Dynamic elevation of the Cordillera, western United States

Anthony R. Lowry, Neil M. Ribe and Robert B. Smith

Presentation by Doug Jones

Purpose of this Paper?

Purpose of this Paper?

• 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

First Step?

• Remove topographic effects of near surface processes

First Step?

• Remove topographic effects of near surface processes– Erosion– Deposition– Volcanic construction– Fault displacement– Strain

Removing near/subsurface processes?

Removing near/subsurface processes?

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

Crustal Mass Variations?

Crustal Mass Variations?

• Found by relating crustal refraction seismic velocities to density

Conductive Thermal Variations?

Conductive Thermal Variations?

• First order approximation from surface heat flow measurements

Dynamic Elevation?

Dynamic Elevation?

• Subtract previous estimates of topographic effects of near surface processes

Yellowstone plume buoyancy?

Yellowstone plume buoyancy?

• Used a 3D numerical convection model– Calculations of both temperature and material

properties vary spatially

Material Properties

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

Lithospheric Thickness

Swell Topography

Thermal Structure

Types of Mantle Buoyancy

• Thermal boundary layer buoyancy• Hotspot swell buoyancy• Magmagenic buoyancy• Others

Thermal Boundary Layer Buoyancy?

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

Hot Spot Swell Buoyancy

• 25% of estimated dynamic elevation

Magmagenic Buoyancy?

Magmagenic Buoyancy?

• 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

Other Sources of Dynamic Buoyancy?

• Superadiabatic upwelling• Phase boundary deflections• Deeper buoyancy

Superadiabatic Upwelling?

Superadiabatic Upwelling?

• 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

Phase Boundary Deflection?

Phase Boundary Deflection?

• Latent heat of recrystalization• Deflection at the 410 & 60 km phase boundary

could have uplifts of 2 and .5 km respectively

Deeper Buoyancy

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

Conclusions

• 95% confidence mantle buoyancy largely contributes (~2 km) to dynamic uplift

• Little insight into relative contributions of different mechanisms for dynamic uplift