Deflection of a compressional stress field by large-scale basement faults. A case study from the Tertiary Almazán basin (Spain)

Abstract

Palaeostresses deduced from brittle mesostructures (more than 50 sites and 1500 structures measured) in the Mesozoic and Tertiary rocks of the Almazán basin, located in the northwestern Iberian Chain, indicate a regional NNE-SSE-directed compression that lasted during all the Tertiary shortening stage. The stress regime corresponding to this compression ranges from thrust to wrench. N-S-directed compression was the responsible for most macrostructures (folds and thrusts forming a S-vergent system with NE-SW to E-W and NW-SE orientations) and mesostructures (mainly faults, stylolytes and impressed pebbles). We propose that the stress trajectories are deflected, following Anderson's model, in the western part of the Almazán basin by the NE-SW-striking left-lateral strike-slip Soria fault. This basement fault was inherited from the late Variscan fracturing stage and controlled the deposition in the Mesozoic basins as well as the location of Tertiary compressional macrostructures. In the eastern border of the Almazán basin palaeostress trajectories were deflected both by right-lateral strike-slip faults and NW-SE-trending thrusts, near which the regional N-S-directed compression becomes perpendicular to them. Three minor compression directions have also been recorded in several sites: NE-SW, E-W and NW-SE. The NE-SW-directed compression is interpreted as a deviation of the main N-S-directed compression at a regional scale, whereas the NW-SE-directed stress field corresponds to the regional field responsible for the main Tertiary shortening stage in the Spanish Central System. The age of compression ranges from Eocene to Miocene.