THE NORTH ANATOLIAN FAULT ZONE AND THE ROLE OF FLUIDS IN SEISMOGENIC DEFORMATION

A study of the North Anatolian Fault Zone (Turkey) indicates significa nt spatial variations of deformation mechanisms and fluid flow along a nd across a major active strike-slip fault. Brittle deformation and la ter dissolution are mostly localized within the fault core, whereas cr ystal plastic deformation and diffusive mass transfer are more widely distributed. The fault underwent a three-stage development: (I) Repeat ed episodes of frictional failure, pressure-solution, fluid migration, and subsequent healing. Cataclasites and veins were formed during thi s stage. We propose that the episodic events are associated with open and closed fluid systems during seismogenic cycles. The progressive br ittle failure led to stage (II), a more open system with increasing di lation connected with dissolution. The dramatic increase of dissolutio n processes terminated the episodic character of deformation and marke d the beginning of a stage (III), mostly characterized by diagenetic p rocesses. The fluid regime is now an open system. In general, our data do not indicate massive fluid redistribution in response to earthquak e cycles of shear stress accumulation and release. Trace element distr ibution, isotopic data, and fluid inclusions suggest that fluids were derived from the adjacent limestones and that hydrostatic fluid pressu re was dominant during fault development. We suggest that meteoric wat er circulates through the fault zone, causing solution transfer and co mpaction, which allows frictional failure at lowered shear stress.