Constraining depth dependent anisotropy in Australia through Ps Receiver function analysis, frequency-dependent shear wave splitting and forward modeling

University Of California-Riverside, Riverside CA

Investigators

Abstract

The continental lithosphere has witnessed much of the Earth's geologic and biologic evolution, giving us an unprecedented view into our planet's history. While our understanding of many aspects of the physical evolution of the Earth has flourished in recent years, fundamental gaps in our knowledge of some processes persist. One particular deficiency is in our understanding of the structure and geologic evolution of continents. While oceanic lithosphere has a life cycle that begins at mid-ocean ridges and ends at subduction zones, we still do not have a clear understanding of how the continental lithosphere first formed, or why the lithosphere has remained stable over billions of years, despite undergoing significant deformation in later tectonic events. In Australia, the oldest parts of the continental lithosphere are dated to about 3.2 billion years old, making it an excellent location for testing out hypotheses related to continental formation and evolution. Detailed geologic studies over decades have placed constraints on the timing of many of Australia's major tectonic events. However, observations of seismic anisotropy, which are used to infer information about tectonic deformation, tell a story about the Australian lithosphere that is significantly more complicated than what is inferred from the surface geology. The goal of this project is to use newly developed seismological methods to provide a more detailed and accurate picture of seismic anisotropy to help infer the tectonic evolution within the Australian continent, in order to better understand the processes of continental formation, assembly and evolution more generally. The character of the anisotropic structure in the mantle lithosphere and asthenosphere beneath Australia represents a long-standing puzzle, largely because inferences gleaned from different data analysis methods (SKS splitting vs. surface wave tomography) often conflict. This proposal will apply frequency-dependent SKS splitting analysis and anisotropic Ps receiver function analysis to place more precise constraints on the lateral and depth variability of anisotropy beneath the Australian continent. The proposed work has three specific goals: 1) Use data from 26 permanent broadband stations to measure frequency-dependent SKS splitting and apply Ps receiver function analysis, and for a subset of stations, carry out detailed forward modeling of the structure and compare results across methods for a subset of these stations. 2) Investigate the origin of depth-dependent anisotropy beneath Australia by analyzing models for anisotropic structure in terms of tectonic history and setting. 3) Understand the origin of the discrepancies between azimuthally anisotropic surface wave models and SKS splitting observations beneath Australia via comparisons with the independent depth-dependent models. Some specific science questions addressed by this work include: How does anisotropy vary with depth beneath the Australian continent? Do variations in anisotropic structure correspond to geologic and/or tectonic structures? Does the anisotropic structure of the mantle lithosphere shed light on past episodes of deformation? Why have previous studies identified discrepancies between surface wave tomography models and SKS splitting observations?