On mylonites in ductile shear zones

White, S. H.; Burrows, S.E.; Carreras, Jordi; Shaw, Napier; Humphreys, F.J.

doi:10.1016/0191-8141(80)90048-6

Cited by 671 publications

(219 citation statements)

References 62 publications

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“…We, therefore, suggest that these mylonites formed mainly during D4 as a result of extreme deformation partitioning and high strain rates and to a lesser extent by repeated reactivation of earlier high-strain zones after D4. In other words, the locally higher strain rates and thereof resulting enhancement of strain softening processes caused relatively high-T mylonitisation more or less simultaneously with the last gneissose deformation phase D4 (White et al, 1980;Passchier & Trouw, 2005). In addition, there are some indications that an earlier (D3) mylonitisation phase may have taken place at ~1.83-1.82 Ga (Torvela et al, 2008).…”

Section: Discussionmentioning

confidence: 99%

Microstructures associated with the Sottunga-Jurmo shear zone and their implications for the 1.83-1.79 Ga tectonic development of SW Finland

Torvela¹,

Ehlers²

2010

Bull Geol Soc Finland

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This petrographic study of rock samples from the area of a large-scale shear zone, the Sottung-Jurmo shear zone, in SW Finland, illuminates the thermal development of and strain distribution within the rocks during the last development stages of the shear zone. The results indicate relatively high temperatures during the deformation phases that created the gneisses and the mylonites, strain partitioning through time and continued transpression from S-SW until at least c. 1.79 Ga. This has implications to the latest tectonic models since the results together with previous studies suggest a compartmentalisation of regional stresses between the area SW of the shear zone and central Fennoscandia. The results also suggest that the uplift rate increased during the late stage of the transpression.

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Section: Discussionmentioning

confidence: 99%

Microstructures associated with the Sottunga-Jurmo shear zone and their implications for the 1.83-1.79 Ga tectonic development of SW Finland

Torvela¹,

Ehlers²

2010

Bull Geol Soc Finland

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“…It could be argued that the reflectors do not follow isochronparallel layering but rather represent secondary shears, similar to shear bands [White et al, 1980], developed in the plastic lower crust near the spreading center [Ranero et al, 1997b]. Such shears might be discrete features with a spacing and length related to the thickness of the sheared plastic layer, and they might be reflective through a combination of crystal alignment (anisotropy) and the concentration of residual melt into the deforming region (e.g., giving fiaser gabbros).…”

Section: Ridgeward Dipping Reflections Inmentioning

confidence: 99%

The structure of Cretaceous oceanic crust of the NW Pacific: Constraints on processes at fast spreading centers

Reston

Ranero

Belykh³

1999

J. Geophys. Res.

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“…However, it is difficult to explain the development of the strong foliation observed in the mafic granulites, which could indicate the existence of a shear zone at these low strain rates. Assuming that a shear strain (γ ) of 5 is needed to form a steady-state mylonitic microstructure (e.g., White et al, 1980), a period of ∼ 300 Myr is required for the development of a strong foliation at a strain rate of 2.9 × 10 −16 s −1 . Such a timeframe for lower-crustal deformation is unrealistic for a ∼ 30 Myr time duration of the Pacific-North American plate boundary.…”

Section: Viscosity Structure Of a Strike-slip Plate Boundarymentioning

confidence: 99%

Constraints on the rheology of the lower crust in a strike-slip plate boundary: evidence from the San Quintín xenoliths, Baja California, Mexico

et al. 2017

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Abstract. The rheology of lower crust and its transient behavior in active strike-slip plate boundaries remain poorly understood. To address this issue, we analyzed a suite of granulite and lherzolite xenoliths from the upper Pleistocene–Holocene San Quintín volcanic field of northern Baja California, Mexico. The San Quintín volcanic field is located 20 km east of the Baja California shear zone, which accommodates the relative movement between the Pacific plate and Baja California microplate. The development of a strong foliation in both the mafic granulites and lherzolites, suggests that a lithospheric-scale shear zone exists beneath the San Quintín volcanic field. Combining microstructural observations, geothermometry, and phase equilibria modeling, we estimated that crystal-plastic deformation took place at temperatures of 750–890 °C and pressures of 400–560 MPa, corresponding to 15–22 km depth. A hot crustal geotherm of 40 ° C km−1 is required to explain the estimated deformation conditions. Infrared spectroscopy shows that plagioclase in the mafic granulites is relatively dry. Microstructures are interpreted to show that deformation in both the uppermost lower crust and upper mantle was accommodated by a combination of dislocation creep and grain-size-sensitive creep. Recrystallized grain size paleopiezometry yields low differential stresses of 12–33 and 17 MPa for plagioclase and olivine, respectively. The lower range of stresses (12–17 MPa) in the mafic granulite and lherzolite xenoliths is interpreted to be associated with transient deformation under decreasing stress conditions, following an event of stress increase. Using flow laws for dry plagioclase, we estimated a low viscosity of 1.1–1.3×1020 Pa ⋅ s for the high temperature conditions (890 °C) in the lower crust. Significantly lower viscosities in the range of 1016–1019 Pa ⋅ s, were estimated using flow laws for wet plagioclase. The shallow upper mantle has a low viscosity of 5.7×1019 Pa ⋅ s, which indicates the lack of an upper-mantle lid beneath northern Baja California. Our data show that during post-seismic transients, the upper mantle and the lower crust in the Pacific–Baja California plate boundary are characterized by similar and low differential stress. Transient viscosity of the lower crust is similar to the viscosity of the upper mantle.

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On mylonites in ductile shear zones

Cited by 671 publications

References 62 publications

Microstructures associated with the Sottunga-Jurmo shear zone and their implications for the 1.83-1.79 Ga tectonic development of SW Finland

Microstructures associated with the Sottunga-Jurmo shear zone and their implications for the 1.83-1.79 Ga tectonic development of SW Finland

The structure of Cretaceous oceanic crust of the NW Pacific: Constraints on processes at fast spreading centers

Constraints on the rheology of the lower crust in a strike-slip plate boundary: evidence from the San Quintín xenoliths, Baja California, Mexico

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