scholarly journals The Hercynian collision in the Armorican Massif : evidence of different lithospheric domains inferred from seismic tomography and anisotropy

2003 ◽  
Vol 174 (1) ◽  
pp. 45-57 ◽  
Author(s):  
Sébastien Judenherc ◽  
Michel Granet ◽  
Jean-Pierre Brun ◽  
Georges Poupinet
Keyword(s):  
Shear Zone ◽  
Seismic Anisotropy ◽  
Shear Zones ◽  
Velocity Model ◽  
P Wave ◽  
Lithospheric Deformation ◽  
Thermal Anomalies ◽  
The North ◽  
Chemical Anomalies ◽  
Armorican Massif

Abstract The Hercynian belt is a continental collision orogen extending from south-west Iberia to the Bohemian Massif in Czech Republic. The successive stages of its formation are dated from 400 to 260 Ma.The Armorican Massif is a preserved segment of this orogen. It presents structures oriented NW-SE, parallel to the general Hercynian trend in this region. The massif is divided into three domains (North, Central, and South-Armorican domains) separated by two main shearzones, the North- and South-Armorican shear zones. As the Armorican Massif escaped from any important tectonic or thermal event since the end of Hercynian times, it is particularly suited for the study of an old collision orogen. Thus, in the framework of the GéoFrance3D-ARMOR2 project, two passive seismological experiments were conducted in 1997 and 1999 in the Armorican Massif. The main goals concerned the characterization of the deep geometry of both shear zones, the understanding of their geodynamic bearing on the long term evolution of the Hercynian belt, the study of the lithospheric deformation, and the 3D imaging of the Champtoceaux nappes.The data allow to model seismic anisotropy and to build a 3D P-wave velocity model beneath the Armorican Massif. Crustal images do not evidence any deep rooting of the Champtoceaux nappes in the lower crust. However, the upper mantle images show a clear signal interpreted as the relic of the northward subduction which lasted until Devonian (≈350 Ma). The results also show that the North-Armorican Shear Zone is limited at depth to the crust and topmost mantle, while the South-Armorican Shear Zone can be traced over the whole lithosphere.The strong velocity contrasts are associated to probable relic thermal anomalies but are also significantly related to chemical anomalies.

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

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.

Des images du systeme lithosphere-asthenosphere sous la France et leurs implications geodynamiques; l'apport de la tomographie telesismique et de l'anisotropie sismique

2000 ◽  
Vol 171 (2) ◽  
pp. 149-167 ◽  
Author(s):  
Michel Granet ◽  
Sebastien Judenherc ◽  
Annie Souriau
Keyword(s):  
Seismic Anisotropy ◽  
Thermal Anomaly ◽  
Rift System ◽  
Low Velocity ◽  
Geological Structures ◽  
Tectonic Event ◽  
Lithospheric Deformation ◽  
Massif Central ◽  
The North ◽  
Armorican Massif

Abstract From seismic tomography and seismic anisotropy, images of the lithosphere-asthenosphere system beneath France for some remarkable tectonic areas have been computed : a continental rift system (the Upper Rhinegraben), an Hercynian structure reactivated by Neogene volcanism (Massif central), a region of a recent continental collision (Pyrenees) and finally a region of an ancient orogeny (Armorican Massif). These images have a horizontal spatial resolution of the order of 10 km and show not only the geometry of the deep geological structures but will also illustrate the link between surface observations and structures detected at depth. The images demonstrate the passive character of the Rhinegraben mainly because no low-velocity was found below the Moho, show the presence of a thermal anomaly beneath the Massif central interpreted as caused by a mantle plume in the decaying phase of its evolution and prove the lithospheric scale of the North Pyrenean fault and of the South-Armorican shear zone. The anisotropic measurements suggest a lithospheric deformation related to the most recent tectonic event. In the Pyrenees, the Armorican Massif or the Rhinegraben areas, the directions of the fast-polarisation azimuth (the polarisation direction of the fast shear wave) are parallel to the tectonic texture of the last events, but suggest also a reactivation of inherited Hercynian discontinuities. In the Massif central, the splitting parameters distinguish between two lithospheric units regions marked by a distinct fast-polarisation azimuth on each side of the Sillon Houiller fault zone.

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

Solid Earth ◽  
2020 ◽  
Vol 11 (4) ◽  
pp. 1441-1455 ◽  
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.

scholarly journals The Dabie Extensional Tectonic System: Structural Framework

2012 ◽  
Vol 2012 ◽  
pp. 1-8 ◽  
Author(s):  
Quanlin Hou ◽  
Hongyuan Zhang ◽  
Qing Liu ◽  
Jun Li ◽  
Yudong Wu
Keyword(s):  
Shear Zone ◽  
Shear Zones ◽  
Ultrahigh Pressure ◽  
Metamorphic Complex ◽  
The South ◽  
Magma Intrusion ◽  
The North ◽  
Extensional Tectonic ◽  
Mechanism Transition ◽  
Tectonic System

A previous study of the Dabie area has been supposed that a strong extensional event happened between the Yangtze and North China blocks. The entire extensional system is divided into the Northern Dabie metamorphic complex belt and the south extensional tectonic System according to geological and geochemical characteristics in our study. The Xiaotian-Mozitan shear zone in the north boundary of the north system is a thrust detachment, showing upper block sliding to the NNE, with a displacement of more than 56 km. However, in the south system, the shearing direction along the Shuihou-Wuhe and Taihu-Mamiao shear zones is tending towards SSE, whereas that along the Susong-Qingshuihe shear zone tending towards SW, with a displacement of about 12 km. Flinn index results of both the north and south extensional systems indicate that there is a shear mechanism transition from pure to simple, implying that the extensional event in the south tectonic system could be related to a magma intrusion in the Northern Dabie metamorphic complex belt. Two 40Ar-39Ar ages of mylonite rocks in the above mentioned shear zones yielded, separately, ~190 Ma and ~124 Ma, referring to a cooling age of ultrahigh-pressure rocks and an extensional era later.

3D Fault Structure Inferred from a Refined Aftershock Catalog for the 2015 Gorkha Earthquake in Nepal

2019 ◽  
Vol 110 (1) ◽  
pp. 26-37 ◽  
Author(s):  
Masumi Yamada ◽  
Thakur Kandel ◽  
Koji Tamaribuchi ◽  
Abhijit Ghosh
Keyword(s):  
Velocity Structure ◽  
Complex Geometry ◽  
Slip Surface ◽  
Hanging Wall ◽  
Velocity Model ◽  
P Wave ◽  
Gorkha Earthquake ◽  
Hypocenter Determination ◽  
The North ◽  
2015 Gorkha Earthquake

ABSTRACT In this article, we created a well-resolved aftershock catalog for the 2015 Gorkha earthquake in Nepal by processing 11 months of continuous data using an automatic onset and hypocenter determination procedure. Aftershocks were detected by the NAMASTE temporary seismic network that is densely distributed covering the rupture area and became fully operational about 50 days after the mainshock. The catalog was refined using a joint hypocenter determination technique and an optimal 1D velocity model with station correction factors determined simultaneously. We found around 15,000 aftershocks with the magnitude of completeness of ML 2. Our catalog shows that there are two large aftershock clusters along the north side of the Gorkha–Pokhara anticlinorium and smaller shallow aftershock clusters in the south. The patterns of aftershock distribution in the northern and southern clusters reflect the complex geometry of the Main Himalayan thrust. The aftershocks are located both on the slip surface and through the entire hanging wall. The 1D velocity structure obtained from this study is almost constant at a P-wave velocity (VP) of 6.0  km/s for a depth of 0–20 km, similar to VP of the shallow continental crust.

Quantitative texture analysis by neutron diffraction in deformed crystalline aggregates: Contín Shear Zone, Morais Complex, N Portugal.

2020 ◽  
Author(s):  
Manuela Durán Oreja ◽  
Jeremie Malecki ◽  
Juan Gómez Barreiro
Keyword(s):  
Neutron Diffraction ◽  
Shear Zone ◽  
Seismic Anisotropy ◽  
Preferred Orientation ◽  
Distribution Functions ◽  
P Wave ◽  
Orientation Data ◽  
Crystal Preferred Orientation ◽  
Two Samples

<p>Two samples of mylonitic-ultramylonitic ortogneisses collected along the Contín shear zone were investigated for crystal preferred orientation and seismic anisotropy. Neutron diffraction data obtained at the D1B beamline at ILL (Institute Laue-Langevin, Grenoble) were analyzed with the Rietveld method as implemented in the code MAUD, to obtain the orientation distribution functions (ODF) of the principal phases (quartz, K-feldspar, plagioclase, phlogopite, muscovite and riebeckite). Texture and microstructure are compatible with the plastic deformation of the aggregates under medium to low-temperature conditions. Kinematic analysis supports a top-to-the SE sense of shear, suggesting a thrust character. Using preferred orientation data and single crystal elastic tensors, P and S-waves velocities and elastic anisotropy have been calculated. We have explored the role of several factors controlling the elastic properties of rocks, particularly the role of strain state and mineral changes in a shear zone. Those factors have a direct impact on the medium impedance and consequently on the interphase reflectivity. P-wave velocities, S-wave splitting and anisotropy increase with muscovite content. Seismic anisotropy is linked with the texture symmetry, which can result in large deviations between actual anisotropy and that measured along Cartesian XYZ sample directions (lineation/foliation reference frame). This is significant for the prediction and interpretation of seismic data. (Research support CGL2016-78560-P)</p>

Microstructural, compositional and petrophysical properties of mylonitic granodiorites from an extensional shear zone (Rhodope Core complex, Greece)

2014 ◽  
Vol 151 (6) ◽  
pp. 1051-1071 ◽  
Author(s):  
ROSALDA PUNTURO ◽  
ROSOLINO CIRRINCIONE ◽  
EUGENIO FAZIO ◽  
PATRIZIA FIANNACCA ◽  
HARTMUT KERN ◽  
...  
Keyword(s):  
Shear Zone ◽  
Seismic Anisotropy ◽  
Eastern Mediterranean ◽  
Tectonic Setting ◽  
P Wave ◽  
S Wave ◽  
Southern Boundary ◽  
Foliation Plane ◽  
Wave Velocities ◽  
Progressive Strain

AbstractAt the southern boundary of the Rhodope Massif, NE Greece, the Kavala Shear Zone (KSZ) represents an example of the Eastern Mediterranean deep-seated extensional tectonic setting. During Miocene time, extensional deformation favoured syntectonic emplacement and subsequent exhumation of plutonic bodies. This paper deals with the strain-related changes in macroscopic, geochemical and microstructural properties of the lithotypes collected along the KSZ, comprising granitoids from the pluton, aplitic dykes and host rock gneisses. Moreover, we investigated the evolution of seismic anisotropy on a suite of granitoid mylonites as a result of progressive strain. Isotropic compressional and shear wave velocities (Vp,Vs) and densities calculated from modal proportions and single-crystal elastic properties at given pressure–temperature (P–T) conditions are compared to respective experimental data including the directional dependence (anisotropy) of wave velocities. Compared to the calculated isotropic velocities, which are similar for all of the investigated mylonites (average values:Vp~ 5.87 km s−1,Vs~ 3.4 km s−1,Vp/Vs= 1.73 and density = 2.65 g cm−3), the seismic measurements give evidence for marked P-wave velocity anisotropy up to 6.92% (at 400 MPa) in the most deformed rock due to marked microstructural changes with progressive strain, as highlighted by the alignment of mica, chlorite minerals and quartz ribbons. The highest P- and S-wave velocities are parallel to the foliation plane and lowest normal to the foliation plane. Importantly,Vpremains constant within the foliation with progressive strain, but decreases normal to foliation. The potential of the observed seismic anisotropy of the KSZ mylonites with respect to detectable seismic reflections is briefly discussed.

Imaging structures below dipping TI media

Geophysics ◽  
1999 ◽  
Vol 64 (4) ◽  
pp. 1239-1246 ◽  
Author(s):  
Robert W. Vestrum ◽  
Don C. Lawton ◽  
Ron Schmid
Keyword(s):  
Seismic Data ◽  
Seismic Anisotropy ◽  
Rocky Mountain ◽  
Transversely Isotropic ◽  
Velocity Model ◽  
P Wave ◽  
Depth Imaging ◽  
Depth Migration ◽  
Kirchhoff Depth Migration ◽  
Ti Media

Seismic anisotropy in dipping shales causes imaging and positioning problems for underlying structures. We developed an anisotropic depth‐migration approach for P-wave seismic data in transversely isotropic (TI) media with a tilted axis of symmetry normal to bedding. We added anisotropic and dip parameters to the depth‐imaging velocity model and used prestack depth‐migrated image gathers in a diagnostic manner to refine the anisotropic velocity model. The apparent position of structures below dipping anisotropic overburden changes considerably between isotropic and anisotropic migrations. The ray‐tracing algorithm used in a 2-D prestack Kirchhoff depth migration was modified to calculate traveltimes in the presence of TI media with a tilted symmetry axis. The resulting anisotropic depth‐migration algorithm was applied to physical‐model seismic data and field seismic data from the Canadian Rocky Mountain Thrust and Fold Belt. The anisotropic depth migrations offer significant improvements in positioning and reflector continuity over those obtained using isotropic algorithms.

scholarly journals Seismic velocity and anisotropy of the uppermost mantle beneath Madagascar from Pn tomography

2020 ◽  
Vol 224 (1) ◽  
pp. 290-305
Author(s):  
Fenitra Andriampenomanana ◽  
Andrew A Nyblade ◽  
Michael E Wysession ◽  
Raymond J Durrheim ◽  
Frederik Tilmann ◽  
...  
Keyword(s):  
Seismic Anisotropy ◽  
Seismic Velocity ◽  
Plate Boundary ◽  
Active Regions ◽  
Shear Zones ◽  
Small Scale ◽  
Uppermost Mantle ◽  
Lithospheric Deformation ◽  
Pan African ◽  
Pn Velocity

SUMMARY The lithosphere of Madagascar records a long series of tectonic processes. Structures initially inherited from the Pan-African Orogeny are overprinted by a series of extensional tectonic and magmatic events that began with the breakup of Gondwana and continued through to the present. Here, we present a Pn-tomography study in which Pn traveltimes are inverted to investigate the lateral variation of the seismic velocity and anisotropy within the uppermost mantle beneath Madagascar. Results show that the Pn velocities within the uppermost mantle vary by ±0.30 km s–1 about a mean of 8.10 km s–1. Low-Pn-velocity zones (<8.00 km s–1) are observed beneath the Cenozoic alkaline volcanic provinces in the northern and central regions. They correspond to thermally perturbed zones, where temperatures are estimated to be elevated by ∼100–300 K. Moderately low Pn velocities are found near the southern volcanic province and along an E–W belt in central Madagascar. This belt is located at the edge of a broader low S-velocity anomaly in the mantle imaged in a recent surface wave tomographic study. High-Pn-velocity zones (>8.20 km s–1) coincide with stable and less seismically active regions. The pattern of Pn anisotropy is very complex, with small-scale variations in both the amplitude and the fast-axis direction, and generally reflects the complicated tectonic history of Madagascar. Pn anisotropy and shear wave (SKS) splitting measurements show good correlations in the southern parts of Madagascar, indicating coherency in the vertical distribution of lithospheric deformation along Pan-African shear zone as well as coupling between the crust and mantle when the shear zones were active. In most other regions, discrepancies between Pn anisotropy and SKS measurements suggest that the seismic anisotropy in the uppermost mantle beneath Madagascar differs from the vertically integrated upper mantle anisotropy, implying a present-day vertical partitioning of the deformation. Pn anisotropy directions lack the coherent pattern expected for an incipient plate boundary within Madagascar proposed in some kinematic models of the region.

scholarly journals P wave anisotropy caused by partial eclogitization of descending crust demonstrated by modeling effective petrophysical properties

2020 ◽  
Author(s):  
Sascha Zertani ◽  
Johannes C. Vrijmoed ◽  
Frederik Tilmann ◽  
Timm John ◽  
Torgeir B. Andersen ◽  
...  
Keyword(s):  
Seismic Anisotropy ◽  
Scale Up ◽  
Geophysical Methods ◽  
Shear Zones ◽  
P Wave ◽  
Crustal Material ◽  
Seismic Velocities ◽  
Initial Anisotropy ◽  
Eclogite Facies ◽  
Geophysical Imaging

<p>Eclogitization occurs deep in subduction and collision zones inaccessible to direct observation. Field-based studies dealing with crustal material previously transformed at eclogite-facies conditions and exhumed to the surface provide information from the micro scale up to a few kilometers. On the other hand, geophysical methods aim at imaging the ongoing processes in-situ. However, these methods are limited by the achievable resolution and typically only sensitive to structures a few kilometers in size, leaving a large gap between the scales at which observations are interpreted. In this study we try to discern the implications of structures mapped in field-based studies to interpretations of geophysical imaging. We therefore calculated effective anisotropic P wave velocities for a suite of representative structural associations using the finite element method. The structural associations are directly extracted from observations of partially eclogitized assemblages on the island of Holsnøy in the Bergen Arcs of western Norway. Physical properties of the constituting lithologies are taken from laboratory measurements of the same rocks and the calculations are performed on a variety of scales, from the 20-m scale up to the kilometer scale to be able to predict how the effective seismic properties change with varying scale. Our results show that the P wave velocity of the effective medium is solely controlled by the volumetric fraction of the constituting lithologies and their elastic properties. We find that the structural relationship of the different lithologies has no significant influence on the resulting seismic velocities. P wave anisotropy, however, is controlled by the constituting lithology with the highest initial anisotropy and to a lesser extent by the modal abundance of the different lithologies. Further, our results show that seismic anisotropy is largely transferable across scales validating the assumptions often made when measuring seismic velocities on centimeter-sized sample volumes. On the kilometer scale, a scale that is potentially resolvable by geophysical methods, our results show that an eclogite-facies shear zone network such as the one exposed on Holsnøy would indeed produce a significant P wave anisotropy on a crustal scale. This anisotropy is produced by the eclogite-facies shear zones themselves even though eclogites are typically considered to be low-anisotropy rocks. Comparison of our results with active settings of continental collision and subduction zones reveals that eclogite-facies shear zones have the potential to produce a significant backazimuthal bias of the retrieved signal in geophysical imaging and underline the significance of seismic anisotropy as a tool to further increase the sensitivity of seismological methods to lithological variations.</p>

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