Contenu du sommaire : Paraglaciaire et changements climatiques

This contribution first reminds the definition of both concepts « paraglacial » and « landsystem » and proposes a new definition of the « paraglacial landsystem » concept, which is defined with a spatial and temporal point of view. It observes how this paraglacial landsystem evolves under climate change, in regards of temporal scales, from instantaneous to million years time span.

The North-Western Highlands of Scotland include large monoclinal ridges showing translational slides on their backslopes (rock block slides). These slides are of various sizes, from compressed and heaved isolated blocks to chaotic blockstreams. They are developed on slopes whose low angles are offset by quartzite structural properties. They respond to paraglacial dynamics (rock mass response to glacial stress, runoff processes) and they contribute to the postglacial erosion of major structural slopes. Reciprocally, they provide an expression of differential paraglacial erosion concept.

In Alpine high mountain area like the Mont Blanc massif the paraglacial period is the time which is needed for the adjustment of side and longitudinal dynamics to the main climate changes, as shown by the formation of large polygenic debris cones at the beginning of the Holocene. Renewed paraglacial response is triggered by secondary climate changes, as illustrated by the gullying of the inner flank of lateral moraines where glaciers retreated since the end of the LIA. Finally, some processes like rock avalanches have generally to be not considered as paraglacial.

For the last 150 years, the Haut Arc glaciers (Savoie, France) follow a quite constant generalized retreat. This glaciological situation releases a wide area recently deglaciated, scene particularly favourable to paraglacial system. This article exposes summarily the glaciological and geomorphological context, and details the main processes reported from this area. The results so obtained show a continuity of processes already active since the end of the Little Ice Age, but also the appearance of new phenomena bound to the speed up of the glacio-fluvial processes and the modification of glaciers' forelands.

Icelandic slopes, in margin of large ice caps, undergo a retreat of glacier fronts since the end of Little Ice Age and are characterized by news developments. The study of several glacier margins in Ôrœfi region (South-East Iceland) permits identification of processes responsible of these developments. It shows that the comprehension of landscapes in a déglaciation context needs to evaluate climatic processes in specific morpho-structural context, as well as paraglacial dynamics.

The current retreat of the glacier fronts induces changes of both discharge and sediment availability. Consequently, the fluvial hydrosystem adjusts its fluvial pattern to these new conditions. However, the relationships between rate of glacial retreat and fluvial pattern adjustment are not direct. Some landscape units linked to glacial activity, like moraines and proglacial lakes, alter locally the sediment balance and disturb the alluvial outwash plain response.

Freissinières catchment is particularly prone to catastrophic floods. Floods are characterized by the influence of temporal dams : the removal of such dams imply massive supply of water and sediments downstream. Dams are due to the large amount of sediments provided from the hill slopes by paraglacial and glacial sediment sources.

Polar regions are very sensitive to climate variability. Glacial environments, such as those of Spitsbergen, respond drastically to climate warming by the disintegration of ice masses, release of huge amounts of free water and rapid evolution of terrestrial landforms. Paraglacial studies have shown that sediment yield decreases with time according to a relaxation curve. This study examines the effects of scaling on rhythms of deglaciation and sediment fluxes in a small catchment. It appears that the paraglacial sequence, known as a morphogenic crisis, can be affected by internal small threshold event which are able to mobilize large quantities of sediment. It means that, at small scale, the curve profile is more irregular than suggested by large scale model.

The Rennes school of geologists has recently addressed the significance of the present-day drainage networks by looking at the relief development on the Armorican Massif basement. It assumes that differential uplift is responsible for the morphological evolution of this structural unit during Quaternary times. By contrast the following paper recalls that the remnants of planation surfaces allow us to reconstruct a history which is not only controlled by the reactivation of inherited fault zones, and suggests that the geomorphological evolution of this massif cannot be restricted to a late rejuvenation of a multi- storeyed topography by trustai uplift.

ABSTRACT. - Steep slopes form the marginal scarp of the semi-arid northern Brazilian "Nordeste", above a low pediplain merging seaward with a coastal erosional-gradational piedmont. Dissected pediments and sparse debris fans visible at the base of some escarpments suggest recent erosive activity and possible slope retreat, although most of these forms are decoupled from major valleys. We describe the morphostructural patterns of chosen escarpments, and the morphodynamic conditions of their evolution since the Miocene. Only a few scarps show strong structural control. Most of them are mainly inherited landforms, initiated during the Early Cretaceous rifting or the later margin uplift. The Neogene clastic sedimentation on piedmonts and coastal areas mainly reflects the occurrence of dry periods inducing widespread stripping of deep soil horizons and erosion of bare rock slopes and surfaces. Dissection stages occurred in periods of more humid climate and/or low sea level. The moderate volumes of Neogene clastic sediments imply overall low uplift and erosion rates until the Present, favourable to morphological and lithological resistance effects in the landscapes.

The analysis of upper Loire and Allier catchments Neogene evolution gives arguments for understanding the status of inherited features with regard to landscape sensitivity question and hits control factors.

The study of some bedrock reaches in the Chassezac watershed, in the Cévennes vivaraises, raises the question of morphogenic processes and their efficacy through time. Bedrock features and their spatial distribution on longitudinal profile have often been explained by the strength of extreme floods. But the structural characteristics of the substratum and the location of the inherited alluvial sheets and the conditions of their dislodgement leads us to relativize the morphogenic role of hydraulic structures. Some bedforms reveal their stability since almost the last interglacial period.

Since the rifting event that occurred during Oligocene to Aquitanian times, landscape development in the southern Massif Central (France) has been driven by lithospheric processes, rather than forced by climatic or eustatic controls. Topography rose in several stages, with a record of accelerated uplift and incision from the Late Miocene onward. However, many unanswered questions still remain, particularly in relation to the endogenous causes of plateau uplift.

Late Neogene culminating high planation surfaces can be observed through all the internal areas of the Eastern Betic chains. A multi-criteria approach allows to interpret these high planation surfaces as quite young landforms, and permit to understand how they develop and persist in the mountainous landscape though an active orogenic regime.

Non-equilibrium mountain topography commonly exhibits elevated erosion surfaces. Such features are still rarely accounted for in geodynamic models and raise the questions of how and when these landforms were generated, and why they have been preserved. We review existing answers and show that alternatives to recent vertical uplift, such as altiplanation, are difficult to generalise even though the endogenous mechanisms for uplift need further investigation. A debated key issue also concerns the palaeoelevation of the flat surfaces and the methods by which to establish it.