The colloidal structure of bitumen: Consequences on the rheology and on the mechanisms of bitumen modification

Geological atlas of western and central Europe

Evolution of the European Cenozoic Rift System: interaction of the Alpine and Pyrenean orogens with their foreland lithosphere

The circum-Mediterranean anorogenic Cenozoic igneous province

More than 100 offshore mass-movement deposits have been studied in Holocene and Pleistocene sediments. The processes can be divided into three main types: slides/slumps, plastic flows, and turbidity currents, of which 13 main varieties have been recognized. The three types are differentiated mainly by motion, architecture, and shape of failure surface. For slides, the morphology of deposits can usually be linked to a process, but for plastic flows and turbidity currents, information about the motion is mainly provided by the sedimentary record. A static classification based on these features is given, and is related to a dynamic classification system to try to underline the morphological transformation of an offshore event from initiation to deposition.

Classification of offshore mass movements

Collapse calderas are frequent in the evolution of volcanic systems, but very few have formed during historical times. Piton de la Fournaise is one of the world's most active basaltic shield volcanoes. The caldera collapse, which occurred during the April 2007 lateral eruption is one of the few large documented collapse events on this volcano. It helps to understand the mode and origin of caldera collapses in basaltic volcanoes. Field observations, GPS and seismic data show that the collapse occurred at an early stage of the eruption. The cyclic seismic signal suggests a step by step collapse that directly influenced the lateral eruption rate. Likely, the caldera results from the combined effect of (i) the progressive collapse of the plumbing system above the magma chamber since 2000, and (ii) the large amount of magma withdrawal during the early stage of the eruption by both a significant intrusion within the edifice and an important emission rate.

/pdf/april-2007-collapse-of-piton-de-la-fournaise-a-new-example-xggufhmyhx.pdf

April 2007 collapse of Piton de la Fournaise: A new example of caldera formation

https://hal.univ-reunion.fr/hal-01382032/document

The Cenozoic evolution of the Roer Valley Rift System integrated at a European scale

In this paper, we use mainly field data from the Massif Central area, which have been presented in a com­panion paper (Michon and Merle, 2001), to discuss the origin and the evolution of the West European Rift system. It is shown that the tectonic event in the Tertiary is two-stage. The overall geological evolution reveal a tectonic paradoxe as the first stage strongly suggests passive rifting, whereas the second stage displays the first stage of active rifting. ln the North, crustal thinning, graben formation and sedimentation at sea level without volcanism during the Lower Oligo­cene, followed by scattered volcanism in a thinned area during Upper Oligocene and Lower Miocene, represent the classical evolution of a rift resulting from extensional stresses within the lithosphere (i.e. passive rifting). In the South, thinning of the lithospheric mantle associated with doming and volcanism in the Upper Miocene, together with the lack of crustal thinning, may be easily interpreted in terms of the first stage of active rifting due to the ascent of a mantle plume. This active rifting process would have been inhibited before stretching of the crust, as asthenospheric rise associated with uplift and volcanism are the only tectonic events observed. The diachronism of these two events is emphasized by two clearly distinct orientations of crustal thinning in the north and mantle lithospheric thinning in the south. To understand this tectonic paradox, a new model is discussed taking into account the Tertiary evolution of the Alpine chain. lt is shown that the formation of a deep lithospheric root may have important mechanical consequences on the adjacent lithosphere. The downward gravitational force acting on the descending slab may induce coeval exten­sion in the surrounding lithosphere. This could trigger graben formation and laguno-marine sedimentation at sea level followed by volcanism as expected for passive rifting. Concurrently, the descending lithospheric flow induces a flow pattern in the asthenosphere which can bring up hot mantle to the base of the adjacent lithosphere. Slow thermal ero­sion of the base of the lithosphere may lead to a late-stage volcanism and uplift as expected for active rifting.

/pdf/the-formation-of-the-west-european-rift-a-new-model-as-1lpxrcse08.pdf

The formation of the West European Rift; a new model as exemplified by the Massif Central area

The Massif Central area is the largest magmatic province of the West-European Rift system.The spa­tial-temporal distribution of Tertiary-Quaternary volcanism in the Massif Central, France, shows that three magmatic phases can be defined, each of them characterized by different volumes and different locations. The first event, termed the pre-rift magmatic event, is very scarce and restricted to the north of the Massif Central. It is suggested that this could result from lithospheric bending of the European lithosphere ahead of the incipient Alpine chain during the Pa­leocene. The second event, termed the rift-related magmatic event, is located in the north of the Massif Central only and is spatially connected with zones of high crustal thinning (i.e. the Limagne graben). It immediately follows Oligo­cene graben formation and associated sedimentation, and is represented by more than 200 scattered monogenic edifices. This second event can be attributed to partial melting as a consequence of lithospheric thinning that affected the north of the Massif Central during the rifting event. The lack of volcanism in the south during the same period of time is probably related to the very slight lithospheric thinning during the Oligocene. The third event, termed the major magmatic event, started first in the South in the upper Miocene at about 15 Ma, well after the end of the sedimentation. lt is unrelated to any extensional event. This major magmatic event reached the North of the Massif Central at about 3.5 Ma, following a pause in volcanism of about 6 Ma after the rift-related magmatic event. These two episodes of the ma­jor magmatic event are spatially and temporally associated with the two main periods of uplift, suggesting a common origin of volcanism and uplift processes. The major magmatic event can be attributed to late thermal erosion of the base of the lithosphere above a mantle diapir, as suggested by seismic tomography data. This general magmatic evolution drawn from data at the Massif Central scale may apply to the Eger graben as well, as the three magmatic events described in this study (pre-rift magmatic event, rifting event and post-Miocene volcanic event) are also reported in the literature. This suggests that a single cause should explain the formation of the entire western European rift surroun­ding the Alpine mountain belt.

https://hal.univ-reunion.fr/hal-01391919/document

The evolution of the Massif Central Rift; spatio-temporal distribution of the volcanism

La Reunion Island is located east of Madagascar, on the eastern rim of the tectonically inactive Mascarene Basin. This island is composed of three shield volcanoes of which only Piton de la Fournaise is currently active. Although the magmatic activity is restricted to Piton de la Fournaise, a scattered seismicity occurs on the whole 200 km wide volcanic edifice and in the underlying oceanic crust. We carried out a multiscale analysis to understand (1) the origin of the seismicity in the geodynamic context and (2) the role of the oceanic lithosphere in the deformation of Piton de la Fournaise and La Reunion Island. Analysis of the magmatic system suggests that the magma ascent is controlled by large N25–30 and N125–130 fracture zones located below the Enclos depression. We also show that the orientation difference between the eruptive fissures and the related dykes result from a rotation of the main principal stress s 1 from vertical to downslope through the surface. Combining a Digital Elevation Model (DEM) analysis, field observations and the geophysical data reveals that the volcano is affected by large fault zones. The fault distribution indicates the predominance of a main N70–80 trend. Magnetic data show the same N80 orientation characterizing the remnant part of the Alizes volcano. Such parallel alignment suggests a control exerted by the underlying Alizes volcano on Piton de la Fournaise. Furthermore, the alignment between the crustal orientations and the structures determined on the island suggests a control of the crustal structures in La Reunion 's volcano-tectonic activity. Contrary to several volcanic islands such as Hawaii and Tenerife, La Reunion volcanoes lie on an upbending crust. Then, we interpret the reactivation of the crustal faults as resulting from a crustal uplift related to the thermal erosion of the base of the lithosphere and/or to strong underplating. The upward deformation may prevent the spreading of the volcanoes, as no evidence of such a mechanism is observed in the bathymetry and the seismic data around the island.

https://hal.archives-ouvertes.fr/hal-01242837/document

Laurent Michon

Papers

April 2007 collapse of Piton de la Fournaise: A new example of caldera formation

The Cenozoic evolution of the Roer Valley Rift System integrated at a European scale

The formation of the West European Rift; a new model as exemplified by the Massif Central area

The evolution of the Massif Central Rift; spatio-temporal distribution of the volcanism

Role of the structural inheritance of the oceanic lithosphere in the magmato-tectonic evolution of Piton de la Fournaise volcano (La Réunion Island)