Kinks in mica: Role of dislocations and (001) cleavage

Bell, Ian A.; Wilson, Christopher J.L.; McLaren, A. C.; Etheridge, M. A.

doi:10.1016/0040-1951(86)90078-8

Cited by 67 publications

(39 citation statements)

References 12 publications

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“…8c, and the latter is inferred where there is no evidence for loss of continuity across grain or subgrain boundaries. Formation of intraclast MKBs is interpreted to have occurred mainly by dislocation glide and climb (Bell et al 1986;Mares and Kronenberg 1993). Microstructures of muscovite in these naturally deformed, polyphase rocks resemble closely the microstructures of experimentally deformed rocks with varying mica contents (Shea and Kronenberg 1993).…”

Section: Feldsparmentioning

confidence: 93%

Textural and microstructural evidence for semi-brittle flow in natural fault rocks with varied mica contents

White

2001

Int J Earth Sci

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Fault rocks from the Siberia Fault Zone (SFZ) in southern New Zealand are derived from schists with varied mica contents. Regional evidence indicates that the rocks are exhumed from depths of 8±10 km and temperatures of 200±250 C. Foliated cataclasites in a zone 5±40 m thick are accompanied by interlaced pseudotachylyte, and are cut through by a late-stage master fault and zones of random fabric cataclasite. Textures and microstructures in the foliated cataclasites reflect contemporaneous or cyclic operation of cataclastic, crystal±plastic and solution transfer deformation mechanisms, partitioned differently between different phases. The deformation regime is interpreted as a form of semi-brittle flow, facilitated by crystal±plastic deformation of phyllosilicate phases in a relatively weak interconnected matrix. Quartz and feldspar are deformed mainly by cataclasis. The presence of pseudotachylyte indicates the fault was seismically active, and non-localised semi-brittle flow was episodically punctuated by high strain-rate earthquake events. Late-stage formation of a discrete master fault probably reflects a change from semi-brittle flow to brittle faulting. The presently exposed level of the fault is thought to represent a section of the mid-crustal brittle±ductile transition in the seismogenic zone. Thus, this study provides a tangible natural example of theoretically and experimentally predicted fault rocks.

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Section: Feldsparmentioning

confidence: 93%

Textural and microstructural evidence for semi-brittle flow in natural fault rocks with varied mica contents

White

2001

Int J Earth Sci

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“…Since climb of dislocations can probably be ruled out at the conditions of our experiments, these arrays presumably form by glide. Regularly spaced walls commonly parallel well-defined kink band boundaries and may act as nuclei for KBBs, as proposed by Bell et al [1986]. Walls appear to be stable structures whose positions are fixed by local stress distributions arising from neighboring dislocations.…”

Section: The Relationship Between Shear Zone Width and Axial Strain Imentioning

confidence: 94%

Rheology and deformation mechanisms of an isotropic mica schist

Shea¹,

Kronenberg²

1992

J. Geophys. Res.

137

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We have investigated the transitional, semibrittle deformation of a mica schist (∼75 % biotite) by shortening cylinders cored at 0°, 45°, and 90° to foliation to varying strains, at confining pressures Pc to 500 MPa, constant strain rates ε from 1.5 × 10−7 to 1.6 × 10−4 s−1 and temperatures T from 25° to 400°C. Deformation is concentrated within one or more throughgoing, millimeter‐wide shear zones at all conditions; these localize at low strains (ε < 2%) through the nucleation and coalescence of dense sets of intragranular microkink bands (MKBs). Despite distinct differences in the relative number of mica grains oriented favorably for slip and kinking in different loading directions, the differential stresses required for shear zone development vary little with fabric orientation. Biotite schist undergoes a transition from strain‐softening to steady strength mechanical response at confining pressures in the range 75 to 150 MPa. The pressure sensitivity of strength (characterized by the slope μ of the Mohr envelope) decreases from μ ∼0.5 (at Pc <100 MPa) to μ < 0.1 at pressures greater than 200 MPa, reflecting the increasing contribution of glide and kinking in biotite at higher pressures. However, dilatancy associated with microcracking and void formation along MKB boundaries persists to at least 500 MPa. Within the pressure‐insensitive regime (200 ≤ Pc ≤ 500 MPa), temperature and strain rate dependencies of strength determined in stepping tests reveal a strong history dependence to flow that cannot strictly be described by a steady state constitutive law. Samples deformed in steps from low to high temperatures or fast to slow strain rates consistently exhibit stronger temperature and strain rate sensitivities than those deformed along T decreasing or ε increasing paths. Path‐dependent effects may reflect differences in the degree to which inherited dislocation substructures are utilized or overprinted during later deformation increments. By assuming an exponential relationship between differential stress σd and strain rate ε of the form ε = C exp(ασd) exp (−Q/RT), we fit the data with two end‐member flow laws with a single activation energy Q = 89 kJ/mol, and exponential constants αss = 0.15 ± 0.01 MPa−1 and αws=0.55 ± 0.04 MPa−1 that account for the different responses observed along stepping paths that are strongly sensitive or weakly sensitive to T and ε, respectively. Application of the results to crustal deformation suggests that mica‐rich aggregates are weaker than other common rock types throughout a broad midcrustal depth range, supporting the inference that retrograde reaction to phyllosilicates may be important in localizing crustal deformations within large faults and shear zones.

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“…Understanding how the mechanical properties of rock-forming minerals depend on P-T-X conditions is fundamental to constraining the rheological behavior and mechanical stratification of the Earth's crust. We have invoked basic crystal chemistry to resolve conflicting strength-composition relationships observed within a very important mineral family in the crust [compare Wilson and Bell, 1979;Bell et al, 1986;Kronenberg et al, 1990;Mares and Kronenberg, 1993]. In so doing, we have provided a simple framework for understanding and predicting mica strengths based upon composition.…”

Section: Discussionmentioning

confidence: 99%

“…Paper number 96JB00326. 0148-0227/96/96JB-00326509.00 muscovite to deformation [Wilson and Bell, 1979;Bell et al, 1986], as well as to fluid-mediated dissolution/neocrystallization, and solid-state recrystallization. Also, in many mylonite-bearing fault zones, muscovitic "mica fish" are preserved whereas accompanying biotite is highly deformed or recrystallized (C.J.L.…”

Section: Introductionmentioning

confidence: 99%

Influence of F(OH)₋₁ substitution on the relative mechanical strength of rock‐forming micas

Dahl

Dorais

1996

J. Geophys. Res.

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Microtextural and experimental studies have yielded conflicting data on the relative mechanical strengths of muscovite and biotite [Wilson and Bell, 1979; Kronenberg et al., 1990; Mares and Kronenberg, 1993]. We propose a crystal‐chemical resolution to this conflict, namely, that (001) dislocation glide in biotite is rate‐limited by its fluorine content. Significant F(OH)−1 substitution, and concomitant removal of hydroxyl H+ directed into the interlayer cavity, potentially increases mechanical strength of biotite in two ways: (1) it eliminates K+‐H+ repulsion, thereby strengthening the interlayer bonds, and (2) it allows K+ to “sink” deeper into the interlayer cavity, the resultant geometry being less favorable to basal slip. In testing this hypothesis we analyzed the naturally deformed biotite studied by Wilson and Bell [1979] and documented its very low F content (XF ≤ 0.02) compared to that of the biotite experimentally deformed by Kronenberg et al. [1990]. Our model and the comparative XF data explain why the biotite of Wilson and Bell [1979] deformed more easily in nature than its coexisting muscovite, whereas the biotite of Kronenberg et al. [1990] was mechanically stronger than muscovite similarly deformed by Mares and Kronenberg [1993]. Our reconciliation of these otherwise conflicting results provides a framework for predicting mechanical strength of natural micas based upon the extent of their F(OH)−1 substitution. Our synthesis highlights the potential role of crystal chemistry in determining mechanical behavior in multicomponent mineral families. Further testing of crystal‐chemical effects on rheology will require mineral specimens of both appropriate composition and sufficient size.

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Kinks in mica: Role of dislocations and (001) cleavage

Cited by 67 publications

References 12 publications

Textural and microstructural evidence for semi-brittle flow in natural fault rocks with varied mica contents

Textural and microstructural evidence for semi-brittle flow in natural fault rocks with varied mica contents

Rheology and deformation mechanisms of an isotropic mica schist

Influence of F(OH)₋₁ substitution on the relative mechanical strength of rock‐forming micas

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Kinks in mica: Role of dislocations and (001) cleavage

Cited by 67 publications

References 12 publications

Textural and microstructural evidence for semi-brittle flow in natural fault rocks with varied mica contents

Textural and microstructural evidence for semi-brittle flow in natural fault rocks with varied mica contents

Rheology and deformation mechanisms of an isotropic mica schist

Influence of F(OH)−1 substitution on the relative mechanical strength of rock‐forming micas

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Product

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Influence of F(OH)₋₁ substitution on the relative mechanical strength of rock‐forming micas