| NSF Org: |
EAR Division Of Earth Sciences |
| Recipient: |
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| Initial Amendment Date: | March 21, 2013 |
| Latest Amendment Date: | March 21, 2013 |
| Award Number: | 1246977 |
| Award Instrument: | Standard Grant |
| Program Manager: |
Luciana Astiz
lastiz@nsf.gov (703)292-4705 EAR Division Of Earth Sciences GEO Directorate for Geosciences |
| Start Date: | April 1, 2013 |
| End Date: | March 31, 2016 (Estimated) |
| Total Intended Award Amount: | $46,499.00 |
| Total Awarded Amount to Date: | $46,499.00 |
| Funds Obligated to Date: |
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| History of Investigator: |
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| Recipient Sponsored Research Office: |
1000 OLD MAIN HL LOGAN UT US 84322-1000 (435)797-1226 |
| Sponsor Congressional District: |
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| Primary Place of Performance: |
UT US 84322-1415 |
| 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, Geophysics |
| 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
This project seeks to illuminate the formation, assembly and subsequent evolution of the US mid-continent. Investigators at two universities and their graduate and undergraduate students will examine composition of the mid-continent crust and upper mantle. A novel combination of gravity and magnetic data with seismic velocity ratios vP/vS estimated from seismic receiver function data and velocity structure from surface wave and body wave tomography will be used to establish variations in temperature as well as thickness and bulk composition of multiple layers within the crust. The novel elements of these efforts include development of a new approach to receiver function imaging using parameter-domain cross-correlation and stacking, and coupling of the seismic images to the potential field data via likelihood filters and velocity-density relationships. Magnetic studies will focus also on understanding the physical properties and tectonic implications of magnetically detected boundaries, with particular emphases on a hidden middle Proterozoic geochemical and magnetic boundary dividing the cratonic core of North America and also on the Tennessee-Illinois-Kentucky lineament (or TIKL) and its unusual banded pattern of magnetization. The geophysical model results will be combined with sparsely sampled basement isotope geochemistry and age data to interpret the history of formation and accretion of lithospheric blocks as well as their subsequent tectonomagmatic modification.
Knowledge about physical properties of the crust and its history of tectonism and volcanism is central to a wide array of solid Earth science topics. Assessments of the likelihood of future earthquakes and the potential for economic mineral deposits at depth are just two examples of applications in which such knowledge plays a vital societal role. In order to evaluate better these risks and economic potential, a multi-disciplinary project involving professors and graduate and undergraduate students at two universities will seek to understand key structures, composition and processes that formed the geologic core of the US mid-continent. In mountainous regions of the western US, mapping variations in physical properties and inferring tectonic history are relatively straightforward because geophysical images of the deep crust and uppermost mantle can be corroborated by studies of rocks exposed and/or brought to the surface by geologic processes. In the mid-continent region of the US however, thick sequences of sedimentary rocks cover the ancient rocks produced by episodes of magmatism. Boreholes reaching below the sedimentary cover are few and far between and that has made it challenging to understand the geologic past that may control the locations of earthquakes and hidden mineral deposits. Consequently, tectonic and seismic hazard maps of this region rely more heavily on geophysical lineaments: linear features found in map-views of geophysical data sets signifying geologic boundaries and weak zones. This study seeks to combine several different types of geophysical data sets, including gravity and magnetic anomalies as well as images made possible by EarthScope?s array of seismometers. The previously unknown characteristics of the mid-continent crust derived from these data will be used to better characterize implications of the geologic boundaries for understanding comprehensively the history and properties of the Earth?s outer layers.
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.
Our understanding of how the Earth deforms leading up to earthquakes and how continents build over time depends fundamentally on geophysical remote sensing of the Earth. A major challenge in geophysics lies in how we determine the relationships between observations we make in remote sensing, such as seismic waves, gravity and magnetic fields, versus the deep rock properties that are important for understanding tectonics (including temperature, mass density, chemistry and the abundance of volatiles such as water in the rocks). This project focused on achieving a better understanding of gravity and magnetic data, which are particularly important tools for examining deep rock structure and identifying locations of possible ancient earthquake fault zones in the Midcontinent region of the United States where basement rocks are deeply buried beneath sediments.
Our approach was to combine seismic imaging of the Earth's crust with gravity and magnetic data to both improve the imaging and more firmly establish the connections between crustal chemistry and density & magnetic properties. We developed a new approach to imaging thicknesses of a multi-layered crust from combinations of sound waves converted to shear waves where seismic velocity changes at layer boundaries, and estimates of the ratio of seismic velocities within the layers. Both are known to be sensitive to changes in rock types: Layer boundaries may occur due to either changes in bulk chemistry of rocks or changes in the packing structure at high pressure and temperature, called "phase transitions", that result in different minerals. Seismic velocity ratios are sensitive to the presence of quartz in the rocks, and correlations of low velocity ratios (implying abundant quartz) with U.S. Cordilleran tectonics had previously been cited as evidence that quartz, which is very weak and fluid at high temperatures, plays an important role in mountain-building.
Our study found that most of the variation in both the thickness and the seismic velocity ratios of the U.S. continental crust relates to variations in thickness and velocity ratio of the lower crustal layer. This is important for two reasons: (1) The relatively uniform thickness of the upper crust implies that the layer boundary we are imaging is a phase transition rather than a change in bulk chemistry, such as that which occurs at the Moho, which separates the crust from the upper mantle. That means small variations in thickness of the upper crust may be used to infer variations in temperature and chemistry of the crust. (2) The lower crustal velocity ratios suggest that quartz abundance can vary significantly in the fluid lower crust. Moreover though, we also found that hydration-- the movement of water through the crystal lattice of deep rocks, which drastically reduces the flow strength of rock-- also increases the abundance of quartz in the rocks. Thus, the correlation of quartz in the lower crust with tectonism may not be because of the weakness of quartz, but rather because hydration both weakens the crust and produces quartz as a by-product. We used these findings to constrain density properties of the crust and mantle and also to compare with magnetic data. Our estimates of compositional variations show only a weak relationship to magnetic field variations, perhaps because the magnetite that dominates the crustal field is a very minor part of the total mineralogy.
This project supported the training, research and mentoring of a PhD student who hopes to find a career in industry, and also contributed to the research training of an undergraduate physics student.
Last Modified: 07/13/2016
Modified by: Anthony R Lowry
