Reply to comment by J. Trampert

Books Reviewed in this Article:
Seismic tomography and mantle circulation Eds O'Nions, R. K. & Parsons, B., Royal Society of London Special Publication , 1989, ISBN 0854 033823.     

Le grand séisme de Huaxian (1556) : quelques documents chinois

The strong earthquake that struck Shaanxi, Shanxi and several other Chinese provinces in 1556 is generally considered as the deadliest of all earthquakes. It is said that the Chinese annals reported 830,000 casualties. We give here a translation into French of the relevant passage of the annals, as well as of a testimony of a survivor Qin Keda, and of a text engraved on a stela.     

Deep inside a neoproterozoic intra-oceanic arc: growth,differentiation and exhumation of the Amalaoulaou complex (Gourma,Mali)

We show here that the Amalaoulaou complex, in the Pan-African belt of West Africa (Gourma, Mali), corresponds to the lower and middle sections of a Neoproterozoic intra-oceanic arc. This complex records a 90–130-Ma-long evolution of magmatic inputs and differentiation above a subducting oceanic slab. Early c. 793 Ma-old metagabbros crystallised at lower crustal or uppermost mantle depths (25–30 km) and have geochemical characteristic of high-alumina basalts extracted from a depleted mantle source slightly enriched by slab-derived sedimentary components ((La/Sm)_N < 1; ε_Nd: +5.4–6.2; ⁸⁷Sr/⁸⁶Sr: 0.7027–0.7029). In response to crustal thickening, these mafic rocks were recrystallised into garnet-granulites (850–1,000°C; 10–12 kbar) and subject to local dehydration–melting reactions, forming trondhjemititic leucosomes with garnet–clinopyroxene–rutile residues. Slightly after the granulitic event, the arc root was subject to strong HT shearing during partial exhumation (detachment faults/rifting or thrusting), coeval with the emplacement of spinel- and garnet-pyroxenite dykes crystallised from a high-Mg andesitic parental magma. Quartz and hornblende-gabbros (700–660 Ma) with composition typical of hydrous volcanic rocks from mature arcs ((La/Sm)_N: 0.9–1.8; ε_Nd: +4.6 to +5.2; ⁸⁷Sr/⁸⁶Sr: 0.7028–0.7031) were subsequently emplaced at mid-arc crust levels (~15 km). Trace element and isotopic data indicate that magmas tapped a depleted mantle source significantly more enriched in oceanic sedimentary components (0.2%). Exhumation occurred either in two stages (700–660 and 623 Ma) or in one stage (623 Ma) with a final exhumation of the arc root along cold P-T path (550°C, 6–9 kbar; epidote–amphibolite and greenschist facies conditions) during the main Pan-African collision event (620–580 Ma). The composition of magmas forming the Cryogenian Amalaoulaou arc and the processes leading to intra-arc differentiation are strikingly comparable to those observed in the deep section of exposed Mezosoic oceanic arcs, namely the Kohistan and Talkeetna complex. This evolution of the Amalaoulaou oceanic arc and its accretion towards the West African craton belong to the life and closure of the Pharusian Ocean that eventually led to the formation of the Greater Gondwana supercontinent, a similar story having occurred on the other side of the Sahara with the Mozambique Ocean.     

Swiss and Alpine geologists between two tectonic revolutions. Part 2: From drifting continents towards plate tectonics

Following the major contributions of Wegener and Argand (Part 1), it was the work of synthesis carried out by R. Staub that represented the major contribution Alpine geology made with respect to that heritage. The research work of young scientists (Gagnebin, Juvet, Wavre, Leuba) who had been influenced by Argand was of lesser importance. Ampferer’s ground breaking contribution, coming along with illuminating graphic illustrations, was all but ignored. Although remaining fairly popular, the theory of continental drift found itself under the heavy fire of criticism from influential geologists in the USA and in Europe. In order to test the validity of the idea, C.E. Wegmann suggested linking geological field work with oceanographic research. He showed that the trajectories of drifting had to be conceived as following the small circles of the sphere. With regard to Alpine geologists of the time, they were renowned for the high quality of their geological mapping. This remained the very special activity in which they excelled, but they focused on topics that were becoming narrower and narrower, and increasingly specialised. The new avenues for research that Holmes and Hess opened up had but little impact on Alpine geologists. In fact, they apparently remained unaware of a note by Holmes written in German and published in a Swiss journal. On the eve of the Second World War, the meeting of the Geologische Vereinigung devoted to the origin of the Atlantic Ocean confirmed that continental drift was being seriously challenged, although a few papers pointed to new developments, e.g. that in Iceland extensional tectonics had been active for the last 5,000 years. Most Alpine geologists were either highly critical of the theory of plate tectonic when it arrived or expressed serious reservations towards the idea. Of the exceptions, first Laubscher and then Bernoulli showed very clearly how important the new theory could be for understanding the evolution of Alpine orogeny. Continental drift and plate tectonics were very much the product of the creative imagination of human minds. Whereas Wegener used a broad range of confirmed results, plate tectonics sprang out of the new research being carried out in the domain of oceans. Graphic illustration was one of the favourite vehicles used to put across these new perspectives. Sometimes their impact remained alive long after their author had withdrawn his backing for the idea (as was the case for Argand’s “embryonic tectonics”); sometimes, even in spite of their very high standard, they were just ignored (which was the case for Ampferer).     

Preface: Proceedings of the 5th International Symposium on Lithographic Limestone and Plattenkalk

Magmatic rocks from the pre-Mesozoic basements of the Sambuco and Maggia nappes have been dated by U–Pb zircon ages with the LA-ICPMS technique. Several magmatic events have been identified in the Sambuco nappe. The mafic banded calc-alkaline suite of Scheggia is dated at 540 Ma, an age comparable to that of mafic rocks in the Austroalpine Silvretta nappe. The Sasso Nero peraluminous augengneiss has an age of 480–470 Ma, like many other “older orthogneisses” in Alpine basement units. It hosts a large proportion of inherited zircons, which were dated around 630 Ma, a Panafrican age indicating the Gondwanan affiliation of the Sambuco basement. The calc-alkaline Matorello pluton yielded ages around 300 Ma, similar to numerous Late Carboniferous intrusions in other basement units of the Lower Penninic (Monte Leone, Antigorio, Verampio) and Helvetic domains (Gotthard and other External Crystalline Massifs). Associated lamprophyric dykes are slightly younger (300–290 Ma), like similar dykes sampled in gneiss blocks included in the sedimentary cover of the underlying Antigorio nappe (290–285 Ma). The Cocco granodiorite and Rüscada leucogranite, both intruding the basement of the neighbouring Maggia nappe, yielded ages of ca. 300–310 Ma, identical within errors to the age of the Matorello pluton. They are significantly older than former age determinations. This age coincidence, coupled with remarkable petrologic similarities between the Cocco and Matorello granodiorites, strongly suggests paleogeographic proximity of the Sambuco and Maggia nappes in Late Carboniferous times. In recent publications these two nappes have been interpreted as belonging to distinct Mesozoic paleogeographic domains: “European” for Sambuco and “Briançonnais” for Maggia, separated by the “Valais” oceanic basin. In this case, the similarity of the Matorello and Cocco intrusions would demonstrate the absence of any significant transcurrent movement between these two continental domains. Alternatively, according to a more traditional view, Sambuco and Maggia might belong to a single large Alpine tectonic unit.     

L’analyse des surfaces enveloppes appliquée à l’étude morpho-structurale de l’est de la France

Résumén

Dans certaines régions où les sédiments récents (plio-quaternaires) sont rares ou inutilisables comme enregistreurs de déformations, seule l’analyse morpho-strcturale peut mettre en évidence des indices de néotectonique. Deux types d’objets morphologiques se révèlent particulièrement utiles : les surfaces (d’érosion et de comblement) et le réseau hydrographique. Leur prise en compte et leur analyse s’effectuent essentiellement à partir des cartes topographiques à différentes échelles (de 1/50 000 à 1/500 000).

* La démarche est la suivante: 1 – Des traitements appropriés sont développés, mettant en évidence des discontinuités et des anomalies au sein des surfaces enveloppes et du réseau hydrographiques;

2 – Ces discontinuités sont analysées puis interprétées afin de déterminer les causes de leur existence dans le cadre d’un modèle d’évolution du relief qui tient compte de facteurs internes (la lithologie, la structure), et de facteurs externes (le climat, la végétation et les actions humaines);

3 – Des travaux de terrain confirment et précisent les caractéristiques de ces facteurs (nature et quantification de la déformation : âge. amplitude, sens des mouvements).

* Les principaux résultats de cette démarche appliquée à l’Kst du Bassin parisien sont les suivants: 1 – Les cuestas orientales du Bassin parisien apparaissent structurées par des accidents NK-SW (prolongement du fossé de Sarre Nahe) et NNW-SSK ( Argonne-Bar-le-Due);

2 – Les discontinuités morpho-structurales sont localisées et caractérisées par des mouvements récents.

     

Estuarine conservation and restoration: the Somme and the Seine case studies (English Channel, France)

Megatidal estuaries such as the Seine and the Somme (North-Western France) are rather well delimited and human impacts on them are well understood. Since the middle of the 19th Century, there has been a slow but irreversible degradation of the state of these English Channel estuaries. However, current conservation and restoration strategies tend to freeze habitats in a particular state, their status being defined, most often, through a patrimonial or utilitarian approach. Connectedness between biotopes (sensu habitat+community) has a tendency to be neglected, especially with regard to main ecological gradients, i.e., salinity. In this paper, evaluation methodologies are proposed with the intention of assessing changes to ecosystem functions, under anthropogenic disturbance, controlled or otherwise. The Seine (a heavily industrialised ecosystem) is compared to the Somme (considered here for its pseudo-natural features) in order to discriminate between oceanic processes (siltation and plugging of estuaries) and anthropogenic influences. Preservation and restoration of habitats rely on a robust scientific methodology. The multi-scale approach adopted in the projects presented here relies on sensitive socio-ecological assessment procedures, tools for evaluating ecological quality, and well-built monitoring programmes based upon pertinent indicators. Such managerial tools were used to refine strategies and make them compatible with the sustainable co-development of resources in a European context. This paper demonstrates how scientists were able to acquire and apply knowledge in the field of rehabilitation and restoration. Jointly with managers and policy-makers, they have brought scientific information and socio-economics together in order to answer questions about the restoration of sites or habitats and to anticipate future propositions in the spirit of Integrated Coastal Zone Management (ICZM).     

Archaean high-K granitoids produced by remelting of earlier Tonalite–Trondhjemite–Granodiorite (TTG) in the Sangmelima region of the Ntem complex of the Congo craton,southern Cameroon

We present a geochemical and isotopic study that, consistent with observed field relations, suggest Sangmelima late Archaean high-K granite was derived by partial melting of older Archaean TTG. The TTG formations are sodic-trondhjemitic, showing calcic and calc-alkalic trends and are metaluminous to peraluminous. High-K granites in contrast show a potassic calc-alkaline affinity that spans the calcic, calc-alkalic, alkali-calcic and alkalic compositions. The two rock groups (TTG and high-K granites) on the other hand are both ferroan and magnesian. They have a similar degree of fractionation for LREE but a different one for HREE. Nd model ages and Sr/Y ratios define Mesoarchaean and slab-mantle derived magma compositions respectively, with Nb and Ti anomalies indicating a subduction setting for the TTG. Major and trace element in addition to Sr and Nd isotopic compositions support field observations that indicate the derivation of the high-K granitic group from the partial melting of the older TTG equivalent at depth. Geochemical characteristics of the high-K granitic group are therefore inherited features from the TTG protolith and cannot be used for determining their tectonic setting. The heat budget required for TTG partial melting is ascribed to the upwelling of the mantle marked by a doleritic event of identical age as the generated high-K granite melts. The cause of this upwelling is related to linear delamination along mega-shear zones in an intracontinental setting.     

Potential Impact of Climate Change on Vegetation in the European Alps: A Review

Based on conclusions drawn from general climatic impact assessmentin mountain regions, the review synthesizes results relevant to the European Alps published mainly from 1994 onward in the fields of population genetics, ecophysiology, phenology, phytogeography, modeling, paleoecology and vegetation dynamics. Other important factors of global change interacting synergistically with climatic factors are also mentioned, such as atmospheric CO₂ concentration, eutrophication, ozone or changes in land-use. Topics addressed are general species distribution and populations (persistence, acclimation, genetic variability, dispersal, fragmentation, plant/animal interaction, species richness, conservation), potential response of vegetation (ecotonal shift – area, physiography – changes in the composition, structural changes), phenology, growth and productivity, and landscape. In conclusion, the European Alps appear to have a natural inertia and thus to tolerate an increase of 1–2 K of mean air temperature as far as plant species and ecosystems are concerned in general. However, the impact of land-use is very likely to negate this buffer in many areas. For a change of the order of 3 K or more, profound changes may be expected.