Stéphane Pochat

Experimental modeling of pressurized subglacial water flow: Implications for tunnel valley formation

Tunnel valleys are elongated hollows commonly found in formerly glaciated areas and interpreted as resulting from subglacial meltwater erosion beneath ice sheets. Over the past two decades, the number of studies of terrestrial tunnel valleys has continuously increased and their existence has been hypothesized also on Mars, but their formation mechanisms remain poorly understood. We introduce here, an innovative experimental approach to examine erosion by circulation of pressurized meltwater within the substratum and at the silicon-substratum interface. We used a permeable substratum (sand) partially covered by a viscous, impermeable and transparent lid (silicon putty), below which we applied a central injection of pure water. Low water pressures led to groundwater circulation in the substratum only, while water pressures exceeding the sum of the glaciostatic and lithostatic pressures led to additional water circulation and formation of drainage landforms at the cap-substratum interface. The formation of these drainage landforms was monitored through time and their shapes were analyzed from digital elevation model obtained by stereo-photogrammetry. The experimental landforms include valleys that are similar to natural tunnel valleys in their spatial organization and in a number of diagnostic morphological criteria, such as undulating longitudinal profiles and “tunnel” shapes. These results are consistent with the hypothesis that overpressurized subglacial water circulation controls the formation of tunnel valleys.

The Landscape and Landforms of the Ogaden, Southeast Ethiopia

The Ogaden region is located on the Somali Plateau, in southeast Ethiopia. Originally a clan-based term, the Ogaden is now commonly used for the entire region below about 1,500 m a.s.l., an area of some 300,000 km2 that encompasses most of the Somali Regional State and includes the southwest portion of Oromia. The climate is hot, arid to semiarid, corresponding to the Ethiopian bereha and kolla climatic zones. Three basic physiographic provinces are recognized: the Genale and Shebele drainage basins and the Eastern Slope and Plains. The two drainage basins include spectacular upstream canyons that witness the vertical movements that have accompanied the succession of rifting events in the Ethiopian Rift, Afar, and the Gulf of Aden. In strong contrast, the Eastern Slopes and Plains is dipping less than 0.4° on average over hundreds of kilometers to the southeast and is mantled by red sands. Several remarkable Ogaden landforms are described and analyzed, including volcanic, fluvial, and gravitational features, some having few equivalents in other areas on Earth. A variety of volcanic landforms are present across the region, reflecting the complex Cenozoic history of the Ogaden’s margins. For instance, meandering basalt hills provide a textbook example of inverted topography by fossilizing paleodrainage networks of various ages. The modern drainage network provides information on the genesis of the mega-geomorphology of the Ogaden and documents its uplift history. In western Ogaden, the deep incision has exposed the Cretaceous evaporites and triggered the development of one of the largest gravitational spreading domains on Earth, the Audo Range.

Can eustatic charts go beyond first order? Insights from the Permian–Triassic

First-order variations of eustatic charts (200–400 m.y.) are in agreement with our understanding of the geodynamic processes that control sea level. By extrapolation, second-order (10–100 m.y.) and third-order (1–10 m.y.) variations are also thought to follow the same rules. However, this assumption may be jeopardized by a closer examination of the Permian–Triassic example, for which climatic and tectonic eustasy fails to explain the variations of the eustatic charts. During this period, eustatic charts peak down to their lowermost Phanerozoic values and display second-order variations at rates of up to 3 m/m.y., which is inconsistent with the expected eustatic signal during the early fragmentation of the Pangean supercontinent and the late Paleozoic melting of ice sheets. Here, we review the possible mechanisms that could explain the apparent sea-level variations. Some of them do modify the eustatic sea level (ESL). In particular, dynamic deflections of Earth’s surface above subduction zones and their locations with respect to continents appear to have been the primary controls of absolute sea level as the Pangean supercontinent formed and broke up. Other mechanisms instead only locally or regionally produced vertical ground motions, either uplifting continents or tilting the margins where the control points were located. We show that (1) the thermal uplift associated with supercontinent insulation and (2) the dynamic uplift associated with the emplacement of a superplume both give rates of sea-level change in the range of long-term changes of ESL. We also show that (3) the dynamic tilt of continental margins not only produces apparent sea-level changes, but it also modifies the absolute sea level, which in turn may end up in the paradoxical situation wherein fingerprints of ESL drop are found in the geological record during actual ESL rise. We conclude that second-order absolute sea-level changes may remain elusive for some time.

Modelled subglacial floods and tunnel valleys control the lifecycle of transitory ice streams

Ice streams are corridors of fast-flowing ice that control mass transfers from continental ice sheets to oceans. Their flow speeds are known to accelerate and decelerate, their activity can switch on and off, and even their locations can shift entirely. Our analogue physical experiments reveal that a life cycle incorporating evolving subglacial meltwater routing and bed erosion can govern this complex transitory behaviour. The modelled ice streams switch on and accelerate when subglacial water pockets drain as marginal outburst floods (basal decoupling). Then they decelerate when the lubricating water drainage system spontaneously organizes itself into channels that create tunnel valleys (partial basal recoupling). The ice streams surge or jump in location when these water drainage systems maintain low discharge but they ultimately switch off when tunnel valleys have expanded to develop efficient drainage systems. Beyond reconciling previously disconnected observations of modern and ancient ice streams into a single life cycle, the modelling suggests that tunnel valley development may be crucial in stabilizing portions of ice sheets during periods of climate change.