Determination of rapid Deccan eruptions across the Cretaceous-Tertiary boundary using paleomagnetic secular variation: 2. Constraints from analysis of eight new sections and synthesis for a 3500-m-thick composite section

International audienceThe present paper completes a restudy of the main lava pile in the Deccan flood basalt province (trap) of India. Chenet et al. (2008) reported results from the upper third, and this paper reports the lower two thirds of the 3500-m-thick composite section. The methods employed are the same, i.e., combined use of petrology, volcanology, chemostratigraphy, morphology, K-Ar absolute dating, study of sedimentary alteration horizons, and as the main correlation tool, analysis of detailed paleomagnetic remanence directions. The thickness and volume of the flood basalt province studied in this way are therefore tripled. A total of 169 sites from eight new sections are reported in this paper. Together with the results of Chenet et al. (2008), these data represent in total 70% of the 3500-m combined section of the main Deccan traps province. This lava pile was erupted in some 30 major eruptive periods or single eruptive events (SEE), each with volumes ranging from 1000 to 20,000 km 3 and 41 individual lava units with a typical volume of 1300 km 3. Paleomagnetic analysis shows that some SEEs with thicknesses attaining 200 m were emplaced over distances in excess of 100 km (both likely underestimates, due to outcrop conditions) and up to 800 km. The total time of emission of all combined SEEs could have been (much) less than 10 ka, with most of the time recorded in a very small number of intervening alteration levels marking periods of volcanic quiescence (so-called ''big red boles''). The number of boles, thickness of the pulses, and morphology of the traps suggest that eruptive fluxes and volumes were larger in the older formations and slowed down with more and longer quiescence periods in the end. On the basis of geochronologic results published by Chenet et al. (2007) and paleontological results from Keller et al. (2008), we propose that volcanism occurred in three rather short, discrete phases or megapulses, an early one at $67.5 ± 1 Ma near the C30r/C30n transition and the two largest around 65 ± 1 Ma, one entirely within C29r just before the K-T boundary, the other shortly afterward spanning the C29r/C29n reversal. We next estimate sulfur dioxide (likely a major agent of environmental stress) amounts and fluxes released by SEEs: they would have ranged from 5 to 100 Gt and 0.1 to 1 Gt/a, respectively, over durations possibly as short as 100 years for each SEE. The chemical input of the Chicxulub impact would have been on the same order as that of a very large single pulse. The impact, therefore, appears as important but incremental, neither the sole nor main cause of the Cretaceous-Tertiary mass extinctions

Etude géochronologique, aspects géomorphologiques et géochimiques du volcanisme de l île de Basse Terre (Guadeloupe), et datation des structures d effondrement de flanc majeures de l arc des Petites Antilles

La technique K-Ar Cassignol-Gillot nous a permis de définir les limites temporelles du volcanisme effusif sub-aérien de l île de Basse Terre, Guadeloupe, et de préciser les périodes d activité des quatre massifs de l île. Le Complexe Basal [2,79+-0,04 - 2,68+-0,04 Ma], la Chaîne Septentrionale [1,81+-0,03 - 1,15+-0,02 Ma], la Chaîne Axiale [1023+-25 - 435+-8 ka], le complexe de Grande-Découverte (GD) [205+-28 - actuel] illustrent la migration du volcanisme à Basse Terre depuis 3 Ma suivant une direction principale N-S et une vitesse de 20 km/Ma. L extraction des magmas serait contrôlée par les systèmes de graben E-W et failles normales en échelon NW-SE affectant la Guadeloupe. Les taux d extrusion calculés sur la base d observations et modélisations morphologiques vont de 1,4 10-5 à 4,5 10-4 km3/yr. Au sud du volcan de GD, les diverses phases d'activité effusive du complexe de Trois-Rivières-Madeleine montrent une contemporanéité avec celles du volcan de GD. Les coulées de Trois-Rivières (87+-5 ka) puis l alignement E-W Madeleine-Le Palmiste (70 à 45 ka) datent une phase de propagation du rift de Marie-Galante. Des coulées <15 ka dans l'environnement du dôme de la Madeleine permettent d étendre à l extrême sud de l île la zone d activité volcanique Holocène. L aléa volcanique n est plus restreint au Massif de Soufrière-GD mais doit être étendu en son sud au massif de Madeleine-Trois-Rivières. Des effondrements de flanc majeurs ont été datés sur les îles de Basse Terre (640 ka, 550 ka), Dominique (100 ka), Martinique (330 ka) et Sainte-Lucie (100 ka). Une relation causale entre la récurrence de ces phénomènes catastrophiques et les changements climatiques globaux est proposée.The K-Ar Cassignol-Gillot technique enabled us to define the temporal bounds of sub-aerial effusive volcanism in Basse Terre, Guadeloupe, and to constrain the periods of volcanic activity of the four massifs of the island. The Basal Complex [2.79+-0.04 2.68+-0.04 Ma], the Septentrional Chain [1.81+-0.03 1.15+-0.02 Ma], the Axial Chain [1023+-25 - 435+-8 ka] and the most recent volcanic complex of Grande-Découverte (GD) [205+-28-present] illustrate the southwards migration of volcanism through time in Basse-Terre since 3 Ma, along a N-S main direction, at a rate of 20 km/Ma. We propose magma extraction to be linked to the E-W striking graben and N-S en-échelon normal fault systems affecting Guadeloupe. Extrusion rates, defined from morphological observations and modeling spread from 1.4x10-5 to 4.5x10-4 km3/yr. South of GD volcano, the several volcanic phases of the Trois Rivières Madeleine Complex show to be contemporaneous with the GD volcano ones. The Trois-Rivières lava flows (87+-5 ka) and subsequent emplacement of E-W alignment Madeleine-Le Palmiste (70 to 45 ka) constrain the age of a Marie-Galante rift propagation phase. Dating of lava flows <15 ka in the surroundings of the Madeleine dome lead to extend the Holocene volcanic zone up to the extreme south of the island, hence to consider volcanic hazards in a larger zone spreading from La Soufrière area up to the Madeleine -Trois Rivières volcanic complex. Finally, dating of major flank-collapse events in Basse Terre (640 and 550 ka), Dominica (100 ka), Martinique (330 ka) and St. Lucia (100 ka) have been realized; we propose a causal link between the recurrence of such catastrophic events and global climate changes.ORSAY-PARIS 11-BU Sciences (914712101) / SudocSudocFranceF

Datation 40Ar/39Ar du Crétacé Supérieur

Dans le cadre du projet Européen GTS Next, nous avons obtenu des nouvelles contraintes sur l âge des étages du Crétacé Supérieur à partir de plusieurs techniques de géochronologie et d interprétations stratigraphiques au Canada et au Japon. Dans le bassin sédimentaire du Western Interior Canada, nous proposons une nouvelle détermination de l âge de la limite Crétacé - Tertiaire (K/Pg) enregistrée dans la coupe de Red Deer River (Alberta). Il a été possible de calibrer par cyclostratigraphie haute-résolution cette série sédimentaire fluviatile non-marine et d identifier 11-12 cycles associés à la précession orbitale de la Terre. En considérant la technique 40Ar/39Ar intercalibrée avec la cyclostratigraphie, l âge apparent de la base du chron magnétique C29r suggère que la limite K/Pg se trouve entre un minimum et un maximum de l excentricité, avec une durée pour C29r de 66.30 +- 0.08 à 65.89 +- 0.08 Ma. En supposant que le cycle contenant le niveau de charbon soit associé à un cycle de précession, l âge révisé de la limite Crétacé - Tertiaire est donné par la plus jeune des populations de zircon datée par U-Pb à 65.75 +- 0.06 Ma.La limite Campanien Maastrichtien est également enregistrée dans ce même bassin canadien, et se trouve à environ 8 m sous le niveau de charbon No. 10 dans la formation de Horseshoe Canyon. L étude cyclostratigraphique montre que le dépôt de cette séquence sédimentaire est directement influencé par les changements du niveau marin dû à la précession et dominés par l excentricité Notre travail montre que la position de la limite Campanien Maastrichtien dans ce bassin sédimentaire du Western Canada est placée à environ 2.5 cycles d excentricité au dessus d un niveau de téphra de la base de la coupe dont l âge U-Pb est donné par la plus jeune population des zircons, et ~4.9 Myr avant la limite Crétacé - Tertiaire. Nous en déduisons un âge absolu de 70.65 +- 0.09 Ma pour la limite Campanien Maastrichtien, ce qui est ~1.4 Myr plus jeune que les études récemment publiées.Enfin, à partir des isotopes du carbone et des foraminifères planctoniques enregistrés au centre d Hokkaido (Pacifique Nord-Ouest), les coupes Crétacé du groupe Yezo ont été corrélée avec les séries européennes et nord-américaines. Plusieurs niveaux de téphra prélevés au sein des coupes de Kotanbetsu et Shumarinai ont été datés par les méthodes 40Ar/39Ar and U-Pb. Deux d entre eux, placés de part et d autre de la limite Turonien Coniacien, ont donné des âges de 89.31 +- 0.11 et 89.57 +- 0.11 Ma, ce qui suggère un âge de 89.44 +- 0.24 Ma pour cette limite. En combinant notre résultat avec les âges récemment publiés, nous pouvons proposer un âge de 89.62 +- 0.04 Ma pour la limite Turonien Coniacien.As part of the wider European GTS Next project, I propose new constraints on the ages of the Late Cretaceous, derived from a multitude of geochronological techniques, and successful stratigraphic interpretations from Canada and Japan. In the Western Canada Sedimentary Basin, we propose a new constraint on the age of the K/Pg boundary in the Red Deer River section (Alberta, Canada). We were able to cyclostratigraphically tune sediments in a non-marine, fluvial environment utilising high-resolution proxy records suggesting a 11-12 precession related cyclicity. Assuming the 40Ar/39Ar method is inter-calibrated with the cyclostratigraphy, the apparent age for C29r suggests that the K/Pg boundary falls between eccentricity maxima and minima, yielding an age of the C29r between 65.89 +- 0.08 and 66.30 +- 0.08 Ma. Assuming that the bundle containing the coal horizon represents a precession cycle, the K/Pg boundary is within the analytical uncertainty of the youngest zircon population achieving a revised age for the K/Pg boundary as 65.75 +- 0.06 Ma. The Campanian - Maastrichtian boundary is preserved in the sedimentary succession of the Horseshoe Canyon Formation and has been placed ~8 m below Coal nr. 10. Cyclostratigraphic studies show that the formation of these depositional sequences (alternations) of all scales are influenced directly by sea-level changes due to precession but more dominated by eccentricity cycles proved in the cyclostratigraphic framework and is mainly controlled by sand horizons, which have been related by autocyclicity in a dynamic sedimentary setting. Our work shows that the Campanian - Maastrichtian boundary in the Western Canada Sedimentary Basin coincides with ~2.5 eccentricity cycles above the youngest zircon age population at the bottom of the section and ~4.9 Myr before the Cretaceous - Palaeogene boundary (K/Pg), and thus corresponds to an absolute age of 70.65 +- 0.09 Ma producing an ~1.4 Myr younger age than recent published ages. Finally, using advances with terrestrial carbon isotope and planktonic foraminifera records within central Hokkaido, Northwest Pacific, sections from the Cretaceous Yezo group were correlated to that of European and North American counterparts. Datable ash layers throughout the Kotanbetsu and Shumarinai section were analysed using both 40Ar/39Ar and U-Pb methods. We successfully dated two ash tuff layers falling either side of the Turonian - Coniacian boundary, yielding an age range for the boundary between 89.31 +- 0.11 Ma and 89.57 +- 0.11 Ma or a boundary age of 89.44 +- 0.24 Ma. Combining these U-Pb ages with recent published ages we are able to reduce the age limit once more and propose an age for the Turonian - Coniacian boundary as 89.62 +- 0.04 Ma.PARIS11-SCD-Bib. électronique (914719901) / SudocSudocFranceF

Last 150 kyr volcanic activity on Mauritius island (Indian ocean) revealed by new Cassignol-Gillot unspiked K–Ar ages

International audienceWe present new K–Ar ages extending the volcanic history of Mauritius Island towards the Holocene. Mauritius volcanism is associated with the activity of the Réunion hotspot, the magmatism of which produced the Deccan Traps across the Cretaceous-Tertiary (KT) boundary and continues up to present on Réunion Island. After shield building from before 8.9 Ma to 4.7 Ma, Mauritius Island underwent an unusual volcanic evolution involving two rejuvenation stages (3.5–1.9 Ma and <0.7 Ma) separated by a 1.2 Myr hiatus. The lower bound of the second rejuvenation stage being poorly constrained, it is not clear whether volcanism in this island has come to a rest. Given the occurrence of uneroded, and yet undated strombolian cones in Mauritius, it is important to better constrain the latest activity of this island in order to assess its volcanic hazard. The importance of dating the end of this stage is also enhanced by the fact that most of the surface of Mauritius is covered by the lavas of the rejuvenation stages, hence indicating a recent volcanic activity that needs to be better characterized for risk assessment.We focus here on the last 150 ka interval which lacked precise age controls with only a few whole-rock and groundmass 40Ar/39Ar plateau ages available (Moore et al., 2011), ranging from 138 ± 29 to 40 ± 48 ka (uncertainties are 1 sigma). Our six new Cassignol-Gillot unspiked K–Ar ages obtained on plagioclase or groundmass range from 113 ± 7 to 14 ± 3 ka, with much lower uncertainties than previous ages available. Our major and trace element analyses of the dated samples highlight the compositional homogeneity of the lavas erupted in the last rejuvenation stage, showing no geochemical evolution.Our results, with ages as young as 14 ± 3 ka, demonstrate that near-Holocene volcanism is present in Mauritius Island, with no evidence of fading magmatism in the last 150 ka. Our second youngest age at 44 ± 5 ka implies that the latest activity occurred after a ∼30 kyr repose interval, with the consequence that Mauritius volcanism may resume at any time

Evolution volcano-tectonique de l'île de la Martinique (arc insulaire des Petites Antilles) (nouvelles contraintes géochronologiques et géomorphologiques)

Nous présentons ici un travail qui combine des études de géochronologie, géochimie et géomorphologie sur l île de la Martinique afin de contraindre l évolution de l activité volcanique de l île. L arc insulaire des Petites Antilles, double dans sa moitié nord, résulte de la subduction vers l ouest de la plaque Atlantique sous la plaque Caraïbe à 2 cm/an. Grâce à sa position centrale où les deux branches nord se rejoignent, la Martinique est la seule île des Petites Antilles où l histoire la plus complète de l arc affleure. Nos âges K-Ar montrent que l Arc Ancien a été actif de 25 à 21 Ma. L Arc Intermédiaire s est ensuite mis en place en milieu sous-marin puis subaérien entre 16 et 7 Ma. Au cours du Miocène, l activité volcanique s est déplacée vers l ouest avec la mise en place de petits volcans monogéniques, alignés le long de failles, entre 3 Ma et 340 ka. Au même moment, le compartiment septentrional s est édifié au niveau du volcan bouclier du Morne Jacob (5.5 1.5 Ma), du Complexe du Carbet (1 Ma 322 ka), du Mont Conil (550 190 ka) et enfin de la Montagne Pelée. Nous avons mis en évidence l étroite relation de l évolution chimique des laves et des taux d éruption avec les modifications de surface, ainsi que l influence de la subduction de rides asismiques sur la migration et la répartition de l activité volcanique à travers et le long de l arc. En conclusion, notre compilation de 56 nouveaux âges K-Ar entre 26 Ma et 190 ka, nous permet de mieux contraindre les différentes phases volcaniques de l île de la Martinique, et d identifier des épisodes de forte production magmatique liés aux changements géodynamiques régionaux et/ou les événements d effondrement de flanc.This thesis combines geochronological, geochemical and geomorphological studies on Martinique Island to constrain the evolution of its volcanic activity. The Lesser Antilles Island arc, which is double in its northern part, was built in response to the subduction of the Atlantic plate under the Caribbean plate. Due to its central position where the two northern arcs merge, Martinique is the island where the most complete history of the arc can be found. Our K-Ar ages show that the Older Arc has been active from 25 to 21 Ma. Then, the intermediate arc was emplaced in a submarine and then subaerial context between 16 and 7 Ma. During the Miocene, volcanic activity migrated westward and small monogenetic volcanoes erupted along faults, between 3 Ma and 340 ka. Simultaneously, the northern compartment has been built within the Morne Jacob shield volcano (5.5 1.5 Ma), the Carbet Complex (1 Ma 322 ka), Mont Conil (550 190 ka), and finally the Mount Pelée. We have shown the relationship between the chemical evolution of the lavas and of the time-average eruptive rates with surface changes, as well as the influence of subducted asismic ridges on the volcanic migration and volcano spacing along and across the arc. To conclude, our dataset of 56 new K-Ar ages ranging from 26 Ma to 190 ka allow us to better constrain the different building stages of Martinique Island, and to identify high magmatic production rates linked to regional geodynamic changes or flank collapse events.ORSAY-PARIS 11-BU Sciences (914712101) / SudocSudocFranceF

Evolution of the East African Rift System from trap-scale to plate-scale rifting

International audienceMany continental rifts are subjected to volcanism in tandem with rifting, which has raised a long-standing debate about whether magmatism is the cause or the consequence of plate fragmentation. To re-evaluate this chickenand-egg question, we took advantage of five decades of research on the East African Rift System (EARS), the largest active continental rift on Earth, to explore the spatial and temporal relationship between rifting and magmatism. By comparing the co-occurrence of tectonics and volcanism since the Eocene with the present-day seismicity, we delimit the EARS as a ~ 5000 km-wide zone of volcano-tectonics made of four branches affecting not only East Africa but also the Mozambique channel and Madagascar. We then developed a quality filtering procedure of published radiometric ages in order to build two independent, robust, and comprehensive age compilations for magmatism and rifting over this extended EARS. Our thorough quality-checked selection of ages reveals that the EARS presents two distinct regimes of volcanism. Since the Upper Eocene, the rift system was affected by (1) pulses of volcanism in 500-1000 km-wide areas, and (2) a discontinuous but remarkably simultaneous volcanic activity, scattered along the four branches of the EARS since 25-27 Ma. Combining this spatio-temporal evolution of volcanism with a critical review of the timing of rifting, we show that the tectonics of the EARS evolves through time from trap-scale to plate-scale rifting. Until the Middle Miocene, extension structures first developed following flood basalt events and plateau uplifts. Then, volcanism resumed synchronously all over the EARS at ca. 12-12.5 Ma, followed by a general extensional deformation. This evolution, which cannot be explained by the sole action of a plume or of tectonics, is therefore interpreted in an intermediate way in which the EARS results from (1) extensive stresses acting on the African lithosphere in the longlived context of the Gondwana breakup and (2) an overall complex mantle upwelling dynamics arising from the African Large Low Shear Velocity Province (LLSVP). We propose that extension stresses affecting the African lithosphere also modulate the melting of mantle anomalies and/or the collection of magma through the Pan-African belts. This influence explains the synchronous occurrence of many magmatic and tectonic events in the EARS and at the boundaries of the Nubia and Somali plates. Finally, our results suggest that the source of extension stresses affecting the African plate probably evolved from a dominant far-field origin to prevailing variations of gravitational potential energy (GPE) and a diverging basal shear of the Nubia and Somali lithosphere. This change would stem from an increase of the mantle flux in the Middle Miocene, yielding a change in the EARS' dynamics from trap-scale to plate scale rifting

Chronologie 40Ar/39Ar et K/Ar de la dislocation du plateau éthiopien et de la déchirure continentale à la corne de l'Afrique depuis 30 Ma

Ce travail de thèse présente le développement et la mise au point d'un procédé original pour la datation par la technique 40_Ar/39_Ar. Le cœur de ce système, un spectromètre de masse utilisant la collection simultanée des cinq isotopes de l'argon, est aujourd'hui unique à l'échelle internationale. Tout d'abord testé et validé sur des échantillons géologiques standard, ce système a permis de réaliser, dans sa configuration actuelle, des analyses de grande qualité sur les roches volcaniques de l'Ethiopie, avec une reproductibilité analytique de l'ordre de 0.2%. Il est maintenant communément admis que les grands traps basaltiques sont en étroite relation avec les grandes déchirures continentales. C'est notamment le cas du point chaud Afar localisé dans un contexte extensif de point triple. Un des objectifs de cette thèse est d'apporter, à la lumière de 28 et 68 nouvelles données acquises, respectivement, en 40_Ar/39_Ar et K/Ar, des contraintes temporelles sur l'épanchement du point chaud et sur les prémices de l'ouverture continentale. Pour cela, des laves et des granites d'ouverture ont été échantillonnées sur les trois zones géographiques aujourd'hui désunies, c'est-à-dire sur les hauts plateaux éthiopiens, yéménites et somaliens, ainsi que le long de leurs escarpements aux marges de la dépression Afar. Nos résultats attestent de l'édification rapide des grands traps (en moins de 1 Ma, pour l'essentiel de la pile volcanique) et d'une initiation du volcanisme autour de 30.2 Ma. Les prémices de l'ouverture apparaissent en conséquence totalement synchrones de cette phase, comme l'indique la mise en place de granites, dès 30 Ma. Les résultats de cette étude sont repris et discutés de manière plus générale, afin de préciser et de proposer un schéma global d'évolution depuis 30 Ma.We present here a new analytical system for 4O_Ar/39_Ar dating that relies onto an original 180ʿ sector multiple collection mass spectrometer with five faraday cups. Results of preliminarily experiments undertaken on geological standard minerals highlight the good behavior of this new instrument for step heating analyses. The age reproducibility of successive steps lead to analytical errors lower than 0.2 %. Such a system appears today unique on the international plan for 40_Ar/39_Ar routine dating. This new penta-collector MS enabled us to perform highly reproducible 40_Ar/39_Ar dating of Ethiopian volcanics. It is now admitted that continental flood basalt volcanism has a strong relationship with continental break-up. The Ethiopian-Afar plume has been linked to the early opening of the Afar depression, where the propagation of Red Sea and Gulf of Aden within the depression is still an ongoing process. The purposes of this study are to constrain the emplacement and duration of the trap series, and to reconstruct the eruptive chronology of the earlier opening stages, since the initiation of the fracturation. Twenty-eight 40_Ar/39_Ar and 68 K/Ar new ages data have been performed on volcanic series and granitic bodies, sampled on the plateaus (Ethiopian, Yemenite, Somalian) and along their boundaries on the Afar margin. Our results support that the onset of traps volcanism took place around 30 Ma, with a duration lower than 1 Myr. The emplacement of granitic bodies as early as 30 Ma argues for a synchronous initiation of the opening. Finally, all results obtained in this study on both plateau and rifted margin areas helped us to propose a global scheme for the evolution of the Afar depression since 30 Ma.ORSAY-PARIS 11-BU Sciences (914712101) / SudocSudocFranceF

Multicomponent magnetization in paleomagnetic records of reversals from continental sediments in Bolivia

A paleomagnetic study of geomagnetic reversals has been attempted from three continental sections of the altiplano in Bolivia. The first section (Viscachani) shows a unique polarity interval with a magnetization carried by magnetite. In the two other sections (Achocalla and Irpavi) the magnetization is characterized by a complex magnetic mineralogy dominated by magnetic sulphides (mostly pyrrhotite) and magnetite. Thermal demagnetization performed under vacuum was found to be much more efficient than in air in isolating the directions of remanence carried by a low temperature (LT) component due to sulphides and a high temperature (HT) component due to magnetite. The directions of magnetization carried by the two minerals have been compared, with particular attention being paid to records of geomagnetic reversals. At Achocalla, the same polarity pattern is revealed by both components. However, the transitional directions of the LT component are located about 50 cm below the abrupt polarity change recorded by magnetite. Such observations are very similar to those from a recent study of Pliocene marine marl~ in Sicily [1]. These characteristics are compatible with the results of a simulation which assumed that a very early acquisition of the remanence was carried by the LT component and that a post-depositional reorientation of the magnetite occurred. In the last section (Irpavi), the observed LT and HT components have opposite polarities. Although the magnetic parameters indicate the presence of sulphides, we cannot rule out the hypothesis that the LT component has a secondary origin due to coarse magnetite grains. If we assume that the hypothesis of sulphides is correct, this observation emphasizes the difficulties in establishing the origin of sulphide minerals

Confounding influence of magnetic fabric on sedimentary records of a field reversal

RECENT compilations of geomagnetic reversal records have generated a controversy as to whether the geomagnetic field is geographically biased during polarity transitions. At present there is general agreement that the virtual geomagnetic poles recorded from Cenozoic sediments preferentially lie over the Americas (or the antipodal longitude), yet such a preference is not statistically established. However, it is intriguing that the claimed preferred paths lie 90° away from the site longitude. Although this may be partly inherent in the very poor geographical distribution of the sites, we prefer not to rely on fortuitous coincidences. Several authors have argued that sedimentary palaeomagnetic records may be modified by artefacts linked to the acquisition of magnetization. Here we report that in two sedimentary records of the Upper Olduvai reversal from Confidence Hills, California, the declinations of the remanent magnetization recorded during the reversal are similar to the directions of the maximum horizontal axes of the ellipsoids of magnetic anisotropy. This supports the idea that, at times of low geomagnetic intensity (such as during a reversal), factors other than the geomagnetic field influence the orientation of elongated grains

Volcanic Evolution of the Back-Arc Pleistocene Payun Matru Volcanic Field (Argentina)

For the first time, about 30 volcanic formations of the back-arc Payun Matru volcanic field (Payun Matru volcanic field, Argentina, 36°S, 69°W) have been sampled for K–Ar geochronology and geochemistry in order to reconstruct the eruptive history of this key province in the Andean back-arc. The Payun Matru volcanic field has been built since final Pleistocene until present with ages ranging from 280 ± 5 to 7 ± 1 ka. Erupted lavas belong to calc-alkaline series, with characteristics of both arc and intraplate magmas. From previous studies, three main units are distinguished: (1) a basaltic field (Los Volcanes), which covers a large surface of the Payun Matru volcanic field, composed of strombolian cones and associated lava flows emitted from 300 ka to Holocene times, (2) the stratovolcano Payun, with intermediate compositions, built around 265 ka, and (3) the shield volcano Payun Matru s.s. characterized by trachytic compositions and a large summit caldera. The earlier stages of the Payun Matru volcano are not dated, but we constrain the major explosive event, related to the eruption of a widespread ignimbrite and to the formation of the caldera, between 168 ± 4 ka (internal wall of caldera) and 82 ± 1 ka (flow within the caldera). Based on the geochemical similarities of the ignimbrite and the upper lava flow of the pre-caldera cone, we suggest that the age of this event is most probably at the older end of this interval. Numerical modeling using a GIS program has been used to reconstruct the morphological evolution for Payun Matru volcano before and after the caldera collapse. The ancient edifice could be modeled as a flattened cone, 2300 m high, with a volume of about 240 km3. The ignimbrite eruption associated with the Payun Matru caldera formation could be related to the regional tectonic environment, which is characterized by multiple Plio-Pleistocene extensional stages during the last 5 Myr. The evolution of the Nazca plate subduction from a flat slab to a normal dip induced an input of fluid mobile elements and asthenosphere plume-like mantle source beneath the Patagonian lithosphere, which yields the observed intraplate signature. We also interpret this geodynamic evolution as the influence of extensive processes in the upper crust leading to caldera-forming eruptions as observed throughout this province