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?
• 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– Erosion– Deposition– Volcanic construction– Fault displacement– Strain
Removing near/subsurface processes?
• Comparing elevations with gravitational potential then removing the undercompensated parts of the topography
Yellowstone plume buoyancy?
• Used a 3D numerical convection model– Calculations of both temperature and material
properties vary spatially
Types of Mantle Buoyancy
• Thermal boundary layer buoyancy• Hotspot swell buoyancy• Magmagenic buoyancy• Others
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
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?
• 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?
• 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