Evidence of Seismic Rupture Directivity from Pseudotachylyte-bearing Fault Vein Networks (Adamello, Southern Alps, Italy)

East-West striking dextral strike-slip paleoseismic faults and branching fractures networks associated to the major Tonale fault are exposed in vast glacially polished outcrops in the Adamello massif (Italian Alps). As faults and fractures are filled by solidified friction-induced melts (pseudotachylytes), the formation of fractures was coseismic. Ambient conditions during faulting were 9-11 km in depth and 250-300° C. Most of the branching of secondary fractures occurs at 30° (consistently with the inferred orientation of the principal horizontal stress) and 90° to the strike of the main faults, and, most interestingly, fractures take off preferentially on one side of the wall rocks, i.e., into the southern block. We interpret these features in terms of stress fluctuations associated to a propagating fracture. With a dynamic faulting numerical simulation we estimate the stress fluctuations at the time of fracture propagation. The numerical model is in agreement with field observations, host rock mechanical properties and other geological constraints. The preferential attitude of the observed fractures is compatible, in terms of the dynamic Coulomb stress fluctuations, with a fracture starting at the western end of the fault (where it connects to the main Tonale fault structure) and propagating toward the East. Numerical modeling together with field observation place several constraints on the friction level and the dynamic stress drop. Non-linear processes occurring in a lateral band off-fault, such as secondary faulting, or diffuse material damage such as breccia and cataclasite formation, should contribute to the amount of energy flow G, the dissipation associated to fracture propagation. In this case we find negligible amount of diffuse material damage, but rather numerous secondary fractures whose contribution to the energy balance may be estimated. Finally, sensible amounts of melt in some portions of the faults, mainly injected into the lateral secondary fractures, yield further information on the coseismic thermal regime and the possible influence of partial melting on friction and pore pressure fluctuations.