Laurent Michon

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

The Roer Valley Rift System (RVRS) is located between the West European rift and the North Sea rift system. During the Cenozoic, the RVRS was characterized by several periods of subsidence and inversion, which are linked to the evolution of the adjacent rift systems. Combination of subsidence analysis and results from the analysis of thickness distributions and fault systems allows the determination of the Cenozoic evolution and quantification of the subsidence. During the Early Paleocene, the RVRS was inverted (Laramide phase). The backstripping method shows that the RVRS was subsequently mainly affected by two periods of subsidence, during the Late Paleocene and the Oligocene–Quaternary time intervals, separated by an inversion phase during the Late Eocene. During the Oligocene and Miocene periods, the thickness of the sediments and the distribution of the active faults reveal a radical rotation of the direction of extension by about 70–80j (counter clockwise). Integration of these results at a European scale indicates that the Late Paleocene subsidence was related to the evolution of the North Sea basins, whereas the Oligocene–Quaternary subsidence is connected to the West European rift evolution. The distribution of the inverted provinces also shows that the Early Paleocene inversion (Laramide phase) has affected the whole European crust, whereas the Late Eocene inversion was restricted to the southern North Sea basins and the Channel area. Finally, comparison of these deformations in the European crust with the evolution of the Alpine chain suggests that the formation of the Alps has controlled the evolution of the European crust since the beginning of the Cenozoic.

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

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 worlds 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.

Morphology of Piton de la Fournaise basaltic shield volcano (La Réunion Island): Characterization and implication in the volcano evolution

The topography of Piton de la Fournaise volcano (PdF) differs from the classic view of basaltic shield volcanoes as it is characterized by (1) several steep slope zones on its flanks and (2) a large U-shaped caldera, the Enclos-Grand Brule structure (EGBS). Most of these structures were previously interpreted as the scars of lateral landslides, the deposits of which cover the submarine flanks of PdF. We carried out a detailed analysis of the morphology of PdF, which reveals that the steep slope zones form two independent, circumferential structures that continue into the caldera. The development of circumferential steep slopes on volcano flanks may have several origins: constructive, destructive, and deformation processes. We interpret those processes acting on PdF as caused by the spreading of the volcanic edifice above a weak hydrothermal core, leading to outward displacements and a summit extensive stress field. The continuity of the steep slope on both sides of the EGBS escarpments suggests that this structure was not caused by a 4.5 ka old giant landslide as it is usually proposed but is due to a mainly vertical collapse. The recent debris avalanche deposits east of the island indicate that this event likely destabilized part of the submarine flank. We propose that the collapse of the Grand Brule, the lower half of the EGBS, was due to the downward drag related to the dense intrusive complex of the Alizes volcano, which is located 1 km below the Grand Brule. The collapse of the Enclos is interpreted as the consequence of the deformation of the hydrothermal system of the pre-Enclos volcano. Although the continuity of the geological and morphological structures between the Enclos and the Grand Brule suggests a narrow link between these two collapse events, their chronology and relationship are still uncertain. Finally, we hypothesize that the persistence of the NE and SE rift zones during the last 150 ka, despite the large changes of the topography related to the recurrent flank destabilizations, is linked to a deep sources, which can be either underlying crustal faults or the continuous downward subsidence of the Alizes intrusive complex.

Volcano destabilization by magma injections in a detachment

We report the discovery of a detachment fault within reach of structural investigation in a shield volcano (Piton des Neiges, La Reunion hotspot). The detachment, exposed by erosion, is located between the top of an extinct gabbroic magma chamber and a cover of debris avalanche deposits. The fault simultaneously concentrated ductile and brittle deformation toward the sea, hydrothermal alteration in the greenschist facies, and repeated sill injections of basic magmas. Our study of this exceptional detachment exposure suggests that two mechanisms participate in volcano flank failure: hydrothermal alteration, likely sustained by the heat of cooling sills, induces slow inter-intrusion deformation, whereas sill injections in the fault may trigger rapid cointrusive slip. This twofold process of destabilization might currently occur on some active volcanoes such as Piton de la Fournaise.

Mode of lithospheric extension: Conceptual models from analogue modeling

Comparison of analogue experiments at crustal and lithospheric scale provides essential information concerning the mode of deformation during lithospheric extension. This study shows that during extension, lithospheric deformation is controlled by the development of shear zones in the ductile parts. At lithospheric scale, the global deformation is initiated by the rupture of the brittle mantle lithosphere. This failure generates the formation of conjugate and opposite shear zones in the lower crust and the ductile mantle lithosphere. The analysis of the internal strain of the ductile layers suggests that the two opposite shear zones located below the asymmetric graben in the lower crust and the ductile mantle lithosphere prevail. Experiments show that from a similar initial stage, the relative predominance of these shear zones originates two different modes of deformation. If the crustal shear zone prevails, a major detachment-like structure crosscuts the whole lithosphere and controls its thinning. In this model named the simple shear mode, the resulting geometry shows that crustal and lithospheric thinning are laterally shifted. If the mantle shear zone predominates, the lithospheric thinning is induced by the coeval activity of the two main shear zones. This process called the necking mode leads to the vertical superposition of crustal and mantle lithospheric thinning. Applied to natural laboratories (West European rift, Red Sea rift and North Atlantic), this conceptual model allows a plausible explanation of the different geometries and evolutions described in these provinces. The North Atlantic and the Red Sea rift systems may result from a simple shear mode, whereas the necking mode may explain part of the evolution of the West European rift especially in the Massif Central and the Eger graben.

The 2007 eruptions and caldera collapse of the Piton de la Fournaise volcano (La Réunion Island) from tilt analysis at a single very broadband seismic station

Seismic records from La Reunion Island very broadband Geoscope station are investigated to constrain the link between the 2007 eruptive sequence and the related caldera collapse of the Piton de la Fournaise volcano. Tilt estimated from seismic records reveals that the three 2007 eruptions belong to a single inflation-deflation cycle. Tilt trend indicates that the small-volume summit eruption of 18 February occurred during a phase of continuous inflation that started in January 2007. Inflation decelerated 24 days before a second short-lived, small-volume eruption on 30 March, almost simultaneous with a sudden, large-scale deflation of the volcano. Deflation rate, which had stabilized at relatively low level, increased anew on 1 April while no magma was erupted, followed on 2 April by a major distal eruption and on 5 April by a summit caldera collapse. Long-term tilt variation suggests that the 2007 eruptive succession was triggered by a deep magma input.

Crustal structures of the Rhinegraben and the Massif Central grabens: An experimental approach

Two of the most improtant segments of the west European Rift, the Rhinegraben and the Massif Central grabens, show in plan and cross section a very different crustal structure.

Deformation of basaltic shield volcanoes under cointrusive stress permutations

We performed a microstructural study of Piton des Neiges (La Reunion Island) to understand how intrusions and stresses control each other in basaltic volcanoes. Our study reveals that three perpendicular intrusions trends coexisted during the 2 Myr history of the volcano: a N120–140°E rift zone, a perpendicular dike trend, and swarms of subhorizontal intrusions hereafter called “sill zones”. Independently, the inversion of fault-slip data shows that incompatible paleostress fields recurrently occurred along with the intrusions: a dominant NNE-SSW extension, a perpendicular extension, and strike-slip or compressional regimes. The orientations of paleostresses are consistent with the orientations of the three perpendicular intrusion populations. We propose that stress accumulation in the edifice under the effect of repeated magma injections resulted in permutations of the principal axes of the stress tensor, causing a reorientation of subsequent intrusions. Stress permutations were cyclical. Each cycle started with dike injections in an extensional stress field, reducing the deviatoric stress and switching the axes of principal stresses, and finished with sill intrusions in a compressional stress field. Sill zones acted as detachment planes, restoring the extensional stress field and initiating a new cycle of permutations. Our model of stress permutations is in agreement with the pattern of eruptions and deformation observed at Piton de la Fournaise. In contrast with the Hawaiian model of spreading on a decollement, stress permutations in La Reunions volcanoes imply that the basal deformation of the edifices, if any, is not sufficient to compensate the reduction of deviatoric stress caused by intrusions.

Cointrusive shear displacement by sill intrusion in a detachment: A numerical approach

Sheared sills are observed at Piton des Neiges (the eroded basaltic volcano of La Reunion) and are a suspected cause of the coeruptive flank displacement of Piton de la Fournaise in 2007. We performed a 2-D numerical study to quantify the perturbation induced by sill injection within a volcanic edifice. Magma is considered as an inviscid and pressurized fluid injected in an initially stable edifice under gravity-controlled extension. Two cases of injection are tested, in an elastic homogeneous edifice or along a detachment controlled by a friction law. We show that sill injection induces tangential displacements. This effect is strongly increased when sills are emplaced along a detachment, producing extension behind the injection and potentially ending in a large-scale flank collapse. Sill injections can thus explain the cointrusive shear deformation observed at Piton des Neiges and the tangential displacements measured at Piton de la Fournaise in 2007.

A volcaniclastic deep-sea fan off La Réunion Island (Indian Ocean): Gradualism versus catastrophism

A new geophysical data set off La Reunion Island (western Indian Ocean) reveals a large volcaniclastic submarine fan developing in an open-ocean setting. The fan is connected to a torrential river that floods during tropical cyclones. Sediment storage at the coast is limited, suggesting that the sediments are carried directly to the basin. The fan morphology and turbidites in cores lead us to classify it as a sand-rich system mainly fed by hyperpycnal flows. In the ancient geological record, there are many examples of thick volcaniclastic successions, but studies of modern analogues have emphasized mechanisms such as debris avalanches or direct pyroclastic flow into the sea. Because the Cilaos deep-sea fan is isolated from any continental source, it provides information on architecture and noncatastrophic processes in a volcaniclastic deep-sea fan.