Geophysical evidence for a large impact structure on the Falkland (Malvinas) Plateau

A large, roughly circular structural basin is recognised on the Falkland (Malvinas) Plateau to the NW of West Falkland (Gran Malvina) Island (S 51°00′, W 62°00′). The basin, seen in seismic‐reflection profiles and evident as a large negative gravity anomaly, has a diameter of ~250 km. The age of the basin is estimated to be Late Palaeozoic. It is completely buried by younger sediments and has no topographic expression on the sea floor. We propose that the basin and geophysical anomalies, especially the combination of a large circular negative gravity anomaly with a rim of positive anomalies, and a marked circular series of positive magnetic anomalies in the same area, may be best explained by the presence of a large buried impact structure.     

Chemical and mineralogical characterization of the Mineo (Sicily,Italy) pallasite: A unique sample

The Mineo pallasite is characterized here for the first time. The only 42 g still available worldwide is part of the collection of the Department of Physics and Geology, University of Perugia. A multianalytical approach was used, joining field-emission scanning electron microscopy, Raman analysis, X-ray powder diffraction, electron-probe microanalysis, and laser ablation inductively coupled plasma mass spectrometry. Results highlighted that (1) the Mineo pallasite belongs to the Main Group pallasites; (2) the silicate component is essentially olivine, with no pyroxene component; (3) the olivine chemical composition varies in terms of both iron and trace elements; (4) the metal phase is essentially kamacite with the taenite mainly found in the plessite structure; (5) phosphide phases are present as schreibersite and barringerite. The observed compositional variability in olivines as well as their occurrence as both angular and rounded crystals suggest that the Mineo pallasite could have been derived from a large impact of a differentiated parent body with a larger solid body. The resulting pallasite conglomerate consists of the compositionally different olivines, likely coming from different areas of the same differentiated parent body, and the residual molten Fe-Ni.     

A statistical model for vesuvius and its volcanological implications

The last 300 years of Vesuvius history are reconstructed as a chronological succession of 4 phenomenological states: i) repose, ii) persistent activity, iii) intermediate eruption and iv) final eruption. It turns out that the times of permanence in each state are distributed according to the same exponential law. Vesuvius activity is then described by a Markov chain of these 4 states, with transition probabilities determined from the previous phenomenological analysis. The model reproduces the Vesuvius activity between 1694 and 1872 and possibly also in the 1872–1944 period. It turns out that, at least between 1694 and 1872, the volcano was behaving like a quasistationary system with 4 equilibrium states, perturbed by a stochastic noise responsible for occasional transitions from an equilibrium state to another. Major physical or structural changes of the volcanic system around 1872 and possibly in the whole subsequent period, are clearly shown by the statistical analysis.     

Some thoughts on the pre-Alpine metamorphic history of the austridic basement of the Eastern Alps

Summary In several places of the old crystalline basement of the Eastern Alps a classification of the pre-Alpine metamorphic effects into an older, high-to intermediate-pressure metamorphism (eclogites, kyanite) and a younger, lower-pressure one (and±ky±cord) is recognizable. Some local geological situations allow a sharp chronological distinction to be made between these two events; and the available radiometric age values demonstrate the Caledonian age (500 m.y.) of the older metamorphism and the Hercynian age (320 m.y.) of the younger one. Elements exist showing that the Caledonian metamorphism, belongs to a complex cycle of geological processes which took place substantially during the Ordivician age (?) and has all the ingredients of the orogenic cycles. This evolutional picture represents a possible model for the whole of the Eastern Alps.

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Überlegungen zur Entwicklung der voralpidischen Metamorphose im Ostalpin

Zusammenfassung An mehreren Stellen im altkristallinen Grundebirge der Ostalpen ist eine Gliederung der voralpidischen metamorphen Ereignisse in eine ältere Metamorphose, die einer hoch-bis mitteldruckbetonten Faziesserie (Eklogite, Disthen) angehört und eine jüngere weniger druckbetonte Metamorphose (Andalusit±Disthen±Cordierit) erkennbar. Einzelne lokale geologische Situationen erlauben eine scharfe chronologische Trennung dieser zwei Ereignisse; das Caledonische Alter ( 500m.y.) der älteren und das Hercynische Alter ( 320 m.y.) der jüngeren Metamorphose wird durch radiometrische Altersdaten demonstriert. Es gibt Hinweise, daß die Caledonische Metamorphose zu einem komplexen geologischen Ereignis mit allen Kennzeichen eines orogenen Zyklusses gehört, das im wesentlichen während des Ordoviziums stattgefunden hat. Diese genetischen Vorstellungen scheinen als mögliches Modell für die gesamten Ostalpen annehmbar.

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With 3 Figures     

15,000-yr pollen record of vegetation change in the high altitude tropical Andes at Laguna Verde Alta, Venezuela

Pollen analysis of sediments from a high-altitude (4215 m), Neotropical (9°N) Andean lake was conducted in order to reconstruct local and regional vegetation dynamics since deglaciation. Although deglaciation commenced 15,500 cal yr B.P., the area around the Laguna Verde Alta (LVA) remained a periglacial desert, practically unvegetated, until about 11,000 cal yr B.P. At this time, a lycopod assemblage bearing no modern analog colonized the superpáramo. Although this community persisted until 6000 cal yr B.P., it began to decline somewhat earlier, in synchrony with cooling following the Holocene thermal maximum of the Northern Hemisphere. At this time, the pioneer assemblage was replaced by a low-diversity superpáramo community that became established 9000 cal yr B.P. This replacement coincides with regional declines in temperature and/or available moisture. Modern, more diverse superpáramo assemblages were not established until 4600 cal yr B.P., and were accompanied by a dramatic decline in Alnus, probably the result of factors associated with climate, humans, or both. Pollen influx from upper Andean forests is remarkably higher than expected during the Late Glacial and early to middle Holocene, especially between 14,000 and 12,600 cal yr B.P., when unparalleled high values are recorded. We propose that intensification of upslope orographic winds transported lower elevation forest pollen to the superpáramo, causing the apparent increase in tree pollen at high altitude. The association between increased forest pollen and summer insolation at this time suggests a causal link; however, further work is needed to clarify this relationship.     

Cretaceous deformation of the southern Central Andes: synorogenic growth strata in the Neuquén Group (35° 30′–37° S)

The Neuquén Group is an Upper Cretaceous continental sedimentary unit exhumed during the latest Miocene contractional phase occurred in the southern Central Andes, allowing a direct field observation and study of the depositional geometries. The identification of growth strata on these units surrounding the structures of the frontal parts of the Andes, sedimentological analyses and U–Pb dating of detrital components, allowed the definition of a synorogenic unit that coexisted with the uplift of the early Andean orogen since ca. 100 Ma, maximum age obtained in this work, compatible with previous assignments and constrained in the top by the deposition of the Malargüe Group, in the Maastrichtian (ca. 72 Ma). The definition of a wedge top area in this foreland basin system, where growth strata were described, permitted to identify a Late Cretaceous orogenic front and foredeep area, whose location and amplitude contrast with previous hypotheses. This wedge top area was mostly fed from the paleo‐Andes with small populations coming from sources in the cratonic area that are interpreted as a recycling in Jurassic and Lower Cretaceous sections, which contrasts with other analyses performed at the foredeep zone that have mixed sources. In particular, Permian sources are interpreted as coming directly from the cores of the basement structures, where Neopaleozoic sections are exposed, next to the synorogenic sedimentation, implying a strong incision in Late Cretaceous times with an exhumation structural level similar to the present. The maximum recognised advance for this Late Cretaceous deformation in the study area is approximately 500 km east of the Pacific trench, which constitutes an anomaly compared with neighbour segments where Late Cretaceous deformations were found considerably retracted. The geodynamic context of the sedimentation of this unit is interpreted as produced under the westward fast moving of South America, colliding with two consecutive mid‐ocean ridges during a period of important plate reorganisation. The subduction of young, anhydrous, buoyant lithosphere would have produced changes in the subduction geometry, reflected first by an arc waning/gap and subsequently by an arc migration that coexisted with synorogenic sedimentation. These magmatic and deformational processes would be the product of a shallow subduction regime, following previous proposals, which occurred in Late Cretaceous times, synchronous to the sedimentation of the Neuquén Group.     

Growth pattern and changes in abundance of the endangered bat star Asterina stellifera

The sea star Asterina stellifera has declined during the last decade and is currently abundant only in the southern limit of its former range. We surveyed this population over 5 years to model individual growth and explore the relationship of changes in local abundance with variation in environmental factors and the reproductive status of individuals. Our results show that A. stellifera is a species with slow growth and a relatively long lifespan. Contrary to expectations for temperate species, growth rates were fairly constant through the year and therefore models including seasonal oscillations were inappropriate. The abundance of this species increased significantly from early spring to early summer, likely due to augmented activity and small‐scale aggregation during the reproductive season that affected our estimates of abundance. No significant recruitment occurred during the 5 years studied. The lack of recruitment during long periods and the slow individual growth rates make A. stellifera particularly vulnerable to local extinction. This study was performed prior to the arrival in the study area of the invasive kelp Undaria pinnatifida and side‐gilled sea slug Pleurobranchaea maculata, species that threaten the community structure where A. stellifera lives. Therefore, the information reported here will be essential to assessing the impacts of these exotic species on this sea star population.