Anisotropy of schists: Contribution of crustal anisotropy to active source seismic experiments and shear wave splitting observations

We have made sets of five independent compressional and shear wave velocity measurements, which with density, allow us to completely characterize the transverse isotropy of samples from five metamorphic belts: the Haast schist terrane (South Island, New Zealand), Poultney slate, Chugach phyllite, Coldfoot schist, and Pelona schist (United States). These velocity measurements include compressional wave velocities for propagation parallel, perpendicular, and at 45° to the symmetry axis, shear wave velocity for propagation and particle motion perpendicular to the symmetry axis, and shear wave velocity for propagation parallel to the symmetry axis. Velocity measurements were made up to pressures of 1 GPa (∼35-km depth) where microcracks are closed and anisotropy is due to preferred mineral orientation. Our samples exhibit compressional wave anisotropy of 9–20% as well as significant shear wave splitting. Metamorphic terranes that are anisotropic to ultrasonic waves may also be anisotropic at the scale of active and passive seismic experiments. Our data suggest that a significant thickness (10–20 km) of appropriately oriented (steeply dipping foliation) schist in the crust could contribute as much as 45% of observed shear wave splitting. Our data set can also be used to model the effects of crustal anisotropy for active source seismic experiments in order to determine if the anisotropy of the terrane is significant and needs to be taken into account during processing and modeling of the data.

[1]  F. Birch.  The velocity of compressional waves in rocks to 10 kilobars: 1. , 1960 .

[2]  E. Zen.  Stratigraphy and Structure at the North End of the Taconic Range in West-Central Vermont , 1961 .

[3]  N. Christensen.  Compressional wave velocities in metamorphic rocks at pressures to 10 kilobars , 1965 .

[4]  N. Christensen.  Shear wave velocities in metamorphic rocks at pressures to 10 kilobars , 1966 .

[5]  Gordon E. Schacher,et al.  Determination of Elastic Constants from Sound‐Velocity Measurements in Crystals of General Symmetry , 1967 .

[6]  D. Okaya,et al.  Crustal anisotropy in the vicinity of the Alpine fault zone , 1995 .

[7]  Some Revisions in the Interpretation of the Taconic Allochthon in West-Central Vermont , 1972 .

[8]  御子柴宣夫.  B. A. Auld : Acoustic Fields and Waves in Solids, Vol. 1 and 2, John Wiley, New York and London, 1973, 2 vols., 23.5×16cm. , 1974 .

[9]  N. Christensen.  Measurements of Dynamic Properties of Rock at Elevated Temperatures and Pressures , 1985 .

[10]  R. Tosdal,et al.  Tectonic setting and lithology of the Winterhaven Formation; a new Mesozoic stratigraphic unit in southeasternmost California and southwestern Arizona , 1985 .

[11]  G. Plafker,et al.  Bedrock geology and tectonic evolution of the Wrangellia, Peninsular, and Chugach Terranes along the Trans‐Alaska Crustal Transect in the Chugach Mountains and Southern Copper River Basin, Alaska , 1989 .

[12]  G. Plafker,et al.  Structural analysis of the Southern Peninsular, Southern Wrangellia, and Northern Chugach Terranes Along the Trans‐Alaska Crustal Transect, Northern Chugach Mountains, Alaska , 1989 .

[13]  N. Christensen,et al.  A high‐resolution seismic reflection/refraction study of the Chugach‐Peninsular Terrane Boundary, southern Alaska , 1989 .

[14]  N. Christensen,et al.  Seismic anisotropy due to preferred mineral orientation observed in shallow crustal rocks in southern Alaska , 1990 .

[15]  P. Silver,et al.  Observations of teleseismic shear‐wave splitting in the basin and range from portable and permanent stations , 1990 .

[16]  Peter O. Koons,et al.  The obliquely-convergent plate boundary in the South Island of New Zealand: implications for ancient collision zones , 1990 .

[17]  Donald F. Winterstein,et al.  Velocity anisotropy terminology for geophysicists , 1990 .

[18]  Effects of point singularities on shear-wave propagation in sedimentary basins , 1991 .

[19]  N. Christensen,et al.  Crustal structure of accreted terranes in southern Alaska, Chugach Mountains and Copper River Basin, from seismic refraction results , 1991 .

[20]  Comment and Reply on "Seismic anisotropy due to preferred mineral orientation observed in shallow crustal rocks in southern Alaska" , 1991 .

[21]  Paul G. Silver,et al.  Shear wave splitting and subcontinental mantle deformation , 1991 .

[22]  N. Christensen,et al.  A seismic velocity-confining pressure relation, with applications , 1991 .

[23]  G. Barruol,et al.  A quantitative evaluation of the contribution of crustal rocks to the shear-wave splitting of teleseismic SKS waves , 1993 .

[24]  J. Moore,et al.  Geology of the southern Alaska margin , 1994 .

[25]  J. Dellinger,et al.  Do traveltimes in pulse‐transmission experiments yield anisotropic group or phase velocities? , 1994 .

[26]  Stuart Crampin,et al.  The fracture criticality of crustal rocks , 1994 .

[27]  P. Silver,et al.  The Interpretation of Shear‐Wave Splitting Parameters In the Presence of Two Anisotropic Layers , 1994 .

[28]  Walter D. Mooney,et al.  Seismic velocity structure and composition of the continental crust: A global view , 1995 .

[29]  N. Christensen,et al.  Seismic anisotropy of shales , 1995 .

[30]  Peter Hubral.  Foundations of Anisotropy for Exploration Seismics , 1995 .

[31]  N. Christensen.  Poisson's ratio and crustal seismology , 1996 .

[32]  P. Silver.  SEISMIC ANISOTROPY BENEATH THE CONTINENTS: Probing the Depths of Geology , 1996 .

[33]  S. Siegesmund,et al.  The Significance of rock fabrics for the geological - interpretation of geophysical anisotropies , 1996 .

[34]  R. Clayton,et al.  Images of crust beneath southern California will aid study of earthquakes and their effects , 1996 .

[35]  Douglas R. Schmitt,et al.  Ultrasonic anisotropic phase velocity determination with the Radon transformation , 1997 .

[36]  T. Moore,et al.  Crustal implications of bedrock geology along the Trans‐Alaska Crustal Transect (TACT) in the Brooks Range, northern Alaska , 1997 .

[37]  P. Davis,et al.  Crustal thickness variations in southern California from Los Angeles Region Seismic Experiment passive phase teleseismic travel times , 1997, Bulletin of the Seismological Society of America.

[38]  A. G. Harris,et al.  Stratigraphic and structural implications of conodont and detrital zircon U—Pb ages from metamorphic rocks of the Coldfoot terrane, Brooks Range, Alaska , 1997 .

[39]  T. Mukerji,et al.  The Rock Physics Handbook , 1998 .

[40]  T. Bohlen,et al.  Shear wave anisotropy of laminated lower crust beneath Urach (SW Germany): a comparison with xenoliths and with exposed lower crustal sections , 1998 .

[41]  D. Okaya,et al.  Preliminary results from a geophysical study across a modern, continent-continent collisional plate boundary — the Southern Alps, New Zealand , 1998 .

[42]  D. Lawton,et al.  A refraction‐seismic field study to determine the anisotropic parameters of shales , 1998 .

[43]  T. Bohlen,et al.  Seismic Velocities and Anisotropy of the Lower Continental Crust: A Review , 1999 .

[44]  M. Savage.  Seismic anisotropy and mantle deformation: What have we learned from shear wave splitting? , 1999 .

[45]  D. Eberhart‐Phillips,et al.  Upper mantle anisotropy in the New Zealand Region , 1999 .