Abstract
Summary. Seismic probing of the upper crust in a catazonal massif in Tras os Montes, Hercynian Iberia, demonstrates the absence of a root towards the lower crust and favours a mechanism of thrust and nappe emplacement. Similar high to low velocity successions in the upper crust are evidenced in particular segments of the Hercynian domain of France. All these occurrences are related to a type region to which a particular meaning has been given in a recent plate tectonics interpretation of the Hercynian orogenesis. This model, implying large-scale nappe displacement and intracrustal thrusts furnishes in turn a general mechanism for the tectonic formation of upper crustal high and low velocity layers of limited extent in orogenic cycles.
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