Alpine deformation and 40Ar/39Ar geochronology of synkinematic white mica in the Siviez-Mischabel Nappe, western Pennine Alps, Switzerland

We explore the timing of deformation and exhumation of the Siviez-Mischabel Nappe (western Swiss Alps), which has been considered a classic example of a midcrustal crystalline nappe since the studies of Argand [1916]. This study presents Ar-40/Ar-39 ages obtained on both synkinematic white mica from Permo-Triassic cover sediments and more complex white mica populations from basement gneisses of the Siviez-Mischabel and middle Pennine Nappes. Primary foliation developed in cover units by nucleation, growth, and rigid rotation of mica grains during noncoaxial Alpine deformation. Although some samples show a crenulation of this primary foliation, mica growth appears to have occurred only during the development of primary foliation, the main phase of greenschist facies deformation related to imbrication of the Siviez-Mischabel Nappe and other middle Pennine Nappes. Good agreement exists between independent estimates of the timing of deformation and reported Ar-40/Ar-39, white mica ages from cover units of the central and southern Siviez-Mischabel Nappe. In cover units from the central and southern Siviez-Mischabel regions of the study area, Ar-40/Ar-39 ages appear to date synkinematic white mica growth. Results suggest that the Siviez-Mischabel :Nappe was emplaced and developed foliation during a 5 m.y. period from 41 to 36 Ma. In cover units from the eastern Siviez-Mischabel, however, Ar-40/Ar-39 white mica ages appear to date postkinematic thermal events. These thermal events may be related to Oligocene magmatic activity in the lower Pennine Nappes or to Miocene development of the Simplon fault zone. Variations in the relation between Alpine age and grain size for cover samples from the central, eastern, and southern Siviez-Mischabel correlate well with the regional variations in temperature inferred from quartz microfabrics and the pattern of regional metamorphism. When considered in concert with other recent isotopic studies on the timing of major tectonic and thermal events in the western Swiss Alps, these data support arguments that the relative timing of events such as thrusting and back thrusting of crystalline nappes in hinterland units and exhumation of high-pressure units in the suture zone of the western Alps are intimately related and synchronous on the scale of a few million years. Copyright 1998 by the American Geophysical Union.

Geochemistry of samples from Valtournenche, Western Italian Alps

Slices of polycyclic metasediments (marbles and meta-cherts) are tectonically amalgamated with the polydeformed basement of the Dent Blanche tectonic system along a major Alpine shear zone in the Western Alps (Becca di Salé area, Valtournenche Valley). A combination of techniques (structural analysis at various scales, metamorphic petrology, geochronology and trace element geochemistry) was applied to determine the age and composition of accessory phases (titanite, allanite and zircon) and their relation to major minerals. The results are used to reconstruct the polyphase structural and metamorphic history, comprising both pre-Alpine and Alpine cycles. The pre-Alpine evolution is associated with low-pressure high-temperature metamorphism related to Permo-Triassic lithospheric thinning. In meta-cherts, microtextural relations indicate coeval growth of allanite and garnet during this stage, at ~ 300 Ma. Textures of zircon also indicate crystallization at HT conditions; ages scatter from 263-294 Ma, with a major cluster of data at ~ 276 Ma. In impure marble, U-Pb analyses of titanite domains (with variable Al and F contents) yield apparent 206Pb/238U dates range from Permian to Jurassic. Chemical and isotopic data suggest that titanite formed at Permian times and was then affected by (extension-related?) fluid circulation during the Triassic and Jurassic, which redistributed major elements (Al and F) and partially opened the U-Pb system. The Alpine cycle lead to early blueschist facies assemblages, which were partly overprinted under greenschist facies conditions. The strong Alpine compressional overprint disrupted the pre-Alpine structural imprint and/or reactivated earlier structures. The pre-Alpine metamorphic record, preserved in these slices of metasediments, reflects the onset of the Permo-Triassic lithospheric extension to Jurassic rifting.

Characteristic reflection patterns in the southeast Canadian cordillera, northern Appalachians and Swiss Alps

There are some striking similarities and some differences between the seismic reflection sections recorded across the fold and thrust belts of the southeast Canadian Cordillera, Quebec-Maine Appalachians and Swiss Alps. In the fold and thrust belts of all three mountain ranges, seismic reflection surveys have yielded high-quality images of. (1) nappes (thin thrust sheets) stacked on top of ancient continental margins; (2) ramp anticlines in the hanging walls of faults that have ramp-flat or listric geometries; (3) back thrusts and back folds that developed during the terminal phases of orogeny; and (4) tectonic wedges and regional decollements. A principal result of the Cordilleran and Appalachian deep crustal studies has been the recognition of master decollements along which continental margin strata have been transported long distances, whereas a principal result of the Swiss Alpine deep crustal program has been the identification of the Adriatic indenter, a crustal-scale wedge that caused delamination of the European lithosphere. Significant crustal roots are observed beneath the fold and thrust belts of the Alps, southeast Canadian Cordillera and parts of the southern Appalachians, but such structures beneath the northern Appalachians have probably been removed by post-orogenic collapse and/or crustal attenuation associated with the Mesozoic opening of the Atlantic Ocean.

Subduction and obduction processes in the Swiss Alps

The significance of the Brianconnais domain in the Alpine orogen is reviewed in the light of data concerning its collision with the active Adriatic margin and the passive Helvetic margin. The Brianconnais which formerly belonged to the Iberian plate, was located on the northern margin of the Alpine Tethys (Liguro-Piemont ocean) since its opening in the early-Middle Jurassic. Together with the Iberian plate the Brianconnais terrane was separated from the European plate in the Late Jurassic-Early Cretaceous, following the northern Atlantic, Bay of Biscay, Valais ocean opening. This was accompanied by the onset of subduction along the northern margin of Adria and the closure of the Alpine Tethys. Stratigraphic and metamorphic data regarding this subduction and the geohistory of the Brianconnais allows the scenario of subduction-obduction processes during the Late Cretaceous-early Tertiary in the eastern and western Alps to be specified. HP-LT metamorphism record a long-lasting history of oceanic subduction-accretion, followed in the Middle Eocene by the incorporation of the Brianconnais as an exotic terrane into the accretionary prism. Middle to Late Eocene cooling ages of the Brianconnais basement and the presence of pelagic, anorogenic sedimentation lasting until the Middle Eocene on the Brianconnais preclude any sort of collision before that time between this domain and the active Adria margin or the Helvetic margin. This is confirmed by plate reconstructions constrained by magnetic anomalies in the Atlantic domain. Only a small percentage of the former Brianconnais domain was obducted, most of the crust and lithospheric roots were subducted. This applies also to domains formerly belonging to the southern Alpine Tethys margin (Austroalpine-inner Carpathian domain). It is proposed that there was a single Palaeogene subduction zone responsible for the Alpine orogen formation (from northern Spain to the East Carpathians), with the exception of a short-lived Late Cretaceous partial closure of the Valais ocean. Subduction in the western Tethyan domain originated during the closure of the Meliata ocean during the Jurassic incorporating the Austroalpine-Carpathian domain as terranes during the Cretaceous. The subduction zone propagated into the northern margin of Adria and then to the northern margin of the Iberian plate, where it gave birth to the Pyrenean-Provencal orogenic belt. This implies the absence of a separated Cretaceous subduction zone within the Austro-Carpathian Penninic ocean. Collision of Iberia with Europe forced the subduction to jump to the SE margin of Iberia in the Eocene, creating the Apenninic orogenic wedge and inverting the vergence of subduction from south- to north-directed. (C) 1998 Elsevier Science B.V. All rights reserved.

40Ar/39Ar dating of Penninic Front tectonic displacement (W Alps) during the Lower Oligocene (31-34 Ma)

Direct absolute dating of the Penninic Frontal Thrust tectonic motion is achieved using the Ar-40/Ar-39 technique in the Pelvoux Crystalline Massif (Western Alps). The dated phengites were formed syn-kinematically in shear zones. They underline the brittle-ductile stretching lineation, pressure-shadow fibres and slickensides consistent with underthrusting of the European continental slab below the propagating Penninic Thrust. Chlorite-phengite thermobarometry yields 10-15 km and T similar to 280 degrees C, while Ar-40/Ar-39 phengite ages mainly range between 34 and 30 Ma, with one younger age at 27 Ma. This Early Oligocene age range matches a major tectonic rearrangement of the Alpine chain. Preservation of prograde Ar-40/Ar-39 ages is ascribed to passive exhumation of the Pelvoux shear zone network, sandwiched between more external thrusts and the Penninic Front reactivated as an E-dipping detachment fault. Partial resetting in the Low Temperature part of argon spectra below 24 Ma is ascribed to brittle deformation and alteration of phengites.

From the Apennines to the Alps: colonization genetics of the naturally expanding Italian wolf (Canis lupus) population

Wolves in Italy strongly declined in the past and were confined south of the Alps since the turn of the last century, reduced in the 1970s to approximately 100 individuals surviving in two fragmented subpopulations in the central-southern Apennines. The Italian wolves are presently expanding in the Apennines, and started to recolonize the western Alps in Italy, France and Switzerland about 16 years ago. In this study, we used a population genetic approach to elucidate some aspects of the wolf recolonization process. DNA extracted from 3068 tissue and scat samples collected in the Apennines (the source populations) and in the Alps (the colony), were genotyped at 12 microsatellite loci aiming to assess (i) the strength of the bottleneck and founder effects during the onset of colonization; (ii) the rates of gene flow between source and colony; and (iii) the minimum number of colonizers that are needed to explain the genetic variability observed in the colony. We identified a total of 435 distinct wolf genotypes, which showed that wolves in the Alps: (i) have significantly lower genetic diversity (heterozygosity, allelic richness, number of private alleles) than wolves in the Apennines; (ii) are genetically distinct using pairwise F(ST) values, population assignment test and Bayesian clustering; (iii) are not in genetic equilibrium (significant bottleneck test). Spatial autocorrelations are significant among samples separated up to c. 230 km, roughly correspondent to the apparent gap in permanent wolf presence between the Alps and north Apennines. The estimated number of first-generation migrants indicates that migration has been unidirectional and male-biased, from the Apennines to the Alps, and that wolves in southern Italy did not contribute to the Alpine population. These results suggest that: (i) the Alps were colonized by a few long-range migrating wolves originating in the north Apennine subpopulation; (ii) during the colonization process there has been a moderate bottleneck; and (iii) gene flow between sources and colonies was moderate (corresponding to 1.25-2.50 wolves per generation), despite high potential for dispersal. Bottleneck simulations showed that a total of c. 8-16 effective founders are needed to explain the genetic diversity observed in the Alps. Levels of genetic diversity in the expanding Alpine wolf population, and the permanence of genetic structuring, will depend on the future rates of gene flow among distinct wolf subpopulation fragments.

Structural analysis, clay mineralogy and K-Ar dating of fault gouges from Centovalli Line (Central Alps) for reconstruction of their recent activity

Between the cities of Domodossola and Locarno, the complex ``Centovalli Line'' tectonic zone of the Central Alps outlines deformation phases over a long period of time (probably starting similar to 30 Ma ago) and under variable P-T conditions. The last deformation phases developed gouge-bearing faults with a general E-W trend that crosscuts the roots of the Alpine Canavese zone and the Finero ultramafic body. Kinematic indicators show that the general motion was mainly dextral associated with back thrusting towards the S. The <2 mu m clay fractions of fault gouges from Centovalli Line consist mainly of illite, smectite and chlorite with varied illite-smectite, chlorite-smectite and chlorite-serpentine mixed-layers. Constrained with the illite crystallinity index, the thermal conditions induced by the tectonic activity show a gradual trend from anchizonal to diagenetic conditions. The <2 and <0.2 mu M clay fractions, and hydrothermal K-feldspar separates all provide K-Ar ages between 14.2 +/- 2.9 Ma and roughly 0 Ma, with major episodes at about 12,8, 6 and close to 0 Ma These ages set the recurrent tectonic activity and the associated fluid circulations between Upper Miocene and Recent. On the basis of the K-Ar ages and with a thermal gradient of 25-30 degrees C/km, the studied fault zones were located at a depth of 4-7 km. If they were active until now as observed in field, the exhumation was approximately 2.5-3.0 km for the last 12 Ma with a mean velocity of 0.4 mm/y. Comparison with available models on the recent Alpine evolution shows that the tectonic activity in the area relates to a continuum of the back-thrusting movements of the Canavese Line, and/or to several late-extensional phases of the Rhone-Simplon line. The Centovalli-Val Vigezzo zone therefore represents a major tectonic zone of the Central-Western Alps resulting from different interacting tectonic events. (C) 2011 Elsevier B.V. All rights reserved.

The Mont Collon mafic complex (Austroalpine Dent Blanche nappe) : permian evolution of the Western European mantle

Résumé: Le complexe du Mont Collon (nappe de la Dent Blanche, Austroalpin) est l'un des exemples les mieux préservés du magmatisme mafique permien des Alpes occidentales. Il est composé d'affleurements discontinus et d'une stratification magmatique en son centre (Dents de Bertol) et est composé à 95% de roches mafiques cumulatives (gabbros à olivine et/ou cpx, anorthositiques, troctolites, wehrlites et wehrlites à plagioclase) et localement de quelques gabbros pegmatitiques. Ces faciès sont recoupés par de nombreux filons acides (aphtes, pegmatites quartziques, microgranodiorites et filons anorthositiques) et mafiques tardifs (dikes mélanocrates riches en Fe et Ti). Les calculs thermométriques (équilibre olivine-augite) montrent des températures de 1070-1120 ± 6°C, tandis que le thermomètre amphibole-plagioclase indique une température de 740 ± 40°C à 0.5 GPa pour les amphiboles magmatiques tardives. La geobarométrie sur pyroxène donne des pressions moyennes de 0.3-0.6 GPa, indiquant un emplacement dans la croûte moyenne. De plus, les températures obtenues sur des amphiboles coronitiques indiquent des températures de l'ordre de 700 ± 40°C confirmant que les réactions coronitiques apparaissent dans des conditions subsolidus. Les âges concordants U/Pb sur zircons de 284.2 ± 0.6 et 282.9 ± 0.6 Ma obtenus sur un gabbro pegmatitique et une pegmatitique quartzique, sont interprétés comme des âges de cristallisation. Les datations 40Ar/39Ar sur amphiboles des filons mélanocrates donnent un âge plateau de 260.2 ± 0.7 Ma, qui est probablement très proche de l'âge de cristallisation. Ainsi, cet age 40Ar/39Ar indique un second évènement magmatique au sein du complexe. Les compositions des roches totales en éléments majeurs et traces montrent peu de variations, ainsi que le Mg# (75-80). Les éléments traces enregistrent le caractère cumulatif des roches (anomalie positive en Eu) et révèlent des anomalies négatives systématiques en Nb, Ta, Zr, Hf et Ti dans les faciès basiques. Le manque de corrélation entre éléments majeurs et traces est caractéristique d'un processus de cristallisation in situ impliquant une quantité variable de liquide interstitiel (L) entre les phases cumulus. Les distributions des éléments traces dans les minéraux sont homogènes, indiquant une rééquilibration .subsolidus entre cristaux et liquide interstitiel. Un modèle quantitatif basé sur les équations de cristallisation in situ de Langmuir reproduisent correctement les concentrations en terres rares légères des minéraux cumulatifs montrant la présence de 0 à 35% de liquide interstitiel L pour des degrés de différenciation F de 0 à 45%, par rapport au faciès les moins évolués du complexe. En outre, les valeurs de L sont bien corrélées avec les proportions modales d'amphibole interstitielle et les concentrations en éléments incompatibles des roches (Zr, Nb). Le liquide parental calculé des cumulats du Mont Collon est caractérisé par un enrichissement relatif en terres rares légères et Th, un appauvrissement en terres rares lourdes typique d'une affinité transitionnelle (T-MORB) et une forte anomalie négative en Nb-Ta. Les roches cumulatives montrent des compositions isotopiques en Nd-Sr proches de la terre globale silicatée (BSE), soit 0.6<εNdi<+3.2, 0.7045<87Sr/86Sri<0.7056. Les rapports initiaux en Pb indiquent une source dans le manteau enrichi subcontinental lithosphérique, préalablement contaminé par des sédiments océaniques. Les dikes mélanocrates Fe-Ti sont représentatifs de liquides et ont des spectres de terres rares enrichis, une anomalie positive en Nb-Ta et des εNdi de +7, des 87Sr/86Sri de 0.703 et des rapports initiaux en Pb, similaires à ceux des basaltes d'île océanique, indiquant une source asthénosphérique modérément appauvrie. Ainsi, la fusion partielle du manteau lithosphérique subcontinental est induite par l'amincissement post-orogénique et la remontée de l'asthénosphère. Les filons mélanocrates proviennent, après délamination du manteau lithosphérique, de la fusion de l'asthénosphère. Abstract The early Permian Mont Collon mafic complex (Dent Blanche nappe, Austroalpine nappe system) is one of the best preserved examples of the Permian mafic magmatism in the Western Alps. It is composed of discontinuous exposures and a well-preserved magmatic layering (the Dents de Bertol cliff) crops out in the center part of the complex. It mainly consists of cumulative mafic rocks, which represent 95 vol-% of the mafic complex (ol- and cpx-bearing gabbros and rare anorthositic layers, troctolites, wehrlites and plagioclase-wehrlites) and locally pegmatitic gabbros. All these facies are crosscut by widespread acidic (aplites, quartz-rich pegmatites, microgranodiorites) and late mafic Fe-Ti melanocratic dikes. Olivine-augite thermometric calculations yield a range of 1070-1120 ± 6°C, while amphibole-plagioclase thermometer yields a temperature of 740 ± 40°C at 0.5 GPa. Pyroxene geobarometry points to a pressure of 0.3-0.6 GPa, indicating a middle crustal level of emplacement. Moreover, temperature calculations on the Mont Conon coronitic amphiboles indicate temperatures of 700 ± 40°C, close to those calculated for magmatic amphiboles. These temperatures confirm that coronitic reactions occurred at subsolidus conditions. ID-TIMS U/Pb zircon ages of 284.2 ± 0.6 and 282.9 ± 0.6 Ma obtained on a pegmatitic gabbro and a quartz-pegmatitic dike, respectively, were interpreted as the crystallization ages of these rocks. 40Ar/39Ar dating on amphiboles from Fe-Ti melanocratic dikes yields a plateau age of 260.2 ± 0.7 Ma, which is probably very close to the crystallization age. Consequently, this 40Ar/P39Ar age indicates a second magmatic event. Whole-rock major- and trace-element compositions show little variation across the whole intrusion and Mg-number stays within a narrow range (75-80). Trace-element concentrations record the cumulative nature of the rocks (e.g. positive Eu anomaly) and reveal systematic Nb, Ta, Zr, Hf and Ti negative anomalies for all basic facies. The lack of correlation between major and trace elements is characteristic of an in situ crystallization process involving variable amounts of interstitial liquid (L) trapped between the cumulus mineral phases. LA-ICPMS measurements show that trace-element distributions in minerals are homogeneous, pointing to subsolidus re-equilibration between crystals and interstitial melts. A quantitative modeling based on Langmuir's in situ crystallization equation successfully reproduced the Rare Earth Element (REE) concentrations in cumulitic minerals. The calculated amounts of interstitial liquid L vary between 0 and 35% for degrees of differentiation F of 0 to 45%, relative to the least evolved facies of the intrusion. Furthermore, L values are well correlated with the modal proportions of interstitial amphibole and whole-rock incompatible trace-element concentrations (e.g. Zr, Nb) of the tested samples. The calculated parental melt of the Mont Collon cumulates is characterized by a relative enrichment in Light REE and Th, a depletion in Heavy REE, typical of a transitional affinity (T-MORB), and strong negative Nb-Ta anomaly. Cumulative rocks display Nd-Sr isotopic compositions close to the BSE (-0.6 < εNdi < +3.2, 0.7045 < 87Sr/86Sri < 0.7056). Initial Pb ratios point to an origin from the melting of an enriched subcontinental lithospheric mantle source, previously contaminated at the source by oceanic sediments. The contrasted alkaline Fe-Ti melanocratic dikes are representative of liquids. They display enriched fractionated REE patterns, a positive Nb-Ta anomaly and εNdi of +7, 87Sr/86Sri of 0.703 and initial Pb ratios, all reminiscent of Ocean Island Basalt-type rocks, pointing to a moderately

Age and geochemistry of mantle peridotites and diorite dykes from the Baldissero body: Insights into the Paleozoic-Mesozoic evolution of the Southern Alps

Trace element and isotopic data obtained for mantle spinel Iherzolites and diorite dykes from the Baldissero massif (Ivrea-Verbano Zone, Western Italy) provide new, valuable constraints on the petrologic and geodynamic evolution of the Southern Alps in Paleozoic to Mesozoic times. Whole rock and mineral chemistry indicates that Baldissero Iherzolites can be regarded as refractory mantle residues following limited melt extraction. In particular, the Light Rare Earth Elements (LREE)-depleted and fractionated compositions of whole rock and clinopyroxene closely match modelling results for refractory residues after low degrees (similar to 4-5%) of near-fractional melting of depleted mantle, possibly under garnet-facies conditions. Following this, the peridotite sequence experienced subsolidus re-equilibration at lithospheric spinel-facies conditions and intrusion of several generations of dykes. However, Iherzolites far from dykes show very modest metasomatic changes, as evidenced by the crystallisation of accessory titanian pargasite and the occurrence of very slight enrichments in highly incompatible trace elements (e.g. Nb). The Re-Os data for Iherzolites far from the dykes yield a 376 Ma (Upper Devonian) model age that is considered to record a partial melting event related to the Variscan orogenic cycle s.l. Dioritic dykes cutting the mantle sequence have whole rock, clinopyroxene and plagioclase characterised by high radiogenic Nd and low radiogenic Sr, which point to a depleted to slightly enriched mantle source. Whole rock and mafic phases of diorites have high Mg# values that positively correlate with the incompatible trace element concentrations. The peridotite at the dyke contact is enriched in orthopyroxene, iron and incompatible trace elements with respect to the Iherzolites far from dykes. Numerical simulations indicate that the geochemical characteristics of the diorites can be explained by flow of a hydrous, silica-saturated melt accompanied by reaction with the ambient peridotite and fractional crystallisation. The composition of the more primitive melts calculated in equilibrium with the diorite minerals show tholeiitic to transitional affinity. Internal Sm-Nd, three-point isochrons obtained for two dykes suggest an Upper Triassic-Lower Jurassic emplacement age (from 204 31 to 198 29 Ma). Mesozoic igneous events are unknown in the southern Ivrea-Verbano Zone (IVZ), but the intrusion of hydrous melts, mostly silica-saturated, have been well documented in the Finero region, i.e. the northernmost part of IVZ and Triassic magmatism with calc-alkaline to shoshonitic affinity is abundant throughout the Central-Eastern Alps. The geochemical and chronological features of the Baldissero diorites shed new light on the geodynamic evolution of the Southern Alps before the opening of the Jurassic Tethys. (C) 2010 Elsevier B.V. All rights reserved.

Geophysical investigations in the European Alps

Joint interpretation of magnetotelluric and geomagnetic depth sounding data in the western European Alps offer new insights into the conductivity structure of the Earth's crust and mantle. This first large scale electromagnetic study in the Alps covers a cross-section from Germany to northern Italy and shows the importance of the alpine mountain chain as an interrupter of continuous conductors. Poor data quality due to the highly crystalline underground is overcome by Remote Reference and Robust Processing techniques. 3d-forward-modelling reveals on the one hand interrupted dipping crustal conductors with maximum conductance of 4960 S and on the other hand a lithosphere thickening up to 208 km beneath the central western Alps. Graphite networks arising from Paleozoic sedimentary deposits are considered to be accountable for the occurrence of high conductivity and the distribution pattern of crustal conductors. The influence of huge sedimentary molasse basins on the electromagnetic data is suggested to be minor compared with the influence of crustal conductors. In conclusion, electromagnetic results can be attributed to the geological, tectonic and palaeogeographical background. Dipping direction (S-SE) and maximum angle (10.1°) of the northern crustal conductor reveal the main thrusting conditions beneath the Helvetic Alps whereas the existence of a crustal conductor in the Briançonnais supports theses about its palaeographic belonging to the Iberian Peninsula.

Tab. 1: Number and percentage of glacier movements in the Austrian Alps between 1890 and 1969

Since 1890 the variations of the fronts of numerous glaciers in the Eastern Alps have been measured for the Österreichischer Alpenverein. The present paper presents the results of these measurements and the percentage of advancing, retreating, and stationary glaciers as calculated for each year from the total number of glaciers measured. The measurements reveal two distinct periods of advance, i. e. one period from 1890 to 1900 with up to 47% of glaciers advancing, and one period from 1909 to 1928 with up to 75% of glaciers advancing. The period from 1928 to 1964 is marked by a strong retreat of glaciers. A comparison with the results of measurements of glaciers in the Western Alps of Switzerland shows good agreement. The behaviour of glaciers corresponds well to the climatic conditions prevailing during the summer months of the 80 years observed.

Relative motions of Africa, Iberia and Europe during Alpine orogeny

A revised kinematic model for the motions of Africa and Iberia relative to Europe since the Middle Jurassic is presented in order to provide boundary conditions for Alpine-Mediterranean reconstructions. These motions were calculated using up-to-date kinematic data predominantly based on magnetic isochrons in the Atlantic Ocean and published by various authors during the last 15 years. It is shown that convergence of Africa with respect to Europe commenced during the Cretaceous Normal Superchron (CNS), between chrons MO and 34 (120-83 Ma). This motion was subjected to fluctuations in convergence rates characterised by two periods of relatively rapid convergence (during Late Cretaceous and Eocene-Oligocene times) that alternated with periods of slower convergence (during the Paleocene and since the Early Miocene). Distinct changes in plate kinematics are recognised in the motion of Iberia with respect to Europe, indicated by: (1) a Late Jurassic-Early Cretaceous left-lateral strike-slip motion; (2) Late Cretaceous convergence; (3) Paleocene quiescence; (4) a short period of right-lateral strike-slip motion; and (5) final Eocene-Oligocene convergence. Based on these results, it is speculated that a collisional episode in the Alpine orogeny at ca. 65 Ma resulted in a dramatic decrease in the relative plate motions and that a slower motion since the Early Miocene promoted extension in the Mediterranean back-arc basins. (C) 2002 Elsevier Science B.V. All rights reserved.

Turnover of plant lineages shapes herbivore phylogenetic beta diversity along ecological gradients.

Understanding drivers of biodiversity patterns is of prime importance in this era of severe environmental crisis. More diverse plant communities have been postulated to represent a larger functional trait-space, more likely to sustain a diverse assembly of herbivore species. Here, we expand this hypothesis to integrate environmental, functional and phylogenetic variation of plant communities as factors explaining the diversity of lepidopteran assemblages along elevation gradients in the Swiss Western Alps. According to expectations, we found that the association between butterflies and their host plants is highly phylogenetically structured. Multiple regression analyses showed the combined effect of climate, functional traits and phylogenetic diversity in structuring butterfly communities. Furthermore, we provide the first evidence that plant phylogenetic beta diversity is the major driver explaining butterfly phylogenetic beta diversity. Along ecological gradients, the bottom up control of herbivore diversity is thus driven by phylogenetically structured turnover of plant traits as well as environmental variables.

The variability of peridotite composition across a mantle shear zone (Lanzo massif, Italy): interplay of melt focusing and deformation

In this paper we present new data on the spatial variability of peridotite composition across a kilometer-scale mantle shear zone within the Lanzo massif (Western Alps, Italy). The shear zone separates the central from the northern part of the massif. Plagioclase peridotite shows gradually increasing deformation towards the shear zone, from porphyroclastic to mylonitic textures in the central body, while the northern body is composed of porphyroclastic rocks. The peridotite displays a large range of compositions, from fertile peridotite to refractory harzburgite and dunite. Deformed peridotites (proto-mylonite and mylonites) tend to be compositionally more homogeneous and fertile than weakly deformed peridotites. The composition of most plagioclase peridotites show rather high and constant (Ce/Yb) (N) ratios, and Yb (N) that cannot be explained by any simple melting model. Instead, refertilization modeling, consisting of melt increments from spinel peridotite sources, particularly with E-MORB melt, reasonably reproduces the plagioclase peridotite whole rock composition. Combined with constraints from Ce-Nb and Ce-Th systematics, we speculate that peridotites such as those from Lanzo record pervasive refertilization processes in the thermal boundary layer. In this scenario, mantle shear zones might act as important areas of melt focusing in the upper mantle that separates the thermal boundary layer from the conductively cooled mantle.

Tectonics of the Simplon massif and Lepontine gneiss dome: deformation structures due to collision between the underthrusting European plate and the Adriatic indenter

this study presents a review of published geological data, combined with original observations on the tectonics of the simplon massif and the Lepontine gneiss dome in the Western Alps. New observations concern the geometry of the Oligocene Vanzone back fold, formed under amphibolite facies conditions, and of its root between Domodossola and Locarno, which is cut at an acute angle by the Miocene, epi- to anchizonal, dextral centovalli strike-slip fault. the structures of the simplon massif result from collision over 50 Ma between two plate boundaries with a different geometry: the underthrusted European plate and the Adriatic indenter. Detailed mapping and analysis of a complex structural interference pattern, combined with observations on the metamorphic grade of the superimposed structures and radiometric data, allow a kinematic model to be developed for this zone of oblique continental collision. the following main Alpine tectonic phases and structures may be distinguished: 1. NW-directed nappe emplacement, starting in the Early Eocene (similar to 50 Ma); 2. W, SW and S- verging transverse folds; 3. transpressional movements on the dextral simplon ductile shear zone since similar to 32 Ma; 4. formation of the Bergell - Vanzone backfolds and of the southern steep belt during the Oligocene, emplacement of the mantle derived 31 - 29 Ma Bergell and Biella granodiorites and porphyritic andesites as well as intrusions of 29-25 Ma crustal aplites and pegmatites; 5. formation of the dextral discrete Rhone-Simplon line and the centovalli line during the Miocene, accompanied by the pull-apart development of the Lepontine gneiss dome - Dent blanche (Valpelline) depression. It is suggested that movements of shortening in fan shaped NW, W and sW directions accompanied the more regular NW- to WNW-directed displacement of the Adriatic indenter during continental collision.