A seismic geotraverse across the Iberian Variscides: Orogenic shortening, collisional magmatism, and orocline development

A crustal geotraverse through the Iberian Variscides is presented by integrating the available geological and seismic profiling data. Different modes of orogenic shortening are identified, with varying degrees of coupling between upper and lower crust. In northern and southern regions of the geotraverse, a decoupling in the middle crust permits the lower crust to subduct/underthrust, thus compensating for strong upper crustal shortening. This behavior does not result in great crustal thickening, except in sectors of crustal underthrusting. In southern Central Iberia, moderate upper crustal shortening is due to a mechanically strong lower crust impeded to subduct/underthrust. In this region, shortening is partitioned between upper and lower crust, deformation being distributed in the upper crust while localized at major fault zones in the lower crust. Finally, the central region of the geotraverse (northern Central Iberia) shows a coupled crustal deformation, having given way to the largest orogenic thickening in the Iberian Variscides. The thermal maturity of this much thickened crust originated voluminous crustal melting, and concomitant normal-fault detachments developed, while shortening dominated in other regions. Theoretical models suggest that compressive stresses may prevail in the lower crust beneath the extending upper crust, thus explaining the efficient syncollisional exhumation in this part of the orogen. A particular feature of the Iberian Variscan geotraverse is the great importance of out-of-section mass movements, mainly left-lateral shear zones concentrated in two suture boundaries, which displaced to the NW (present coordinates) central and northern Iberia with respect to southern Iberia.