Two stages of Coseismic Rupturing separated by a stage of Postseismic Creep indicated by deformed pseudotachylytes from the Silvretta basal thrust

Decoupling and movement of large crystalline nappes during orogenesis are controlled by episodic deformation at changing strain-rate and stress conditions. Deformed pseudotachylytes from the Silvretta basal thrust in the Central Alps, Austria, record two stages of pseudotachylyte formation separated by a stage of creep with quartz dynamic recrystallization at decreasing stresses at greenschist facies conditions. Amphibole (-101) mechanical twins indicate high strain rates and transient high differential stresses of >400 MPa during the formation of pseudotachylytes in amphibole-rich gneisses. Undeformed pseudotachylytes contain isometric rounded amphibole-gneiss clasts and quartz-rich clasts. The pseudotachylytes can be gradually transformed into ultramylonites containing deformed quartz clasts that are elongated parallel to the foliation of the fine-grained ultramylonitic matrix. The deformed quartz clasts are characterized by aggregates with an oblique shape preferred orientation and crystallographic preferred orientation indicating dislocation creep during ultramylonite formation. In other samples, pseudotachylyte injection veins within quartz-rich layers that underwent dislocation creep were foliated and folded. These deformed pseudotachylytes can be cross-cut by undeformed pseudotachylytes. The two stages of pseudotachylyte formation are reflecting coseismic rupturing. The intermediate stage of creep with quartz dynamic recrystallization at decreasing stresses is interpreted to represent postseismic creep with relatively slow stress relaxation, as opposed to a separate deformation stage. However, also quartz-rich host rocks deformed related to pseudotachylytes occur that show growth of new grains at quasi-isostatic conditions along the damage zone of coseismically generated cleavage cracks, without any evidence of creep after pseudotachylyte formation and thus representing relatively fast stress relaxation.