Control of the geometry of precursor brittle structures on the type of ductile shear zone in the Adamello tonalites, Southern Alps (Italy)

Amphibolite facies ductile shear zones developed during the immediate post-intrusive cooling history of the Adamello tonalites (Southern Alps, Italy). Shear zones include: (i) thin (a few mm's thick) fault-like shear zones that accommodate shear strain values up to several 100’s (the dominant type); (ii) mylonitic horizons (dm's thick) in sharp contact with the undeformed wall rock; (iii) continuous shear zones with sigmoidal-shaped S or composite S–C′ foliations. A transition between the different types occurs along strike over short distances. Discontinuous shear zones are also localized on pegmatite dykes and quartz veins, and both (i) and (ii) are discontinuously outlined by quartz layers. Detailed surface mapping reveals that most fault-like shear zones are arranged en-échelon, mainly forming contractional steps. Markers crosscut by shear zones allow the displacement to be measured at several positions along shear zones and this reveals very steep displacement gradients close to the shear zone tips. Differential displacement is mainly accommodated at contractional steps by the development of foliated domains. Geochemical analyses of major and trace elements show that there is no compositional change along strain gradients. The overall features are consistent with nucleation of shear zones on former sets of en-échelon joints, in many cases intruded by pegmatite dykes or filled with quartz. Reactivation of joints produced strongly localized shear zones, whereas broader foliated zones evolved from the contractional jogs between adjacent stepped joint terminations during progressive shearing. These jogs were progressively involved in the accommodation of shear displacement and overall shear zone development.