Retrogression of eclogite-facies shear zones by short-lived fluid infiltration during the Caledonian orogeny, Lofoten islands, Norway

Fournier, Herbert; Lee, J. K. W.; Camacho, Alfredo; Creaser, Robert A.

doi:10.1144/sp390.9

Cited by 8 publications

(33 citation statements)

References 75 publications

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“…They show that the gneisses are of Gothian (1680-1640 Ma) age, but underwent local granulite-facies metamorphism at 1100 Ma before reacting, again only locally, during the Caledonian UHP eclogite event at about 400 Ma. Fournier et al (2013) used the Ar-Ar and Rb -Sr systems in an effort to date eclogitefacies shear zones in Lofoten (Fig. 1).…”

Section: New Results On Other Caledonian High-pressure Rocksmentioning

confidence: 99%

New perspectives on the Caledonides of Scandinavia and related areas: introduction

Corfú

Gasser

Chew

2014

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Section: New Results On Other Caledonian High-pressure Rocksmentioning

confidence: 99%

New perspectives on the Caledonides of Scandinavia and related areas: introduction

Corfú

Gasser

Chew

2014

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“…P‐T conditions of pseudotachylyte formation and mylonitization were estimated at 650°C–750°C and 0.7–0.8 GPa, corresponding to an approximate depth range of 24–30 km (Menegon et al, ). 40 Ar‐ 39 Ar dating of amphibole from localized amphibolite facies shear zones in the Nusfjord area with similar orientation to the Nusfjord east network yielded an age range of 433–413 Ma (Fournier et al, ; Steltenpohl et al, ). Given the similarity of structures, mineral assemblages and metamorphic conditions, this age range is also taken as representative of the Nusfjord east shear zone network described here.…”

Section: Geological Setting and Sample Descriptionmentioning

confidence: 99%

Transient High Strain Rate During Localized Viscous Creep in the Dry Lower Continental Crust (Lofoten, Norway)

Campbell

Menegon

2019

JGR Solid Earth

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Understanding the ability of the lower crust to support transient changes in stresses and strain rates during the earthquake cycle requires a detailed investigation of the deformation mechanisms and rheology of deep crustal fault rocks. Here, we show that lower crustal pseudotachylyte-bearing shear zones are able to accommodate short-term episodes of high strain rate and high stress deformation by accelerated viscous creep, followed by a reduction in stresses to some ambient deformation condition. Quartz microstructure within pseudotachylyte-bearing shear zones in otherwise undeformed granulites from Lofoten, Norway, indicates that dynamic recrystallization occurred during viscous creep under rapid strain rates and high stresses of~10 −9 s −1 and~100 MPa, respectively. Lower stress microstructures (i.e., foam textures) are also recorded in the shear zones, indicating spatial and temporal variations of stress and strain rate during deformation cycles. Both the high and lower stress quartz recrystallization took place under granulite facies conditions of 650°C-750°C and 0.7-0.8 GPa and represented a record of highly localized viscous creep within the lower crust. This implies that lower crustal pseudotachylytes are potentially able to form extremely localized weak zones within strong lower crust, enabling a deep mechanical response to perturbations in stress and strain rate such as those experienced during the seismic cycle, for example, seismogenic loading followed by subsequent postseismic relaxation.Plain Language Summary Detailed investigation of the strength and deformation style of fault rocks sourced from the Earth's lower crust is important to understand how the lower crust reacts to shortterm variations in stress and strain rate, which can occur, for example, between earthquakes. Here, we show that solidified pseudotachylytes (initially melts produced due to frictional heating along the fault plane during an earthquake) occurring at depths of 25-30 km in the lower crust can accommodate deformation at particularly high strain rates and high stresses via solid-state creep. We look at pseudotachylytes formed in lower crustal shear zones that are now exhumed in Lofoten, Norway. Deformation microstructures in quartz within these pseudotachylytes have recorded rapid strain rates and high stresses. These microstructures are occasionally transformed into lower stress versions, indicating that during the deformation the stress and strain rate varied through both time and space. Both stages, however, record the same deformation temperatures and pressures, indicating that these are snapshots of ongoing deformation within the lower crust. We conclude that, when the lower crust is strong, pseudotachylytes will form important weak zones that accommodate deformation even during rapid variations in the deformation conditions, for example, as occurs during the postseismic period immediately after an earthquake.

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“…Hacker, Kelemen, & Behn, ), models of shear zone evolution in mafic rocks offer important constraints on deformation within lithologies that are potentially volumetrically dominant at deeper levels. Examples of shear zone studies in mafic rocks include eclogite (Austrheim, ; Austrheim & Griffin, ; Fournier, Lee, Camacho, & Creaser, ), amphibolite (Getsinger et al., ) and anorthosite (Okudaira, Shigematsu, Harigane, & Yoshida, ).…”

Section: Introductionmentioning

confidence: 99%

Fracturing, fluid flow and shear zone development: Relationships between chemical and mechanical processes in Proterozoic mafic dykes from southwestern Montana, USA

Condit

Mahan

2017

Journal Metamorphic Geology

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Fluids can play an important role in the localization of deformation in the deep crust, yet the specific mechanisms active during the complex interactions between metasomatism, metamorphism and deformation remain elusive. Precambrian metagabbronorite dykes in southwest Montana contain fractures filled with Hbl±Grt and discrete cm‐scale shear zones with well‐preserved strain gradients. This system offers an ideal opportunity to constrain the chemical and mechanical processes that facilitated strain localization. An early M1 assemblage of Grt1+Cpx1+Pl1+Qz developed at conditions of 0.51–0.85 GPa and 500–700°C and is preserved largely as a static replacement of relict igneous phases (Opx, Pgt, Pl) in coronitic textures. An M2 assemblage characterized by Grt2+Pl2±Cpx2+Hbl+Scp+Qz developed at 0.86–1.00 GPa and 660–730°C coincided with fluid flow and deformation associated with shear zone development. Microstructural observations in marginal protomylonite/mylonite and laminated ultramylonite suggest a shear zone evolution that involved (1) nucleation from pre‐existing fractures that were sites for major fluid infiltration, (2) initial widening coincident with grain‐size reduction by microfracturing, dislocation creep, and synkinematic metamorphic reaction by solution transfer, and (3) a switch in the dominant deformation mechanisms active in the ultramylonite from grain‐size insensitive mechanisms to grain‐size sensitive granular flow accommodated by fluid‐assisted diffusion. Throughout this evolution, the effective bulk compositions of the rock volumes responding to metamorphism changed through a combination of mechanical and metasomatic processes.

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Retrogression of eclogite-facies shear zones by short-lived fluid infiltration during the Caledonian orogeny, Lofoten islands, Norway

Cited by 8 publications

References 75 publications

New perspectives on the Caledonides of Scandinavia and related areas: introduction

New perspectives on the Caledonides of Scandinavia and related areas: introduction

Transient High Strain Rate During Localized Viscous Creep in the Dry Lower Continental Crust (Lofoten, Norway)

Fracturing, fluid flow and shear zone development: Relationships between chemical and mechanical processes in Proterozoic mafic dykes from southwestern Montana, USA

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