedral quartz indicates that late fluids were mostly aqueous with very low salinity (0–1.7 wt% eq.) and with homogenization temperatures ranging between 150 and 270 °C. Together with very low δ18O values of some euhedral quartz, down to −2‰, these features argue for a surface origin. Corresponding δ18Ofluid values estimated near −11‰ are probably related to the high palaeo-elevation of meteoric precipitation. Scarce, but significant, H2O–CO2 fluid inclusions in euhedral quartz indicate a metamorphic contribution. These were probably sourced from the exhumed metamorphic basement in the southern part of the region.

Macroscale dynamics of experimental landscapes

Abstract We review results from laboratory-scale modelling of erosion and relief dynamics under variable uplift and rainfall rates. Under constant values of these forcing parameters an experimental landscape typically evolves towards a steady-state between uplift and erosion, and we show how the geometry of the steady-state landscape adjusts to the rates of uplift and rainfall. The comparison between these laboratory-scale landscapes and the natural ones is not straightforward because contrary to analogue modelling in tectonics, natural conditions of relief evolution cannot be downscaled to the laboratory without any scale distortions. Laboratory-scale modelling in geomorphology is therefore only experimental, not analogue. Despite these limitations, experimental models may be used to provide physical tests for numerical models and they give insights into first-order behaviours and directions for future research.

Erosion in the Chilean Andes between 27°S and 39°S: tectonic, climatic and geomorphic control

Abstract The effect of mean precipitation rate on erosion is debated. Three hypotheses may explain why the current erosion rate and runoff may be spatially uncorrelated: (1) the topography has reached a steady state for which the erosion rate pattern is determined by the uplift rate pattern; (2) the erosion rate only depends weakly on runoff; or (3) the studied catchments are experiencing different transient adjustments to uplift or to climate variations. In the Chilean Andes, between 27°S and 39°S, the mean annual runoff rates increase southwards from 0.01 to 2.6 m a−1 but the catchment averaged rates of decadal erosion (suspended sediment) and millennial erosion (10Be in river sand) peak at c. 0.25 mm a−1 for runoff c. 0.5 m a−1 and then decrease while runoff keeps increasing. Erosion rates increase non-linearly with the slope and weakly with the square root of the runoff. However, sediments trapped in the subduction trench suggest a correlation between the current runoff pattern and erosion over millions of years. The third hypothesis above may explain these different erosion rate patterns; the patterns seem consistent with, although not limited to, a model where the relief and erosion rate have first increased and then decreased in response to a period of uplift, at rates controlled by the mean precipitation rate.