Samantha E. Hansen

Crustal structure beneath the Northern Transantarctic Mountains and Wilkes Subglacial Basin: Implications for tectonic origins

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¯s) are resolved within ±4 km and ±0.1 km/s, respectively. The crust thickens from ~20 km near the Ross Sea coast to ~46 km beneath the northern TAMs, which is somewhat thicker than that imaged in previous studies beneath the central TAMs. The crust thins to ~41 km beneath the WSB. V¯s ranges from ~3.1–3.9 km/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 ~1 km of additional topography in the northern TAMs compared to the central TAMs.

Variable thermal loading and flexural uplift along the Transantarctic Mountains, Antarctica

The high elevations of the Transantarctic Mountains (TAMs) have been suggested to be flexural in origin, but to date, the thermal contribution to uplift has yet to be quantified. Here, we present new P- and S-wave tomography images of the upper-mantle seismic structure beneath the central to northern TAMs, which reveal two, focused low-velocity anomalies beneath Ross Island and Terra Nova Bay that laterally extend beneath the TAMs front and that are connected by a low-velocity region constrained offshore within the Victoria Land Basin. The focused low velocities are interpreted as shallow regions of partial melt, connected by a broad region of slow (warm) upper mantle associated with Cenozoic extension along the Terror Rift. Thermal loading constraints based on our tomographic results are used to update flexural uplift models for the TAMs. Our findings confirm that thermal buoyancy is a principal component leading to the uplift of the TAMs but suggest that the thermal loading is variable along the TAMs front.

Seismic evidence for lithospheric foundering beneath the southern Transantarctic Mountains, Antarctica

The 3000-km-long Transantarctic Mountains (TAMs), which separate cratonic East Antarctica from tectonically active West Antarctica, remain one of the least understood of Earth’s major mountain ranges. The tectonic mechanism that generates the high elevation, as well as the processes that produce major differences between various sectors of the TAMs, are still uncertain. Here we present newly constructed seismic images of the crust and uppermost mantle beneath central Antarctica derived from recently acquired seismic data, indicating ongoing lithospheric foundering beneath the southern TAMs. These images reveal an absence of thick, cold cratonic lithosphere beneath the southern TAMs. Instead, an uppermost-mantle slow seismic anomaly extends across the mountain front and 350 km into East Antarctica, beneath a high plateau near the South Pole. Under the slow anomaly, a relatively high-wavespeed root is found at ~200 km depth, connected with the East Antarctic lithosphere, suggesting that sinking lithosphere has been replaced at shallow depths by warm, slow-velocity asthenosphere. A mantle lithosphere foundering model is proposed to interpret these images, which best explains the present large area of high elevation and the uplift of the TAMs, as well as Miocene-age volcanism in the Mount Early region.

Upper mantle seismic anisotropy beneath the northern transantarctic mountains, Antarctica from PKS, SKS, and SKKS splitting analysis

Using data from the new Transantarctic Mountains Northern Network, this study aims to constrain azimuthal anisotropy beneath a previously unexplored portion of the Transantarctic Mountains (TAMs) to assess both past and present deformational processes occurring in this region. Shear-wave splitting parameters have been measured for PKS, SKS, and SKKS phases using the eigenvalue method within the SplitLab software package. Results show two distinct geographic regions of anisotropy within our study area: one behind the TAMs front, with an average fast axis direction of 42 ± 3° and an average delay time of 0.9 ± 0.04 s, and the other within the TAMs near the Ross Sea coastline, with an average fast axis oriented at 51 ± 5° and an average delay time of 1.5 ± 0.08 s. Behind the TAMs front, our results are best explained by a single anisotropic layer that is estimated to be 81-135 km thick, thereby constraining the anisotropic signature within the East Antarctic lithosphere. We interpret the anisotropy behind the TAMs front as relict fabric associated with tectonic episodes occurring early in Antarcticas geologic history. For the coastal stations, our results are best explained by a single anisotropic layer estimated to be 135-225 km thick. This places the anisotropic source within the viscous asthenosphere, which correlates with low seismic velocities along the edge of the West Antarctic Rift System. We interpret the coastal anisotropic signature as resulting from active mantle flow associated with rift-related decompression melting and Cenozoic extension. This article is protected by copyright. All rights reserved.

An Assessment of Crustal and Upper‐Mantle Velocity Structure by Removing the Effect of an Ice Layer on the P‐Wave Response: An Application to Antarctic Seismic Studies

Standard P ‐wave receiver function analyses in polar environments can be difficult because reverberations in thick ice coverage often mask important P ‐to‐ S conversions from deeper subsurface structure and increase ambient noise levels, thereby significantly decreasing the signal‐to‐noise ratio of the data. In this study, we present an alternative approach to image the subsurface structure beneath ice sheets. We utilize downward continuation and wavefield decomposition of the P ‐wave response to obtain the up‐ and downgoing P and S wavefield potentials, which removes the effects of the ice sheet. The upgoing P wavefield, computed from decomposition of the waveform at a reference depth, is capable of indicating ice layer thickness. This simple step removes the necessity of modeling ice layer effects during iterative inversions and hastens the overall velocity analysis needed for downward continuation. The upgoing S wave is employed and modeled using standard inversion techniques as is done with receiver functions at the free surface using a least‐squares approximation. To illustrate our proof of concept, data from several Antarctic networks are examined, and our results are compared with those from previous investigations using P ‐ and S ‐wave receiver functions as well as body‐ and surface‐wave tomographic analyses. We demonstrate how our approach satisfactorily removes the ice layer, thus creating a dataset that can be modeled for crustal and upper‐mantle structure. Solution models indicate crustal thicknesses as well as average crustal and upper‐mantle shear‐wave velocities. [Electronic Supplement:][1] Figure of measured data, the vertical‐component stack used in deconvolution, and the resultant vertical, radial, and tangential transfer functions. [1]: http://www.bssaonline.org/lookup/suppl/doi:10.1785/0120160262/-/DC1