GEOSCIENCES UASCIENCE T HE U NIVERSITY OF Arizona ® C. G. Chase Department of Geosciences University of Arizona, David Coblentz & Aviva Sussman LANL Geoid Anomalies: Aspen Anomaly region and the Colorado Plateau

GEOSCIENCES UASCIENCE T HE U NIVERSITY OF Arizona ® 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”

GEOSCIENCES UASCIENCE T HE U NIVERSITY OF Arizona ® 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

GEOSCIENCES UASCIENCE T HE U NIVERSITY OF Arizona ® Global Geoid: GGM02, Nonhydrostatic reference figure

GEOSCIENCES UASCIENCE T HE U NIVERSITY OF Arizona ® GGM02, Nonhydrostatic reference figure l,m (order, degree) = 55 km

GEOSCIENCES UASCIENCE T HE U NIVERSITY OF Arizona ® Global Geoid: GGM02, Filtered high pass, 7-11 cosine taper

GEOSCIENCES UASCIENCE T HE U NIVERSITY OF Arizona ® 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

GEOSCIENCES UASCIENCE T HE U NIVERSITY OF Arizona ® 1-D Geoid to Topography Ratio: Calculated

GEOSCIENCES UASCIENCE T HE U NIVERSITY OF Arizona ® Western North America: Regional elevation

GEOSCIENCES UASCIENCE T HE U NIVERSITY OF Arizona ® Filtered geoid

GEOSCIENCES UASCIENCE T HE U NIVERSITY OF Arizona ® Geoid to Topography Ratio: Observed

GEOSCIENCES UASCIENCE T HE U NIVERSITY OF Arizona ® 1-D Geoid to Topography Ratio: Calculated

GEOSCIENCES UASCIENCE T HE U NIVERSITY OF Arizona ® 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

GEOSCIENCES UASCIENCE T HE U NIVERSITY OF Arizona ® Western US geophysics TopographyP-wave tomography

GEOSCIENCES UASCIENCE T HE U NIVERSITY OF Arizona ® Western US geoid “slices”

GEOSCIENCES UASCIENCE T HE U NIVERSITY OF Arizona ® Western US geoid “slices”

GEOSCIENCES UASCIENCE T HE U NIVERSITY OF Arizona ® Do the results register? Geoid is north of topography is north of V p 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-wavelength topography VpVp

GEOSCIENCES UASCIENCE T HE U NIVERSITY OF Arizona ® 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 V p not exact Lateral density contrasts in both crust and upper mantle implied Halfway to being a Yellowstone anomaly

GEOSCIENCES UASCIENCE T HE U NIVERSITY OF Arizona ®