An official website of the United States government
Abstract Several hypotheses have been proposed to explain intriguing circular shear wave splitting patterns in the Pacific Northwest, invoking either 2‐D entrained flows or 3‐D return flows. Here, we present some hitherto unidentified, depth‐dependent anisotropic signatures to reconcile different conceptual models. At depths shallower than 200 km, the fast propagation directions of seismic waves to the west of the Rocky Mountain are aligned sub‐parallel to the subduction direction of the Juan de Fuca and Gorda Plates. This pattern is consistent with previous onshore/offshore shear wave splitting measurements and indicates that 2‐D entrained flows dominate at shallower depths. From 300 to 500 km, two large‐scale return flows are revealed, one circulating around Nevada and Colorado and the other running around the edge of the descending Juan de Fuca slab. These observations suggest the development of toroidal‐mode mantle flows, driven by the fast rollback of the narrow, fragmented Juan de Fuca and Gorda slabs.
more »
« less
Depth‐Dependent Azimuthal Anisotropy Beneath the Juan de Fuca Plate System
Abstract We use surface wave measurements to reveal anisotropy as a function of depth within the Juan de Fuca and Gorda plate system. Using a two‐plane wave method, we measure phase velocity and azimuthal anisotropy of fundamental mode Rayleigh waves, solving for anisotropic shear velocity. These surface wave measurements are jointly inverted with constraints fromSKSsplitting studies using a Markov chain approach. We show that the two data sets are consistent and present inversions that offer new constraints on the vertical distribution of strain beneath the plates and the processes at spreading centers. Anisotropy of the Juan de Fuca plate interior is strongest (~2.4%) in the low‐velocity zone between ~40‐ to 90‐km depth, with ENE direction driven by relative shear between plate motion and mantle return flow from the Cascadia subduction zone. In disagreement withPnmeasurements, weak (~1.1%) lithospheric anisotropy in Juan de Fuca is highly oblique to the expected ridge‐perpendicular direction, perhaps connoting complex intralithospheric fabrics associated with melt or off‐axis downwelling. In the Gorda microplate, strong shallow anisotropy (~1.9%) is consistent withPninversions and aligned with spreading and may be enhanced by edge‐driven internal strain. Weak anisotropy with ambiguous orientation in the low‐velocity zone can be explained by Gorda's youth and modest motion relative to the Pacific. Deeper (≥90 km) fabric appears controlled by regional flow fields modulated by the Farallon slab edge: anisotropy is strong (~1.8%) beneath Gorda, but absent beneath the Juan de Fuca, which is in the strain shadow of the slab.
more »
« less
- Award ID(s):
- 1658214
- PAR ID:
- 10363953
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Journal of Geophysical Research: Solid Earth
- Volume:
- 125
- Issue:
- 8
- ISSN:
- 2169-9313
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
We estimate seismic azimuthal anisotropy for the Juan de Fuca ‐ Gorda plates from inversion of a new 10–80 s period Rayleigh wave dataset, resulting in a two‐layer model to 80 km depth. In the lithosphere, most anisotropy patterns reflect the kinematics of plate formation, as approximated from seafloor‐age‐based paleo‐spreading, except for regions close to propagator wakes and near plate boundaries. In the asthenosphere, the fast propagation orientations align with convective shear as inferred from the NUVEL1A plate motion model, which is indicative of a ∼3 Myr average, rather than with the more recent, ∼0.8 Myr, motions inferred from MORVEL. Regional anisotropy of this young plate system thus records convection like older plates such as the Pacific. On smaller scales, anisotropy imaging provides insights into dynamics of plate generation and can further elucidate plate reorganizations and changes in boundary loading.more » « less
-
Palin, Richard (Ed.)A 3D crustal model for the central Cascadia continental shelf and Coast Range between 44°N and 45°N shows that the crystalline crust of the forearc wedge beneath the coastline is characterized by a NW-trending, vertical slab of high-velocity rock interpreted to represent the dike complex that fed the Yachats Basalt, which was intruded into the forearc approximately 37 million years ago. A spatial correlation is observed between downward deflection of the crust of the subducting Juan de Fuca plate, inferred from inversion of PmP arrivals to image the Moho surface, and the high velocity (and consequently high density) anomaly underlying the Yachats Basalt. Apparent subsequent rebound of the subducting plate at greater depth suggests a primarily elastic response of the subducting plate to this load. Calculations for a range of plausible values for the magnitude of the load and the width and depth of the depression indicate that the effective elastic thickness of the subducted Juan de Fuca plate is < 6 km. Although our simple analytical models do not include partial support of the load of the slab by the adjacent upper plate crust or time dependence to account for the motion of the slab beneath the load, incorporation of those effects should decrease rather than increase the apparent strength of the subducted plate. We conclude that the subducted Juan de Fuca plate beneath the central Oregon margin is elastically thin and has the potential to store elastic strain energy before rupturing. Our model of a well-defined, focused and static upper plate load that locally deforms the subducted plate within the nominally seismogenic or transitional part of the Cascadia plate boundary may be unique in providing a relatively straightforward scenario for estimating the mechanical properties of the subducted Juan de Fuca plate. We extrapolate from these results to speculate that elastic deformation of the subducting plate may contribute to the low level of seismicity throughout much of the Cascadia forearc in the inter-seismic period between great earthquakes but note that our local results do not preclude faulting or elasto-plastic deformation of a thin and weak plate as it subducts. These results also suggest that the subducting plate should deform in response to larger scale variations in upper plate thickness and density.more » « less
-
SUMMARY A new amphibious seismic data set from the Cascadia subduction zone is used to characterize the lithosphere structure from the Juan de Fuca ridge to the Cascades backarc. These seismic data are allowing the imaging of an entire tectonic plate from its creation at the ridge through the onset of the subduction to beyond the volcanic arc, along the entire strike of the Cascadia subduction zone. We develop a tilt and compliance correction procedure for ocean-bottom seismometers that employs automated quality control to calculate robust station noise properties. To elucidate crust and upper-mantle structure, we present shoreline-crossing Rayleigh-wave phase-velocity maps for the Cascadia subduction zone, calculated from earthquake data from 20 to 160 s period and from ambient-noise correlations from 9 to 20 s period. We interpret the phase-velocity maps in terms of the tectonics associated with the Juan de Fuca plate history and the Cascadia subduction system. We find that thermal oceanic plate cooling models cannot explain velocity anomalies observed beneath the Juan de Fuca plate. Instead, they may be explained by a ≤1 per cent partial melt region beneath the ridge and are spatially collocated with patches of hydration and increased faulting in the crust and upper mantle near the deformation front. In the forearc, slow velocities appear to be more prevalent in areas that experienced high slip in past Cascadia megathrust earthquakes and generally occur updip of the highest-density tremor regions and locations of intraplate earthquakes. Beneath the volcanic arc, the slowest phase velocities correlate with regions of highest magma production volume.more » « less
-
Abstract To explore seismic structures beneath the Australian continents and subduction zone geometry around the Australian plate, we introduce a new radially‐anisotropic shear‐wavespeed model, AU21. By employing full‐waveform inversion on data from 248 regional earthquakes and 1,102 seismographic stations, we iteratively refine AU21, resulting in 32,655 body‐wave and 35,897 surface wave measurements. AU21 reveals distinct shear‐wavespeed contrasts between the Phanerozoic eastern continental margin and the Precambrian western and central Australia, with the lithosphere‐asthenosphere boundary estimated at 250–300 km beneath central and western Australia. Notably, a unique weak radial anisotropy layer at 80–150 km is identified beneath the western Australian craton, possibly due to alignments of dipping layers or tilted symmetry axes of anisotropic minerals. Furthermore, slow anomalies extending to the uppermost lower mantle beneath the east of New Guinea, Tasmania, and the Tasman Sea indicate deep thermal activities, likely contributing to the formation of a low wavespeed band along the eastern Australian margin. In addition, our findings demonstrate the stagnant Tonga slab within the mantle transition zone and the Kermadec slab's penetration through the 660‐km discontinuity into the lower mantle.more » « less
