PALEOSTRESS 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 fractu ring and/or reactivation of previous structures took place from 300 to - at least - 100 Ma, in the Sierra del Guadarrama, which is part of t he crystalline axis of the Iberian Hercynian Fold Belt (Central-Iberia n Zone). One of these episodes led to the formation of episyenites, wh ich are de-quartzified and alkalinized granites. Episyenite formation took place on a regional scale and in a short period (approx. at 277 M a). The episyenites were formed by the action of fluids at temperature s between 350 degrees C and 650 degrees C, at depths of about 6.5 km, and in microfractured dilatancy zones developed under a regional exten sional regime. These zones are crosscut by normal faults, developed du ring the progressive deformation process accompanying the formation of the episyenites. The calculated regional palaeostress tensor has sigm a(1) close to vertical and sigma(3) between N10-20E and an average val ue of the stress ratio (Phi) of 0.19 [Phi = (sigma(2) - sigma(3))/(sig ma(1) - sigma(3))]. Because sigma(1) is close to vertical the stress t ensor is compatible with an extensional deformation field. The analysi s also shows that most of the faults that slip under this stress field have an average coefficient of friction of 0.8. This extensional regi me was probably accompanied by a regional thermal anomaly, as suggeste d 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 Alpin e Cycle. The episode was preceded by a wrench-faulting event, equivale nt to the Late Variscan event of Arthaud and Matte (1977), for which a n age of - at least - 300-290 Ma is indicated by recent radiometric da ta, in its turn, this event was preceded by the regional extensional g ravitative collapse of the Hercynian orogen. A correlation between evi dence from the cover (stratigraphy and volcanism) and evidence from th e basement (hydrothermal alterations, dyke injection episodes and gran itic magmatism) is attempted on the basis of new available radiometric data.