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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

80

60

40

20

0

-20

-40

-60

-80

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

80

60

40

20

0

-20

-40

-60

-80

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

80

60

40

20

0

-20

-40

-60

-80

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

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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Western North America:Regional elevation

-130 -125 -120 -115 -110 -105 -100

50

45

40

35

30

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

45

40

35

30

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

4

4

6

4

8

16

12

8

8

4

-130 -125 -120 -115 -110 -105 -100

50

45

40

35

30

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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