From embryonic tectonics and geosynclines to modern concepts of rifting, spreading, and Alpine orogenesis: a summary of contributions to the 9th Alpine Workshop (Cogne, 2009)

Abstract

The 9th Alpine Workshop was held in September of 2009 in the village of Cogne (Italy) amid, the spectacular geology and scenery of the Western Alps. This volume of the International Journal of Earth Science comprises a selection of papers read at the workshop, which in keeping with the workshop’s tradition, encompassed a broad range of themes on the processes and evolution of Alpine-type mountain belts. The workshop itself was dedicated to the memory of Rudolph Trumpy (1921–2009), a key figure in Alpine Geology and a frequent participant of past Alpine workshops. The keynote paper by Sengor and Bernoulli reviews Trumpy’s scientific oevre in the context of his time, especially his landmark paleogeographic reconstructions of Alpine Tethys and his early recognition of the role of strike-slip tectonics in the evolution of the Alps. Beyond these contributions, their paper provides insight into how Trumpy inspired colleagues and students alike to look carefully at rocks and to think critically about the geologic record, even as he acknowledged the need for a deductive approach to further our understanding of orogenesis. One of Trumpy’s many insights was that Alpine Tethys resulted from Mesozoic extension (rather than shortening) and that the structure of the adjacent European and Adriatic continental margins did not coincide with the impressive nappe structures that formed later during Alpine subduction and collision. It is therefore fitting that several papers in this volume deal with this early evolution and its effect on Alpine orogenic structure. Mohn, Manatschal, Massini, and Muntener present a revised interpretation of Austroalpine units that represent the distal Adriatic continental margin as preserved at the junction of Eastern and Central/ Western Alps in eastern Switzerland. In a series of E-W oriented palinspastic reconstructions, the authors show how Early Mesozoic rift-related structures of this margin determined the complex sequence of nappe stacking during late Cretaceous east–west shortening. Further complications arose during Tertiary north–south shortening, yet the original pre-orogenic structure of the passive margin is shown to be remarkably well preserved. In the Western Alps, Loprieno, Bousquet, Bucher, Ceriani, Fugenschuh, and Schmid present new structural and stratigraphic evidence for the existence of Valais oceanic lithosphere in Cretaceous time. Internal Valais units (Versoyen ophiolites and underlying magmatic and sedimentary rocks) are shown to have originated at the ocean–continent transition in contrast with external Valais units that did not have an oceanic lithosphere and probably derived from the European distal continental margin. This interpretation will be controversial to some Alpine geologists, as it refutes previous interpretations of the ‘‘Versoyen ophiolites’’ as a Carboniferous-age complex in tectonic (rather than stratigraphic) contact with post-rift sediments of the internal Valais unit. The Early Mesozoic paleogeography of the eastern part of the European margin of Alpine Tethys has been relatively poorly constrained until now. By careful dating of pre-Mesozoic basement rocks in the western part of the Tauern Window, Vesela, Sollner, Finger, and Gerdes correlate the sub-Penninic ‘‘Zentral Gneiss’’ there with the External basement Massifs in the Western Alps of Switzerland and France. Synsedimentary rhyodacitic and rhyolitic horizons (310–304 Ma) in these units along the Alpine chain are interpreted to mark the onset of sedimentation in a post-Variscan extensional basins, just prior to the onset of Early Perman intracontinental rifting. M. R. Handy C. L. Rosenberg (&) Freie Universitat Berlin, Berlin, Germany e-mail: cla@zedat.fu-berlin.de