Extensional Flow during Gravitational Collapse: A Tool for Setting Plate Convergence (Padrón Migmatitic Dome, Variscan Belt, NW Iberia)

Abstract

Plate convergence analysis in collisional orogens is usually based on the study of major contractional structures and
strike-slip shear zones. Here we show how the structural analysis of extensional structures may report the regional
or far stress field during relatively local, gravity-driven extensional collapse of a thickened continental crust and how
this information may be used to constrain the broad vectors of plate convergence at that time. The Padro´n migmatitic
dome is a synconvergent extensional system developed in the axial zone of the Variscan belt exposed in the NW part
of the Iberian Massif of Spain. This system affected the allochthonous and autochthonous sequences involved in
Pangaea’s assembly in Southern Europe. It includes three major extensional shear zones, which have been analyzed
in detail to provide a wide ground data set for the discussion of the proposed model. The tectonic flow in the Padrón
migmatitic dome and in other coeval structures is characterized by vectors ranging from parallel to oblique, in the
latter case with a counterclockwise azimuth in relation to the trend of the orogenic belt. Our model suggests that
the extensional collapse of the Variscan belt inNWIberia would have developed if the convergence between Gondwana
and Laurussia had not stopped and that it would have included a dextral component.