High-stress crystal plasticity of titanite and other minerals recording coseismic off-fault damage in strike-slip and thrust tectonic settings

• Mechanically 〈110〉 twins in titanite record transient high stress deformation. • Twinning occurred before cataclasis and emplacement of pseudotachylytes. • Deformation during transient high-stress events controls subsequent creep. • Thrust fault rocks reveal fast unloading from differential stresses of hundreds of MPa. • Strike-slip mylonites reveal creep after peak stress deformation at hundreds of MPa. This study compares coseismic off-fault damage in a strike-slip (Sesia zone) and a thrust (Silvretta) tectonic setting to evaluate the different stress-strain histories. High-stress crystal plasticity at greenschist-facies conditions is recorded by mylonites from the Sesia zone and pseudotachylyte-bearing gneisses from the Silvretta basal thrust, European Alps. Twinned titanite occurring in both fault rocks highlights the similarities and differences in the recorded deformation. Fine-lamellar (< 1 µm) 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 with 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 of 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.