An official website of the United States government
Abstract Anatolian tectonics are associated with slab retreat in the west and gravitational potential energy and continental collision in the east, leading to westward motion of Anatolia relative to Eurasia, partially accommodated on the North and East Anatolian transform faults. We construct a three‐layer, azimuthal seismic anisotropy model using new Rayleigh wave measurements and compare results against geodetic strain‐rates and new receiver function analysis for the crust, as well as new and existing shear wave splitting for the mantle to link surface and deep dynamics. In the crust, anisotropy is primarily controlled by recent tectonics, especially along major fault zones. In the lithospheric mantle, anisotropy reflects ongoing tectonic deformation in western Anatolia, whereas eastern Anatolia mainly preserves prior deformation signatures. In the asthenosphere, anisotropy reflects mantle convection, including the effects of Hellenic slab retreat. Our model provides new constraints on lithospheric deformation and geodynamics in the Eastern Mediterranean.
more »
« less
Crustal and Uppermost Mantle Azimuthal Seismic Anisotropy of Antarctica From Ambient Noise Tomography
Abstract Seismic anisotropy provides essential information for characterizing the orientation of deformation and flow in the crust and mantle. The isotropic structure of the Antarctic crust and upper mantle has been determined by previous studies, but the azimuthal anisotropy structure has only been constrained by mantle core phase (SKS) splitting observations. This study determines the azimuthal anisotropic structure of the crust and mantle beneath the central and West Antarctica based on 8—55 s Rayleigh wave phase velocities from ambient noise cross‐correlation. An anisotropic Rayleigh wave phase velocity map was created using a ray—based tomography method. These data are inverted using a Bayesian Monte Carlo method to obtain an azimuthal anisotropy model with uncertainties. The azimuthal anisotropy structure in most of the study region can be fit by a two‐layer structure, with one layer at depths of 0–15 km in the shallow crust and the other layer in the uppermost mantle. The azimuthal anisotropic layer in the shallow crust of West Antarctica, where it coincides with strong positive radial anisotropy quantified by the previous study, shows a fast direction that is subparallel to the inferred extension direction of the West Antarctic Rift System. Fast directions of upper mantle azimuthal anisotropy generally align with teleseismic shear wave splitting fast directions, suggesting a thin lithosphere or similar lithosphere‐asthenosphere deformation. However, inconsistencies in this exist in Marie Byrd Land, indicating differing ancient deformation patterns in the shallow mantle lithosphere sampled by the surface waves and deformation in the deeper mantle and asthenosphere sampled more strongly by splitting measurements.
more »
« less
- PAR ID:
- 10519982
- Publisher / Repository:
- American Geophysical Union
- Date Published:
- Journal Name:
- Journal of Geophysical Research: Solid Earth
- Volume:
- 129
- Issue:
- 1
- ISSN:
- 2169-9313
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
Abstract The 2021–2023 OHANA ocean‐bottom seismometer deployment in the northeast Pacific Ocean provides a rich data set for seismic studies to explore the crust, lithosphere and asthenosphere in a 600 km wide region about 1,500 km northeast of Hawaii, west of the Moonless Mountains. The study area covers mainly 40‐to‐55 Myr‐old Pacific lithosphere. A fundamental question to be addressed is whether this particular area has the signature of typical oceanic lithosphere that has a normal plate cooling history or if there is evidence for plate rejuvenation from a secondary mantle‐dynamic process. We present the analysis of path‐averaged low‐frequency (10–60 mHz) Rayleigh‐wave phase‐velocity curves from earthquake records retrieved for 210 two‐station pairs, and its inversion for depth‐dependent shear velocity,Vs. Forward modeling and the inversion of the average phase velocity indicate lower shear velocities than anticipated for 50‐Myr old lithosphere. Velocities in the mid‐to‐lower lithosphere and in the asthenosphere are 2%–3% lower than expected. We also observe significant and internally consistent azimuthal anisotropy of up to 5%, where the “fast direction” places between the fossil and modern plate motion directions. A preliminary analysis of ambient‐noise cross‐correlation functions reveals similar patterns for the fundamental mode as well as the first overtone.more » « less
-
Niu, F (Ed.)The 2021-2023 OHANA ocean-bottom seismometer deployment in the northeast Pacific Ocean provides a rich dataset for seismic studies to explore the crust, lithosphere and asthenosphere in a 600 km wide region about 1500 km northeast of Hawaii, west of the Moonless Mountains. The study area covers mainly 40-to-55 Myr-old Pacific lithosphere. A fundamental question to be addressed is whether this particular area has the signature of typical oceanic lithosphere that has a normal plate cooling history or if there is evidence for plate rejuvenation from a secondary mantle-dynamic process. We present the analysis of path-averaged low-frequency (10 - 60 mHz) Rayleigh-wave phase-velocity curves from earthquake records retrieved for 210 two-station pairs, and its inversion for depth-dependent shear velocity, Vs. Forward modeling and the inversion of the average phase velocity indicate lower shear velocities than anticipated for 50-Myr old lithosphere. Velocities in the mid-to-lower lithosphere and in the asthenosphere are 2-3% lower than expected. We also observe significant and internally consistent azimuthal anisotropy of up to 5%, where the ‘fast direction’ places between the fossil and modern plate motion directions. A preliminary analysis of ambient-noise cross-correlation functions reveals similar patterns for the fundamental mode as well as the first overtone.more » « less
-
Abstract Seismic azimuthal anisotropy beneath Australia is investigated using splitting of the teleseismic PKS, SKKS, and SKS phases to delineate asthenospheric flow and lithospheric deformation beneath one of the oldest and fast‐moving continents on Earth. In total 511 pairs of high‐quality splitting parameters were observed at 116 seismic stations. Unlike other stable continental areas in Africa, East Asia, and North America, where spatially consistent splitting parameters dominate, the fast orientations and splitting times observed in Australia show a complex pattern, with a slightly smaller than normal average splitting time of 0.85 ± 0.33 s. On the North Australian Craton, the fast orientations are mostly N‐S, which is parallel to the absolute plate motion (APM) direction in the hotspot frame. Those observed in the South Australian Craton are mostly NE‐SW and E‐W, which are perpendicular to the maximum lithospheric horizontal shortening direction. In east Australia, the observed azimuthal anisotropy can be attributed to either APM induced simple shear or lithospheric fabric parallel to the strike of the orogenic belts. The observed spatial variations of the seismic azimuthal anisotropy, when combined with results from depth estimation utilizing the spatial coherency of the splitting parameters and seismic tomography studies, suggest that the azimuthal anisotropy in Australia can mostly be related to simple shear in the rheologically transition layer between the lithosphere and asthenosphere. Non‐APM parallel anisotropy is attributable to modulations of the mantle flow system by undulations of the bottom of the lithosphere, with a spatially variable degree of contribution from lithospheric fabric.more » « less
-
Abstract Seismic anisotropy beneath eastern North America, as expressed in shear wave splitting observations, has been attributed to plate motion‐parallel shear in the asthenosphere, resulting in fast axes aligned with the plate motion. However, deviations of fast axes from plate motion directions are observed near major tectonic boundaries of the Appalachians, indicating contributions from lithospheric anisotropy associated with past tectonic processes. In this study, we conduct anisotropic receiver function (RF) analysis using data from a dense seismic array traversing the New England Appalachians in Connecticut to examine anisotropic layers in the crust and upper mantle and correlate them with past tectonic processes as well as present‐day mantle flow. We use the harmonic decomposition method to separate directionally‐dependent variations of RFs and focus on features with the same harmonic signals observed across multiple stations. Within the crust, there are multiple features that may be correlated with stratification in the Hartford Basin, faults in the Taconic thrust belt, shear zones formed during Salinic/Acadian terrane accretion events, and orogen‐parallel crustal flow in the Acadian orogenic plateau. We apply a Bayesian inversion method to obtain quantitative constraints on the direction and strength of intra‐crustal anisotropy beneath the Hartford Basin. In the upper mantle, we identify a fossil shear zone possibly formed during oblique subduction of Rheic Ocean lithosphere. We also find evidence for a plate motion‐parallel flow zone in the asthenosphere that is likely disturbed by mantle upwelling near the southern margin of the Northern Appalachian Anomaly in the eastern part of the study area.more » « less
- Free Publicly Accessible Full Text
-
Accepted Manuscript
- Journal Article:
- https://doi.org/10.1029/2023JB027556
