Foreword

Abstract

This issue of Ofioliti presents a selection of the ophiolite-oriented communications presented at the Giulio Elter memorial workshop ãDalla Tetide alle Alpi: genesi, metamorfismo e tettonica del bacino ofiolitico ligure-piemontese e dei suoi margini continentali (From Tethys to the Alps: birth, metamorphism and deformation of the Piemontese-Ligurian ophiolite basin and its continental margins). The workshop was held in Cogne (Valle dâAosta) on June 21st and 22nd, 2001, and was attended by some 60 partecipants. The workshop was opened by an appreciation of Giulio Elter by G.V Dal Piaz (University of Padua). The communications were arranged in five discussion topics: - Continental rifting - Formation of the oceanic lithosphere, sedimentation and intra-oceanic tectonics - Subduction and exhumation - Continental collision and emplacement of the ophiolite nappes - Evolution of the Alpine relief The workshop was followed by one-day excursion to the Valnontey section led by R.Compagnoni (University of Torino) in which structures and metamorphic assemblages in the ophiolite units of the Piemonte Zone and the continental crust of Gran Paradiso Massif could be observed. A second excursion, guided by Giorgio Elter, was devoted to major landslides and Quaternary deposits in the Cogne valley. The Editors wish to thank the authors and the reviewers of the papers for their contribution and the Comune di Cogne in the person of the Mayor, Mr. Osvaldo Ruffier for hosting the workshop in their ãMaison de la Cultureä. Financial support was provided by Assessorato allâIstruzione e alla Cultura, Regione Autonoma Valle d'Aosta, Dipartimento di Scienze Mineralogiche e Petrologiche, the University of Torino, Dipartimento per lo Studio del Territorio e delle sue Risorse, the University of Genova and C.N.R., C.S. Geodinamica Catene Collisionali, Torino. The help of Sandro Borghi, Franco Rolfo, Simona Ferrando (University of Torino), Giorgio Elter and the Elter Family in organizing and running the Workshop is gratefully acknowledged. IN MEMORY OF GIULIO ELTER (1922-1999) Giulio Elter was born on October 18 1922 in Cogne, a mountain village south of Aosta, north-west Italy, where his father Franz was the geologist and general manager of the local iron mines, and where Giulio spent a happy childhood. After schooling in Aosta, during the Second World War he began to study Chemistry at the Universities of Turin and Pavia.In September 1943, the Germans occupied Italy. Giulio and his younger brother Giorgio, in order to avoid military service under the Fascist Republic of Salò, left the Valle dâAosta for Switzerland, where they were interned in a work camp for political refugees. On August 17 1944, Giorgio Elter returned to Cogne and joined the Italian partisans. On September 6 1944, he was killed fighting against the Nazi-Fascists. Giulio only learnt of his brotherâs death in November 1944, when his parents, with their younger children Piero and Orsetta, joined him in Switzerland, crossing the snow-covered Gran San Bernardo pass on foot. After the war, Giulio completed his studies in geological sciences at the University of Geneva (diplome) and Milan (degree, 1950), where he graduated with a dissertation on the geology of the Cogne area.In late 1950, Giulio Elter was appointed assistant at the Geological Institute of the University of Turin. In 1962, he left the university and moved to the Italian Research Council (CNR), where he was appointed researcher and then (1978) director of the ãCentro di studio sui problemi dellâOrogeno delle Alpi occidentaliä, Turin, until his retirement in November1987.He since lived during summer and autumn in his beloved Cogne, where his home was always open to Alpine geologists and village friends alike. Giulio Elter died peacefully on February 20, 1999, at his home in Turin.Giulio Elterâs inclination towards geology probably went back to his childhood, when he followed his father in his frequent visits to the Cogne magnetite mines, where Giulio learnt the rudiments of geological work. His geological knowledge and the way he faced and resolved problems clearly reveal the scientific rigour he had acquired in Geneva, inspired as it was by the classic concepts of Emile Argand and his kinematic reconstructions on an Alpine scale - attitudes which were evident in his degree thesis and which he later developed in full autonomy. He followed Argand in his belief that tectonics, without stratigraphic bases, was reduced to mere geometry, and the influence of the Swiss tectonic school may also be seen in the ability with which he conceived and drew three-dimensional structures by means of serial profiles and geological panoramas.As well as loving field work, which he especially liked because it put him in contact with nature, Giulio Elter was clearly oriented towards large-scale tectonics, a subject which was definitely congenial to him, although he was also an excellent stratigrapher and was particularly talented at analysing sedimentary facies changes with the aim of creating paleogeographic reconstructions in the tradition of the great French and Swiss masters of the time. He also had a deep and extensive knowledge of the geological literature on the Alps and Northern Apennines, which all his life he liberally shared with younger geologists and colleagues alike.As can be seen from the list at the end of this paper, Giulio Elterâs work has touched upon several sides of the Alpine geology, ranging from the study of the Mesozoic-Tertiary sequences of the Langhe-Monferrato hills to the lithostratigraphy of the Hercynian basement reworked during the Alpine orogeny. However, it has been mainly on the Penninic stack of metamorphic nappes in the Aosta valley and surrounding areas, trying to unravel the deformation history of the nappes back to their birth in Mesozoic Tethys, that his work was mainly focussed. In this field, Giulio Elterâs major contributions during the 1960s were: i) a memoir on the Penninic zone in the Aosta valley (1960), in which he carefully reviewed Emile Argandâs concepts and later developments on the structure and stratigraphy of the nappe pile (including the results of his own extensive fieldwork), and discussed currently debated problems and research directions aimed at solving them; ii) a 1:25.000 geological map and monograph on the Piccolo San Bernardo area, published (together with his brother Piero) in 1965, dealing with the outer Penninic units (Briançonnais, Versoyen ophiolite, Tarentaise Breccia) and with the underlying Ultrahelvetic-Helvetic décollement nappes and cover units; iii) a grandiose paleostructural reconstruction of the Ligurian domain from the northern Apennine to the French-Swiss Alps, through the Monferrato-Langhe area, prepared together with Piero Elter, Carlo Sturani and Marc Weidmann (1966). This paper, like those of 1971 and 1972, was read by a large international audience and exerted a deep influence on the subsequent development of the plate tectonic paradigm in the Western Alps.In 1971-1972, Giulio Elter published two major memoirs on the ophiolitic Piedmont zone between the Aosta valley and Gran Paradiso massif and on the contiguous basement and cover units issuing from the European continental margin. In these papers a previously unrecognized distinction was made between the true Schistes lustrés and the Liassic Schistes lustrés. The former consist of the sedimentary series which was originally associated with the ophiolites, and the latter are characterized by a complete lack of any primary relationship with the ophiolites. Giulio Elter was able to demonstrate that the ophiolite-bearing Schistes lustrés are later than the Liassic Schistes lustrés, most likely being of Neojurassic and Cretaceous age. Study of their relationships with the ophiolites showed that they behaved as a stratigraphic cover for the ophiolites. Their deposition was either contemporaneous to or later than the submarine volcanic outpourings, and took place over the latterâs gabbroic-peridotitic basement. Therefore, the ophiolites of the gabbro-peridotite group acted as a basement for the eugeosynclinal sediments and volcanic outpourings when their deposition began at the end of the Jurassic period. Field relationships and reconstruction of the paleotectonic relationships between the Briançonnais platform and the Piedmont ocean during the Jurassic thus led Giulio Elter to envisage the existence of a ãSima gapä between the Briançonnais and Austroalpine domains, corresponding to the gabbro and serpentinised peridotite basement of both the sediments and volcanic outpourings.In 1981, Giulio Elter, pursuing a lifelong interest in the geology of the Cogne iron mines, contributed to a new genetic interpretation of this well known (high quality) magnetite deposit, until then considered as a unique type of Fe-mineralisation: a hydrothermal alteration process of the host peridotite with Fe mobilization was suggested, which precipitated the ore body close to ocean floor during formation of the Mesozoic Western Tethyan basin (Compagnoni et al., 1981).His last major accomplishment in the field of Alpine geology was the ãCarte géologique de la Vallée dâAoste, échelle 1:100.000ä (1987, and in Dal Piaz G.V., 2001, Commemorazione di Giulio Elter. Mem. Sci. Geol., 53, 153-162), a synthetical view of one of the classical areas of Alpine geology, in which, developing and extending to the whole Val dâAosta his earlier syntheses of 1960 and 1972, he assembled and updated previous sheets of the Geological Map of Italy, incorporating and critically reviewing new advances in field mapping and interpretations of the 1970s and 1980s. SELECTED PAPERS BY GIULIO ELTER Elter G., 1950 - Geologia della regione di Cogne. Unpublish. Thesis, Univ. Milano, 91 pp. Elter G., 1956 - Osservazioni sulla tettonica del Monferrato orientale. Mem. Ist. Geol. Univ. Padova, 20, 21 pp. Elter G., 1960 - Osservazioni preliminari sullâetà dei terreni preoligocenici del Monferrato. Rend. Acc. naz. Lincei, Cl. Sci. fis., ser. VIII, 29/6, 573-578. Elter G., 1960 - La zona Pennidica dellâalta e media Valle dâAosta e le unità limitrofe. Mem. Ist. Geol. Min. Univ. Padova, 22, 113 pp. Cita M.B. e Elter G., 1961 - La posizione stratigrafica delle marne a Pteropodi delle Langhe e della Collina di Torino ed il significato cronologico del Langhiano. Rend. Acc. Naz. Lincei, Cl. Sci. fis., ser. VIII, 29/5, 360-369. Compagnoni R., Elter G. and Sturani C., 1964 - Segnalazione di Albiano fossilifero nel tratto valdostano della zona delfinese-elvetica. Rend. Acc. Naz. Lincei, Cl. Sci. fis., ser. VIII, 36, 2-5. Elter G. and Elter P., 1965 - Carta geologica della regione del Piccolo S. Bernardo (versante italiano). Note illustrative. Mem. Ist. Geol. Min. Univ. Padova, 25, 53 pp. Elter G., Elter P., Sturani C. and Weidmann M., 1966 - Sur la prolongation du domain ligure de lâApennin dans le Montferrat et les Alpes et sur lâorigine de la nappe de la Simme s.l. des Préalpes romandes et chablaisiennes. Archives Sci. Genève, 19, 279-378. Elter G., 1971 - Schistes lustrés et ophiolites de la zone piémontaise entre Orco et Doire Baltée (Alpes Graies). Hypothèses sur lâorigine des ophiolites. Geol. Alpine, 47, 147-169. Elter G., 1972 - Contribution à la connaissance du briançonnais interne et de la bordure piémontaise dans les Alpes Graies nordorientales et considérations sur les rapports entre les zones du briançonnais et des schistes lustrés. Mem. Ist. Geol. Min. Univ. Padova, 28, 19 pp. Compagnoni R., Elter G. and Lombardo B., 1976 - Eterogeneità stratigrafica del complesso degli ãGneiss minutiä nel massiccio cristallino del Gran Paradiso. Atti colloquio ãLâorogenesi Ercinica nelle Alpiä, Mem. Soc. Geol. It., 13, suppl. 1 (1974), 227-239. Cassinis G., Elter G., Rau A. and Tongiorgi M., 1979 - Verrucano: a tectofacies of the Alpine-Mediterranean southern Europe. Mem. Soc. Geol. It., 20, 135-149. Bearth P., Dal Piaz G.V., Elter G., Gosso G., Martinotti G. and Nervo R., 1980 - Il lembo di ricoprimento del Monte Emilius, Dent Blanche s.l.. Osservazioni preliminari. Atti Acc. Sci. Torino, 114, 227-241. Compagnoni R., Elter G., Fiora L., Natale P. and Zucchetti S., 1981 - Magnetite deposits in serpentinized lherzolites from the ophiolitic belt of the Western Alps, with special reference to the Cogne deposit (Aosta Valley). Proceedings Intern. Symp. Mafic Ultramafic Complexes, Athens 9-11 October 1980, 3, 376-394. Elter G., 1987 - Carte géologique de la Vallée dâAoste, échelle 1:100.000. Centro di Studio sui problemi dellâorogeno delle Alpi Occidentali, C.N.R. Torino, S.E.L.C.A., Firenze. Also in:Dal Piaz G.V., 2001 - Commemorazione di Giulio Elter. Mem. Sci. Geol., 53, 153-162.