Role of extensional structures on the location of folds and thrusts duringtectonic inversion (northern Iberian Chain, Spain)

The Aguilon Subbasin (NE Spain) was originated during the Late Jurassic-Ear ly Cretaceous rifting due to the action of large normal faults, probably in herited from Late Variscan fracturing. WNW-ESE normal faults limit two majo r troughs filled by continental deposits (Valanginian to Early Barremian). NE-SW faults control the location of subsidiary depocenters within these tr oughs. These basins were weakly inverted during the Tertiary with folds and thrusts striking E-W to WNW-ESE involving the Mesozoic-Tertiary cover with a maximum estimated shortening of about 12%. Tertiary compression did not produce the total inversion of the Mesozoic basin but extensional structure s are responsible for the location of major Tertiary folds. Shortening of t he cover during the Tertiary involved both reactivation of some normal faul ts and development of folds and thrusts nucleated on basement extensional s teps. The inversion style depends mainly on the occurrence and geometry of normal faults limiting the basin. Steep normal faults were not reactivated but acted as buttresses to the cover translation. Around these faults, affe cting both basement and cover, folds and thrusts were nucleated due to the stress rise in front of major faults. Within the cover, the buttressing aga inst normal faults consists of folding and faulting implying little shorten ing without development of cleavage or other evidence of internal deformati on. (C) Elsevier, Paris.