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Digitale Medien

Structural softening of the lithosphere (2005)

Schmalholz, Stefan M. ; Podladchikov, Yuri Y. ; Jamtveit, Bjørn

Oxford, UK : Blackwell Science Ltd

Terra nova 17 (2005), S. 0

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ISSN: 1365-3121

Quelle: Blackwell Publishing Journal Backfiles 1879-2005

Thema: Geologie und Paläontologie

Notizen: Structural softening is a decrease in the amount of stress needed to deform the lithosphere at a particular rate because of its structural reorganization while all true rheological properties remain constant. Structural softening is fundamentally different than material softening, where the decrease in stress is generated by a change in rheological properties with progressive deformation, such as grain size reduction resulting from large shearing strain. We study structural softening generated by folding of the crust-mantle boundary, which is a structural instability that inevitably develops during compression of the mechanically layered lithosphere. For ductile rheologies, the stress decrease represents a decrease of the effective lithospheric viscosity, which is proportional to the ratio of stress to lithospheric shortening strain rate. We present analytical and numerical results quantifying the decrease in stress and effective viscosity that occur during shortening at a constant rate. The decrease in effective viscosity can be up to 10-fold.

Materialart: Digitale Medien

URL: http://dx.doi.org/10.1111/j.1365-3121.2004.00585.x

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Widening of Hydrous Shear Zones During Incipient Eclogitization of Metastable Dry and Rigid Lower Crust—Holsnøy, Western Norway (2021)

Kaatz, Lisa ; Zertani, Sascha ; Moulas, Evangelos ; [weitere]

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Publikationsdatum: 2022-08-26

Beschreibung: The partially eclogitized crustal rocks on Holsnøy in the Bergen Arcs, Norway, indicate that eclogitization is caused by the interplay of brittle and ductile deformation promoted by fluid infiltration and fluid‐rock interaction. Eclogitization generated an interconnected network of millimeter‐to‐kilometer‐wide hydrous eclogite‐facies shear zones, which presumably caused transient weakening of the mechanically strong lower crust. To decipher the development of those networks, we combine detailed lithological and structural mapping of two key outcrops with numerical modeling. Both outcrops are largely composed of preserved granulite with minor eclogite‐facies shear zones, thus representing the beginning phases of eclogitization and ductile deformation. We suggest that deformation promoted fluid‐rock interaction and eclogitization, which gradually consumed the granulite until fluid‐induced reactions were no longer significant. The shear zones widen during progressive deformation. To identify the key parameters that impact shear zone widening, we generated scale‐independent numerical models, which focus on different processes affecting the shear zone evolution: (i) rotation of the shear zones caused by finite deformation, (ii) mechanical weakening due to a limited amount of available fluid, and (iii) weakening and further hydration of the shear zones as a result of continuous and unlimited fluid supply. A continuous diffusion‐type fluid infiltration, with an effective diffusion coefficient around D=10−16m2s, coupled with deformation is prone to develop structures similar to the ones mapped in field. Our results suggest that the shear zones formed under a continuous fluid supply, causing shear zone widening, rather than localization, during progressive deformation.

Beschreibung: Key Points: Continuous fluid supply causes shear zone widening. Shear zones widen during strain accumulation.

Beschreibung: Deutsche Forschungsgemeinschaft (DFG) http://dx.doi.org/10.13039/501100001659

Beschreibung: Norges Forskningsråd http://dx.doi.org/10.13039/501100005416

Schlagwort(e): ddc:551.8

Sprache: Englisch

Materialart: doc-type:article

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Contributions of Grain Damage, Thermal Weakening, and Necking to Slab Detachment (2020)

Thielmann, Marcel ; Schmalholz, Stefan M.

Frontiers Media

In: Frontiers in Earth Science. 2020; 8: Published 2020 Jul 16. doi: 10.3389/feart.2020.00254.

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Publikationsdatum: 2020-07-16

Digitale ISSN: 2296-6463

Thema: Geologie und Paläontologie

Publiziert von Frontiers Media

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Phase Velocity Dispersion and Attenuation of Seismic Waves due to Trapped Fluids in Residual Saturated Porous Media (2012)

Steeb, Holger ; Kurzeja, Patrick S. ; Frehner, Marcel ; Schmalholz, Stefan M.

Soil Science Society of America (SSSA)

In: Vadose Zone Journal 11: vzj2011.0121.

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Publikationsdatum: 2012-08-01

Beschreibung: Propagation of seismic waves in partially saturated porous media depends on various material properties, including saturation, porosity, elastic properties of the skeleton, viscous properties of the pore fluids, and, additionally, capillary pressure and effective permeability. If the wetting fluid is in a discontinuous state (i.e., residual saturated configuration), phase velocities and frequency-dependent attenuation additionally depend on microscopical (pore-scale) properties such as droplet and/or ganglia size. To model wave propagation in residual saturated porous media, we developed a three-phase model based on an enriched continuum mixture theory capturing the strong coupling between the micro- and the macroscale. The three-phase model considers a continuous and a discontinuous part. The continuous part exhibits similar behavior as the poroelastic model introduced by Biot. The discontinuous part describes the movement of blobs/clusters of the wetting fluid and is based on an oscillator rheology. In comparison with other three-phase models, the presented one accounts for the heterogeneity of the discontinuous fluid clusters by use of their dynamic properties, i.e., their statistically distributed inertia, eigenfrequency, and damping effects. This heterogeneous and discontinuous distribution of the wetting fluid in the form of single blobs or fluid clusters is represented by a model-embedded distribution function of the cluster sizes. We define a dimensionless parameter that determines if the overall motion of the residual fluid is dominated by oscillations (underdamped, resonance) or not (overdamped). Our results show that the residual fluid has a significant impact on the velocity dispersion and attenuation no matter if it oscillates or not. For long wavelengths our model coincides with the Biot–Gassmann equations. We show under which conditions and how the classical biphasic models can be used to approximate the dynamic behavior of residual saturated porous media.

Digitale ISSN: 1539-1663

Thema: Geologie und Paläontologie , Land- und Forstwirtschaft, Gartenbau, Fischereiwirtschaft, Hauswirtschaft

Publiziert von Soil Science Society of America (SSSA)

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Time-reverse imaging with limited S-wave velocity model information (2011)

Steiner, Brian ; Saenger, Erik H. ; Schmalholz, Stefan M.

Society of Exploration Geophysicists (SEG)

In: Geophysics 76: MA33-MA40.

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Publikationsdatum: 2011-09-01

Beschreibung: Time-reverse imaging is a wave propagation algorithm for locating sources. Signals recorded by synchronized receivers are reversed in time and propagated back to the source location by elastic wavefield extrapolation. Elastic wavefield extrapolation requires a P-wave as well as an S-wave velocity model. The velocity models available from standard reflection seismic methods are usually restricted to only P-waves. In this study, we use synthetically produced time signals to investigate the accuracy of seismic source localization by means of time-reverse imaging with the correct P-wave and a perturbed S-wave velocity model. The studies reveal that perturbed S-wave velocity models strongly influence the intensity and position of the focus. Imaging the results with the individual maximum energy density for both body wave types instead of mixed modes allows individual analysis of the two body waves. P-wave energy density images render stable focuses in case of a correct P-wave and incorrect S-wave velocity model. Thus, P-wave energy density seems to be a more suitable imaging condition in case of a high degree of uncertainty in the S-wave velocity model.

Print ISSN: 0016-8033

Digitale ISSN: 1942-2156

Thema: Geologie und Paläontologie , Physik

Publiziert von Society of Exploration Geophysicists (SEG)

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Impact of fluid saturation on the reflection coefficient of a poroelastic layer (2011)

Quintal, Beatriz ; Schmalholz, Stefan M. ; Podladchikov, Yuri Y.

Society of Exploration Geophysicists (SEG)

In: Geophysics 76: N1-N12.

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Publikationsdatum: 2011-03-01

Beschreibung: The impact of changes in saturation on the frequency-dependent reflection coefficient of a partially saturated layer was studied. Seismic attenuation and velocity dispersion in partially saturated (i.e., patchy saturated) poroelastic media were accounted for by using the analytical solution of the 1D White's for wave-induced fluid flow. White's solution was applied in combination with an analytical solution for the normal-incidence reflection coefficient of an attenuating layer embedded in an elastic or attenuating background medium to investigate the effects of attenuation, velocity dispersion, and tuning on the reflection coefficient. Approximations for the frequency-dependent quality factor, its minimum value, and the frequency at which the minimum value of the quality factor occurs were derived. The approximations are valid for any two alternating sets of petrophysical parameters. An approximation for the normal-incidence reflection coefficient of an attenuating thin (compared to the wavelength) layer was also derived. This approximation gives insight into the influence of contrasts in acoustic impedance and/or attenuation on the reflectivity of a thin layer. Laboratory data for reflections from a water-saturated sand layer and from a dry sand layer were further fit with petrophysical parameters for unconsolidated sand partially saturated with water and air. The results showed that wave-induced fluid flow can explain low-frequency reflection anomalies, which are related to fluid saturation and can be observed in seismic field data. The results further indicate that reflection coefficients of partially saturated layers (e.g., hydrocarbon reservoirs) can vary significantly with frequency, especially at low seismic frequencies where partial saturation may often cause high attenuation.

Print ISSN: 0016-8033

Digitale ISSN: 1942-2156

Thema: Geologie und Paläontologie , Physik

Publiziert von Society of Exploration Geophysicists (SEG)

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Pore fluid effects on S-wave attenuation caused by wave-induced fluid flow (2012)

Quintal, Beatriz ; Steeb, Holger ; Frehner, Marcel ; Schmalholz, Stefan M. ; Saenger, Erik H.

Society of Exploration Geophysicists (SEG)

In: Geophysics 77: L13-L23.

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Publikationsdatum: 2012-05-01

Beschreibung: We studied seismic attenuation of P- and S-waves caused by the physical mechanism of wave-induced fluid flow at the mesoscopic scale. Stress relaxation experiments were numerically simulated by solving Biot’s equations for consolidation of 2D poroelastic media with finite-element modeling. The experiments yielded time-dependent stress-strain relations that were used to calculate the complex moduli from which frequency-dependent attenuation was determined. Our model consisted of periodically distributed circular or elliptical heterogeneities with much lower porosity and permeability than the background media, which contained 80% of the total pore space of the media. This model can represent a hydrocarbon reservoir, where the porous background is fully saturated with oil or gas and the low-porosity regions are always saturated with water. Three different saturation scenarios were considered: oil-saturated (80% oil, 20% water), gas-saturated (80% gas, 20% water), and fully water-saturated media. Varying the dry bulk and shear moduli in the background and in the heterogeneities, a consistent tendency was observed in the relative behavior of the S-wave attenuation among the different saturation scenarios. First, in the gas-saturated media the S-wave attenuation was very low and much lower than in the oil-saturated or in the fully water-saturated media. Second, at low frequencies the S-wave attenuation was significantly higher in the oil-saturated media than in the fully water-saturated media. The P-wave attenuation exhibited a more variable relative behavior among the different saturation degrees. Based on the mechanism of wave-induced fluid flow and on our numerical results, we suggest that S-wave attenuation could be used as an indicator of fluid content in a reservoir. Additionally, we observed that impermeable barriers in the background can cause a significant increase in S-wave attenuation. This suggests that S-wave attenuation could also be an indicator of permeability changes in a reservoir due to, for example, fracturing operations.

Print ISSN: 0016-8033

Digitale ISSN: 1942-2156

Thema: Geologie und Paläontologie , Physik

Publiziert von Society of Exploration Geophysicists (SEG)

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Necking of the Lithosphere: A Reappraisal of Basic Concepts With Thermo-Mechanical Numerical Modeling (2018)

Chenin, Pauline ; Schmalholz, Stefan M. ; Manatschal, Gianreto ; [weitere]

American Geophysical Union

In: Journal of Geophysical Research: Solid Earth. 2018; 123(6): 5279-5299. Published 2018 Jun 22. doi: 10.1029/2017jb014155.

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Publikationsdatum: 2018-06-22

Beschreibung: We investigate lithosphere necking using two-dimensional thermo-mechanical numerical simulations without strain softening or weakening mechanisms. The models have an initial small sinusoidal perturbation of the Moho depth, whose wavelength corresponds to the model width. Applied boundary conditions (constant extension velocity or bulk extension rate) and initial model width significantly impact the necking dynamics. For constant bulk extension rates, wider models generate more intense necking with locally higher strain rates, whereas for constant velocity extension, models evolution is similar independent on their initial width. However, the width of the final necking zones ranges consistently between 45 and 105 km, independent on the type of applied boundary conditions and the initial Moho wavelength. The modeled widths are similar to along dip necking zones widths of natural rifted margins that formed during a single, unidirectional, and relatively continuous extensional event (e.g., Iberia-Newfoundland margins, Porcupine Basin, Gulf of Aden). When the crust is mechanically decoupled from the mantle by a weak ductile lower crust, models exhibit three characteristic stages: (1) distributed thinning and extension associated with progressive subsidence; (2) upper mantle necking compensated by flow of the weak lower crust, which hampers both crustal thinning and subsidence at the rift center; and (3) crustal necking associated with fast subsidence after the mantle has necked. Decoupled models display regions of relatively thick crust on one or both sides of the rift center, comparable to the Galicia, Rockall, Hatton, and Porcupine Banks along the North Atlantic rifted margins. ©2018. American Geophysical Union. All Rights Reserved.

Print ISSN: 2169-9313

Digitale ISSN: 2169-9356

Thema: Geologie und Paläontologie , Physik

Publiziert von American Geophysical Union

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