Browsing by Author "Nyblade, Andrew A."
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Item 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 ParkThe 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.Item Determining crustal thickness beneath the Transantarctic Mountains and the Wilkes Subglacial Basin using S-wave receiver functions(University of Alabama Libraries, 2014) Kenyon, Lindsey M.; Hansen, Samantha E.; University of Alabama TuscaloosaThe Transantarctic Mountains (TAMs) are a ~4,000 km long mountain range, with elevations up to 4,500 m, which separate East and West Antarctica. Given the lack of compressional structures in the TAMs, the origin for these mountains is unclear, and many possible uplift mechanisms have been suggested. The formation of the Wilkes Subglacial Basin (WSB), which is situated inland and parallel to the TAMs, has also been widely debated. A key characteristic to distinguish between different origin models for the TAMs and the WSB is the thickness of the crust beneath these areas. A new 15-station seismic array deployed in the northern TAMs, called the Transantarctic Mountains Northern Network (TAMNNET), as well as 5 stations operated by the Korean Polar Research Institute (KOPRI), are used to investigate the crustal structure beneath a previously unexplored portion of the TAMs and the WSB. Data from the combined TAMNNET and KOPRI networks are analyzed using S-wave receiver functions (SRFs) to estimate the crustal thicknesses. Using both the timing of the conversion from the crust-mantle interface obtained with the SRFs and Rayleigh wave phase velocities, a grid search procedure is used to determine the crustal thickness and velocity beneath each station. Results indicate that the crust is 12-27 km thick near the Ross Sea coast, increasing to a maximum thickness of ~47 km beneath some portions of the TAMs. Further inland, beneath the East Antarctic craton and the WSB, the crust has an average thickness of ~42 km. Average crustal S-wave velocities range from 3.3-3.8 km/s, with the slowest velocities near the coast. These results support a flexural origin model, which jointly explains the uplift of the TAMs and the down-warp of the WSB. Small variations in the crustal thickness may contribute to locally high topography, but crustal isostasy does not appear to play a major role in the overall support of the TAMs.Item Determining the upper mantle seismic structure beneath the northern transantarctic mountains, Antarctica, form regional p- and s-wave tomography(University of Alabama Libraries, 2016) Brenn, Gregory; Hansen, Samantha E.; University of Alabama TuscaloosaStretching ~3,500 km across Antarctica, with peak elevations up to 4,500 m, the Transantarctic Mountains (TAMs) are the largest non-compressional continental mountain range on Earth and represent a tectonic boundary between the East Antarctica (EA) craton and the West Antarctic Rift System. The origin and uplift mechanism associated with the TAMs is controversial, and multiple models have been proposed. Seismic investigations of the TAM’s subsurface structure can provide key constraints to help evaluate these models, but previous studies have been primarily focused only on the central TAMs near Ross Island. Using data from the new 15-station Transantarctic Mountain Northern Network as well as data from several smaller networks, this study investigates the upper mantle velocity structure beneath a previously unexplored portion of the northern TAMs through regional body wave tomography. Relative travel-times were calculated for 11,182 P-wave and 8,285 S-wave arrivals from 790 and 581 Mw ≥ 5.5 events, respectively, using multi-channel cross correlation, and these data were then inverted for models of the upper mantle seismic structure. Resulting P- and S-wave tomography images reveal two focused low velocity anomalies beneath Ross Island (RI; δVP ≈ -2.0%; δVS ≈ -1.5% to -4.0%) and Terra Nova Bay (TNB; δVP ≈ -1.5% to -2.0%; δVS ≈ -1.0% to -4.0%) that extend to depths of ~200 and ~150 km, respectively. The RI and TNB slow anomalies also extend ~50-100 km laterally beneath the TAMs front and sharply abut fast velocities beneath the EA craton (δVP ≈ 0.5% to 2%; δVS ≈ 1.5% to 4.0%). A low velocity region (δVP ≈ -1.5%), centered at ~150 km depth beneath the Terror Rift (TR) and primarily constrained within the Victoria Land Basin, connects the RI and TNB anomalies. The focused low velocities are interpreted as regions of partial melt and buoyancy-driven upwelling, connected by a broad region of slow (presumably warm) upper mantle associated with Cenozoic extension along the TR. Dynamic topography estimates based on the imaged S-wave velocity perturbations are consistent with observed surface topography in the central and northern TAMs, thereby providing support for uplift models that advocate for thermal loading and a flexural origin for the mountain range.Item Rayleigh wave constraints on the structure and tectonic history of the Gamburtsev Subglacial Mountains, East Antarctica(American Geophysical Union, 2013-05-10) Heeszel, David S.; Wiens, Douglas A.; Nyblade, Andrew A.; Hansen, Samantha E.; Kanao, Masaki; An, Meijan; Zhao, Yue; Washington University (WUSTL); University of California System; University of California San Diego; Scripps Institution of Oceanography; Pennsylvania Commonwealth System of Higher Education (PCSHE); Pennsylvania State University; Pennsylvania State University - University Park; University of Alabama Tuscaloosa; Research Organization of Information & Systems (ROIS); National Institute of Polar Research (NIPR) - Japan; Chinese Academy of Geological SciencesThe Gamburtsev Subglacial Mountains (GSM), located near the center of East Antarctica, remain one of the most enigmatic mountain ranges on Earth. A lack of direct geologic samples renders their tectonic history almost totally unconstrained. We utilize teleseismic Rayleigh wave data from a 2 year deployment of broadband seismic stations across the region to image shear velocity structure and analyze the lithospheric age of the GSM and surrounding regions. We solve for 2-D phase velocities and invert these results for 3-D shear velocity structure. We perform a Monte Carlo simulation to improve constraints of crustal thickness and shear velocity structure. Beneath the core of the GSM, we find crustal thickness in excess of 55km. Mantle shear velocities remain faster than global average models to a depth of approximately 250km, indicating a thick lithospheric root. Thinner crust and slower upper mantle velocities are observed beneath the Lambert Rift System and the Polar Subglacial Basin. When compared with phase velocity curves corresponding to specific tectonothermal ages elsewhere in the world, average phase velocity results for the GSM are consistent with regions of Archean-Paleoproterozoic origin. Combined with radiometric ages of detrital zircons found offshore, these results indicate a region of old crust that has undergone repeated periods of uplift and erosion, most recently during the Mesozoic breakup of Gondwana. Lower crustal seismic velocities imply a moderately dense lower crust beneath the core of the GSM, but with lower density than suggested by recent gravity models.Item Upper mantle seismic structure beneath central East Antarctica from body wave tomography: Implications for the origin of the Gamburtsev Subglacial Mountains(American Geophysical Union, 2013-04-17) Lloyd, Andrew J.; Nyblade, Andrew A.; Wiens, Douglas A.; Shore, Patrick J.; Hansen, Samantha E.; Kanao, Masaki; Zhao, Dapeng; Pennsylvania Commonwealth System of Higher Education (PCSHE); Pennsylvania State University; Pennsylvania State University - University Park; Washington University (WUSTL); University of Alabama Tuscaloosa; Research Organization of Information & Systems (ROIS); National Institute of Polar Research (NIPR) - Japan; Tohoku UniversityThe Gamburtsev Subglacial Mountains (GSM), located near the center of East Antarctica, are the highest feature within the East Antarctic highlands and have been investigated seismically for the first time during the 2007/2008 International Polar Year by the Gamburtsev Mountains Seismic Experiment. Using data from a network of 26 broadband seismic stations and body wave tomography, the P and S wave velocity structure of the upper mantle beneath the GSM and adjacent regions has been examined. Tomographic images produced from teleseismic P and S phases reveal several large-scale, small amplitude anomalies (Vp=1.0%, Vs=2.0%) in the upper 250 km of the mantle. The lateral distributions of these large-scale anomalies are similar in both the P and S wave velocity models and resolution tests show that they are well resolved. Velocity anomalies indicate slower, thinner lithosphere beneath the likely Meso- or Neoproterozoic Polar Subglacial Basin and faster, thicker lithosphere beneath the likely Archean/Paleoproterozoic East Antarctic highlands. Within the region of faster, thicker lithosphere, a lower amplitude (Vp=0.5%, Vs=1.0%) slow to fast velocity pattern is observed beneath the western flank of the GSM, suggesting a suture between two lithospheric blocks possibly of similar age. These findings point to a Precambrian origin for the high topography of the GSM, corroborating other studies invoking a long-lived highland landscape in central East Antarctica, as opposed to uplift caused by Permian/Cretaceous rifting or Cenozoic magmatism. The longevity of the GSM makes them geologically unusual; however, plausible analogs exist, such as the 550 Ma Petermann Ranges in central Australia. Additional uplift may have occurred by the reactivation of pre-existing faults, for example, during the Carboniferous-Permian collision of Gondwana and Laurussia.Item Upper-mantle low-velocity zone structure beneath the Kaapvaal craton from S-wave receiver functions(Oxford University Press, 2009) Hansen, Samantha E.; Nyblade, Andrew A.; Julia, Jordi; Dirks, Paul H. G. M.; Durrheim, Raymond J.; Pennsylvania Commonwealth System of Higher Education (PCSHE); Pennsylvania State University; Pennsylvania State University - University Park; University of Witwatersrand; University of Alabama TuscaloosaP>The southern African Plateau is marked by anomalously high elevations, reaching 1-2 km above sea level, and there is much debate as to whether this topography is compensated by a lower mantle source or by elevated temperatures in the upper mantle. In this study, we use S-wave receiver functions (SRFs) to estimate the lithospheric thickness and sublithospheric mantle velocity structure beneath the Kaapvaal craton, which forms the core of the Plateau. To fit the SRF data, a low-velocity zone (LVZ) is required below a similar to 160-km-thick lithospheric lid, but the LVZ is no thicker than similar to 90 km. Although the lid thickness obtained is thinner than that reported in previous SRF studies, neither the lid thickness nor the shear velocity decrease (similar to 4.5%) associated with the LVZ is anomalous compared to other cratonic environments. Therefore, we conclude that elevated temperatures in the sublithospheric upper mantle contribute little support to the high elevations in this region of southern Africa.Item 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 TuscaloosaFor 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.