Mechanically twinned titanite records stress-strain histories during strike-slip and thrust faulting at greenschist-facies conditions

Mechanically twinned titanite in mylonites from the Sesia zone (strike-slip regime) and pseudotachylyte-bearing gneisses from the Silvretta basal thrust record the stress-strain histories at greenschist-facies conditions in the two different tectonic regimes. Twinned titanite in both fault rocks was investigated by analytical scanning electron microscopy, including electron backscatter diffraction (EBSD) and U-stage measurements. It highlights the similarities and differences in the recorded deformation history. Fine-lamellar (< 1µm) mechanical <110> twins in titanite from the Sesia mylonites with twin planes close to {221} show densities of 0.5 µm^-1. Consistent with twinned jadeite, the differential stresses indicated are on the order of 0.5 GPa. In the Silvretta fault rocks, the twin density is higher, at 2.5 µm^-1 and additionally, twin planes close to { ̅1 ̅1 2} occur, indicating higher stress/strain-rate conditions, consistent with twinned amphibole and ilmenite as well as the presence of pseudotachylytes. The Silvretta fault rocks do not record subsequent creep, indicating rapidly decreasing stresses. In contrast, in the Sesia mylonites, subsequent creep of the surrounding quartz matrix at decreasing stresses resulted in sets of subparallel intragranular fractures in titanite, garnet, jadeite and zircon oriented at angles between 60° and 80° to the mylonitic foliation. The similarities in high-stress crystal plasticity in both settings, with twinning at high differential stresses, as well as the differences with pseudotachylyte formation in the Silvretta fault rocks and creep at more slowly decreasing stresses in the Sesia zone mylonites, demonstrate the importance of deformation at transient high stresses for the subsequent stress-strain history.