September 7, marks the anniversary of the publication of an important paper, "Magnetic Anomalies Over Oceanic Ridges" (1964) describes the discovery of parallel stripes of magnetized igneous rocks along the ocean floor. These stripes are formed when lava pours out along the Mid-Ocean-Ridges, cools and solidifies and pushes aside older oceanic and continental crust. This observation finally provided geologists with a mechanism that could move continents, vindicating Wegener's early Continental-Drift Hypothesis and upgrading it to the Plate-Tectonics Theory.

Plate tectonics doesn't only explain the shape of oceanic basins, but also how mountain belts, like the Alps, form. So why was plate tectonics not discovered earlier in the Alps, studied by geologists already for more than 200 years?

Outcrops with deformed rocks and sediments are widespread and early naturalists realized that the material was once deposited in the open sea - but the mechanism how these rocks could be moved from the ocean floor to the highest peaks remained unclear.

Fig.1. Deformed and folded marbles of the inner Alps.

German geologist Leopold von Buch (1774-1835) proposed a theory that seemingly explained a lot of geological observations. He proposed large "volcanic bubbles", uplifting and bending metamorphic and sedimentary rocks to form a symmetrical mountain belt.

The Central-Gneiss (Zentralgneiss) in the inner eastern Alps is a large magmatic intrusion, in direct contact to a variety of metamorphic rocks, like crystalline schists, calcareous marbles and even basaltic rocks - all rocks once deposited or formed on the ocean floor. This succession seemed to confirm without doubt von Buch's theory and soon became a popular model for early geologists to interpret other mountain belts (like the Pyrenees).

Fig.2. Geologic map and section of the Pyrenees, after the "Berghaus-Atlas", a supplement to explorer and naturalist Alexander von Humboldt's masterpiece "Kosmos" (1845-1862). This example shows how early geologists imagined the inner structure of mountains - magmatic rocks are found always in the core, surrounded like an onion by layers of sedimentary rocks. In the section the rocks from inside to the outside are described as "Granitic rocks and general basement" - "transition mountains/rocks" - "secondary mountains". In the map Granite=pink, Basaltic rocks=green, Schist= grey, Clastic rocks/ Limestone= blue, Sandstone= red, Secondary Limestone= yellow, Tertiary rocks= dark-green

Fig.3. Contact between massive Central-Gneiss (background) with large pegmatitic quartz vein and metamorphic marbles (foreground) with visible foliation.

However according to this model the age of the layers should decrease towards the borders of a mountain belt (as the horizontal, undisturbed layers - according to Steno's law with the oldest on the bottom and youngest on the top - are simply bended). Unfortunately metamorphic rocks can't be dated with fossils, but only with radiometric dating methods, available and reliable only after 1920.

When the first metamorphic rocks were dated a problem with the tectonic evolution of the Eastern Alps emerged. In the Alps the Central-Gneiss and its sedimentary hull is completely surrounded, even in parts covered, by older, metamorphic rocks. This was not possible according to von Buch's "volcanic uplift" model. Layers should simply bend around the central intrusive body and the general order, from old to young, should be preserved. But now we had a succession of young rocks on the bottom and old rocks on the top.

The introduction of tectonic nappes in the geological literature between 1909-1934 helped to explain this apparent contradiction.

The magmatic Central-Gneiss is an 250 million years old intrusion into the crust (the basaltic rocks) and marine sediments (schist and marbles) of an ancient ocean. During the Alpine Orogeny (30-23 million years ago) surrounding older metamorphic rock-units (460-380 million years) were first uplifted and then thrusted as nappes onto the Central-Gneiss/former Ocean-Floor units. These movements implied horizontal, rather than vertical movements, like the spreading of the ocean floor could provide.

Bibliography:

DalPIAZ, G.V. (2001): History of tectonic interpretations of the Alps. Journal of Geodynamics 32: 99-114

TRÜMPY, R. (2001): Why plate tectonics was not invented in the Alps. Int. J. Earth Sciences 90: 477-483