Seismic anisotropy in metamorphic rocks from the COSC-1 borehole, Sweden: A cross-scale investigation from thin section analysis to seismic scales

Metamorphic and deformed rocks in thrust zones show particularly high seismic anisotropy causing challenges for seismic imaging and interpretation. A good example is the Seve Nappe Complex in J&#228;mtland, Sweden, an exhumed orogenic thrust zone characterized by a strong but incoherent seismic reflectivity and considerable seismic anisotropy. However, only little is known about the origin of the anisotropy in relation to composition, structural influences, and implications for measurements at different seismic scales. We present an integrative study of the seismic anisotropy at different scales combining mineralogical composition, microstructural analyses and seismic laboratory experiments from samples of the 2.5 km-deep COSC-1 borehole. While there is a pronounced crystallographic preferred orientation in most of the core samples, variations in anisotropy correlate strongly with bulk mineral composition and dominant core lithology. Based on three major lithologic different facies (felsic gneiss, amphibole-rich rocks, and mica schists), we propose an anisotropy model for the full length of the borehole, which indicates two prevailing anisotropic units. Comparison of laboratory seismic measurements and electron-backscatter diffraction (EBSD) data reveals a strong scale-dependence, which is more pronounced in the highly deformed, heterogeneous samples. This highlights the need for comprehensive cross-validation of microscale anisotropy analyses with additional lithological data when integrating seismic anisotropy through seismic scales.

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Correlation of core and downhole seismic velocities in high-pressure metamorphic rocks: A case study for the COSC-1 borehole, Sweden

10.5194/egusphere-egu2020-13955 ◽

2020 ◽

Author(s):

Felix Kästner ◽

Simona Pierdominici ◽

Judith Elger ◽

Christian Berndt ◽

Alba Zappone ◽

...

Keyword(s):

Seismic Anisotropy ◽

Seismic Velocity ◽

Metamorphic Rocks ◽

Seismic Velocities ◽

Mafic Rocks ◽

Seismic Properties ◽

The Core ◽

Nappe Complex ◽

Thrust Zone ◽

Mountain Belts

Deeply rooted thrust zones are key features of tectonic processes and the evolution of mountain belts. Exhumed and deeply-eroded orogens like the Scandinavian Caledonides allow to study such systems from the surface. Previous seismic investigations of the Seve Nappe Complex have shown indications for a strong but discontinuous reflectivity of this thrust zone, which is only poorly understood. The correlation of seismic properties measured on borehole cores with surface seismic data can help to constrain the origin of this reflectivity. In this study, we compare seismic velocities measured on cores to in situ velocities measured in the borehole. The core and downhole velocities deviate by up to 2 km/s. However, velocities of mafic rocks are generally in close agreement. Seismic anisotropy increases from about 5 to 26 % at depth, indicating a transition from gneissic to schistose foliation. Differences in the core and downhole velocities are most likely the result of microcracks due to depressurization of the cores. Thus, seismic velocity can help to identify mafic rocks on different scales whereas the velocity signature of other lithologies is obscured in core-derived velocities. Metamorphic foliation on the other hand has a clear expression in seismic anisotropy. To further constrain the effects of mineral composition, microstructure and deformation on the measured seismic anisotropy, we conducted additional microscopic investigations on selected core samples. These analyses using electron-based microscopy and X-ray powder diffractometry indicate that the anisotropy is strongest for mica schists followed by amphibole-rich units. This also emphasizes that seismic velocity and anisotropy are of complementary importance to better distinguish the present lithological units. Our results will aid in the evaluation of core-derived seismic properties of high-grade metamorphic rocks at the COSC-1 borehole and elsewhere.

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Correlation of core and downhole seismic velocities in high-pressure metamorphic rocks: a case study for the COSC-1 borehole, Sweden

Solid Earth ◽

10.5194/se-11-607-2020 ◽

2020 ◽

Vol 11 (2) ◽

pp. 607-626

Author(s):

Felix Kästner ◽

Simona Pierdominici ◽

Judith Elger ◽

Alba Zappone ◽

Jochem Kück ◽

...

Keyword(s):

Seismic Anisotropy ◽

Metamorphic Rocks ◽

Seismic Velocities ◽

Seismic Properties ◽

The Core ◽

Nappe Complex ◽

Thrust Zone ◽

Seve Nappe Complex ◽

Mountain Belts

Abstract. Deeply rooted thrust zones are key features of tectonic processes and the evolution of mountain belts. Exhumed and deeply eroded orogens like the Scandinavian Caledonides allow us to study such systems from the surface. Previous seismic investigations of the Seve Nappe Complex have shown indications of a strong but discontinuous reflectivity of this thrust zone, which is only poorly understood. The correlation of seismic properties measured on borehole cores with surface seismic data can constrain the origin of this reflectivity. To this end, we compare seismic velocities measured on cores to in situ velocities measured in the borehole. For some intervals of the COSC-1 borehole, the core and downhole velocities deviate by up to 2 km s−1. These differences in the core and downhole velocities are most likely the result of microcracks mainly due to depressurization. However, the core and downhole velocities of the intervals with mafic rocks are generally in close agreement. Seismic anisotropy measured in laboratory samples increases from about 5 % to 26 % at depth, correlating with a transition from gneissic to schistose foliation. Thus, metamorphic foliation has a clear expression in seismic anisotropy. These results will aid in the evaluation of core-derived seismic properties of high-grade metamorphic rocks at the COSC-1 borehole and elsewhere.

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Correlation of core and downhole seismic velocities in high-pressure metamorphic rocks: A case study for the COSC-1 borehole, Sweden

10.5194/se-2019-161 ◽

2019 ◽

Author(s):

Felix Kästner ◽

Simona Pierdominici ◽

Judith Elger ◽

Alba Zappone ◽

Jochem Kück ◽

...

Keyword(s):

Seismic Anisotropy ◽

Seismic Velocity ◽

Metamorphic Rocks ◽

Seismic Velocities ◽

Mafic Rocks ◽

Seismic Properties ◽

The Core ◽

Nappe Complex ◽

Thrust Zone ◽

Mountain Belts

Abstract. Deeply rooted thrust zones are key features of tectonic processes and the evolution of mountain belts. Exhumed and deeply-eroded orogens like the Scandinavian Caledonides allow to study such systems from the surface. Previous seismic investigations of the Seve Nappe Complex have shown indications for a strong but discontinuous reflectivity of this thrust zone, which is only poorly understood. The correlation of seismic properties measured on borehole cores with surface seismic data constrains the origin of this reflectivity. In this study, we compare seismic velocities measured on cores to in situ velocities measured in the borehole. The core and downhole velocities deviate by up to 2 km/s. However, velocities of mafic rocks are generally in close agreement. Seismic anisotropy increases from about 5 to 26 % at depth, indicating a transition from gneissic to schistose foliation. We suggest that differences in the core and downhole velocities are most likely the result of microcracks mainly due to depressurization. Thus, seismic velocity can help to identify mafic rocks on different scales whereas the velocity signature of other lithologies is obscured in core-derived velocities. Metamorphic foliation on the other hand has a clear expression in seismic anisotropy. These results will aid in the evaluation of core-derived seismic properties of high-grade metamorphic rocks at the COSC-1 borehole and elsewhere. In particular, they show that core log seismic integration via synthetic seismograms requires wireline logging data in any but mafic lithologies.

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Kinematic and tectonic significance of microstructures and crystallographic fabrics within quartz mylonites from the Assynt and Eriboll regions of the Moine thrust zone, NW Scotland

Transactions of the Royal Society of Edinburgh Earth Sciences ◽

10.1017/s0263593300010774 ◽

1986 ◽

Vol 77 (2) ◽

pp. 99-125 ◽

Cited By ~ 98

Author(s):

R. D. Law ◽

M. Casey ◽

R. J. Knipe

Keyword(s):

Strain Path ◽

Extensional Flow ◽

Crystallographic Preferred Orientation ◽

X Ray ◽

Tectonic Significance ◽

Thrust Zone ◽

Thrust Sheets ◽

Strain Paths ◽

Tectonic Loading ◽

Microstructural Studies

ABSTRACTUsing a combination of optical microscopy and X-ray texture goniometry, an integrated microstructural and crystallographic fabric study has been made of quartz mylonites from thrust sheets located beneath, but immediately adjacent to, the Moine thrust in the Assynt and Eriboll regions of NW Scotland. A correlation is established between shape fabric symmetry and pattern of crystallographic preferred orientation, a particularly clear relationship being observed between shape fabric variation and quartza-axis fabrics.Coaxial strain paths dominate the internal parts of the thrust sheets and are indicated by quartzc- anda-axis fabrics which are symmetrical with respect to foliation and lineation. Non-coaxial strain paths are indicated within the more intensely deformed quartzites located near the boundaries of the sheets by asymmetricalc- anda-axis fabrics. These kinematic interpretations are supported by microstructural studies. At the Stack of Glencoul in the northern part of the Assynt region, the transition zone between these kinematic (strain path) domains is located at approximately 20 cm beneath the Moine thrust and is marked by a progression from symmetrical cross-girdlec-axis fabrics (30cm beneath the thrust), through asymmetrical cross-girdlec-axis fabrics to asymmetrical single girdlec-axis fabrics (0·5 cm beneath the thrust).Tectonic models (incorporating processes such as extensional flow, gravity spreading and tectonic loading) which may account for the presence of strain path domains within the thrust sheets are considered, and their compatibility with local thrust sheet geometries assessed.

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Quantifying Intrinsic and Extrinsic Contributions to Elastic Anisotropy Observed in Seismic Tomography Models

10.5194/egusphere-egu21-507 ◽

2021 ◽

Author(s):

John Keith Magali ◽

Thomas Bodin ◽

Navid Hedjazian ◽

Yanick Ricard ◽

Yann Capdeville

Keyword(s):

Seismic Tomography ◽

Large Scale ◽

Seismic Anisotropy ◽

Texture Evolution ◽

Scaling Law ◽

Preferred Orientation ◽

Elastic Model ◽

Effective Anisotropy ◽

Crystallographic Preferred Orientation ◽

Radial Anisotropy

Large-scale seismic anisotropy inferred from seismic observations has been loosely interpreted either in terms of intrinsic anisotropy due to Crystallographic Preferred Orientation (CPO) development of mantle minerals or extrinsic anisotropy due to rock-scale Shape Preferred Orientation (SPO). The coexistence of both contributions misconstrues the origins of seismic anisotropy observed in seismic tomography models. It is thus essential to discriminate CPO from SPO in the effective anisotropy of an upscaled/homogenized medium, that is, the best possible elastic model recovered using finite-frequency seismic data assuming perfect data coverage. In this work, we investigate the effects of upscaling an intrinsically-anisotropic and highly-heterogeneous Earth's mantle. The problem is applied to a 2-D marble cake model of the mantle with a binary composition in the presence of CPO obtained from a micro-mechanical model. We compute the long-wavelength effective equivalent of this mantle model using the 3D non-periodic elastic homogenization technique. Our numerical findings predict that overall, upscaling purely intrinsically anisotropic medium amounts to the convection-scale averaging of CPO. As a result, it always underestimates the anisotropy, and may only be overestimated due to the additive extrinsic anisotropy from SPO. Finally, we show analytically (in 1D) and numerically (in 2D) that the full effective radial anisotropy &#958;* is approximately just the product of the effective intrinsic radial anisotropy &#958;*CPO and the extrinsic radial anisotropy &#958;SPO:&#958;*&#160;= &#958;*CPO &#215; &#958;SPOBased on the above relation, it is imperative to homogenize a texture evolution model first before drawing interpretations from existing anisotropic tomography models. Such a scaling law can therefore be used as a constraint to better estimate the separate contributions of CPO and SPO from the effective anisotropy observed in tomographic models.

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Seismic anisotropy reveals crustal flow driven by mantle vertical loading in the Pacific NW

Science Advances ◽

10.1126/sciadv.abb0476 ◽

2020 ◽

Vol 6 (28) ◽

pp. eabb0476

Author(s):

Jorge C. Castellanos ◽

Jonathan Perry-Houts ◽

Robert W. Clayton ◽

YoungHee Kim ◽

A. Christian Stanciu ◽

...

Keyword(s):

Pacific Northwest ◽

Upper Mantle ◽

Seismic Anisotropy ◽

Azimuthal Anisotropy ◽

Mantle Flow ◽

Near Surface ◽

Vertical Loading ◽

The Pacific ◽

Anisotropy Model ◽

Lithosphere Dynamics

Buoyancy anomalies within Earth’s mantle create large convective currents that are thought to control the evolution of the lithosphere. While tectonic plate motions provide evidence for this relation, the mechanism by which mantle processes influence near-surface tectonics remains elusive. Here, we present an azimuthal anisotropy model for the Pacific Northwest crust that strongly correlates with high-velocity structures in the underlying mantle but shows no association with the regional mantle flow field. We suggest that the crustal anisotropy is decoupled from horizontal basal tractions and, instead, created by upper mantle vertical loading, which generates pressure gradients that drive channelized flow in the mid-lower crust. We then demonstrate the interplay between mantle heterogeneities and lithosphere dynamics by predicting the viscous crustal flow that is driven by local buoyancy sources within the upper mantle. Our findings reveal how mantle vertical load distribution can actively control crustal deformation on a scale of several hundred kilometers.

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Inherited Fabric in an Omphacite Symplectite: Reconstruction of Plastic Deformation under Ultra-High Pressure Conditions

Microscopy and Microanalysis ◽

10.1017/s1431927613001451 ◽

2013 ◽

Vol 19 (4) ◽

pp. 942-949 ◽

Cited By ~ 3

Author(s):

Florian Heidelbach ◽

Michael P. Terry

Keyword(s):

Orientation Relationship ◽

Electron Backscatter Diffraction ◽

Pure Shear ◽

Additional Component ◽

Western Gneiss Region ◽

Crystallographic Preferred Orientation ◽

Ultra High Pressure ◽

Shear Component ◽

Electron Backscatter ◽

Backscatter Diffraction

AbstractWe investigated an eclogitic gneiss from the Western Gneiss Region in Norway, which underwent subduction as part of Baltica lithosphere beneath Laurentia during the Scandian orogeny. Petrologic data indicate that the eclogite was deformed plastically at about 4 GPa and 800°C producing a strong macroscopic foliation and lineation. Whereas garnet remained largely stable during the retrograde uplift, omphacite was transformed statically into a symplectite consisting of lamellar diopside and plagioclase with more equant grains of hornblende and orthopyroxene. Measurements of the crystallographic preferred orientation with electron backscatter diffraction show that diopside and hornblende, as well as orthopyroxene, have a systematic orientation relationship with the macroscopic fabric, as well as the (presumed) orientation of the host omphacite. The orientation relationship between the chain silicates is very sharp with the crystallographic forms {100}, {010}, and ⟨001⟩ being parallel. Their bulk texture shows a maximum of ⟨001⟩ parallel to the lineation and girdles of {010} and {110} perpendicular to the lineation with maxima subparallel to the foliation corresponding to an L-type texture of the original omphacite and indicating constrictional strain with an additional component of pure shear/simple shear component.

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