Post on 07-Jan-2016
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GEOSCIENCES
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C. G. ChaseDepartment of GeosciencesUniversity of Arizona, David Coblentz & Aviva SussmanLANL
Geoid Anomalies: Aspen Anomaly region and the Colorado Plateau
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Outline Geoid anomalies
Potential field versus gravity proper Filters and what they tell us (or not) Isostatic compensation
Colorado Plateau Geoid signal Elevation
Aspen Anomaly Yellowstone anomaly much larger in both amplitude and wavelength, could be deep
Aspen anomaly has to be “shallow”
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Geoid Anomalies Global geoid anomaly: departure of
gravitational equipotential surface (equivalent to sea level) from simple reference figure. Units: m
Geoid sees deeper than gravity anomalies (1 / r versus 1 / r 2)
That’s good news and bad news Bad - also sensitive to mantle
anomalies Good - sensitive to isostatic
compensation
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Global Geoid: GGM02, Nonhydrostatic reference figure
0 20 40 60 80 100 120 140 160 180 200 220 240 260 280 300 320 340 360
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-20
-40
-60
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Longitude
Latitude
-100 -50 0 50 100Nonhydrostatic geoid height, m
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GGM02, Nonhydrostatic reference figurel,m (order, degree) <= 360: half-wavelength >=
55 km
0 20 40 60 80 100 120 140 160 180 200 220 240 260 280 300 320 340 360
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0
-20
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Longitude
Latitude
-100 -50 0 50 100Nonhydrostatic geoid height, m
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Global Geoid: GGM02, Filtered high pass,7-11 cosine taper
0 20 40 60 80 100 120 140 160 180 200 220 240 260 280 300 320 340 360
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-20
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Lon
Latitude
-30 -20 -10 0 10 20World geoid, high pass 7/11 filter
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Geoid, isostasy, and models Non-uniqueness of potential fields
For a specified density distribution, you can only determine a maximum depth
Sharp, deep versus broad, shallow One-dimensional geoid models
Geoid anomaly mass dipole moment More topography, more anomaly Deeper compensation, more anomaly
Full three-dimensional models Models don’t have to be isostatic Spherical harmonics “easy” to work
with Need more data and representation
geometry
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1-D Geoid to Topography Ratio: Calculated
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0 10 20 30 40 50 60 70 80 90 100
Geoid/elevation ratio, m/km
Depth of Moho compensation, km
Airy compensation
Pratt compensation
For elevation of 2 km
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Western North America:Regional elevation
-130 -125 -120 -115 -110 -105 -100
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Longitude
Latitude
-10000 -5000 0 5000Elevation, contour interval 1000 m
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Filtered geoid
10
20
0
-130 -125 -120 -115 -110 -105 -100
50
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35
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Longitude
Latitude
-10 -5 0 5 10 15 20 25Geoid anomaly, filtered 7/11 to 60/65, contour interval 5m
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Geoid to Topography Ratio: Observed
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16
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-130 -125 -120 -115 -110 -105 -100
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Longitude
Latitude
5 6 8 10 11 13 14 16Geoid/elevation ratio, contour interval 2m/km
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1-D Geoid to Topography Ratio: Calculated
0
2
4
6
8
10
12
14
0 10 20 30 40 50 60 70 80 90 100
Geoid/elevation ratio, m/km
Depth of Moho compensation, km
Airy compensation
Pratt compensation
For elevation of 2 km
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Interpretation of 1-D profile
Effective compensation depth ~50 km Colorado Plateau now seems to have
crustal structure appropriate to elevation Why was Plateau at sea level pre-
Tertiary? Possible causes of elevation change:
Crustal thickening or removal of dense lithosphere in K
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Western US geophysics
Topography P-wave tomography
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Western US geoid “slices”
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Western US geoid “slices”
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Do the results register? Geoid is north of
topography is north of Vp
Excess mass on the Colo-Wyo border
Depth <= 75 km In their different
ways, geoid and topography are quite exact
What’s the state of the art for shallow P & S wave tomography?
Long-wavelengthtopography
Vp
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Conclusions Geoid is useful but requires care in
interpreting The right data The right filters The “right” interpretation
Compensation under the Colorado Plateau is shallow Definitely upper lithospheric, could be
crustal Under the Aspen anomaly, less clear
In detail, correlation of geoid with elevation and Vp not exact
Lateral density contrasts in both crust and upper mantle implied
Halfway to being a Yellowstone anomaly
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