Geometry and deformation mechanisms of the central iberian zone and the iberian central system

Abstract

The Iberian Central System is an intraplate mountain range located in the Central Iberian Zone. It divides the Iberian Inner Plateau in two sectors: the northern Duero Basin and the Tajo Basin to the S. The topography of the area is highly variable, with the Tajo Basin having an average altitude of 400 m while the Duero Basin presents a higher average elevation of 750–800 m. The Iberian Central System is characterized by a thick-skin pop-up and pop-down configuration formed by the reactivation of Variscan structures during the Alpine Orogeny. The high topography is, most probably, the response of a tectonically thickened crust that should be also identified by i) the geometry of the Moho discontinuity, ii) an imbricated crustal architecture, and/or iii) the rheological properties of the lithosphere. Shedding some light about these features is the main target of the current investigation. In this work, we present a lithospheric-scale model across this part of the Iberian Massif. We have used data from the CIMDEF project, which consists of 1 to 2 months of recordings of ambient noise and teleseismic events along an almost-linear array of 69 short-period seismic stations, which define a ≃ 320 km long transect. We have applied different approaches of Seismic Interferometry techniques in an effort to achieve the best resolved images of the area. The processing of the datasets provides an approximation of the zero-offset reflection response of a single station to a vertical source sending (near) vertical seismic energy. Results indeed reveal a clear thickening of the crust below the Iberian Central System resulting from an imbrication of the lower crust that probably affects the upper crust too. This implies that the geometry of the mountain range is similar to that of other Alpine cordilleras (e.g., the Pyrenees) and that, contrarily to previous models, buckling has played a minor role in deformation. In addition, the late Variscan evolution of NW Iberia, characterized by gravitational collapse, extension and melting, has left an imprint in the crustal structure of the Central Iberian Zone that has constrained its present characteristics. On the one side, the crust is highly re-mobilized and thinner in this area, as witnessed by the calculation of the Curie-Depth Point maps presented here. Furthermore, the orogen scale mid-crustal discontinuity characterizing the Iberian Massif is probably erased or redefined in Central Iberia. This allows shallow thrusts to continue in depth, cutting across the entire crust, connecting with the lower crustal imbrication and allowing the upper crust to partly underthrust and sink. This process is, most likely, responsible of the contrasting altitude observed in the mesetas to the N and S of the Iberian Central System. Finally, the mantle in the Central Iberia Zone is not magnetic, as it is deeper than the 580◦C isotherm. However, it displays some sparse reflectivity that is tentatively related to the, already identified to the S of the CIMDEF profile, Hales discontinuity.