Nanograins form carbonate fault mirrors

Siman‐Tov, Shalev; Aharonov, Einat; Sagy, Amir; Emmanuel, Simon

doi:10.1130/g34087.1

Cited by 161 publications

(155 citation statements)

References 23 publications

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“…Atomic force microscopy (AFM) data showed the isotropic regime for a single carbonate fault, but it did not extend to large enough scales to capture the transition (Siman-Tov et al, 2013). Other AFM work showed nanofibers, which have smaller aspect ratios and are distinct from the deeper grooves studied here (Verberne et al, 2014).…”

Section: Observation Of the Minimum Scale Of Groovingmentioning

confidence: 99%

The minimum scale of grooving on faults

Candela

Brodsky

2016

Geology

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At the field scale, nearly all fault surfaces contain grooves generated as one side of the fault slips past the other. Grooves are so common that they are one of the key indicators of principal slip surfaces. Here, we show that at sufficiently small scales, grooves do not exist on fault surfaces. A transition to isotropic roughness occurs at 4-500 mm. Although the scale of the transition can vary even between locales on a single fault, the aspect ratio of the roughness at the transition is well defined for a given fault. We interpret the transition between grooved and ungrooved scales as a transition in deformation mode of asperities on the slip surface. Grooves can form when a hard indenter slides past a softer surface. At small scales, the asperities appear to yield plastically and therefore do not generate grooves as hard indenters. The plastic yielding can be a consequence of the high shear strains required to deform the surfaces at small scales where the aspect ratio (roughness) is high. The transition to plastic yielding is predicted to occur at a specific aspect ratio for each fault, as observed. The new observation both shows a limit to one of the most commonly observed features of faults and suggests a change in the mode of failure of faults as a function of scale.

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Section: Observation Of the Minimum Scale Of Groovingmentioning

confidence: 99%

The minimum scale of grooving on faults

Candela

Brodsky

2016

Geology

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“…Rocks sheared at sub-seismic sliding velocities (<<1 mm/s) but temperatures spanning the entire range typical of frictionally generated temperatures (bulk estimates up to 1200˝C) typically had relatively large Byerlee's friction coefficient values (µ = 0.6) (e.g., [34]). Moreover, observations on natural and experimental faults reported grain size reduction up to the nanoscale (i.e., [35][36][37]) and solid state amorphization [38] which could not solely be explained by an increase in temperature. As a consequence, there must be a relation between temperature rise, grain size reduction, and slip-rate having profound implications for the mechanics of earthquakes.…”

Section: Introductionmentioning

confidence: 98%

Dislocation Motion and the Microphysics of Flash Heating and Weakening of Faults during Earthquakes

et al. 2016

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Abstract:Earthquakes are the result of slip along faults and are due to the decrease of rock frictional strength (dynamic weakening) with increasing slip and slip rate. Friction experiments simulating the abrupt accelerations (>>10 m/s 2 ), slip rates (~1 m/s), and normal stresses (>>10 MPa) expected at the passage of the earthquake rupture along the front of fault patches, measured large fault dynamic weakening for slip rates larger than a critical velocity of 0.01-0.1 m/s. The dynamic weakening corresponds to a decrease of the friction coefficient (defined as the ratio of shear stress vs. normal stress) up to 40%-50% after few millimetres of slip (flash weakening), almost independently of rock type. The microstructural evolution of the sliding interfaces with slip may yield hints on the microphysical processes responsible for flash weakening. At the microscopic scale, the frictional strength results from the interaction of micro-to nano-scale surface irregularities (asperities) which deform during fault sliding. During flash weakening, the visco-plastic and brittle work on the asperities results in abrupt frictional heating (flash heating) and grain size reduction associated with mechano-chemical reactions (e.g., decarbonation in CO 2 -bearing minerals such as calcite and dolomite; dehydration in water-bearing minerals such as clays, serpentine, etc.) and phase transitions (e.g., flash melting in silicate-bearing rocks). However, flash weakening is also associated with grain size reduction down to the nanoscale. Using focused ion beam scanning and transmission electron microscopy, we studied the micro-physical mechanisms associated with flash heating and nanograin formation in carbonate-bearing fault rocks. Experiments were conducted on pre-cut Carrara marble (99.9% calcite) cylinders using a rotary shear apparatus at conditions relevant to seismic rupture propagation. Flash heating and weakening in calcite-bearing rocks is associated with a shock-like stress release due to the migration of fast-moving dislocations and the conversion of their kinetic energy into heat. From a review of the current natural and experimental observations we speculate that this mechanism tested for calcite-bearing rocks, is a general mechanism operating during flash weakening (e.g., also precursory to flash melting in the case of silicate-bearing rocks) for all fault rock types undergoing fast slip acceleration due to the passage of the seismic rupture front.

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“…In the advanced stages of slip, the PSZ exhibits a densely stacked polygonal aggregate of small (few tens to hundreds of nanometres) crystal grains, with structure typical of grain-boundary sliding plasticity, a regime where superplastic behaviour (the capacity to accommodate finite plastic strain under high deformation rate) has been reported for ceramics. Such structures have been observed in experimental simulated faults after sliding at seismic slip velocity [8,16] but also on samples of natural faults from tectonic areas involving carbonate rocks [7,16].…”

Section: Challenging Observations (A) Dissipation: Is It Only Friction?mentioning

confidence: 59%

From slow to fast faulting: recent challenges in earthquake fault mechanics

Nielsen

2017

Phil. Trans. R. Soc. A.

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Faults—thin zones of highly localized shear deformation in the Earth—accommodate strain on a momentous range of dimensions (millimetres to hundreds of kilometres for major plate boundaries) and of time intervals (from fractions of seconds during earthquake slip, to years of slow, aseismic slip and millions of years of intermittent activity). Traditionally, brittle faults have been distinguished from shear zones which deform by crystal plasticity (e.g. mylonites). However such brittle/plastic distinction becomes blurred when considering (i) deep earthquakes that happen under conditions of pressure and temperature where minerals are clearly in the plastic deformation regime (a clue for seismologists over several decades) and (ii) the extreme dynamic stress drop occurring during seismic slip acceleration on faults, requiring efficient weakening mechanisms. High strain rates (more than 10 4 s −1 ) are accommodated within paper-thin layers (principal slip zone), where co-seismic frictional heating triggers non-brittle weakening mechanisms. In addition, (iii) pervasive off-fault damage is observed, introducing energy sinks which are not accounted for by traditional frictional models. These observations challenge our traditional understanding of friction (rate-and-state laws), anelastic deformation (creep and flow of crystalline materials) and the scientific consensus on fault operation. This article is part of the themed issue ‘Faulting, friction and weakening: from slow to fast motion’.

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Nanograins form carbonate fault mirrors

Cited by 161 publications

References 23 publications

The minimum scale of grooving on faults

The minimum scale of grooving on faults

Dislocation Motion and the Microphysics of Flash Heating and Weakening of Faults during Earthquakes

From slow to fast faulting: recent challenges in earthquake fault mechanics

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