A Seismic Investigation of the Lithospheric Structure Beneath the U.S. Midcontinent Using Surface Wave Tomography and S-to-P Converted Waves

Chen Chen, Purdue University

Abstract

In order to improve our understanding of the lithospheric structure of stable continents, this study focuses on the U.S. midcontinent by analyzing teleseismic earthquakes recorded by the EarthScope Transportable Array and the OIINK (Ozark-Illinois-Indiana-Kentucky) Flexible Array. I constructed a shear wave velocity model using Rayleigh wave tomography and imaged lithospheric discontinuities using S-to-P converted waves. My results demonstrate the heterogeneous nature of the continental lithosphere, and highlight that the anomalous features at depth spatially correlate with the geological structures at the surface. Specifically, relatively high crustal shear wave velocities characterize the southern Illinois Basin and the Reelfoot Rift. The high crustal velocities may correspond to high-density mafic intrusions emplaced into the crust during the development of the rift. In addition to high crustal velocities, the Reelfoot Rift exhibits low mantle velocities at depths > 80 km and the top of this low velocity zone coincides with a negative Sp phase imaged using converted waves. Considering the lack of heat flow anomaly in the region, I interpret this low velocity zone as a mechanically and chemically rejuvenated mantle emplaced during rifting. The imaged negative Sp phase at 80 km depth then represents a paleo-LAB beneath which the lithosphere could have been removed during rifting. Additionally, my results reveal anomalous structure that may result from accretionary events. Double negative Sp phases extend along the Nd-line, an isotopically defined crustal boundary. I hypothesize these multiple arrivals to be a manifestation of stacked or imbricated lithospheric blocks that formed during convergence and accretion of lithosphere less than 1.55 Ga onto lithosphere older than 1.55 Ga. At the base of the lithosphere inferred from earthquake tomography, my results reveal a general absence of negative Sp phases at this depth across much of the central US, which suggests a gradual velocity decrease between the lithosphere and asthenosphere. This corroborates the hypothesis that the cratonic LAB is largely governed by temperature. The station setup of the Transportable Array and the OIINK Array not only enables us to closely inspect the lithospheric structure of the region, but also provides an ideal setting to investigate the influences of different parameters on resolved velocity structures. I restricted the number of earthquakes and stations used in the analysis in different ways and inverted for Rayleigh wave phase velocities. My results show that using half the amount of the waveforms retrieves similar phase velocity structures, with a slightly increased uncertainty, and that limited azimuthal coverage causes smearing in the phase velocities. I also found that excluding data recorded by the dense spacing Flexible Array stations in the analysis has small impact on the resolved phase velocity structures.

Degree

Ph.D.

Advisors

Flesch, Purdue University.

Subject Area

Geophysics

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