COUPLING OF SHEAR ZONES AND MAGMA ASCENT: A REVIEW OF THE OLIGOCENE MAGMATISM IN THE ALPS

Plutonic bodies of Oligocene age in the European Alps are spatially and temporally associated with the activity of the Periadriatic Fault System, an orogen-parallel, crustal-scale transpressive mylonitic belt. A wealth of structural, seismic, petrological, geochronological, geochemical, and paleomagnetic data collected over the last decades allow one to constrain the relationship between deformation, ascent and emplacement of magmas. Most plutons have tonalitic to granodioritic composition and all crystallization ages are comprised in a time interval of approximately 10 Ma. All plutons show a similar geochemical signature, resulting from a mixture of lithospheric mantle and lower crust. The source region of the early differentiates of the plutons is inferred to be at a depth greater than 50 km. Hence magmas ascended across most of the crust before final emplacement. The Periadriatic Fault System and related plutons are exposed at crustal levels varying from 28 km depth to near the surface, indicating that the fault plane is associated with melts from the surface down to the middle-lower crust. Therefore, the ascent of the melts occurred along the fault plane. Emplacement mechanisms differ from one pluton to the other, showing good examples of ballooning, fracture-controlled intrusion, and stoping, suggesting that the Periadriatic Fault only controlled the ascent, but not the final emplacement of the magmas. The presence of plutons along fault segments affected by different shear senses and shear directions suggests that the ascent of magmas was controlled by the fault plane, but not by its kinematic regime. The occurrence of some plutons along segments of the fault system that cannot be followed down to the source region (as indicated by seismic sections) shows that magmas were channelled during ascent and did not initially form along the plane of the Periadriatic Fault.