1 / 19

Geoid Anomalies: Aspen Anomaly region and the Colorado Plateau

Geoid Anomalies: Aspen Anomaly region and the Colorado Plateau. C. G. Chase Department of Geosciences University of Arizona, David Coblentz & Aviva Sussman LANL. Outline. Geoid anomalies Potential field versus gravity proper Filters and what they tell us (or not) Isostatic compensation

waldo
Download Presentation

Geoid Anomalies: Aspen Anomaly region and the Colorado Plateau

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Geoid Anomalies: Aspen Anomaly region and the Colorado Plateau C. G. Chase Department of Geosciences University of Arizona, David Coblentz & Aviva Sussman LANL

  2. 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”

  3. 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

  4. Global Geoid: GGM02, Nonhydrostatic reference figure

  5. GGM02, Nonhydrostatic reference figurel,m (order, degree) <= 360: half-wavelength >= 55 km

  6. Global Geoid: GGM02, Filtered high pass,7-11 cosine taper

  7. 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

  8. 14 For elevation of 2 km 12 10 Airy compensation 8 Geoid/elevation ratio, m/km Pratt compensation 6 4 2 0 0 10 20 30 40 50 60 70 80 90 100 Depth of Moho compensation, km 1-D Geoid to Topography Ratio: Calculated

  9. Western North America:Regional elevation

  10. Filtered geoid

  11. Geoid to Topography Ratio: Observed

  12. 14 For elevation of 2 km 12 10 Airy compensation 8 Geoid/elevation ratio, m/km Pratt compensation 6 4 2 0 0 10 20 30 40 50 60 70 80 90 100 Depth of Moho compensation, km 1-D Geoid to Topography Ratio: Calculated

  13. 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

  14. Western US geophysics Topography P-wave tomography

  15. Western US geoid “slices”

  16. Western US geoid “slices”

  17. Do the results register? Long-wavelength topography 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? Vp

  18. 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

More Related