| NSF Org: |
EAR Division Of Earth Sciences |
| Recipient: |
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| Initial Amendment Date: | February 9, 2009 |
| Latest Amendment Date: | April 13, 2011 |
| Award Number: | 0838180 |
| Award Instrument: | Continuing Grant |
| Program Manager: |
Robin Reichlin
EAR Division Of Earth Sciences GEO Directorate for Geosciences |
| Start Date: | April 1, 2009 |
| End Date: | March 31, 2014 (Estimated) |
| Total Intended Award Amount: | $171,368.00 |
| Total Awarded Amount to Date: | $171,368.00 |
| Funds Obligated to Date: |
FY 2010 = $71,676.00 FY 2011 = $78,411.00 |
| History of Investigator: |
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| Recipient Sponsored Research Office: |
1 SILBER WAY BOSTON MA US 02215-1703 (617)353-4365 |
| Sponsor Congressional District: |
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| Primary Place of Performance: |
1 SILBER WAY BOSTON MA US 02215-1703 |
| 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): | Geophysics |
| Primary Program Source: |
01001011DB NSF RESEARCH & RELATED ACTIVIT 01001112DB NSF RESEARCH & RELATED ACTIVIT |
| 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
This project will investigate the thermal and compositional state, melting processes, and deformation of the upper mantle beneath the Atlantic basin. These properties will be inferred from new, high-resolution, three-dimensional tomographic models of seismic velocity, attenuation, and anisotropy of the Atlantic upper mantle. Much of what is currently known about the oceanic upper mantle comes from regional seismic models of the Pacific basin. However, differences in the spreading rates and plate velocities of the Atlantic and Pacific oceans suggest that different dynamical processes are occurring in the mantle beneath the two basins. The new models will be constrained by large data sets of fundamental and higher-mode surface-wave phase delay and amplitude from events and stations located within or on the margins of the Atlantic basin. This research seeks to address three key issues: (1) What controls the seismic structure as a function of depth and seafloor age? Can these properties be attributed solely to temperature, or must composition and melt also be considered? (2) What controls along-axis and off-axis seismic anomalies? Do they reflect a variable mantle source, and what roles do temperature and composition play? (3) Is the mantle fabric associated with slower plate velocities weaker and more variable than that produced by faster spreading rates? Each of these questions invites a comparison with the faster-spreading Pacific upper mantle, and together they present an opportunity to investigate possible spreading-rate dependence of seismic structure as well as the mechanisms that might produce it. The final seismic models will ultimately be used to infer temperature, composition, partial melt content, and deformation state, aided by constraints from mineral-physics experiments and other data sets such as basalt chemistry, bathymetry, and geoid height.
Non-Technical Description of Research
According to the theory of plate tectonics, the Earth?s rigid outer shell is divided into tectonic plates that slowly move relative to one another, driven by convection currents within hot, weak rocks in the mantle beneath the plates (the asthenosphere). We have a poor understanding of the processes that control this abrupt transition from rigid rocks within the plate to weak deforming rocks beneath the plate. Is it simply that the asthenospheric rocks are hotter? Are they partially molten, or do they contain compositional components (water or other volatiles) that weaken them? Because seismic waves generated by earthquakes are sensitive to temperature and other weakening processes, we can use seismic imaging of mantle structure to address these questions. Distinguishing between these processes will allow us to better understand how the Earth?s tectonic system developed and evolved over time, and also illuminates the weakening and melting processes that produce geologic hazards such as fault zones and volcanic systems.
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 Earth’s surface is divided into rigid tectonic plates (the lithosphere) that slowly move relative to one another, driven by convection currents within hot, weak rocks in the mantle beneath the plates (the asthenosphere). We have a poor understanding of the processes that control this abrupt transition from rigid rocks within the plate to weak deforming rocks beneath the plate. Is it simply that the asthenospheric rocks are hotter? Are they partially molten, or do they contain compositional components (water or other volatiles) that weaken them? Seismic waves generated by earthquakes travel through the Earth’s crust and mantle, and we can record them with sensitive instruments called seismometers. Similar to how a CT scan can be used to image the properties inside a human body, seismic waves can be used to image the properties inside the Earth. Because seismic waves generated by earthquakes are sensitivite to temperature and other weakening processes, we can use seismic imaging of mantle structure to answer questions about the transition from the lithosphere to the asthenosphere. This allows us to better understand how the Earth’s tectonic system developed and evolved over time, and it illuminates the weakening and melting processes that produce geologic hazards such as fault zones and volcanic systems. It also helps us to understand the interiors of other planets in our solar system.
We have performed such a study beneath the Atlantic Ocean, using seismic waves from earthquakes generated beneath the Atlantic seafloor. Our results show that temperatures in the mantle are hottest beneath the mid-ocean ridge that runs down the center of the Atlantic and coldest at the perimeter, close to the edge of the continents. The cold temperatures on the perimeter, however, are not as cold as our models for conductive cooling predict, suggesting that a mechanism must exist to provide additional heat to the lithosphere in the Atlantic ocean. This is consistent with earlier studies of the Pacific seafloor and indicates that this reheating may be a ubiquitous feature of the mantle under oceans.
Last Modified: 07/03/2014
Modified by: Colleen Dalton
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