Plate Tectonics and the Alpine Orogeny: Implications of Thermometric and Kinematic Analyses of the Upper and Lower Boundaries of the Pennine Zone in the Central Alps

Abstract

Mesoscopic structural measurements near the top and bottom of the Pennine Zone in the Central Alps of eastern Switzerland indicate multiple, spatially heterogeneous directions of Tertiary movement relative to the Austroalpine allochthon above and the Helvetic zone below. At the top of the Pennine Zone in the Oberhalbstein Valley, motion varies mainly from top-E to top-SSE. At the bottom of the Pennine Zone in the Val Lumnezia area, Ultrahelvetic units exhibit distributed top-NW and top-N shear overprinted by relatively brittle top-NE shear localized just beneath the contact with Penninic units in the Peidener shear zone, which we interpret largely to postdate juxtaposition of Penninic and Helvetic units. Where observed in the Chur Rhine Valley, just 35 km ENE of Val Lumnezia, movement within the basal Pennine units is exclusively top-N. The contrast in movement directions, from top-N to top-NW at the base, to top-E to -SSE at the top, supports the interpretation, drawn from thermochronological data, that the Pennine Zone was tectonically interposed between Adria and Europe as a 20-km-thick "piston" or "mega-pip" from ca. 29 to 18 Ma, driven by its buoyancy contrast with surrounding deep crust and mantle. Emplacement occurred after "docking" of Adria with cratonic Europe at ca. 35 Ma (i.e., continent-continent collision), raising the question of whether the formation of Alpine nappe structure, high Alpine topography, and the peripheral Molasse and Lombardy basins require significant coeval plate convergence.

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