Claudio Rosenberg (Sorbonne Université, ISTeP) donnera, dans l'amphitéâtre de l'OSUC, un séminaire intitulé : History of Alpine cross sections: a mirror of evolving tectonic concepts since the 18th century.
L'enregistrement du séminaire est disponible ici.
The very first geological cross sections of the Alps are sketched in the early 18th century. A subtle transition takes place from representations of natural landscapes in which bedding and tectonic structures are exposed, to sections which intentionally focus on geological structures, and eventually those that reproduce the lateral continuity of structures over longer distances. The aim of these sections passes from the simple documentation of peculiar structures until the end of the 18th century, to the assessment of lateral and vertical spatial relationships between geological units in the very first years of the 19th century. Throughout the 1st half of the 19th century, cross sections only represent the observable part of valley flanks and evolve from panorama-type of drawings, including topographic effects that reflect the specific point of observation of the author, to sections representing continuous beds of constant thickness having an objective value.
It is only in the 2nd half of the 19th century that geologists dare to expand their interpretations to areas located above and below the topographic surface. This graphic change coincides with the need to clarify long-standing debates on the interpretation of structures, which are not entirely exposed, in specific Alpine areas. In the last decades of the century, the discovery of large-scale nappes indicates that the orogen consists of piles of gently-dipping units, whose representation requires to increase the vertical dimension of cross section. They no longer terminate at the sea level and they reach several km above the topographic surface.
Since the beginning of the 20th century cross sections are also used to synthetise the bulk structure of the Alpine chain. It is only at this time that geologists reduce the complexity of local areas to the very first-order structures of the Chain, identified as piles of large-scale nappes. The lateral projection of surface structures along fold axes and the understanding of the nappe structure of the orogen allow for the construction of sections that reach up to 20 km depth and 10 km height above the surface. The direct observation of geological structures is entirely replaced by map-based constructions, and sections of the same area can be extremely different based on the author's interpretation. By the third decade of the 20th century orogen-scale cross sections of the Alpine crust are quite precise and not very different from those that are drawn at the end of the same century. Innovative Alpine cross sections only arrive around 1970, after the theory of Plate Tectonics. The strong concepts on the deep-mantle structure of orogens provided by the latter theory, long before geophysical data could image such structures, lead to the drawing of extremely conceptual cross sections, following the newly born theory of subduction. It is very emblematic for that period to have cross sections illustrating the evolution of Alpine orogeny through time, clearly avoiding the illustration of the present-day structure, the only one that needs to be based on observations instead of concepts. The merging of such concepts with observations in order to assess the deep structure of the orogen is a slow and still ongoing process and a major goal of the last 50 years of Alpine research.
On vous attend nombreux !