Mantle Structure and Seismicity of Antarctica
Restricted (Penn State Only)
- Author:
- Lucas, Erica
- Graduate Program:
- Geosciences
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- November 04, 2022
- Committee Members:
- Andrew Nyblade, Chair & Dissertation Advisor
Charles Ammon, Major Field Member
Sridhar Anandakrishnan, Major Field Member
Victor Sparrow, Outside Unit & Field Member
Donald Fisher, Program Head/Chair - Keywords:
- seismology
tectonics
geophysics
Antarctica
mantle - Abstract:
- In this dissertation, I investigate upper mantle structure and characterize seismicity in Antarctica using data recorded on broadband seismic stations over more than two decades. My investigations of upper mantle structure address outstanding questions about the tectonic history and neotectonic setting of Antarctica, and my investigations of seismicity advance our understanding of modern glacial, tectonic, and volcanic processes in West Antarctica. In Chapter 2, shear wave splitting measurements are used to examine mantle anisotropy across Antarctica. Largely northeast – southwest oriented fast polarization directions observed across much of the continent likely arise from regionally variable contributions of both lithospheric and sublithospheric fabrics. Lithospheric fabrics are best associated with Precambrian tectonism in East Antarctica and Jurassic – Cenozoic tectonic activity elsewhere on the continent. Sub-lithospheric mantle fabrics can be attributed to either plate motion-induced asthenospheric flow or small-scale convection regimes. New P- and S-wave velocity models of the upper mantle beneath central West Antarctica are presented in Chapter 3. The models show significant heterogeneity in upper mantle structure throughout the region. High velocity anomalies imaged throughout the central portion of the West Antarctic Rift System are consistent with lithosphere unmodified by tectonic activity since the late Cretaceous to early Cenozoic formation of the rift system, while localized low velocity anomalies are suggestive of focused Cenozoic rifting. A low velocity anomaly imaged beneath the Pine Island Glacier and the mouth of the Thwaites Glacier is interpreted as a rift-related thermal structure that may include warm mantle flowing from Marie Byrd Land. The heterogenous upper mantle velocity structure imaged in Chapter 3 indicates that upper mantle temperatures beneath central West Antarctica could vary by 100 K or more over distances of 100 km or less, suggestive of significant lateral variability in upper mantle viscosity throughout the region. In Chapter 4, I locate 117 previously undetected seismic events originating from glacial, tectonic, volcanic processes in central West Antarctica. Seismicity is clustered in four geographic regions, including the Ellsworth Mountains, Thwaites Glacier, Pine Island Glacier, and Mount Takahe. Using Pn-wave travel times from select seismic events, fast uppermost mantle velocities are imaged throughout central West Antarctica, consistent with the presence of a tectonically stable mantle lid. A study on repeating glacial seismicity at the Foundation Ice Stream in West Antarctica is presented in Chapter 5. Two clusters of seismicity are observed at the Foundation Ice Stream, one located near the grounding line and second further upstream. Seismicity at the grounding line shows clear tidal modulation, with seismic events primarily occurring during spring tide and, more specifically, concurrent with rising tide preceding daily maximum high tide. The second cluster of seismicity is located upstream Foundation Ice Stream near a prominent subglacial ridge, and the observed events most likely reflect basal stick-slip processes associated with the topographic high.