Two-stage metamorphic evolution of the Bemarivo Belt of northern Madagascar: constraints from reaction textures and in situ monazite dating

New results on the pressure–temperature–time evolution, deduced from conventional geothermobarometry and in situ U‐Th‐total Pb dating of monazite, are presented for the Bemarivo Belt in northern Madagascar. The belt is subdivided into a northern part consisting of low‐grade metamorphic epicontinental series and a southern part made up of granulite facies metapelites. The prograde metamorphic stage of the latter unit is preserved by kyanite inclusions in garnet, which is in agreement with results of the garnet (core)‐alumosilicate‐quartz‐plagioclase (inclusions in garnet; GASP) equilibrium. The peak metamorphic stage is characterized by ultrahigh temperatures of ～900–950 °C and pressures of ～9 kbar, deduced from GASP equilibria and feldspar thermometry. In proximity to charnockite bodies, garnet‐sillimanite‐bearing metapelites contain aluminous orthopyroxene (max. 8.0 wt% Al₂O₃) pointing to even higher temperatures of ～970 °C. Peak metamorphism is followed by near‐isothermal decompression to pressures of 5–7 kbar and subsequent near‐isobaric cooling, which is demonstrated by the extensive late‐stage formation of cordierite around garnet. Internal textures and differences in chemistry of metapelitic monazite point to a polyphasic growth history. Monazite with magmatically zoned cores is rarely preserved, and gives an age of c. 737 ± 19 Ma, interpreted as the maximum age of sedimentation. Two metamorphic stages are dated: M1 monazite cores range from 563 ± 28 Ma to 532 ± 23 Ma, representing the collisional event, and M2 monazite rims (521 ± 25 Ma to 513 ± 14 Ma), interpreted as grown during peak metamorphic temperatures. These are among the youngest ages reported for high‐grade metamorphism in Madagascar, and are supposed to reflect the Pan‐African attachment of the Bemarivo Belt to the Gondwana supercontinent during its final amalgamation stage. In the course of this, the southern Bemarivo Belt was buried to a depth of >25 km. Approximately 25–30 Myr later, the rocks underwent heating, interpreted to be due to magmatic underplating, and uplift. Presumably, the northern part of the belt was also affected by this tectonism, but buried to a lower depth, and therefore metamorphosed to lower grades.     

Protection of marine birds and turtles at St Brandon’s Rock,Indian Ocean,requires conservation of the entire atoll

A survey of seabirds and turtles at St Brandon’s Rock, 400 km north of Mauritius, was undertaken in 2010. We estimated that 1 084 191 seabirds comprising seven breeding species and excluding non-breeders were present at the archipelago and we counted 279 turtle tracks and nesting pits of green turtles Chelonia mydas. Hawksbill turtles Eretmochelys imbricata were also present. Analyses of 30 different islets that make up the atoll showed that the seabird species mostly partitioned their use of islets based on islet size, with four species preferring larger islets and two species preferring smaller islets. Alien species introduced historically are still present and other threats, such as shipwrecks, remain. We propose conservation and other measures that should adequately protect the birds, turtles and coral reef by treating the atoll as a system.     

The Darfur Dome,western Sudan: the product of a subcontinental mantle plume

Field investigations, K-Ar age determinations and chemical data were used to describe the development of an intraplate volcanic province, the Darfur Dome, Sudan. Magmatism started 36 Ma ago at a small subvolcanic complex (Jebel Kussa) in the center of the dome and was active in the same area between 26 and 23 Ma. Two major volcanic fields (Marra Mountains and Tagabo Hills) developed between 16 and 10 Ma. Volcanism started again at 6.8 Ma with a third volcanic field (Meidob Hills) and at 4.3 Ma in the Marra Mountains and with the reactivation of the center. Activity then continued until the late Quaternary. Having started in the center of the Darfur Dome, volcanism moved in 36 Ma 200 km towards the NNE and 100 km SSW No essential difference in the alkaline magma types (basanitic to phonolitic-trachytic, with different amounts of assimilation of crustal material) in the different fields, was observed. Magmatism is thought to have been produced by a rising mantle plume and volcanism was triggered by stress resolution along the Central African Fault Zone.     

Interactions between a cushion plant (Azorella selago) and surface sediment transport on sub-Antarctic Marion Island

On sub-Antarctic Marion Island cushions of the dominant vascular plant species, Azorella selago, interact with the geomorphology of fellfield landscapes by affecting sediment distribution and ultimately terrace formation. Here, to understand the consequences of Azorella cushions for substrate movement and sorting, we quantified the size and shape of Azorella cushions and the grain size distribution of sediment surrounding these cushions, using a combination of image analysis approaches. Results show that as cushions become larger, they tend to become more elongated and grow more perpendicular to the slope. Mean and variance of grain size were greater upslope of Azorella cushions, while the number of particles was higher downslope of cushions, although these differences were not significant at all sites studied. Differences between upslope and downslope particle sizes were, however, not related to cushion elongation or growth angle as had been expected. The observed sediment partitioning is likely caused by a combination of frost-related sediment transport and Azorella cushions acting as sediment obstructions. Understanding these interactions between Azorella cushions and the landscape is especially important in the light of recent warming and drying on the island, as particle size affects soil properties such as water-holding capacity and frost susceptibility.     

Relics of the Mozambique Ocean in the central East African Orogen: evidence from the Vohibory Block of southern Madagascar

The Vohibory Block of south‐western Madagascar is part of the East African Orogen, the formation of which is related to the assembly of the Gondwana supercontinent. It is dominated by metabasic rocks, which have chemical compositions similar to those of recent basalts from a mid‐ocean ridge, back‐arc setting and island‐arc setting. The age of formation of protolith basalts has been dated at 850–700 Ma by U–Pb SHRIMP analysis of magmatic cores in zircon, pointing to an origin related to the Neoproterozoic Mozambique Ocean. The metabasic rocks are interpreted as representing components of an island arc with an associated back‐arc basin. In the early stage of the Pan‐African orogeny, these rocks experienced high‐pressure amphibolite to granulite facies metamorphism (9–12 kbar, 750–880 °C), dated at 612 ± 5 Ma from metamorphic rims in zircon. The metamorphism was most likely related to accretion of the arc terrane to the margin of the Azania microcontinent (Proto‐Madagascar) and closure of the back‐arc basin. The main metamorphism is significantly older than high‐temperature metamorphism in other tectonic units of southern Madagascar, indicating a distinct tectono‐metamorphic history.     

A computational finite‐element program for hybrid simulation

The essence of real time hybrid simulation (RTHS) is the reliance on a physical test (virtual finite element) in support of a numerical simulation, which is unable to properly simulate it numerically. Hence, the computational support for a hybrid simulation is of paramount importance, and one with anything less than a state of the art computational support may defeat the purpose of such an endeavor. A critical, yet often ignored, component of RTHS is precisely the computational engine, which unfortunately has been a bottleneck for realistic studies. Most researches have focused on either the control or on the communication (mostly in distributed, non‐real time hybrid simulation) leaving the third leg of RTHS (computation) ignored and limited to the simulation of simple models (small number of degrees of freedom and limited nonlinearities). This paper details the development of a specialized software written explicitly to perform, single site, hybrid simulation ranging from pseudo‐dynamic to hard real time ones. Solution strategy, implementation details, and actual applications are reported. Copyright © 2011 John Wiley & Sons, Ltd.     

Biogeomorphology: understanding different research approaches

Geomorphologists and ecologists commonly employ different scientific approaches to data collection and validation. Ecologists often employ a more statistics‐based approach, whereas geomorphologists place less emphasis on replication and significance testing and more often focus on multiple‐parameter measurements and theoretical reasoning. These differences in approach could slow down collaboration between the fields and hinder integration in biogeomorphology. Such integrated research is necessary, however, if both fields are to benefit from the knowledge and experience of the other. Copyright © 2010 John Wiley & Sons, Ltd.