Friction of phyllosilicates-rich faults sheared in their in situ geometry is significantly lower than friction of their powdered equivalents.
Many rock deformation experiments used to characterize the frictional properties of tectonic faults are performed on powdered fault rocks or on bare rock surfaces. These experiments have been fundamental to document the frictional properties of granular mineral phases and provide evidence for crustal faults characterized by high friction. However, they cannot entirely capture the frictional properties of faults rich in phyllosilicates.
Numerous studies of natural faults have documented fluid-assisted reaction softening promoting the replacement of strong minerals with phyllosilicates that are distributed into continuous foliations. To study how these foliated fabrics influence the frictional properties of faults we have: 1) collected foliated phyllosilicate-rich rocks from natural faults; 2) cut the fault rock samples to obtain solid wafers 0.8-1.2 cm thick and 5 cm x 5 cm in area with the foliation parallel to the 5x5cm face of the wafer; 3) performed friction tests on both solid wafers sheared in their in situ geometry and powders, obtained by crushing and sieving and therefore disrupting the foliation of the same samples; 4) recovered the samples for microstructural studies from the post experiment rock samples; and 5) performed microstructural analyses via optical microscopy, scanning and transmission electron microscopy.
Mechanical data show that the solid samples with well-developed foliation show significantly lower friction in comparison to their powdered equivalents. Micro- and nano-structural studies demonstrate that low friction results from sliding along the foliation surfaces composed of phyllosilicates. When the same rocks are powdered, frictional strength is high, because sliding is accommodated by fracturing, grain rotation, translation and associated dilation. Friction tests indicate that foliated fault rocks may have low friction even when phyllosilicates constitute only a small percentage of the total rock volume, implying that a significant number of crustal faults are weak.
The overall goal of this procedure is to test the frictional properties of intact phyllosilicate-rich faults sheared in their in situ geometry and to show that their friction is significantly lower than friction obtained from experiments conducted on powders of the same material.
Numerous geological studies have documented fluid-assisted reaction softening during the long-term evolution of tectonic faults. Softening occurs by the replacement of strong minerals, like quartz, feldspar, calcite, dolomite, olivine, pyroxene, with weak phyllosilicates1,2,3,4,5,6,7,8,9,10. This weakening originates at the grain-scale and is mainly due to sliding, at very low friction, along the phyllosilicate foliae that act together to produce a form of lubrication. From the grain-scale, fault weakening is transmitted to the entire fault zone via the interconnectivity of the phyllosilicate-rich zones11. To capture the role of frictional sliding along interconnected phyllosilicate foliae, intact solid wafers of natural fault-rock samples have been sheared in their in situ geometry during rock deformation experiments12,13,14. At the end of the experiment, microstructural studies on the tested samples have been performed to check if effectively the deformation was accommodated by frictional sliding along the phyllosilicate foliae.
In comparison with traditional friction tests performed on powdered materials obtained from crushing and sieving the fault rock, experiments on intact wafers can capture the frictional sliding along the interconnected phyllosilicate-rich layers formed by fluid assisted reaction softening. In fact, during the process of powder preparation, the crushing and sieving of the fault rock disrupts the connectivity of the phyllosilicate layers and when the material is sheared in the laboratory, the absence of continuous phyllosilicate horizons favors a deformation mainly consisting of grain crushing, rotation and translation resulting in high friction.
Experiments on solid wafers show a significantly lower friction in comparison to experiments on powdered material obtained from the same rock type, particularly when the percent of the phyllosilicates is < 40%15. With increasing phyllosilicate abundance, a reduction in friction has been documented also for tests on powdered material, since in this case the large volume of phyllosilicates is sufficient to promote interconnectivity of the weak mineral phases through the entire experimental fault16,17,18,19,20,21,22. Alternatively, to simulate frictional sliding on the interconnected weak layers, other types of friction tests have been performed on powders composed of 100% weak mineral phases23,24,25.
Geometrical fault weakening promoted by rock fabric in deformation experiments at high temperature, and therefore representative of the ductile lithosphere, has been well known for many years26. The results obtained from the procedure presented here indicate that phyllosilicate fabric promotes fault weakening also for a large number of faults contained within the seismogenic upper crust.
1. Rock sample collection
2. Sample preparation for friction experiments in the double direct shear configuration
3. Friction experiments
4. Post-experimental sample collection
5. Microstructural analysis
In a diagram of normal stress vs. shear stress both solid foliated and powdered samples plot along a line consistent with a brittle failure envelope, but the solid wafers have friction values significantly lower than their powdered analogues30. For example, in the specific case of a talc-rich foliation, foliated fault rocks at each normal stress have a friction coefficient that is 0.2-0.3 lower than the powders made from them (Figure 2 and12). The lower friction is explained by microstructural studies of the tested rocks showing that the sliding surfaces of the foliated solid wafers occur along the pre-existing phyllosilicate-rich foliation. TEM images show that slip is mainly accommodated by (001) easy sliding associated with interlayer delamination. In contrast, experimental microstructures from the powdered material show that significant deformation is accommodated by grain-size reduction and localization.
Although the foliated wafers of intact fault rocks and their powders have identical mineralogical compositions, the foliated samples show friction that is significantly lower than their powdered analogues. Microstructural studies indicate that the lower friction (i.e., fault weakness) of the foliated fault rocks is due to the reactivation of the pre-existing natural phyllosilicate-rich surfaces that are absent in the powdered samples since the sample preparation steps (2.2 – 2.4) disrupt them.
Figure 1: Representative images of the tested fault rocks: solid foliated vs. powdered material. (left) Solid foliated samples sheared parallel to the natural foliation marked by the arrows. (right) Powders obtained by crushing and sieving the solid foliated rock. Please click here to view a larger version of this figure.
Figure 2: Friction tests on the same material (talc-rich foliation) but solid foliated samples vs. powdered rock. Each dataset plot along a line consistent with a brittle failure envelope, but the solid foliated rocks are characterized by friction significantly lower than their powdered analogue, friction, μ = 0.3 and μ = 0.57 respectively. Please click here to view a larger version of this figure.
Figure 3: Natural vs. laboratory reactivated foliation. In the left picture an example of a natural talc-rich foliation with surrounding sigmoidal clasts of calcite31. The right picture shows the same foliation at the end of a friction test on wafers32. Note that during the friction test most of the slip occurs by frictional sliding along the phyllosilicate layers and that the original microstructure is preserved. Please click here to view a larger version of this figure.
An important point worth mentioning is that with this procedure we characterize the steady state fault frictional strength, measured with experiments at low sliding velocities (i.e., 0.01 µm/s < v < 100 µm/s). The measured low values of friction demonstrate the weakness of phyllosilicate-rich faults resulting from long-term fluid assisted reaction softening and foliation development1,4,5,6,7,8,9,10,11,12,30. This low frictional strength can be used as a proxy to evaluate the fault strength at steady-state or during the pre-seismic phases of the seismic cycle. Therefore, the important dynamic weakening mechanisms that occur at high slip velocities (i.e., > 10 cm/s) and induced by temperature rise33 are not considered in our analysis.
The critical steps in the protocol regard the sample collection and preparation. Since phyllosilicates are characterized by very low tensile strength in the direction perpendicular to the (001) basal planes (i.e., in the direction perpendicular to the foliation), during the work with the hammer and chisel in the field or with the hand-grinder in the lab, quite often the rock samples fall apart and the shaping process has to restart. Therefore, it is strongly recommended to collect more samples than those strictly required to run experiments and arm yourself with patience.
Before integrating mechanical with microstructural data, it is important to check that the frictional sliding along the phyllosilicate-rich foliae observed along natural fault rocks is reproduced in the lab, or in other words that the natural fault rock microstructure is similar to the one obtained from shearing the wafer (Figure 3).
In experiments on solid wafers characterized by thin networks of phyllosilicates, the continuous layers of weak mineral phases can be consumed during significant shearing (displacement > 12 mm). At this stage the deformation is accommodated by a combination of cataclasis of the strong mineral phases and sliding along the phyllosilicates. This coincides with a phase of strain hardening with an increase in friction of about 0.1 or more13.
The majority of rock deformation experiments, aimed at the characterization of the frictional properties of tectonic faults, are performed on millimetric rock layers that are composed by powders obtained by crushing and sieving natural fault rocks24,27 or on fault rocks that are pre-cut34. These types of experiments are fundamental to characterize the frictional properties of faults where the deformation occurs on fault gouges35 or along sharp slipping planes of localized deformation36. For faults rich in phyllosilicates, low friction and hence fault weakness is related to the interconnectivity of the phyllosilicate-rich networks, which in the field is manifested by multiple anastomosing principal slip zones. This indicate that even a small quantity of phyllosilicates can induce significant fault weakening if their interconnectivity is very high37,38. Therefore, the final goal of our laboratory experiments on solid wafers is to preserve the natural continuity of the phyllosilicate-rich layers during friction tests.
Other laboratory experiments on powdered mixtures of strong and weak mineral phases have documented fault weakening with the addition of the weak phases18,19,20,21,22. It has been observed that amounts of 40-50% of phyllosilicates induce a significant reduction in friction because during shearing they become interconnected. This suggests that for large percentages of phyllosilicates (i.e., > 40%), experiments on wafers or powders are similar25.
A compilation of friction tests conducted on a large number of natural fault rocks rich in phyllosilicates, wafers or powdered material with phyllosilicates percentages > 40%, under a wide range of experimental conditions show that friction is in the range of 0.1-0.330. This implies that a significant number of crustal faults are weak.
The authors have nothing to disclose.
We kindly acknowledge Marco Albano for providing the video dealing with optical microscope and SEM and Domenico Mannetta for the rock cutting procedure. This research has been supported by the ERC Grant GLASS n° 259256 and TECTONIC n° 835012. This contribution was greatly improved by the comments of three anonymous reviewers and by the editorial production suggestions on the video.
disk mill | Plenty of companies | none | Standard disk mills to pulverize rocks |
fault rock | Natural outcrops | none | All the outcrops rich in phyllosilicates worldwide |
hammer and chisel | Plenty of companies | none | Standard hammer and chisel used by geologists |
optical microscope | Plenty of companies | none | Standard microscope used for mineralogy |
rock deformation apparatus | we use prototypes like BRAVA & BRAVA2.0 | none | Eock deformation apparatusses (Marone et al., 1998; Collettini et al., 2014) |
saw to cut rocks | Plenty of companies | none | Standard saws to cat fault rocks |
SEM, scanninc electron microscope | Plenty of companies | none | Microscope to investigate microstructures at the micron scale |
TEM, transmission electron microscope | Plenty of companies | none | Microscope to investigate microstructures at the nano scale |