Abrupt global events in the Earth's history: a physics perspective

The timeline of the Earth's history reveals quasi-periodicity of the geological record over the last 542 Myr, on timescales close, in the order of magnitude, to 1 Myr. What is the origin of this quasi-periodicity? What is the nature of the global events that define the boundaries of the geological time scale? I propose that a single mechanism is responsible for all three types of such events: mass extinctions, geomagnetic polarity reversals, and sea-level fluctuations. The mechanism is fast, and involves a significant energy release. The mechanism is unlikely to have astronomical causes, both because of the energies involved, and because it acts quasi-periodically. It must then be sought within the Earth itself. And it must be capable of reversing the Earth's magnetic field. The last requirement makes it incompatible with the consensus model of the origin of the geomagnetic field - the hydromagnetic dynamo operating in the Earth's fluid core. In the second part of the paper, I show that a vast amount of seemingly unconnected geophysical and geological data can be understood in a unified way if the source of the Earth's main magnetic field is a ~200-km-thick lithosphere, repeatedly magnetized as a result of methane-driven oceanic eruptions, which produce ocean flow capable of dynamo action. The eruptions are driven by the interplay of buoyancy forces and exsolution of dissolved gas, which accumulates in the oceanic water masses prone to stagnation and anoxia. Polarity reversals, mass extinctions, and sequence boundaries are consequences of these eruptions. Unlike the consensus model of geomagnetism, this scenario is consistent with the paleomagnetic data showing that "directional changes during a [geomagnetic polarity] reversal can be astonishingly fast, possibly occurring as a nearly instantaneous jump from one inclined dipolar state to another in the opposite hemisphere".Comment: Final journal version. New title, significant changes. Supersedes v.

Fluid release from the subducted Cocos plate and partial melting of the crust deduced from magnetotelluric studies in southern Mexico: implications for the generation of volcanism and subduction dynamics

In order to study electrical conductivity phenomena that are associated with subduction related fluid release and melt production, magnetotelluric (MT) measurements were carried out in southern Mexico along two coast to coast profiles. The conductivity-depth distribution was obtained by simultaneous two-dimensional inversion of the transverse magnetic and transverse electric modes of the magnetotelluric transfer functions. The MT models demonstrate that the plate southern profile shows enhanced conductivity in the deep crust. The northern profile is dominated by an elongated conductive zone extending >250 km below the Trans-Mexican Volcanic Belt (TMVB). The isolated conductivity anomalies in the southern profile are interpreted as slab fluids stored in the overlying deep continental crust. These fluids were released by progressive metamorphic dehydration of the basaltic oceanic crust. The conductivity anomalies may be related to the main dehydration reactions at the zeolite → blueschist → eclogite facies transitions and the breakdown of chlorite. This relation allows the estimation of a geothermal gradient of ∼8.5°C/km for the top of the subducting plate. The same dehydration reactions may be recognized along the northern profile at the same position relative to the depth of the plate, but more inland due to a shallower dip, and merge near the volcanic front due to steep downbending of the plate. When the oceanic crust reaches a depth of 80–90 km, ascending fluids produce basaltic melts in the intervening hot subcontinental mantle wedge that give rise to the volcanic belt. Water-rich basalts may intrude into the lower continental crust leading to partial melting. The elongated highly conductive zone below the TMVB may therefore be caused by partial melts and fluids of various origins, ongoing migmatization, ascending basaltic and granitic melts, growing plutons as well as residual metamorphic fluids. Zones of extremely high conductance (>8000 S) in the continental crust on either MT profile might indicate extinct magmatism

Spatial and temporal facies evolution of a Lower Jurassic carbonate platform, NW Tethyan margin (Mallorca, Spain)

The variety of depositional facies of a Lower Jurassic carbonate platform has been investigated on the island of Mallorca along a transect comprising six stratigraphic profiles. Twenty-nine facies and sub-facies have been recognized, grouped into seven facies associations, ranging in depositional environment from supratidal/terrestrial and peritidal to outer platform. Spatial and temporal (2D) facies distribution along the transect reflects the evolution of the carbonate platform with time showing different facies associations, from a broad peritidal platform (stage 1) to a muddy open platform (stage 2), and finally to a peritidal to outer carbonate platform (stage 3). Stage 1 (early Sinemurian to earliest late Sinemurian) corresponds to a nearly-flat peritidal-shallow subtidal epicontinental platform with facies belts that shifted far and fast over the whole study area. The evolution from stage 1 to stage 2 (late Sinemurian) represents a rapid flooding of the epicontinental shallow platform, with more open-marine conditions, and the onset of differential subsidence. During stage 3 (latest Sinemurian), peritidal and shallow-platform environments preferentially developed to the northeast (Llevant Mountains domain) with a rapid transition to middle-outer platform environments toward the northwest (Tramuntana Range domain). Stages 1 and 3 present facies associations typical of Bahamian-type carbonates, whereas stage 2 represents the demise of the Bahamian-type carbonate factory and proliferation of muddy substrates with suspension-feeders. The described platform evolution responded to the interplay between the initial extensional tectonic phases related to Early Jurassic Tethyan rifting, contemporaneous environmental perturbations, and progressive platform flooding related to the Late Triassic–Early Jurassic worldwide marine transgression and associated accommodation changes

La symétrie des feldspaths potassiques dans les gneiss du massif de Menderes (Asie Mineure)

Les gneiss œillés du massif de Menderes comportent deux variétés de feldspaths potassiques : d'une part, les yeux constitués en grande majorité d'orthose (déterminé aux rayons X) et de l'autre, des petits grains de microcline quadrillé présents dans la matière intersticielle. Or le faciès métamorphique des gneiss, dans lesquels ces échantillons de feldspaths ont été recueillis, correspond à « l'Épidote˗Amphibolite faciès » d'Eskola ou à un niveau élevé de « l'Amphibolite faciès » pour lesquels les quelques données de la bibliographie reconnaissent le microcline comme feldspath potassique en équilibre. Sous réserve que de nouvelles recherches viennent s'y opposer, l'orthose dans les gneiss du Menderes peut être considérée comme héritée d'équilibres antérieurs plus profonds, que n'a pu totalement effacer une phase de métamorphisme ultérieure, normalement microclinisante.Graciansky Pierre Charles de. La symétrie des feldspaths potassiques dans les gneiss du massif de Menderes (Asie Mineure). In: Bulletin de la Société française de Minéralogie et de Cristallographie, volume 89, 3, 1966. pp. 362-366

Le château de La Bâtie (Savoie). Changement de distributions résidentielles et défensives

International audienceLe château de La Bâtie, grand ensemble inédit assis sur une échine dominant la combe de Chambéry, était fief des Seyssel, proches conseillers du duc de Savoie, puis des d'Oncieu ; son étude bénéficie ainsi de sa continuité familiale, avec archives et mobilier, mais aussi du relevé tout neuf que nous avons produit récemment.Archétypal de l'évolution de la distribution aristocratique de 1300 à 1850, le château va connaître trois étapes principales à partir d'un noyau composé d'une salle à grosse tour circulaire, précédée d'une porterie avec chapelle. La campagne des années 1480 voit l'inversion de l'accès et de son système de défense, du col à l'ouest vers celui de l'est, la surélévation des niveaux du logis avec tour de garde-robes accolée au nord, la centralisation des distributions verticales par une grande vis, plus tard prolongée d'une galerie reliant la grosse tour en façade ; la chronologie fine de cette campagne est permise par l'évolution des orifices de tir pour armes à feu. Le XIXe siècle va poser l'ensemble sur une vaste terrasse d'agrément, reprendre intégralement les combles, remodeler les espaces grâce au couloir et adosser une aile de cuisines au corps principal ; le goût d'antiquaire des maîtres des lieux voit l'intégration de mosaïques antiques et de couronnements troubadour dans le programme