Rayleigh wave constraints on shear-wave structure and azimuthal anisotropy beneath the Colorado Rocky Mountains (original) (raw)
Related papers
Models of seismic anisotropy in the deep continental lithosphere
Physics of The Earth and Planetary Interiors, 1993
Seismological observations (SKS-wave polarizations, systematic P-residual variations, azimuthal dependence of P n- and surface-wave velocities or a dispersion of surface waves) are not consistent with isotropic, if laterally heterogeneous, upper-mantle structure. Therefore, an anisotropy should be considered as an a priori aspect of future large-scale studies of mantle structure. Most studies of anisotropy, however, have assumed horizontal or vertical axes of symmetry, but such orientations cannot explain bipolar patterns of spatial variations of P residuals, which we have observed at many seismological stations. On the basis of the petrophysical properties of real upper-mantle rocks we consider anisotropy formed either by hexagonal or by orthorhombic aggregates composed of olivine, orthopyroxene, and clinopyroxene. Rotations of the aggregates about vertical and horizontal axes allow us to find the three-dimensional orientations of symmetry axes that fit combinations of both P and S seismological observations in Central Europe and in western North America. The orientations with plunging symmetry axes (velocity extremes) seem to be consistent across large, spatially uniform tectonic units and change abruptly at important suture zones.
USArray shear wave splitting shows seismic anisotropy from both lithosphere and asthenosphere
Geology, 2015
North America provides an important test for assessing the cou- pling of large continents with heterogeneous Archean- to Cenozoic- aged lithospheric provinces to the mantle flow. We use the unprec- edented spatial coverage of the USArray seismic network to obtain an extensive and consistent data set of shear wave splitting intensity measurements at 1436 stations. Overall, the measurements are con- sistent with simple shear deformation in the asthenosphere due to viscous coupling to the overriding lithosphere. The fast directions agree with the absolute plate motion direction with a mean differ- ence of 2° with 27° standard deviation. There are, however, devia- tions from this simple pattern, including a band along the Rocky Mountain front, indicative of flow complication due to gradients in lithospheric thickness, and variations in amplitude through the cen- tral United States, which can be explained through varying contri- butions of lithospheric anisotropy. Thus, seismic anisotropy may be sourced in both the asthenosphere and lithosphere, and variations in splitting intensity are due to lithospheric anisotropy developed during deformation over long time scales.
Geophysical Journal International, 2011
Using a combination of long period seismic waveforms and SKS splitting measurements, we have developed a 3-D upper-mantle model (SAWum_NA2) of North America that includes isotropic shear velocity, with a lateral resolution of ∼250 km, as well as radial and azimuthal anisotropy, with a lateral resolution of ∼500 km. Combining these results, we infer several key features of lithosphere and asthenosphere structure.
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.
Bulletin of the Seismological Society of America, 2008
Shear-wave splitting measurements from SKS and SKKS phases show fast polarization azimuths that are subparallel to North American absolute plate motion within the central Rio Grande Rift (RGR) and Colorado Plateau (CP) through to the western rim of the CP, with anisotropy beneath the CP and central RGR showing a remarkably consistent pattern with a mean fast azimuth of 40° 6°E of N. Approaching the rim from the southeast, fast anisotropic directions become northnortheast-south-southwest (NNE-SSW), rotate counter clockwise to north-south in the CP-GB transition, and then to NNW-SSE in the western Great Basin (GB). This change is coincident with uppermost mantle S-wave velocity perturbations that vary from 4% beneath the western CP and the eastern edge of the Marysvale volcanic field to about 8% beneath the GB. Corresponding delay times average 1.5 sec beneath the central CP, decrease to approximately 0.8 sec near the CP-GB transition, and increase to about 1.2 sec beneath the GB. For the central CP, we suggest anisotropy predominantly controlled by North American plate motion above the asthenosphere. The observed pattern of westward-rotating anisotropy from the western CP through the CP-GB transition may be influenced to asthenospheric flow around a CP lithospheric keel and/or by vertical flow arising from edge-driven small-scale convection. The anisotropic transition from the CP to the GB thus marks a first-order change from absolute plate motion dominated lithosphere-asthenosphere shear to a new regime controlled by regional flow processes. The NNW-SSE anisotropic fast directions of split SKS waves in the eastern GB area are part of a broad circular pattern of seismic anisotropic fast direction in the central GB that has recently been hypothesized to be due to toroidal flow around the sinking Juan de Fuca-Gorda slab.
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.
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.
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.
Keywords: lithosphere–asthenosphere boundary seismic anisotropy LPO development The depth of the oceanic lithosphere–asthenosphere boundary (LAB), as inferred from shear wave velocities, increases with lithospheric age, in agreement with models of cooling oceanic lithosphere. On the other hand, the distribution of radial anisotropy under oceanic plates is almost age-independent. In particular, radial anisotropy shows a maximum positive gradient at a depth of ∼70 km, which, if used as a proxy, indicates an age-independent LAB depth. These contrasting observations have fueled a controversy on the seismological signature of the LAB. To better understand the discrepancy between these observations, we model the development of lattice preferred orientation (LPO) in upper mantle crystal aggregates and predict the seismic anisotropy produced by plate-driven mid-ocean ridge flows. The model accounts for the progressive cooling of the lithosphere with age and can incorporate both diffusion and dislocation creep deformation mechanisms. We find that an age-independent distribution of radial anisotropy is the natural consequence of these simple flows. The depth and strength of anisotropy is further controlled by the deformation regime – dislocation or diffusion creep – experienced by crystals during their ascent towards, and subsequent motion away from, the ridge axis. Comparison to surface wave tomography models yield constraints on rheological parameters such as the activation volume. Although not excluded, additional mechanisms proposed to explain some geophysical signatures of the LAB, such as the presence of partial melt or changes in water content, are not required to explain the radial anisotropy proxy. Our prediction, that the age-independent radial anisotropy proxy marks the transition to flow-induced asthenospheric anisotropy, provides a way to reconcile thermal, mechanical and seismological views of the LAB.