We present an upper mantle P-wave velocity model for the Ross Sea Embayment (RSE) region of West Antarctica, constructed by inverting relative P-wave travel-times from 1881 teleseismic earthquakes recorded by two temporary broadband seismograph deployments on the Ross Ice Shelf, as well as by regional ice- and rock-sited seismic stations surrounding the RSE. Faster upper mantle P-wave velocities (∼ +1%) characterize the eastern part of the RSE, indicating that the lithosphere in this part of the RSE may not have been reheated by mid-to-late Cenozoic rifting that affected other parts of the Late Cretaceous West Antarctic Rift System. Slower upper mantle velocities (∼ −1%) characterize the western part of the RSE over a ∼500 km-wide region, extending from the central RSE to the Transantarctic Mountains (TAM). Within this region, the model shows two areas of even slower velocities (∼ −1.5%) centered beneath Mt. Erebus and Mt. Melbourne along the TAM front. We attribute the broader region of slow velocities mainly to reheating of the lithospheric mantle by Paleogene rifting, while the slower velocities beneath the areas of recent volcanism may reflect a Neogene-present phase of rifting and/or plume activity associated with the formation of the Terror Rift. Beneath the Ford Ranges and King Edward VII Peninsula in western Marie Byrd Land, the P-wave model shows lateral variability in upper mantle velocities of ±0.5% over distances of a few hundred km. The heterogeneity in upper mantle velocities imaged beneath the RSE and western Marie Byrd Land, assuming no significant variation in mantle composition, indicates variations in upper mantle temperatures of at least 100◦C. These temperature variations could lead to differences in surface heat flow of ∼ ±10 mW/m2 and mantle viscosity of 102 Pa s regionally across the study area, possibly influencing the stability of the West Antarctic Ice Sheet by affecting basal ice conditions and glacial isostatic adjustment.
more »
« less
The Seismic Structure of the Antarctic Upper Mantle
Abstract The deployment of seismic stations and the development of ambient noise tomography and new analysis methods provide an opportunity for higher resolution imaging of Antarctica. Here we review recent seismic structure models and describe their implications for the dynamics and history of the Antarctic upper mantle. Results show that most of East Antarctica is underlain by continental lithosphere to depths of ∼ 200 km. The thickest lithosphere is found in a band 500-1000 km west of the Transantarctic Mountains, representing the continuation of cratonic lithosphere with Australian affinity beneath the ice. Dronning Maud Land and the Lambert Graben show much thinner lithosphere, consistent with Phanerozoic lithospheric disruption. The Transantarctic Mountains mark a sharp boundary between cratonic lithosphere and the warmer upper mantle of West Antarctica. In the Southern Transantarctic Mountains, cratonic lithosphere has been replaced by warm asthenosphere, giving rise to Cenozoic volcanism and an elevated mountainous region. The Marie Byrd Land volcanic dome is underlain by slow seismic velocities extending through the transition zone, consistent with a mantle plume. Slow velocity anomalies beneath the coast from the Amundsen Sea Embayment to the Antarctic Peninsula likely result from upwelling of warm asthenosphere during subduction of the Antarctic-Phoenix spreading center.
more »
« less
- NSF-PAR ID:
- 10275822
- Date Published:
- Journal Name:
- Geological Society, London, Memoirs
- ISSN:
- 0435-4052
- Page Range / eLocation ID:
- M56-2020-18
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
Many seismic tomography investigations have imaged the East Antarctic lithosphere as a thick and continuous cratonic structure that is separated from the thinner lithosphere of the adjacent West Antarctic Rift System by the Transantarctic Mountains. However, recent studies have painted a more complicated picture, suggesting, for instance, a separate cratonic fragment beneath Dronning Maud Land and possible lithospheric delamination beneath the southern Transantarctic Mountains. In addition, patterns of intracratonic seismicity have been identified near the Gamburtsev Subglacial Mountains in East Antarctica, indicating possible rift zones in this region. That said, detailed imaging of the subsurface structure has remained challenging given the sparse distribution of seismic stations and the generally low seismicity rate throughout the interior of East Antarctica. Therefore, new approaches that can leverage existing seismic datasets to elucidate the Antarctic cratonic structure are vital. We are utilizing records of ambient seismic noise recorded by numerous temporary, moderate-term, and long-term seismic networks throughout Antarctica to improve the imaging of the lithospheric structure. Empirical Green’s Functions with periods of 40-340 seconds have been extracted using a frequency-time normalization approach, and these data are being used to constrain our full-waveform inversion. A finite-difference approach with a continental-scale, spherical grid is employed to numerically model synthetic seismograms, and a scattering integral method is used to construct the associated sensitivity kernels. Our initial results suggest that some portions of East Antarctica, particularly those beneath the Wilkes Subglacial Basin and the Aurora Basin, may have reduced shear-wave velocities that potentially indicate regions of thinner lithosphere. Further, possible segmentation may be present in the vicinity of the Gamburtsev Subglacial Mountains. Our new tomographic results will allow for further assessment of the East Antarctic tectonic structure and its relation to local seismicity.more » « less
-
null (Ed.)The thick ice coverage and harsh climatic conditions in East Antarctica hinder detailed investigations of tectonic features, leading to debates regarding the origin and evolution of the Gamburtsev Subglacial Mountains (GSM), the Wilkes Subglacial Basin (WSB), the Aurora Subglacial Basin (ASB), and the Transantarctic Mountains (TAMs). Present tomographic models lack resolution and consistency given the minimal seismic coverage in East Antarctica. To further such investigations, we are using full-waveform ambient noise tomography to model shear-wave velocities and to constrain the crustal and upper mantle structure beneath East Antarctica. This approach utilizes Empirical Green’s functions (EGFs), which provides information about the Earth structure between recording stations and is an alternative approach compared to many traditional tomographic models. EGFs from ambient seismic noise between periods of 15-340 secs are extracted using a frequency-time normalization approach, and synthetic waveforms are simulated through a three-dimensional heterogeneous Earth model using a finite-difference wave propagation method with a grid spacing of 0.025º (~ 2.25 km). Phase delays are computed by cross correlating EGFs and the synthetics, and sensitivity kernels are constructed using a scattering integral approach. Preliminary results show slow velocities beneath both the WSB and ASB, possibly reflecting old rift systems or other inherited tectonic structures. A transition from slow to fast velocities beneath the Northern Victoria Land portion of the TAMs is consistent with thermal loading beneath the mountain range. Slow velocities beneath the GSM may be due to rifting associated with the extended Lambert Rift System. These preliminary results are currently being updated using a larger EGF dataset; our final model will be used to assess East Antarctic tectonic structures and to resolve the ambiguity associated with their origin models.more » « less
-
more » « less
Abstract We examine upper mantle anisotropy across the Antarctic continent using 102 new shear wave splitting measurements obtained from teleseismic SKS, SKKS, and PKS phases combined with 107 previously published results. For the new measurements, an eigenvalue technique is used to estimate the fast polarization direction and delay time for each phase arrival, and high‐quality measurements are stacked to determine the best‐fit splitting parameters at each seismic station. The ensemble of splitting measurements shows largely NE‐SW‐oriented fast polarization directions across Antarctica, with a broadly clockwise rotation in polarization directions evident moving from west to east across the continent. Although the first‐order pattern of NE‐SW‐oriented polarization directions is suggestive of a single plate‐wide source of anisotropy, we argue the observed pattern of anisotropy more likely arises from regionally variable contributions of both lithospheric and sub‐lithospheric mantle sources. Anisotropy observed in the interior of East Antarctica, a region underlain by thick lithosphere, can be attributed to relict fabrics associated with Precambrian tectonism. In contrast, anisotropy observed in coastal East Antarctica, the Transantarctic Mountains (TAM), and across much of West Antarctica likely reflects both lithospheric and sub‐lithospheric mantle fabrics. While sub‐lithospheric mantle fabrics are best associated with either plate motion‐induced asthenospheric flow or small‐scale convection, lithospheric mantle fabrics in coastal East Antarctica, the TAM, and West Antarctica generally reflect Jurassic—Cenozoic tectonic activity.
-
null (Ed.)The origins of tectonic structures in East Antarctica, such as the Gamburtsev Subglacial Mountains (GSMs), the Wilkes Subglacial Basin (WSB), the Aurora Subglacial Basin (ASB), and the Transantarctic Mountains (TAMs), are not clearly understood. Previous investigations have proposed multiple origin models to explain the formation of these structures; however, existing tomographic images lack resolution and consistency given the sparse seismic coverage in East Antarctica. We use full-waveform ambient noise tomography to model the shear-wave velocity structure beneath East Antarctica to further investigate these features. We extract Rayleigh-wave Empirical Green’s Func-tions (EGFs) between periods of 15 and 340 secs from ambient seismic noise using a frequency time normalization technique. Synthetic waveforms are simulated through a 3-D heterogenous Earth model with a lateral grid spacing of 0.025º (~2.25 km) using a finite-difference wave propagation method. The synthetic seismograms are cross-correlated with the EGFs to measure the phase delays. The fi-nite-frequency sensitivity kernels are calculated using the scattering-integral approach and the shear-wave velocity model is computed by inverting the phase delays using a sparse damped least-square inversion method. Preliminary results show fast seismic velocities beneath the WSB, which may be associated with thick and stable lithosphere, and slow velocities beneath the ASB, possibly reflecting old rift systems or other inherited tectonic structures. Slow upper mantle velocities are also observed beneath the TAMs, possibly associated with a thermal load that contributes to the uplift of the moun-tain range. Slow shear-wave velocities in the vicinity of the GSMs may be associated with rifting along the extended Lambert Rift System. Our final tomographic model and associated tectonic inter-pretations will be shared.more » « less
- Free Publicly Accessible Full Text
-
Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1144/M56-2020-18
