Palaeostress and geotectonic interpretation of the Alpine cycle onset in the Sierra del Guadarrama (eastern Iberian central system), based on evidence from episyenites

Several episodes of hydrothermal activity related to periods of fracturing and/or reactivation of previous structures took place from 300 to - at least - 100 Ma, in the Sierra del Guadarrama, which is part of the crystalline axis of the Iberian Hercynian Fold Belt (Central-Iberian Zone). One of these episodes led to the formation of episyenites, which are de-quartzified and alkalinized granites. Episyenite formation took place on a regional scale and in a short period (approx. at 277 Ma). The episyenites were formed by the action of fluids at temperatures between 350 degrees C and 650 degrees C, at depths of about 6.5 km, and in microfractured dilatancy zones developed under a regional extensional regime. These zones are crosscut by normal faults, developed during the progressive deformation process accompanying the formation of the episyenites. The calculated regional palaeostress tensor has sigma(1) close to vertical and sigma(3) between N10-20E and an average value of the stress ratio (Phi) of 0.19 [Phi = (sigma(2) - sigma(3))/(sigma(1) - sigma(3))]. Because sigma(1) is close to vertical the stress tensor is compatible with an extensional deformation field. The analysis also shows that most of the faults that slip under this stress field have an average coefficient of friction of 0.8. This extensional regime was probably accompanied by a regional thermal anomaly, as suggested by the high temperature of the fluids involved, which are amagmatic. This thermo-tectonic episode is interpreted as representative of the generalized extensional regime corresponding to the onset of the Alpine Cycle. The episode was preceded by a wrench-faulting event, equivalent to the Late Variscan event of Arthaud and Matte (1977), for which an age of - at least - 300-290 Ma is indicated by recent radiometric data, in its turn, this event was preceded by the regional extensional gravitative collapse of the Hercynian orogen. A correlation between evidence from the cover (stratigraphy and volcanism) and evidence from the basement (hydrothermal alterations, dyke injection episodes and granitic magmatism) is attempted on the basis of new available radiometric data.