From ductile to brittle: Evolution and localization of deformation below a crustal detachment (Tinos, Cyclades, Greece)

Abstract

The Cycladic Oligo‐Miocene detachment of Tinos island is an example of a flat‐lying extensional shear zone evolving into a low‐angle brittle detachment. A clear continuum of extensional strain from ductile to brittle regime is observed in the footwall. The main brittle structures marking extension are shallow‐ and steeply dipping normal faults associated with subvertical extensional joints and veins. The earliest brittle structures are low‐angle normal faults which commonly superimpose on, and reactivate, earlier (precursory) ductile shear bands, but newly formed low‐angle normal faults could also be observed. Low‐angle normal faults are cut by late steeply dipping normal faults. The inversion of fault slip data collected within, and away from, the main detachment zone shows that the direction of the minimum stress axis is strictly parallel to the NE‐SW stretching lineation and that the maximum principal stress axis remained subvertical during the whole brittle evolution, in agreement with the subvertical attitude of veins throughout the island. The high angle of σ1 to the main detachment suggests that the detachment was weak. This observation, together with the presence of a thick layer of cataclasites below the main detachment and the kinematic continuum from ductile to brittle, leads us to propose a kinematic model for the formation of the detachment. Boudinage at the crustal scale induces formation, near the brittle‐ductile transition, of ductile shear zones near the edges of boudins. Shear zones are progressively exhumed and replaced by shallow‐dipping cataclastic shear zones when they reached the brittle field. Most of the displacement is achieved through cataclastic flow in the upper crust and only the last increment of strain gives rise to the formation of brittle faults. The formation of the low‐angle brittle detachment is thus “prepared” by the ductile shear zone and the cataclasites and favored by the circulation of surface‐derived fluids in the shear zone.