Deformation and kinematics in a melt-bearing shear zone from the Western Betic Cordilleras (Southern Spain)

This work deals with the structural evolution and kinematics of the basal dynamothermal aureole of the Ronda peridotites, located in the Sierra Bermeja massif of the Western Betic Cordilleras, in southern Spain. This aureole is characterized by: 1) a marked strain gradient towards the contact with the overlying peridotites; 2) the great angular dispersion, ≈90°, of stretching lineations; and 3) the intrusion of thin syn-kinematic granite sheets. These features are consistent with the aureole being a low-viscosity shear zone formed at temperatures high enough, ≈725 °C, to promote the partial melting of crustal metamorphic rocks. The high-temperature deformation is demonstrated by the activation of c-slip in quartz. We consider that the heat source came from the thrusting of the hot subcontinental mantle over metamorphic crustal rocks. The interaction of alternating events of melt-enhanced deformation and melt-enhanced embrittlement explains the heterogeneity of this ductile shear zone and the strong narrowing, to only 30 meters, of the deformed domain. The small thickness of this shear zone is in contradiction with classical conceptual models of shear zones suggesting a broadening of ductile shear zones with increasing depths and temperatures.