Seismic azimuthal anisotropy beneath the eastern United States and its geodynamic implications (original) (raw)
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Complex and variable crustal and uppermost mantle seismic anisotropy in the western United States
Nature Geoscience, 2011
The orientation and depth of deformation in the Earth is characterized by seismic anisotropy 1 -variations in the speed of passing waves caused by the alignment of minerals under strain into a preferred orientation. Seismic anisotropy in the western US has been well studied 2-11 and anisotropy in the asthenosphere is thought to be controlled by plate motions and subduction 6-9 . However, anisotropy within the crust and upper mantle and the variation of anisotropy with depth are poorly constrained. Here, we present a three-dimensional model of crustal and upper mantle anisotropy based on new observations of ambient noise 12 and earthquake 13 data that reconciles surface wave and body wave 9 data sets. We confirm that anisotropy in the asthenosphere reflects a mantle flow field controlled by a combination of North American plate motion and the subduction of the Juan de Fuca and Farallon slab systems 6-9 . We also find that seismic anisotropy in the upper mantle and crust are largely uncorrelated: patterns of anisotropy in the crust correlate with geological provinces, whereas anisotropy in the upper mantle is controlled by temperature variations. We conclude that any coupling between anisotropy in the crust and mantle must be extremely complex and variable.
Mantle deformation and tectonics: constraints from seismic anisotropy in the western United States
Physics of the earth and planetary interiors, 1993
We have examined shear-wave splitting in teleseismic shear waves (SKS, SKKS, S) from 15 stations in the western United States, based on analysis of 123 records from 67 events . The varied past and present tectonic styles in this region (subduction, transform faulting, extension, and stable domains) are expected to cause a wide variety of anisotropic behavior and therefore make it an excellent natural laboratory. Fast polarization azimuths (0) vary from E-W to nearly N-S and time delays (St) range from being barely detectable (less than 0 .6 s) to 1 .6 s . Most stations yielded consistent measurements independant of station-event geometry . The exceptions were stations situated very close to the San Andreas Fault, which yielded well-constrained but inconsistent splitting parameters . These have been successfully modeled by two anisotropic layers with different horizontal symmetry axes. The upper layer has a fast direction parallel to the fault and St of about 1 s . The lower layer, with St from 0.6 to 0.9 s, is oriented E-W near the San Francisco Bay Area and NE-SW in the Mojave Desert. Other measurements of E-W fast 46 are observed as far east as western Nevada, with large delay times of 1 .3-1 .5 s, but disappear to the north . Stations in the northwestern Basin and Range have values of 0 oriented at about + 70°with delay times ranging from 0.7 to 1 .2 s. 0 varies in other regions of the Basin and Range, from -70°in eastern Nevada to +20°in the transition zone between the Colorado Plateau and the Basin and Range, with St of 1 .0 s.
2008
Keywords: seismic anisotropy surface wave stratified anisotropy shear-wave anisotropy lithospheric deformation Evolution of continental lithosphere during orogenies and the following periods of relative stability is poorly understood, largely because of the lack of relevant observational constraints. Measurements of seismic anisotropy provide such constraints, but due to limitations in the resolving power of available data sets and, more generally, of various data types, detailed mapping of lithospheric anisotropy has remained elusive. Here we apply surface-wave array analysis to data from the East-central U.S. and determine the layering of azimuthal anisotropy beneath the Grenville-Appalachian orogen in the entire lithosphere-asthenosphere depth range. Combined measurements of Rayleigh-wave phase velocities along 60 interstation paths constrain phasevelocity maps with statistically significant anisotropy. Distinct anisotropy patterns in three different period ranges point to the existence of three distinct layers beneath the orogen, with different anisotropic fabric within each. We invert phase-velocity maps and, alternatively, pairs of selected measured dispersion curves for anisotropic shear-velocity structure. The results confirm that three anisotropic layers with different fabric within each are present, two in the lithosphere (30-70 km; 70-150 km depths) and another in the asthenosphere beneath (N150 km). Directions of fast wave propagation in the upper lithosphere are parallel to the Grenville and Appalachian fronts, suggesting that the region-scale anisotropy pattern reflects the pervasive deformation of the lower crust and uppermost mantle during the continental collisions. The fast-propagation azimuth within the lower lithosphere is different, parallel to the NNW direction of North America's motion after the orogeny (~160-125 Ma). This suggests that the lithosphere, 70-km thick by the end of the Appalachian orogeny, gradually thickened to the present 150-km while inheriting the fabric from the sheared asthenosphere below, as the plate moved NNW. Below 150 km, the fast-propagation direction is parallel to the present plate motion, indicating fabric due to recent asthenospheric flow. Anisotropy in narrower depth ranges beneath the region has been sampled previously. Published results (from observations of P n and SKS and waveform tomography) can be accounted for and reconciled by the three-layered model of anisotropy for the lithosphere-asthenosphere depth range constrained in this study. In particular, the anisotropy we detect in the asthenosphere can account for the magnitude of SKS-wave splitting, with the fast wave-propagation directions inferred from SKS and surface-wave data also consistent, both parallel to the current plate motion.
Mapping P-wave azimuthal anisotropy in the crust and upper mantle beneath the United States
Physics of the Earth and Planetary Interiors, 2013
Much progress has been made on revealing seismic structure and mantle dynamics beneath the United States (US) with the EarthScope/USArray project. Seismic anisotropy revealed by shear-wave splitting studies provides important constraints on constructing geodynamic models with regard to the seismic images, but the shear-wave splitting observations have poor vertical resolution and so their interpretations are often not unique. In this work we used a large number of arrival-time data from local and distant earthquakes recorded by the USArray to determine the first P-wave azimuthal anisotropy tomography of the crust and upper mantle beneath the US. Our results show that fast velocity directions (FVDs) in the lithosphere under the tectonically active areas correlate well with the surface tectonic features, suggesting that the P-wave anisotropy mainly reflects the present deformation. A circular pattern of the FVDs centered in the Great Basin is revealed, which is well consistent with the specific circular shear-wave splitting observations there, suggesting that the anisotropy occurs in the crust and uppermost mantle. In contrast, beneath the stable cratonic region, the FVDs revealed by this study differ from the shear-wave splitting observations but consistent with the features of gravity and magnetic anomalies, indicating that the P-wave FVDs mainly reflect the fossil anisotropy in the lithosphere, whereas the Swave splitting observations mainly reflect the significant anisotropy in the asthenosphere. The present results shed new light on the seismic anisotropy in the crust and upper mantle and provide new constraints on constructing geodynamic models beneath the US.
Western U.S. seismic anisotropy revealing complex mantle dynamics
Earth and Planetary Science Letters, 2018
The origin of the complex pattern of SKS splitting over the western United States (U.S.) remains a long-lasting debate, where a model that simultaneously matches the various SKS features is still lacking. Here we present a series of quantitative geodynamic models with data assimilation that systematically evaluate the influence of different lithospheric and mantle structures on mantle flow and seismic anisotropy. These tests reveal a configuration of mantle deformation more complex than ever envisioned before. In particular, we find that both lithospheric thickness variations and toroidal flows around the Juan de Fuca slab modulate flow locally, but their coexistence enhances large-scale mantle deformation below the western U.S. The ancient Farallon slab below the east coast pulls the western U.S. upper mantle eastward, spanning the regionally extensive circular pattern of SKS splitting. The prominent E-W oriented anisotropy pattern within the Pacific Northwest reflects the 2 existence of sustaining eastward intrusion of the hot Pacific oceanic mantle to beneath the continental interior, from within slab tears below Oregon to under the Snake River Plain and the Yellowstone caldera. This work provides an independent support to the formation of intra-plate volcanism due to intruding shallow hot mantle instead of a rising mantle plume.
Journal of Geophysical Research, 2003
We have obtained shear velocity structure beneath the northeastern United States and southeastern Canada using Rayleigh wave phases and amplitudes. Thin crust (36-42 km) is observed along the Atlantic coast and in the eastern Appalachian orogen, and thick crust (42-46 km) is imaged in the western Appalachians and in the western New York portion of the Grenville Province. The variation of crustal thickness correlates well with the observed Bouguer gravity anomalies. In the upper mantle, the high-velocity continental keel of cratonic North America is present in the western part of the study area, while a broad low-velocity region is imaged in New England from the Hudson River valley to the White Mountains. This low-velocity anomaly is probably the consequence of past heating of the lithospheric mantle associated with the Monteregian hotspot and may represent intrusion of asthenosphere into the edge of the keel. In addition to lateral variations in velocity, we estimate the azimuthal dependence of phase velocity. Strong and relatively uniform shear wave splitting is observed in the study region, but at periods of 100 s or less, the average azimuthal anisotropy of Rayleigh waves is less than 1% and is not significantly different from zero at any individual period. This small degree of azimuthal anisotropy is not consistent with a substantial contribution to shear wave splitting from fossil anisotropy in the lithosphere. Much of the source of the shear wave splitting must lie deeper than 200 km.
Global patterns of azimuthal anisotropy and deformations in the continental mantle
Geophysical Journal International, 1992
We present a summary of measurements of azimuthal anisotropy in the continental mantle based on the SKS technique and performed mostly with the active participation of the authors. The directions of polarization of the fast quasi-shear wave and the time delays between the quasi-shear waves are obtained at nearly 70 locations in all continents, except Antarctica. These data are interpreted in terms of lattice-preferred orientation of olivine which is caused by deformations in the mantle. The depth interval responsible for anisotropy is unknown but the data suggest that it may reach at least 300 km. The fast directions in SKS do not show clear correlation with the fast directions of the teleseismic P at the same seismograph stations. In the regions of present-day convergence the fast direction of anisotropy usually aligns with the plate boundary. This correlation implies that the direction of shortening is the same in the crust and the upper mantle. In the regions of rifting, the inferred direction of mantle flow usually aligns with the direction of extension in the crust. Outside the regions of recent tectonic activity we, most likely, observe a combined effect of frozen anisotropy in the subcrustal lithosphere and of recently formed anisotropy in the asthenosphere. On a global scale, in these regions there is a positive correlation between the absolute plate velocity directions and the fast directions of anisotropy. The correlation is especially strong in central and eastern parts of North America. A clear absence of any evidence of large-scale azimuthal anisotropy in the data of long-range refraction profiling for the upper 100 km of the mantle of that region implies that the effect in SKS is generated mainly at greater depths, in the asthenosphere. Orientation of olivine at these depths reflects recent and present-day flow in the mantle rather than processes of a distant geologic past.
Variable Azimuthal Anisotropy in Earth's Lowermost Mantle
Science, 2004
A persistent reversal in the expected polarity of the initiation of vertically polarized shear waves that graze the Dµ layer (the layer at the boundary between the outer core and the lower mantle of Earth) in some regions starts at the arrival time of horizontally polarized shear waves. Full waveform modeling of the split shear waves for paths beneath the Caribbean requires azimuthal anisotropy at the base of the mantle. Models with laterally coherent patterns of transverse isotropy with the hexagonal symmetry axis of the mineral phases tilted from the vertical by as much as 20-are consistent with the data. Small-scale convection cells within the mantle above the Dµ layer may cause the observed variations by inducing laterally variable crystallographic or shapepreferred orientation in minerals in the Dµ layer.