Fluid-rock reactions and resulting change in rheological behavior of a composite granitoid: the Archean Mooshla stock, Canada

The composite Mooshla stock displays clear evidence of variations in style and intensity of strain that are closely related to its internal lithological heterogeneity. Gabbro-diorite, quartz diorite, and tonalite rocks are weakly foliated and characterized by brittle and brittle-ductile small-scale shear zones, whereas leucotonalitic rocks are strongly foliated and transected by numerous wide and extensive ductile shear zones. Increasing degrees of penetrative deformation and marked changes of strain style in the pluton, from the more mafic rocks to the more felsic ones, are interpreted to reflect metamorphism-related rheological contrasts, rather than differences in the physical conditions of deformation. Metamorphism of the stock is characterized by an intensive hydration of the igneous rocks that has greatly enhanced their original heterogeneities. Petrographic, microstructural, and chemical studies show that the least deformed rocks are characterized by abundant albite-oligoclase (65-80%) with a matrix of minor quartz (5-10%) and actinolitic amphibole. The resistant plagioclase laths, although altered and replaced, form a stress-supporting framework that has protected the interstitial weak minerals, such as quartz, chlorite, and biotite, from deformation. However, the least deformed leucotonalites are characterized by low albite (35-45%) and high quartz contents (up to 65%). Extensive metamorphic hydration of these rocks produces quartz and phyllitic minerals that had enhanced significantly the ductility of the leucotonalites. Characterization of the chemical changes and the thermochemical conditions of the fluid, using microstructure and measurement of stable isotopes, indicates that fluid-rock interactions during metamorphism and syntectonic hydrothermal alteration have played an important role in creating the contrasting deformation of the composite granitoid.

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Reaction of Precambrian high-grade gneisses to mid-crustal ductile deformation in western Dove Bugt, North-East Greenland

Rapport Grønlands Geologiske Undersøgelse ◽

10.34194/rapggu.v162.8248 ◽

1994 ◽

Vol 162 ◽

pp. 53-70

Author(s):

B Chadwick ◽

C.R.L Friend

Keyword(s):

Shear Zones ◽

Ductile Deformation ◽

Small Scale ◽

High Grade ◽

East Greenland ◽

North West ◽

North East ◽

Basin Development ◽

Ductile Shear Zones ◽

Partial Melts

Mid-crustal deformation of an Early Proterozoic high-grade gneiss complex in western Dove Bugt gave rise to at least two sets of nappes. Structures in mylonites in low-angle ductile shear zones associated with the younger nappes indicate north-easterly-directed displacements. The nappes and mylonites are folded by upright to inclined folds that verge north-west and which appear to be associated with decollements that dip south-east. Hornblende, sillimanite and anatectic partial melts that developed with the nappes, mylonites and younger folds show that deformation took place under amphibolite facies conditions. Several lines of evidence suggest that the younger nappes, the mylonites and the upright to inclined folds formed during the Caledonian orogeny. Some pre-Caledonian deformation may be represented by the oldest isoclinal folds. Numerous, small-scale, ductile extensional shear zones and more brittIe fractures that were superimposed across the Caledonian structures are believed to have formed during orogen-parallel collapse which may be related IO Devonian basin development farther south in central East Greenland. Younger fauIts and major joints are correlated with Carboniferous, Mesozoic and Tertiary basin development in North-East Greenland.

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Characterizing a decametre-scale granitic reservoir using GPR and seismic methods – A case study for preparing hydraulic stimulations

10.5194/se-2020-40 ◽

2020 ◽

Author(s):

Joseph Doetsch ◽

Hannes Krietsch ◽

Cedric Schmelzbach ◽

Mohammadreza Jalali ◽

Valentin Samuel Gischig ◽

...

Keyword(s):

Rock Mass ◽

Seismic Anisotropy ◽

Shear Zones ◽

Velocity Model ◽

P Wave ◽

Fracture Density ◽

Ductile Shear Zones ◽

Ductile Shear ◽

Geophysical Characterization

Abstract. Ground-penetrating radar (GPR) and seismic imaging have proven to be important tools for the characterization of rock volumes. Both methods provide information about the physical rock mass properties and geology structures away from boreholes or tunnel walls. Here, we present the results from a geophysical characterization campaign that was conducted in preparation for a decametre-scale hydraulic stimulation experiment in the crystalline rock volume at the Grimsel Test Site (Central Switzerland). For this characterization experiment, we used tunnel based GPR reflection imaging as well as seismic traveltime tomography to investigate the volumes between several tunnels and boreholes. The interpretation of the GPR data with respect to geological structures is based on the unmigrated and migrated images. For the tomographic analysis of the seismic first-arrival traveltime data, we inverted for an anisotropic velocity model described by the Thomsen parameters v0, ϵ and δ to account for the rock mass foliation. Subsequently, the GPR and seismic images were interpreted in combination with the geological model of the test volume and the known in-situ stress states. We found that the ductile shear zones are clearly imaged by GPR and show an increase in seismic anisotropy due to a stronger foliation, while they are not visible in the P-wave (v0) velocity model. Regions of decreased seismic p-wave velocity, however, correlate with regions of high fracture density. For geophysical characterization of potential deep geothermal reservoirs, our results imply that wireline compatible borehole GPR should be considered for shear zone characterization, and that seismic anisotropy and velocity information are desirable to acquire in order to gain information about ductile shear zones and fracture density, respectively.

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Characterizing a decametre-scale granitic reservoir using ground-penetrating radar and seismic methods

Solid Earth ◽

10.5194/se-11-1441-2020 ◽

2020 ◽

Vol 11 (4) ◽

pp. 1441-1455 ◽

Cited By ~ 3

Author(s):

Joseph Doetsch ◽

Hannes Krietsch ◽

Cedric Schmelzbach ◽

Mohammadreza Jalali ◽

Valentin Gischig ◽

...

Keyword(s):

Ground Penetrating Radar ◽

Seismic Anisotropy ◽

Shear Zones ◽

Velocity Model ◽

P Wave ◽

Fracture Density ◽

Geological Structures ◽

Ductile Shear Zones ◽

Ground Penetrating

Abstract. Ground-penetrating radar (GPR) and seismic imaging have proven to be important tools for the characterization of rock volumes. Both methods provide information about the physical rock mass properties and geological structures away from boreholes or tunnel walls. Here, we present the results from a geophysical characterization campaign that was conducted as part of a decametre-scale hydraulic stimulation experiment in the crystalline rock volume of the Grimsel Test Site (central Switzerland). For this characterization experiment, we used tunnel-based GPR reflection imaging as well as seismic travel-time tomography to investigate the volumes between several tunnels and boreholes. The interpretation of the GPR data with respect to geological structures is based on the unmigrated and migrated images. For the tomographic analysis of the seismic first-arrival travel-time data, we inverted for an anisotropic velocity model described by the Thomsen parameters v0, ϵ and δ to account for the rock mass foliation. Subsequently, the GPR and seismic images were interpreted in combination with the geological model of the test volume and the known in situ stress states. We found that the ductile shear zones are clearly imaged by GPR and show an increase in seismic anisotropy due to a stronger foliation, while they are not visible in the p-wave (v0) velocity model. Regions of decreased seismic p-wave velocity, however, correlate with regions of high fracture density. For geophysical characterization of potential deep geothermal reservoirs, our results imply that wireline-compatible borehole GPR should be considered for shear zone characterization, and that seismic anisotropy and velocity information are desirable to acquire in order to gain information about ductile shear zones and fracture density, respectively.

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Small-scale ductile shear zones as transposed rheologically weak domains: A numerical modeling investigation and practical application

Journal of Structural Geology ◽

10.1016/j.jsg.2013.06.003 ◽

2013 ◽

Vol 54 ◽

pp. 184-198 ◽

Cited By ~ 5

Author(s):

Biwei Xiang ◽

Dazhi Jiang

Keyword(s):

Numerical Modeling ◽

Shear Zones ◽

Small Scale ◽

Practical Application ◽

Ductile Shear Zones ◽

Ductile Shear

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Kinematic evolution of metre-scale shear zones in foliated low-grade tectonites: an example from the Bousquet gold district, Quebec

Canadian Journal of Earth Sciences ◽

10.1139/e94-112 ◽

1994 ◽

Vol 31 (8) ◽

pp. 1301-1308 ◽

Cited By ~ 1

Author(s):

Ghislain Tourigny ◽

Francis Chartrand

Keyword(s):

Shear Bands ◽

Shear Zones ◽

Acute Angle ◽

Small Scale ◽

Low Grade ◽

Plan View ◽

Kinematic Indicators ◽

Kinematic Evolution ◽

Dextral Shear ◽

Scale Shear

Small-scale subvertical shear zones developed parallel to a regional preexisting S2 schistosity exhibit evidence of a complex shearing history recorded by conflicting kinematic indicators in both crosssection and plan view. The concordant schistosity internal to the shear zones contains a steeply plunging stretching lineation. Coexisting kinematic indicators of non-coaxial deformation parallel to this lineation are compatible with reverse dip-slip. This earliest shearing event was characterized by (1) the development of several shear discontinuities along selected preexisting S2 foliation surfaces, (2) subvertical transposition of both bedding and the oldest (S1) flat-lying foliation, and (3) by the emplacement of shear veins along the S2 foliation planes. The youngest shearing event reactivated the foliation-parallel shear discontinuities as dextral shear planes, thereby causing concomitant subhorizontal retransposition, east–west subhorizontal stretching, and emplacement of en echelon extension veins. A single set of shear bands occurring at a clockwise acute angle to the slipping foliation indicates that small-scale shear zones were transpressional during the late dextral shearing.

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Small-scale ductile shear zones: Neither extending, nor thickening, nor narrowing

Earth-Science Reviews ◽

10.1016/j.earscirev.2018.06.004 ◽

2018 ◽

Vol 184 ◽

pp. 1-12 ◽

Cited By ~ 16

Author(s):

G. Pennacchioni ◽

N.S. Mancktelow

Keyword(s):

Shear Zones ◽

Small Scale ◽

Ductile Shear Zones ◽

Ductile Shear

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High-Resolution Optical Observations of Interstellar Absorption Lines

Highlights of Astronomy ◽

10.1017/s1539299600017196 ◽

2005 ◽

Vol 13 ◽

pp. 808-810

Author(s):

Daniel E. Welty

Keyword(s):

High Resolution ◽

Isotopic Ratio ◽

Optical Observations ◽

Small Scale ◽

Molecular Gas ◽

Interstellar Clouds ◽

Physical Conditions ◽

Molecular Abundances

AbstractWe briefly note several current topics concerning the properties of interstellar clouds for which high-resolution optical spectra play a significant role: (1) the recognition and characterization of small-scale (sub-pc) structure in both atomic and molecular gas; (2) the discovery of variations in the 7Li/6Li isotopic ratio in the nearby Galactic ISM; (3) the determination of atomic and molecular abundances and physical conditions for heavily reddened (“translucent”) Galactic sightlines; and (4) studies of interstellar clouds in the LMC and SMC.

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