Rheology and fault slip behavior of Triassic Evaporites: an experimental study

The Triassic Evaporites (TE), made of alternating dolostones and anhydrites, hosted the mainshocks and most of the aftershocks of the Mw 6.0 1997 Colfiorito and Mw 6.5 2016 Norcia earthquakes in central Italy. Their complex rheology features elasto-frictional behavior and ductile deformation. The latter is highlighted by the spatio-temporal evolution of the Norcia aftershock sequence showing widespread distributed seismicity within a large crustal volume, suggesting a rheological embrittlement of the whole evaporitic layer.

 

To understand the main factors causing rheology variations of TE, we performed triaxial compression experiments on TE borehole samples varying strain rate, confining pressure, lithology and fabric. We tested pure anhydrite, foliated anhydrite-dolostone and mixed-chaotic dolostone-rich specimens. Samples, subjected to confining pressures of 10 and 20 MPa, were loaded up to failure at strain rates of 10-4 and 10-5 s-1, and then reloaded after a holding time of 1000 s to evaluate fault reactivation characteristics.  We also performed friction experiments on TE gouge at normal stress of 20 MPa and load-point velocity of 10 μm/s, to capture fault structure development starting from randomly distributed anhydrite-dolostone particles. Mechanical measurements were coupled by microstructural analyses to elucidate the deformation processes operating at different boundary conditions.

 

In triaxial experiments, all samples exhibited brittle shear failure with associated stress drop. Upon rock failure, we observed a spectrum of fault slip behaviors ranging from slow (< 50 μm/s) to fast (> 600 μm/s) fault slip.  We observe that dynamic faulting occurred preferentially at 10 MPa, and at higher strain rate in dolostone-rich samples . Upon fault reactivation, we recorded fault slip instabilities, mainly for the same type of conditions: dolostone-rich samples and at confining pressure of 10 MPa. On the contrary, neither fabric nor textural heterogeneities appeared to influence rock failure properties or fault reactivation behavior. During friction experiments on gouge, we measured similar friction coefficients between anhydrite and dolostone (μ ∼ 0.65), detecting minor fault slip instabilities within dolostone-rich samples. Microstructural investigations revealed the enrichment of dolostone within the experimentally developed fault slip zones, characterized by grain size reduction and shear localization.

 

The analysis of mechanical data suggests that rheological embrittlement of TE is facilitated by low confining pressure, high strain rate and high dolostone content. The simultaneous occurrence of these conditions promotes dynamic faulting upon rock failure and fault slip instabilities during fault reactivation. Shear localization favors dolostone concentration along slip planes, implying that the shear strength of TE-hosted faults is primarily controlled by frictional properties of dolostone, which create favorable conditions for the development of slip instabilities. When upscaling laboratory results to the crustal scale, we can speculate that low effective pressure is given by pressurized fluids while mainshock-induced stress changes facilitated a strain rate increase. These processes together contribute to the embrittlement of the evaporitic layer, explaining the distributed seismicity observed after the Norcia mainshock.