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Dynamic elevation of the Cordillera, western United States Anthony R. Lowry, Neil M. Ribe and Robert B. Smith Presentation by Doug Jones

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Purpose of this Paper?

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To better understand the relative importance of the contributions of different sources to elevation of the western USA Isolate topographic expression of each process that influences elevation

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First Step? Remove topographic effects of near surface processes

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First Step? Remove topographic effects of near surface processes – Erosion – Deposition – Volcanic construction – Fault displacement – Strain

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Removing near/subsurface processes?

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Comparing elevations with gravitational potential then removing the undercompensated parts of the topography

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Crustal Mass Variations?

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Found by relating crustal refraction seismic velocities to density

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Conductive Thermal Variations?

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First order approximation from surface heat flow measurements

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Dynamic Elevation?

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Subtract previous estimates of topographic effects of near surface processes

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Yellowstone plume buoyancy?

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Used a 3D numerical convection model – Calculations of both temperature and material properties vary spatially

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Material Properties Expressed in terms of mantle activation energy (H* m )

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Lithospheric Thickness

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Swell Topography

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Thermal Structure

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Types of Mantle Buoyancy Thermal boundary layer buoyancy Hotspot swell buoyancy Magmagenic buoyancy Others

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Thermal Boundary Layer Buoyancy?

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Thermal Boundary Layer Buoyancy Thinning of thermal boundary layer contributes to raised elevation 15% of total isostatic response to mantle buoyancy Not sufficient to offset effects of crustal thinning

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Hot Spot Swell Buoyancy 25% of estimated dynamic elevation

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Magmagenic Buoyancy?

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When partially melted, both the melt and residuum are less dense than the original aggregate Aggregate density change after %5 partially melted same as 500K change in temperature Dynamic elevation is dynamic, not compositional – Partial melt only contributes slightly to elevation

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Other Sources of Dynamic Buoyancy? Superadiabatic upwelling Phase boundary deflections Deeper buoyancy

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Superadiabatic Upwelling?

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Upwelling in Basin-Range as passive response to rifting If isentropic (no change in entropy) no thermal anomaly would be produced If upwelling material was anomalously hot, then anomaly would be produced

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Phase Boundary Deflection?

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Latent heat of recrystalization Deflection at the 410 & 60 km phase boundary could have uplifts of 2 and.5 km respectively

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Deeper Buoyancy The small scale anomalies studied in this paper would not be affected significantly by deeper buoyancy sources

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Conclusions 95% confidence mantle buoyancy largely contributes (~2 km) to dynamic uplift Little insight into relative contributions of different mechanisms for dynamic uplift

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