Initiation, geometry and mechanics of brittle faulting in exhuming metamorphic rocks: insights from the northern Cycladic islands (Aegean, Greece)

Initiation, geometry and mechanics of brittle faulting in exhuming metamorphic rocks are discussed on the basis of a synthesis of field observations and tectonic studies carried out over the last decade in the northern Cycladic islands. The investigated rocks have been exhumed in metamorphic domes partly thanks to extensional detachments that can be nicely observed in Andros, Tinos and Mykonos. The ductile to brittle transition of the rocks from the footwall of the detachments during Aegean post-orogenic extension was accompanied by the development of asymmetric sets of meso-scale low-angle normal faults (LANFs) depending on the distance to the detachments and the degree of strain localization, then by conjugate sets of high-angle normal faults. This suggests that rocks became progressively stiffer and isotropic and deformation more and more coaxial during exhumation and localization of regional shearing onto the more brittle detachments. Most low-angle normal faults result from the reactivation of precursory ductile or semi-brittle shear zones; like their precursors, they often initiate between or at the tips of boudins of metabasites or marbles embedded within weaker metapelites, emphasizing the role of boudinage as an efficient localizing factor. Some LANFs are however newly formed, which questions the underlying mechanics, and more generally rupture mechanisms in anisotropic rocks. The kinematics and the mechanics of the brittle detachments are also discussed in the light of recent field and modeling studies, with reference to the significance of paleostress reconstructions in anisotropic metamorphic rocks.