Upper mantle structure Upper mantle structure beneath the eastern beneath the eastern

Colorado Plateau and Rio Colorado Plateau and Rio Grande rift revealed by Grande rift revealed by

Bouguer gravity, seismic Bouguer gravity, seismic velocities and xenolith datavelocities and xenolith data

Mousumi Roy, Jonathan K. MacCarthy, Mousumi Roy, Jonathan K. MacCarthy, and Jane Selverstoneand Jane Selverstone

G3, October 2005G3, October 2005

Angela Magee

ASU Earthscope Seminar

April 16, 2007

MotivationMotivation

Bouguer anomalies Bouguer anomalies under the Colorado under the Colorado Plateau and Rio Plateau and Rio Grande RiftGrande Rift

Most interested in the Most interested in the longer wavelength low longer wavelength low gravity anomaly under gravity anomaly under east edge of plateau east edge of plateau and the rift which and the rift which can’t be compeltely can’t be compeltely explained by upliftexplained by uplift

Data and MethodsData and Methods Use LA RISTRA Use LA RISTRA

teleseimic array, a teleseimic array, a compilation, and compilation, and CRUST 2.0 2X2 crustal CRUST 2.0 2X2 crustal model for the crustal model for the crustal thickness datathickness data

Crust thickness cannot Crust thickness cannot explain all the Bouguer explain all the Bouguer anomalyanomaly

Remove the Remove the contribution of crust contribution of crust thickness and thickness and background trend of background trend of density from the datadensity from the data

Data and Methods Data and Methods ContinuedContinued

Forward model Forward model density anomalies in density anomalies in upper crust and upper crust and upper mantleupper mantle

Upper crust: high Upper crust: high pass gravity data, pass gravity data, match up what can to match up what can to geological geological informationinformation

Also tried lower crust, Also tried lower crust, but upper mantle but upper mantle more reasonable more reasonable density contrasts density contrasts

ResultsResults Also compare Also compare

density model with density model with densities estimated densities estimated from xenolith data from xenolith data and with seismic and with seismic velocity datavelocity data

Low density Low density anomaly:anomaly: Broader at north, Broader at north,

narrower and narrower and stronger at southstronger at south

Oblique to and offset Oblique to and offset from Rio Grande Rift from Rio Grande Rift

ImplicationsImplications

Middle Tertiary ignimbrite flare-up Middle Tertiary ignimbrite flare-up and late tertiary magatismand late tertiary magatism Basalt extractionBasalt extraction Partial melt Partial melt High heat flowHigh heat flow

Broad upwarping of asthenosphere Broad upwarping of asthenosphere beneath thinned lithospherebeneath thinned lithosphere

Puzzle of relationship with Rio Puzzle of relationship with Rio Grande RiftGrande Rift

Additional ThoughtsAdditional Thoughts

Puzzle of relationship with Rio Puzzle of relationship with Rio Grande RiftGrande Rift

DepthDepth Heat flowHeat flow