Department of Geological Sciences

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Browsing Department of Geological Sciences by Subject "Antarctica"

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    Crustal and upper-mantle structure beneath ice-covered regions in Antarctica from S-wave receiver functions and implications for heat flow
    (Oxford University Press, 2016) Ramirez, C.; Nyblade, A.; Hansen, S. E.; Wiens, D. A.; Anandakrishnan, S.; Aster, R. C.; Huerta, A. D.; Shore, P.; Wilson, T.; Pennsylvania Commonwealth System of Higher Education (PCSHE); Pennsylvania State University; Pennsylvania State University - University Park; University of Alabama Tuscaloosa; Washington University (WUSTL); Colorado State University; Central Washington University; University System of Ohio; Ohio State University
    S-wave receiver functions (SRFs) are used to investigate crustal and upper-mantle structure beneath several ice-covered areas of Antarctica. Moho S-to-P (Sp) arrivals are observed at similar to 6-8 s in SRF stacks for stations in the Gamburtsev Mountains (GAM) and Vostok Highlands (VHIG), similar to 5-6 s for stations in the Transantarctic Mountains (TAM) and the Wilkes Basin (WILK), and similar to 3-4 s for stations in the West Antarctic Rift System (WARS) and the Marie Byrd Land Dome (MBLD). A grid search is used to model the Moho Sp conversion time with Rayleigh wave phase velocities from 18 to 30 s period to estimate crustal thickness and mean crustal shear wave velocity. The Moho depths obtained are between 43 and 58 km for GAM, 36 and 47 km for VHIG, 39 and 46 km for WILK, 39 and 45 km for TAM, 19 and 29 km for WARS and 20 and 35 km for MBLD. SRF stacks for GAM, VHIG, WILK and TAM show little evidence of Sp arrivals coming from upper-mantle depths. SRF stacks for WARS and MBLD show Sp energy arriving from upper-mantle depths but arrival amplitudes do not rise above bootstrapped uncertainty bounds. The age and thickness of the crust is used as a heat flow proxy through comparison with other similar terrains where heat flow has been measured. Crustal structure in GAM, VHIG and WILK is similar to Precambrian terrains in other continents where heat flow ranges from similar to 41 to 58 mW m(-2), suggesting that heat flow across those areas of East Antarctica is not elevated. For the WARS, we use the Cretaceous Newfoundland-Iberia rifted margins and the Mesozoic-Tertiary North Sea rift as tectonic analogues. The low-to-moderate heat flow reported for the Newfoundland-Iberia margins (40-65 mW m(-2)) and North Sea rift (60-85 mW m(-2)) suggest that heat flow across the WARS also may not be elevated. However, the possibility of high heat flow associated with localized Cenozoic extension or Cenozoic-recent magmatic activity in some parts of the WARS cannot be ruled out.
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    Crustal structure beneath the Northern Transantarctic Mountains and Wilkes Subglacial Basin: Implications for tectonic origins
    (American Geophysical Union, 2016-02-13) Hansen, Samantha E.; Kenyon, Lindsey M.; Graw, Jordan H.; Park, Yongcheol; Nyblade, Andrew A.; University of Alabama Tuscaloosa; Korea Institute of Ocean Science & Technology (KIOST); Korea Polar Research Institute (KOPRI); Pennsylvania Commonwealth System of Higher Education (PCSHE); Pennsylvania State University; Pennsylvania State University - University Park
    The Transantarctic Mountains (TAMs) are the largest noncollisional mountain range on Earth. Their origin, as well as the origin of the Wilkes Subglacial Basin (WSB) along the inland side of the TAMs, has been widely debated, and a key constraint to distinguish between competing models is the underlying crustal structure. Previous investigations have examined this structure but have primarily focused on a small region of the central TAMs near Ross Island, providing little along-strike constraint. In this study, we use data from the new Transantarctic Mountains Northern Network and from five stations operated by the Korea Polar Research Institute to investigate the crustal structure beneath a previously unexplored portion of the TAMs. Using S wave receiver functions and Rayleigh wave phase velocities, crustal thickness and average crustal shear velocity ((V)overbar(s)) are resolved within 4km and 0.1km/s, respectively. The crust thickens from similar to 20km near the Ross Sea coast to similar to 46km beneath the northern TAMs, which is somewhat thicker than that imaged in previous studies beneath the central TAMs. The crust thins to similar to 41km beneath the WSB. (V)overbar(s) ranges from similar to 3.1-3.9km/s, with slower velocities near the coast. Our findings are consistent with a flexural origin for the TAMs and WSB, where these features result from broad flexure of the East Antarctic lithosphere and uplift along its western edge due to thermal conduction from hotter mantle beneath West Antarctica. Locally, thicker crust may explain the similar to 1km of additional topography in the northern TAMs compared to the central TAMs.
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    Investigating Tectonic Structures in East Antarctica Using Full Waveform Ambient Noise Tomography
    (University of Alabama Libraries, 2021) Kumar, Ashish; Hansen, Samantha; University of Alabama Tuscaloosa
    Previous investigations have proposed multiple origin models to explain the formation of major tectonic structures, such as the Gamburtsev Subglacial Mountains (GSMs), the Wilkes Subglacial Basin (WSB), the Aurora Subglacial Basin (ASB), and the Transantarctic Mountains (TAMs) in East Antarctica. However, existing tomographic images lack resolution and consistency given the sparse seismic coverage across the continent, particularly in East Antarctica. In this thesis, I use full-waveform ambient noise tomography to model the shear-wave velocity structure beneath East Antarctica to further investigate these tectonic features and to provide new insights into existing origin models. This technique has been shown to provide improved resolution of the seismic structure in geographic regions with limited station coverage compared to more traditional tomographic approaches. Rayleigh-wave Empirical Green’s Functions are extracted from ambient noise using a frequency-time normalization technique. Synthetic waveforms are simulated with a lateral grid spacing of 0.025º (~2.25 km) and are cross-correlated with the EGFs to measure phase delays. The shear-wave velocity model is computed by inverting the phase delays using a sparse, damped least-squares inversion method. The new tomographic model shows fast velocities beneath the GSMs that extend to ~250 km depth, suggesting Archean or Proterozoic lithosphere beneath the mountain range. The perseverance of thick ancient crust beneath the GSMs support the high topography of the mountain range. Slow upper mantle velocities are observed beneath the TAMs, likely associated with hot upper mantle material that provides a thermal load beneath the mountain range, consistent with a flexural uplift model. Beneath the WSB, fast seismic velocities are attributed to thick, stable lithosphere, also consistent with a flexural origin model. Slow velocities beneath the ASB, which is an area of particular interest since it has not been studied extensively, may reflect a zone of lithospheric weakness, associated with the reactivation of a major fault system. By providing new evidence that further constrains the origin models for tectonic structures in East Antarctica, my full-waveform ambient noise model helps to elucidate the geologic history of this remote continent. These seismic constraints could also inform cryospheric models, which require lithospheric and mantle characteristics to assess ice-sheet evolution.
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    Using S wave receiver functions to estimate crustal structure beneath ice sheets: An application to the Transantarctic Mountains and East Antarctic craton
    (American Geophysical Union, 2009) Hansen, Samantha E.; Julia, Jordi; Nyblade, Andrew A.; Pyle, Moira L.; Wiens, Douglas A.; Anandakrishnan, Sridhar; Pennsylvania Commonwealth System of Higher Education (PCSHE); Pennsylvania State University; Pennsylvania State University - University Park; Washington University (WUSTL); University of Alabama Tuscaloosa
    For seismic stations deployed on ice sheets, determining crustal structure using P wave receiver functions can be difficult since ice reverberations may mask P-to-S (Ps) conversions from the crust-mantle boundary (Moho). In this study, we assess the usefulness of S wave receiver functions (SRFs), which are not affected by ice multiples, for investigating crustal structure beneath ice sheets by analyzing broadband seismic data recorded across the Transantarctic Mountains (TAMs) and the East Antarctic (EA) craton. Clear S-to-P (Sp) conversions from the Moho are obtained using standard SRF processing methods and are easier to interpret than the corresponding Ps conversion on PRFs. When the Sp-S times are modeled together with 16-20 s Rayleigh wave group velocities, we obtain Moho depth estimates of similar to 40-45 km for the EA craton, consistent with average Precambrian crustal thickness found globally but similar to 9 km thicker than previously reported estimates. A somewhat thinner crust (similar to 35-40 km) is obtained beneath the TAMs, suggesting that crustal buoyancy is at most a minor contributor to the uplift of the mountain range in this region.