A Discussion on natural strain and geological structure - Strain and anisotropy in rocks

The evaluation of finite strain in naturally deformed rocks is restricted by the limited occurrence of good natural strain indicators which are also homogeneous with respect to the matrix. This problem is overcome by establishing the relation between measured finite strain and those physical behaviour characteristics of rocks that are dependent upon the anisotropy resulting from deformation. Accordingly, the strain measured from natural indicators is calibrated against ( degree of preferred orientation, (b) magnetic susceptibility anisotropy, and (r) seismic anisotropy. This _ will permit three approaches to be used independently for the evaluation of strain, provided that a minimal number of actual strains are available. The relation between measured strain and the degree of preferred orientation of layer silicates as revealed by X-ray transmission goniometry is established for a group of fine grained tectonites of dominantly planar fabric which have an average deformation ellipsoid of form 1.6:1 :,0.26. The strains measured from the degree of preferred orientation are in remarkable agreement with those measured from natural strain indicators. The measured deformation ellipsoids for a wide range of strains are also compared to the correlative ellipsoids of magnetic susceptibility anisotropy. The axes of both sets of ellipsoids are coincidental and the shape relationship between deformation and magnetic susceptibility ellipsoids is established by linear regression. Finally, the anisotropy of seismic velocities is determined by measuring the pseudocompressional velocity and two orthogonally polarized pseudo shear wave velocities for each of a minimum of nine non-coplanar directions. The velocity surfaces thus obtained define an elastic or seismic velocity anisotropy ellipsoid, the axes of which are also precisely coincidental with those of the finite deformation ellipsoid. The influence of rock fabric upon seismic velocities is such that for a rock which has undergone a principal finite extension of 135 % and a finite shortening of 65 %, the difference of compressional and shear wave velocities between these two directions is in the ratio 1.26:1 for P waves and 1.33:1 for S waves.

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Seismic velocities of unconsolidated sands: Part 1 — Pressure trends from 0.1 to 20 MPa

Geophysics ◽

10.1190/1.2399459 ◽

2007 ◽

Vol 72 (1) ◽

pp. E1-E13 ◽

Cited By ~ 76

Author(s):

Michael A. Zimmer ◽

Manika Prasad ◽

Gary Mavko ◽

Amos Nur

Keyword(s):

Shear Wave ◽

Pressure Dependence ◽

Glass Bead ◽

Theoretical Models ◽

Compressional Wave ◽

P Wave ◽

Seismic Velocities ◽

Unconsolidated Sands ◽

Wave Velocities ◽

Shear Wave Velocities

Knowledge of the pressure dependences of seismic velocities in unconsolidated sands is necessary for the remote prediction of effective pressures and for the projection of velocities to unsampled locations within shallow sand layers. We have measured the compressional- and shear-wave velocities and bulk, shear, and P-wave moduli at pressures from [Formula: see text] in a series of unconsolidated granular samples including dry and water-saturated natural sands and dry synthetic sand and glass-bead samples. The shear-wave velocities in these samples demonstrate an average pressure dependence approximately proportional to the fourth root of the effective pressure [Formula: see text], as commonly observed at lower pressures. For the compressional-wave velocities, theexponent in the pressure dependence of individual dry samples is consistently less than the exponent for the shear-wave velocity of the same sample, averaging 0.23 for the dry sands and 0.20 for the glass-bead samples. These pressure dependences are generally consistent over the entire pressure range measured. A comparison of the empirical results to theoretical predictions based on Hertz-Mindlin effective-medium models demonstrates that the theoretical models vastly overpredict the shear moduli of the dry granular frame unless the contacts are assumed to have no tangential stiffness. The models also predict a lower pressure exponent for the moduli and velocities [Formula: see text] than is generally observed in the data. We attribute this discrepancy in part to the inability of the models to account for decreases in the amount of slip or grain rotation occurring at grain-to-grain contacts with increasing pressure.

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Using Volcanic Geochemistry and Seismic Tomography to Refine Global Models of Mantle Temperature and Plate Thickness

10.5194/egusphere-egu21-3572 ◽

2021 ◽

Author(s):

Patrick Ball ◽

Nicky White ◽

John Maclennan ◽

Simon Stephenson

Keyword(s):

Volcanic Rock ◽

Shear Wave ◽

Plate Thickness ◽

Volcanic Rocks ◽

Seismic Velocities ◽

Intraplate Volcanism ◽

Lithospheric Thickness ◽

Wave Velocities ◽

Shear Wave Velocities ◽

Mantle Temperature

<p>The thermochemical structure of lithospheric and asthenospheric mantle exert primary controls on surface topography and volcanic activity. Volcanic rock compositions and mantle seismic velocities provide indirect observations of this structure. Here, we compile and analyze a global database of the distribution and composition of Neogene-Quaternary intraplate volcanic rocks. By integrating this database with seismic tomographic models, we show that intraplate volcanism is concentrated in regions characterized by slow upper mantle shear-wave velocities and by thin lithosphere (i.e. < 100 km). We observe a negative correlation between shear-wave velocities at depths of 125-175 km and melt fractions inferred from volcanic rock compositions. Furthermore, mantle temperature and lithospheric thickness estimates obtained by geochemical modeling broadly agree with values determined from tomographic models that have been converted into temperature. Intraplate volcanism often occurs in regions where uplifted (but undeformed) marine sedimentary rocks are exposed. Regional elevation of these rocks can be generated by a combination of hotter asthenosphere and lithospheric thinning. Therefore, the distribution and composition of intraplate volcanic rocks through geologic time will help to probe past mantle conditions and surface processes.</p>

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Global influence of mantle temperature and plate thickness on intraplate volcanism

Nature Communications ◽

10.1038/s41467-021-22323-9 ◽

2021 ◽

Vol 12 (1) ◽

Author(s):

P. W. Ball ◽

N. J. White ◽

J. Maclennan ◽

S. N. Stephenson

Keyword(s):

Volcanic Rock ◽

Shear Wave ◽

Plate Thickness ◽

Volcanic Rocks ◽

Seismic Velocities ◽

Intraplate Volcanism ◽

Lithospheric Thickness ◽

Wave Velocities ◽

Shear Wave Velocities ◽

Mantle Temperature

AbstractThe thermochemical structure of lithospheric and asthenospheric mantle exert primary controls on surface topography and volcanic activity. Volcanic rock compositions and mantle seismic velocities provide indirect observations of this structure. Here, we compile and analyze a global database of the distribution and composition of Neogene-Quaternary intraplate volcanic rocks. By integrating this database with seismic tomographic models, we show that intraplate volcanism is concentrated in regions characterized by slow upper mantle shear-wave velocities and by thin lithosphere (i.e. <100 km). We observe a negative correlation between shear-wave velocities at depths of 125–175 km and melt fractions inferred from volcanic rock compositions. Furthermore, mantle temperature and lithospheric thickness estimates obtained by geochemical modeling broadly agree with values determined from tomographic models that have been converted into temperature. Intraplate volcanism often occurs in regions where uplifted (but undeformed) marine sedimentary rocks are exposed. Regional elevation of these rocks can be generated by a combination of hotter asthenosphere and lithospheric thinning. Therefore, the distribution and composition of intraplate volcanic rocks through geologic time will help to probe past mantle conditions and surface processes.

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