Mega-Folding of a Basement During Incipient Intra-Plate Continental Subduction (Alpine Central Iberia)

Metamorphic basements are usually considered rigid and isotropic at a large scale. However, basements contain inherited weaknesses that may potentially accommodate superimposed contraction (e.g., fault reactivation), and that favor fold nucleation (e.g., penetrative foliations). If these conditions are met, what could be the factors that impede the development of basement folds or their recognition? Actual basement folding is rarely documented, especially for large dimensions. Here we provide a case example, discussed from the perspective of structural analysis of surface data and sustained by geophysical data. The basement of the Spanish-Portuguese Central System is defined by an Alpine mega-fold (Hiendelaencina Antiform) that trends parallel to this mountain range and affects the basement and its sedimentary cover, collectively. The wavelength of this fold matches or even surpasses the thickness of the crust that hosts it (36–41 km). The Moho under this mega-fold is displaced by an Alpine fault that accounts for incipient intraplate continental subduction. The topography of the mantle may reflect an up-warping compatible with the mega-fold observed on the surface. Mega-folding is observed in the hanging wall of the Berzosa Fault, which emerges as a SE-dipping, Variscan (Paleozoic), extensional fault reactivated as a basal decollement upon Alpine (Cenozoic) contraction. The mega-fold was formed after well-oriented planar anisotropies in the basement (foliation and bedding). The development of this fold was assisted by heterogeneous shearing (coeval thrusting) plus the buttressing effect of pre-existing, near-vertical, crustal-scale faults (Somolinos and Somosierra), which inhibited slip-upsection through the basal decollement (Berzosa Fault).