Análisis paleopiezométrico y tasas de deformación de la zona de cizalla dúctil extensional de Santa María de la Alameda (Sistema Central)

Abstract

The differential stress that caused the extensional ductile shear o f Santa Ma de la Alameda during the collapse of the Variscan orogen in the Sistema Central (central Iberia) has been estimated using the dynamically recrystallized grain size of quartz. An estimate of the strain rate has been calculated by the flow laws for ductile strain of quartz knowing the deformation temperature. The stress history obtained for this shear band features 3 stages of 22-31 MPa, 34-65 MPa and 53-67 MPa. The increase o f the differential stress is caused by the concentration of the strain in smaller bands during mylonitization. The first stage can be correlated to the initial deformation of a wide shear band, the second stage can be related to the formation of two main mylonitic shears, and finally the third stage correspond to the deformation of thin (<1 mm) shear bands between grains in granitic synkinematic dikes. The strain rate deduced for the temperature interval of 300-350Q C ranges from 10-11 s-1 to 10-13 s-1. An small fault displacement can be assumed based on the low difference of the metamorphic grade, this allows us to discuss different duration for mylonitization according to different strain rates. The strain rates more successful range from 10-12 s-1 to 10-13 s-1 giving respectively a mylonitization period o f 1,6 ma. and 16 ma. for the observed thickness ot 280 m of mylonites.