The pseudotachylyte-­mylonite association: an insight into the mechanics of deep earthquakes

The aim of this thesis is to investigate the association of fault rocks formed by seismic deformation with coeval ductile deformation. The only recognised geological record of these two concurrent deformation mechanisms is represented by the relationship between coeval pseudotachylytes (quenched melts produced during seismic slip) and mylonites (high-­‐strain rocks deformed in high-temperature ductile flow). The scientific importance of this association of rocks lies in the fact that their existence in rocks exhumed from below the long-­‐term brittle/ductile transition is a compelling evidence that rheology of the deep crust cannot be treated simply in terms of brittle and ductile models. By investigating associations of coeval pseudotachylytes and mylonites we aim at giving an original and meaningful contribution to the understanding of the mechanisms involved in the interplay in space and time between seismic deformation and ductile flow. A model proposed to explain such association of fault rocks is “self-­‐localising thermal runaway”, assuming a spontaneous acceleration of localized slip in a ductile shear zone eventually leading to seismic slip and melting. This model is supported by numerical modelling, but its application to nature is disputed and speculative. We analyse a pseudotachylyte-­ultramylonite association in exhumed lower crustal, quartz-­‐rich metapelites from the Mont Mary nappe of the Western Italian Alps, representing a possible candidate for thermal runaway instability, to find evidence in support or against this process. • We document by detailed electron backscatter diffraction (EBSD) analysis of quartz-­‐rich layers, the progressive microstructural evolution, at nearly constant temperature conditions (550 °C), to high differential stresses (> 200 MPa) and high strain rates (10-­‐9 s-­‐1) within the most strongly deformed portions of the ultramylonite hosting the pseudotachylyte. This microstructural evolution is associated with a switch in deformation mechanism from grain-­size-insensitive to grain-­size-­sensitive creep assisted by grain boundary sliding and creep cavitation. These latest recorded stages of deformation were still aseismic, as the rate-­controlling process was precipitation of oriented biotite in cavitation pores. • We calculate, by calibrated numerical models, the critical conditions for thermal runaway instability in quartz for a wide range of temperature/strain-­‐rate combinations, and determine that deformation in the studied ultramylonite occurred close to the conditions for the instability to occur. At the same time, we estimate that deformation occurred proximal to brittle-­ductile transition for such high strain rates. • We conclude that the observed pseudotachylyte-­mylonite association is best explained by transient downward propagation of seismic rupture from the nearby, overlying base of the seismogenic crust; or by earthquake nucleation below the long-­term brittle/ductile transition permitted by the downward deflection of the transition after a large seismic event in the upper crust. Based on the study of wall-­rock garnet coseismic fragmentation in the Mont Mary pseudotachylyte-­ultramylonite and on garnet preferential melting within the pseudotachylyte, we suggest a general process for garnet disappearance due to thermal shock fragmentation during co-­seismic frictional heating. We show that garnet has the lowest thermal shock resistance between the host rock minerals (garnet, plagioclase, quartz, and sillimanite, in an increasing sequence of resistance), and thus underwent extreme comminution leading to total melting within the frictional melt. Our analysis highlights the critical role of thermal shock as a general process in mineral comminution during the initial stages of co-seismic slip preceding (and promoting) extensive frictional melting. We also present the preliminary results (mechanical and microstructural) of rotary shear experiments designed to reproduce the formation of Mont Mary pseudotachylytes in the lab. We extend the study of deep crustal pseudotachylytes and pseudotachylyte-­mylonite associations to the Calabrian lower crust (Southern Italy) in a preliminary study that aims at paving the way to a further in-­depth analysis of Calabrian pseudotachylytes, which represent a unique information source about the rheology of the granulitic continental lower crust. We present microstructural evidence for cyclic pseudotachylyte and mylonite development in the dry lower crust and document the first finding of low-­‐p/high T, cordierite-­bearing, peraluminous pseudotachylytes featuring sillimanite microlites.