Transient deformation and long-term shear zone activity
Mineralogy, Petrology and Geochemistry
Final Report Abstract
A deeper understanding of the deformation at hypocentral depth is vital for several first-order questions in Geosciences, including how crustal blocks are transposed to each other during orogeny and how stresses can build up leading to hazardous large earthquakes. Hypocentres of large earthquakes in the continental crust are confined to ca. 300 °C at 10-15 km depth, which is the base of the seismogenic zone. At these greenschist facies conditions, high differential stresses can transiently be realized coseismically. Yet, quantitative constraints on the stress history at hypocentral depths are scarce, as these depths are not directly accessible. Here, we use exhumed fault rocks of the Silvretta basal thrust, central Alps, to evaluate the deformation and stress history. Pseudotachylytes are common in amphibole-bearing gneisses and formed coseismically associated with quartz and amphibole high-stress crystal plasticity at differential stresses >400 MPa, as indicated by twinned amphiboles. Mylonitic quartz clasts enclosed within deformed pseudotachylytes and mylonitic vein-quartz, hosting folded pseudotachylyte injection veins, reflect creep at lower stresses (~100 MPa) after a stage seismic rupturing. Deformed pseudotachylytes can be crosscut by pristine pseudotachylytes, indicating a second, independent stage of coseismic rupturing after creep. Similar ambient greenschist facies conditions during all deformation stages are indicated by dynamic dislocation creep of quartz before and after pseudotachylyte formation, as well as the presence of stilpnomelane and epidote associated with all fault rocks. As the evidence of quartz dislocation creep occurs spatially unrelated to pseudotachlylytes, it represents an independent deformation stage ruling out a major influence of thermal runaway. Fast stress relaxation from peak stresses is indicated by strain-free new grains decorating cleavage fractures in quartz clasts in undeformed pseudotachylytes. The intermediate stage of creep is interpreted to represent deformation at large distance to the propagating thrust tip. The pristine pseudotachylytes formed during the last stage of rupturing eventually leading to nappe decoupling from its basement. Isometric gneiss clasts in the ultramylonitic matrix of deformed pseudotachylytes reveal that pseudotachylytes have a lower strength during creep. In contrast, during coseismic highstress crystal plasticity, the coarse gneisses accumulate a higher amount of strain than the pseudotachylytes. This strength-relationship explains that only those rocks rupture, which have not been previously deformed before. The results of this project demonstrate the importance of different strengths of crustal rocks at specific stress- and strain-rate conditions during transient deformation and long-term creep in dependence on the distance to the propagating fault tip.
Publications
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Stresses during pseudotachylyte formation -Evidence from deformed amphibole and quartz in fault rocks from the Silvretta basal thrust (Austria). Tectonophysics, 817, 229046.
Brückner, Lisa M. & Trepmann, Claudia A.
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Rheology Dependent on the Distance to the Propagating Thrust Tip—(Ultra‐)Mylonites and Pseudotachylytes of the Silvretta Basal Thrust. Tectonics, 42(10).
Brückner, Lisa M.; Trepmann, Claudia A. & Kaliwoda, Melanie
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Quartz cleavage fracturing and subsequent recrystallization along the damage zone recording fast stress unloading. Journal of Structural Geology, 178, 105008.
Brückner, Lisa M.; Dellefant, Fabian & Trepmann, Claudia A.
