Publication: Palaeostress and geotectonic interpretation of the Alpine Cycle
onset in the Sierra del Guadarrama (eastern Iberian Central
System), based on evidence from episyenites
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Publication Date
1996
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Elsevier Science B.V., Amsterdam
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Abstract
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°C and 650°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 ~r I close to vertical and σ3 between NI0-20E and an average value of the
stress ratio (Ф) of 0.19 [Ф = (σ2 - σ3)/(σ1 - σ3)]. Because σ1 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.