Cecilia Viti

Fault zone fabric and fault weakness

Geological and geophysical evidence suggests that some crustal faults are weak compared to laboratory measurements of frictional strength. Explanations for fault weakness include the presence of weak minerals, high fluid pressures within the fault core and dynamic processes such as normal stress reduction, acoustic fluidization or extreme weakening at high slip velocity. Dynamic weakening mechanisms can explain some observations; however, creep and aseismic slip are thought to occur on weak faults, and quasi-static weakening mechanisms are required to initiate frictional slip on mis-oriented faults, at high angles to the tectonic stress field. Moreover, the maintenance of high fluid pressures requires specialized conditions and weak mineral phases are not present in sufficient abundance to satisfy weak fault models, so weak faults remain largely unexplained. Here we provide laboratory evidence for a brittle, frictional weakening mechanism based on common fault zone fabrics. We report on the frictional strength of intact fault rocks sheared in their in situ geometry. Samples with well-developed foliation are extremely weak compared to their powdered equivalents. Micro- and nano-structural studies show that frictional sliding occurs along very fine-grained foliations composed of phyllosilicates (talc and smectite). When the same rocks are powdered, frictional strength is high, consistent with cataclastic processes. Our data show that fault weakness can occur in cases where weak mineral phases constitute only a small percentage of the total fault rock and that low friction results from slip on a network of weak phyllosilicate-rich surfaces that define the rock fabric. The widespread documentation of foliated fault rocks along mature faults in different tectonic settings and from many different protoliths suggests that this mechanism could be a viable explanation for fault weakening in the brittle crust.

Fault lubrication and earthquake propagation in thermally unstable rocks

Experiments performed on dolomite or Mg-calcite gouges at seismic slip rates ( v > 1 m/s) and displacements (d > 1 m) show that the frictional coefficient μ decays exponentially from peak values (m p ≈ 0.8, in the Byerlee9s range), to extremely low steady-state values (μ ss ≈ 0.1), attained over a weakening distance D w . Microstructural observations show that discontinuous patches of nanoparticles of dolomite and its decomposition products (periclase and lime or portlandite) were produced in the slip zone during the transient stage (d w ). These observations, integrated with CO 2 emissions data recorded during the experiments, suggest that particle interaction in the slip zone produces flash temperatures that are large enough to activate chemical and physical processes, e.g., decarbonation reactions ( T = 550 °C). During steady state (d ≥ D w ), shear strength is very low and not dependent upon normal stresses, suggesting that pressurized fluids (CO 2 ) may have been temporarily trapped within the slip zone. At this stage a continuous layer of nanoparticles is developed in the slip zone. For d >> D w , a slight but abrupt increase in shear strength is observed and interpreted as due to fluids escaping the slip zone. At this stage, dynamic weakening appears to be controlled by velocity dependent properties of nanoparticles developed in the slip zone. Experimentally derived seismic source parameter W b (i.e., breakdown work, the energy that controls the dynamics of a propagating fracture) (1) matches W b values obtained from seismological data of the A.D. 1997 M6 Colfiorito (Italy) earthquakes, which nucleated in the same type of rocks tested in this study, and (2) suggests similar earthquake-scaling relationships, as inferred from existing seismological data sets. We conclude that dynamic weakening of experimental faults is controlled by multiple slip weakening mechanisms, which are activated or inhibited by physicochemical reactions in the slip zone.

Development of interconnected talc networks and weakening of continental low-angle normal faults

Fault zones that slip when oriented at large angles to the maximum compressive stress, i.e., weak faults, represent a significant mechanical problem. Here we document fault weakening induced by dissolution of dolomite and subsequent precipitation of calcite + abundant talc along a low-angle normal fault. Within the fault core, talc forms an interconnected foliated network that deforms by frictional sliding along 50–200-nm-thick talc lamellae. The low frictional strength of talc, combined with dissolution-precipitation creep, can explain slip on low-angle normal faults. In addition, the stable sliding behavior of talc is consistent with the absence of strong earthquakes along such structures. The development of phyllosilicates such as talc by fluid-assisted processes within fault zones cutting Mg-rich carbonate sequences may be widespread, leading to profound and long-term fault weakness.

Heterogeneous distribution of metal nanocrystals in glazes of historical pottery

It has been recently shown that lustre decoration of medieval and renaissance pottery consists of silver and copper nanocrystals, dispersed within the glassy matrix of the ceramic glaze. Lustre surfaces show peculiar optical effects such as metallic reflection and iridescence. In many cases, lustre appears overlapped to colored drawings. Here we report the findings of a study on glazes, pigments and lustre of several shards belonging to Deruta and Gubbio pottery of XVI century. The components of glazes and pigments have been identified. Lustre is confirmed to be characterised by silver and copper metal nanocrystals inhomogeneously dispersed in the glassy matrix of the glaze. In the case of lustre overlapped to colored decorations, we found two contradictory cases. The first consists of a lustre surface successfully applied over a blue smalt geometrical drawing. The second consists of a lustre surface, unsuccessfully applied over a yellow lead-antimonate pigment. The yellow pigment hinders the formation of lustre and removes crystals of tin dioxide, normally present in the glaze as opacifier.

Thermal decomposition along natural carbonate faults during earthquakes

Earthquake slip is facilitated by a number of thermally activated physicochemical processes that are triggered by temperature rise during fast fault motion, i.e., frictional heating. Most of our knowledge on these processes is derived from theoretical and experimental studies. However, additional information can be provided by direct observation of ancient faults exposed at the Earth’s surface. Although fault rock indicators of earthquake processes along ancient faults have been inferred, the only unambiguous and rare evidence of seismic sliding from natural faults is solidified friction melts or pseudotachylytes. Here we document a gamut of natural fault rocks produced by thermally activated processes during earthquake slip. These processes occurred at 2–3 km depth, along a thin (0.3–1.0 mm) principal slip zone of a regional thrust fault that accommodated several kilometers of displacement. In the slip zone, composed of ultrafine-grained fault rocks made of calcite and minor clays, we observe the presence of relict calcite and clay, numerous vesicles, poorly crystalline/amorphous phases, and newly formed calcite skeletal crystals. These observations indicate that during earthquake rupture, frictional heating induced calcite decarbonation and phyllosilicate dehydration. These microstructures may be diagnostic for recognizing ancient earthquakes along exhumed faults.

Serpentine minerals discrimination by thermal analysis

Abstract This paper reports a complete set of TG, DTG, and DTA data, coupled with emitted gas analysis, for well-constrained, almost pure serpentine samples. Serpentine dehydroxylation takes place between 550 and 800 °C, with DTG and DTA peak temperatures progressively decreasing from antigorite (720 and 715 °C, respectively) to lizardite (708 and 714 °C), polygonal serpentine (685 and 691 °C), and chrysotile (650 and 654 °C). Antigorite has an additional diagnostic signal at ~740-760 °C, always absent in the other serpentines, and dependent on antigorite superperiodicity (T shift of ~20 °C from 36 to 49 Å modulation wavelength). A sharp exothermic peak occurs at extremely constant temperatures (~820 °C), independently from the starting serpentine structure. The high-T mineral assemblage is always represented by forsterite and enstatite. Based on the observed relationships between serpentine structures and DTG/DTA dehydroxylation temperatures, thermal analysis may represent a useful tool for serpentine identification, particularly in the case of natural massive samples where different varieties are mutually intermixed. The accurate definition of serpentine mineralogy would have obvious implications in both geological-petrological and health-related issues.

Frictional strength and healing behavior of phyllosilicate‐rich faults

[1] We study the mechanisms of frictional strength recovery for tectonic faults with particular focus on fault gouge that contains phyllosilicate minerals. We report laboratory and microstructural work from fault rocks associated with a regional, low-angle normal fault in Central Italy. Experiments were conducted in a biaxial deformation apparatus at room temperature and humidity, nominally dry, under constant normal stresses of 20 and 50 MPa, and at a sliding velocity of 10 mm/s. Our results for nominally dry conditions show good agreement with previous work conducted under controlled pore fluid pressure. The phyllosilicate contents of our samples, which include clay, talc and chlorite range from 0 to 52 weight %. We study both intact rock samples, sheared in their in situ geometry, and powders made from the same rocks to address the role of fabric in fault healing. We measured frictional healing, Dm, using slide-hold-slide tests with hold periods ranging from 3 to 3000 s. Phyllosilicate-free materials show friction values of m ≈ 0.6 and healing rates that are larger in powdered samples, b ≈ 0.006 (Dm per decade in time, s) compared to intact wafers of fault rock, b ≈ 0.004. For phyllosilicate-bearing materials, healing rates are low, b < 0.002, and independent of fabric, phyllosilicate content and normal stress. We observe that frictional strength decreases systematically with increasing phyllosilicate content. Intact, phyllosilicate-bearing fault rock is consistently weaker than its powdered equivalent (0.2 < m < 0.3 versus 0.4 < m < 0.5, respectively). We compare our data to results from experiments conducted on a wide range of materials and conditions. Deformation microstructures show localized slipping along sub-parallel shear planes. We suggest that low values of frictional strength and near zero healing rates will combine to exacerbate the weakness of phyllosilicate-bearing faults and promote stable, aseismic creep.

Insights into the antigorite structure from Mössbauer and FTIR spectroscopies

Pure, selected samples of antigorite (#7 and #18, from Elba Island veins, Italy, with superperiodicities of 38 and 49 A, respectively) have been analyzed by Mossbauer and infrared spectroscopies. Mossbauer data indicate that most of the iron is present as ferrous iron (88.6 % Fe 2+ in #7 and 83.2 % Fe 2+ in #18). Both ferrous and ferric iron occur in octahedral coordination; ferric iron in tetrahedral coordination has not been detected. The infrared spectra of antigorites #7 and #18 are similar, with minor shifts in peak positions. More in general, the comparison with other vein antigorites from the Elba suite (#2, #4, #11, #16) rules out any relation between modulation wavelength and IR behaviour. Evident differences arise from the comparison between antigorite and lizardite spectra. The absorption bands corresponding to stretching of the basal Si-O bonds are systematically shifted towards higher wavenumbers in antigorite with respect to lizardite (from 951 to 979–991 cm -1 ), suggesting higher energy of the bridging bonds. In contrast, antigorite and lizardite show the same IR patterns in the apical Si-O stretching vibrations (1073–1084 cm -1 ). The OH stretching region (3700–3400 cm -1 ) indicates similar structural arrangement for the inner O-H in antigorite and lizardite, whereas the absence of the broad band at ∼ 3440 cm -1 in antigorite indicates the lack of important hydrogen bonding in the interlayer. Other IR differences ( e.g. , absence of Si-O bending and of external OH bending in lizardite) are explained as due to different symmetries (monoclinic antigorite vs. trigonal lizardite). We conclude that antigorite and lizardite share common features (similar iron coordination and disordered distribution within the magnesium octahedra), but differ in the oxidation state (more reduced antigorite), in the tetrahedral sheet size (basal Si-O bond shrinked by 0.009 A in antigorite) and in the interlayer connections mechanism (absence of hydrogen bond in antigorite).

Re-equilibration of glass and CO2 inclusions in xenolith olivine: A TEM study

Abstract CO2-rich fluid inclusions were observed in olivine from mantle xenoliths from the Island of Tenerife, Canary Islands. Inclusions that are present in deformed olivine porphyroclasts consist of CO2 fluids + minor high-alkali, silica-rich glass ± Ni-Fe sulfides. Homogenization temperature distributions reveal that most of the inclusions (originally trapped at mantle conditions) re-equilibrated to lower density values. Transmission electron microscope (TEM) studies indicate that most fluid inclusions appear as perfectly euhedral negative crystals, with variable shape (from prismatic to equant), size (from <0.02 to 0.15 μm), and inner texture. Different kinds of negative crystals may coexist in the same trail of inclusions. Inclusions are commonly connected to structural defects (dislocation arrays formed after fracture healing), which represent a possible path for leakage of the fluid phase. These microstructures, undetectable by optical microscopy, could have modified the original composition and/or density of the inclusions through CO2 diffusion; consequently, they should be taken into account for the correct interpretation of microthermometric results.

Hydrogen positions and thermal expansion in lizardite-1T from Elba; a low-temperature study using Rietveld refinement of neutron diffraction data

Abstract The structure of lizardite-1T from Monte Fico, Elba, was refined in space group P31m using neutron diffraction data, measured at 8, 150, and 294 K, and full-profile Rietveld refinement techniques. The lattice parameters at 8 K [a = 5.3267(2), c = 7.2539(6) Å], 150 K [a = 5.3260(2), c = 7.2574(6) Å], and 294 K [a = 5.3332(2), c = 7.2718(6) Å] show nonlinear expansion, with nearly all volume change above 150 K. H positions were precisely refined at 8 K. The inner H4 atom deviates from the idealized O,O,z positions and is disordered over three symmetry-related positions 0.24 Å away from the ternary axis. The outer H3 atom location is consistent with the previous single-crystal X-ray structure refinement. On the basis of the present thermal expansion data and previous compressibility measurements, the following equation of state for lizardite-1T is proposed: VP,T = V0[1 + 32.8 × 10-6(T - 294) - 15.5 × 10-4(P - 0.001)]. Accordingly, the constant volume condition requires geothermal gradients on the order of 15 °C/km.