Continuous deformation, ductile thrusting and backfolding of cover and basement in the Sierra de Guadarrama, Hercynian orogen of central Spain

Abstract

Hercynian deformation has affected two different crustal ensembles, a crystalline basement and a metasedimentary cover, in the Sierra de Guadarrama. The rheological basement is a medium- to high-grade metamorphic complex composed mostly of granitic orthogneisses, with minor amounts of sediments. The cover consists of a thick sequence of low- to high-grade metasediments, with minor gneissic bodies, ranging in age from Late Proterozoic to Devonian. The deformation in the cover was analysed mainly in the eastern part of the Sierra de Guadarrama, whereas the crystalline basement was analysed in the Segovia area, to the west. Both cover and basement deformation started with a stage (D 1) of continuous, penetrative deformation. The deformational regime has a strong component of subhorizontal simple shear, with east vergence. A second phase (D 2) of shear deformation gave rise to a broad, subhorizontal shear zone and a thrust in the cover, and to narrow ductile shear zones in the basement. The transition between the two first phases of deformation seems to have been gradual. The third phase (D 3) is a generalized backfolding episode, which induced an intense and penetrative strain in broad areas of both ensembles. Finally, large open folds with steep axial surfaces developed in two mutually perpendicular systems, giving rise to “dome and basin” interference patterns, and some subvertical ductile shear zones and faults were also created. The structural evolution of the basement and cover points to a thicker-skinned model of deformation, giving rise to an important crustal thickening. A salient feature of the model arose during the second phase, when a thick detachment level developed in the lower part of the cover. The generalized existence of this level may account for the scarcity of basement outcrops in the Spanish Hercynian belt.