| NSF Org: |
EAR Division Of Earth Sciences |
| Recipient: |
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| Initial Amendment Date: | May 13, 2016 |
| Latest Amendment Date: | May 13, 2016 |
| Award Number: | 1614066 |
| Award Instrument: | Standard Grant |
| Program Manager: |
Dennis Geist
EAR Division Of Earth Sciences GEO Directorate for Geosciences |
| Start Date: | June 1, 2016 |
| End Date: | May 31, 2019 (Estimated) |
| Total Intended Award Amount: | $187,296.00 |
| Total Awarded Amount to Date: | $187,296.00 |
| Funds Obligated to Date: |
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| History of Investigator: |
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| Recipient Sponsored Research Office: |
1 PROSPECT ST PROVIDENCE RI US 02912-9100 (401)863-2777 |
| Sponsor Congressional District: |
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| Primary Place of Performance: |
RI US 02912-1846 |
| Primary Place of
Performance Congressional District: |
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| Unique Entity Identifier (UEI): |
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| Parent UEI: |
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| NSF Program(s): | EARTHSCOPE-SCIENCE UTILIZATION |
| Primary Program Source: |
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| Program Reference Code(s): |
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| Program Element Code(s): |
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| Award Agency Code: | 4900 |
| Fund Agency Code: | 4900 |
| Assistance Listing Number(s): | 47.050 |
ABSTRACT
Throughout Earth?s history, the tectonic plates that make up the lithosphere have converged to form super-continents and then pulled apart over time-scales of hundreds of millions of years. The southern U.S. near the Gulf Coast and the eastern U.S. bordering the Atlantic experienced two cycles of super-continent formation and break-up. These episodes of collision and rifting altered the crust and mantle lithosphere of the North American plate, thickening these layers in some regions, thinning them in others, and producing distinctive structures by aligning mineral grains and melting small amounts of rock. In this project we will use earthquake waves to construct three-dimensional models of the crust and mantle lithosphere, and we will interpret these models to better understand how continental collision and rifting affect continental lithosphere.
The goal of this work is to improve resolution of crust and mantle structure in the southern and eastern U.S. in order to better understand lithospheric evolution and modification through the last Wilson cycle. The southern and eastern margins of the North America continent experienced two complete Wilson cycles of orogeny and rifting, leaving varied signatures of deformation and magmatism in the crust and mantle lithosphere of these regions. However, many aspects of crust and mantle structure remain poorly known at the scales relevant to distinguishing the effects of orogeny, rifting, and other processes such as hotspot interaction. Using data from the NSF EarthScope USArray/Transportable Array (TA), now completed in the contiguous U.S., we propose to image crust and mantle seismic structure in new detail. We will focus on two regions: one spanning the southern craton edge, the entire Ouachita orogeny and the Gulf of Mexico coastal plain, and the other containing the eastern craton edge, the central and northern Appalachians, and the Atlantic coast. We will use seismic data from TA stations, supplemented by other temporary and permanent broadband stations, to image crustal and mantle discontinuities using Sp and Ps receiver functions, construct 3-D anisotropic models based on Rayleigh wave, Love wave and shear-wave-splitting analyses, and we will integrate these data and methods through joint analyses and inversions. The seismic models will be used constrain the style and geometry of deformation in the crust and mantle lithosphere due to the last episodes of orogeny and rifting. This interpretation will be guided by geological and active source indicators of deformation in the shallow crust and by the type and age of magmatism across the region, bearing in mind that some facets of crust and mantle structure may reflect events that predated or followed the last cycle of orogeny and continental rifting. Variations in the impact of orogeny and rifting on the lithosphere will be compared between and within the study regions. The proposed project will support the education and career development of graduate students at the University of Houston and at Brown. It will also contribute to undergraduate senior theses, lab exercises and group research projects in undergraduate and graduate courses, and to science outreach activities with local public schools. Students from diverse backgrounds will be recruited to participate in this research.
PUBLICATIONS PRODUCED AS A RESULT OF THIS RESEARCH
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PROJECT OUTCOMES REPORT
Disclaimer
This Project Outcomes Report for the General Public is displayed verbatim as submitted by the Principal Investigator (PI) for this award. Any opinions, findings, and conclusions or recommendations expressed in this Report are those of the PI and do not necessarily reflect the views of the National Science Foundation; NSF has not approved or endorsed its content.
The goal of this research project is to measure the properties of the lithosphere in eastern North America, in order to understand how the lithosphere has been altered by its tectonic history, including its collisions with other tectonic plates, its rifting (that led to the creation of the Atlantic ocean), and its passage over upwelling hot mantle (hotspots). We used the seismic waves that emanated from distant earthquakes and were recorded by seismic stations in eastern North America, including the stations from the NSF EarthScope Transportable Array, to measure features of the structure of the upper mantle. These features include the thickness of the lithosphere, the seismic wave velocities internal to the lithosphere, and the wave velocities in the asthenosphere (the layer of mantle that lies directly beneath the lithosphere). We measured both isotropic wave velocities, and anisotropic (directionally-dependent) wave velocities. We also developed new methods for analyzing how earthquake waves that convert from one mode of vibration (S) to another (P) can be used to measure the properties of the boundaries where these conversions occur.
We found that while the base of the lithosphere in the western U.S. is marked by a very rapid decrease in velocity with depth, and the base of the lithosphere in the cratonic (ancient) central U.S. is characterized by a very gradual decrease in velocity with depth, the base of the lithosphere in the eastern U.S. has variable properties that lie in between these end-member velocity gradients. Beneath two regions of the eastern U.S. (approximately New England-New York and Virginia) where seismic waves also indicate that the lithosphere is unusually thin and the asthenosphere is unusually hot, the sharp velocity gradient at the base of the lithosphere is consistent with the presence of partially molten rock in the shallow asthenosphere. Assuming that anisotropy in seismic wave velocities is produced by the alignment of olivine a-axes due to mantle deformation, anisotropy in these regions is consistent with quasi-horizontal shear in the asthenosphere. In addition, the shape of the New England-New York low velocity anomaly aligns with the track of a hotspot that the plate moved over 110-140 million years ago. These results suggest that hotspots are likely related to the initiation of the low velocity anomalies, the New England-New York anomaly in particular. Hotspots may have eroded the base of the continental lithosphere to form a channel or localized thinning, and the asthenosphere could have been dragged horizontally but also upward into these channels by North American plate motion.
Beneath the eastern U.S. as a whole, seismic waves indicate distinct differences in seismic wave velocity anisotropy between the lithosphere and asthenosphere. Olivine a-axis azimuths in the asthenosphere typically align with absolute North American plate motion in the Pacific hotspot reference frame, consistent with shearing of the asthenosphere due to recent plate motion. Olivine a-axis azimuths in the lithosphere approximately align with the strike of the Appalachian orogen, consistent with lithospheric compression during accretion, except for southern New England where a-axes are more consistent with Mesozoic extension. These results highlight that lithospheric anisotropy is dominated by relatively simple patterns of strain related to plate collisions and rifting.
At Brown, this project contributed to the education and career development of two Ph.D. students, two post-doctoral researchers, and one undergraduate. It also reached a broader audience through its use as examples in undergraduate and graduate courses.
Last Modified: 11/06/2019
Modified by: Karen M Fischer
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