Geology 56606660 Applied Geophysics 27 Apr 2018 Last

Geology 5660/6660 Applied Geophysics 27 Apr 2018 Last Time: Roots of Cordilleran Elevation (for fun) • Buoyancy under Cordilleran uplift combines crustal (strain, magmatic addition, hydration-related), thermal (rifting), and asthenospheric (convection = Yellowstone? , hydration? ) • High elevation in the northern Basin & Range has an asthenospheric source (shown by removing elevation due to surface processes, crustal thickness/buoyancy, lithospheric thermal mass) • Early estimates looked similar to modeling of plume swell sheared by an extending, variable thickness lithosphere; latest estimates are also high in SBR and Rio Grande rift • Colorado Plateau is some combination of crustal thickening (melts at edges + hydration) & regional asthenospheric support from outside the CP © A. R. Lowry 2018

A bit on Cordilleran elevations… Schmandt et al. GRL 2015 Contractile strain in the Cordillera was likely greatest in the Sevier orogeny, a possibly Andean-like orogeny focused west of the modern Intermountain Seismic Belt… Contraction in Laramide structures is mostly small & not well correlated to crustal thickness variations…

A bit on Cordilleran elevations… Jones USU MSc thesis Schmandt et al. GRL 2015 Strongest evidence for magmatic thickening of the crust is in the Snake River plain (track of the Yellowstone hotspot); ~15 km of gabbro added

A bit on Cordilleran elevations… Schmandt et al. GRL 2015 Which happens to have lower elevation because gabbro is denser than typical Cordilleran crust

A bit on Cordilleran elevations… Schmandt et al. GRL 2015 Different crust/mantle density contrasts in east versus west?

A bit on Cordilleran elevations… Ma & Lowry Tectonics 2018 Note hydration reduces density (by consumption of garnet into melt in the lower crust) and reduces v. P/v. S (by consuming feldspar and adding to quartz in the upper crust)

Lowry et al. , J. Geophys. Res. 2000

Becker et al. , EPSL 2014

Schmandt et al. , EPSL 2012

Geology 5660/6660 Applied Geophysics 25 Apr 2016 Last Time: Yellowstone dynamics (for fun) • High elevation in the northern Basin & Range has an asthenospheric source (shown by removing elevation due to surface processes, crustal thickness/buoyancy, lithospheric thermal mass) • Early estimates looked similar to modeling of plume swell sheared by an extending, variable thickness lithosphere; latest estimates are also high in SBR and Rio Grande rift • Early seismic imaging suggested melt in upper mantle, “cumulates” in a high velocity midcrust of SRP, magma chamber at ~ 8 km depth, hydrothermal alteration to surface • Most recent imaging indicates significant fraction of melt is in the lower crust © A. R. Lowry 2016

Hot? Wet? Higher H 2 O Property Higher Temp. Cold? Dry? Slow Seismic Velocity Fast High Electrical Conductivity Low Mass Density ! Flow Strength ! Partial Melt ! These are important for the dynamics…

“Dry” mantle peridotite “Hydrous” mantle serpentinite

Kanda et al. , in prep, 2016

Schmandt & Humphreys EPSL 2010

v. P v. S v. P/v. S Schmandt & Humphreys EPSL 2010

Meqbel et al. EPSL 2014

Meqbel et al. EPSL 2014

Crossey et al. EPSL 2016

Kennedy & van Soest (Science 2007): ~ Minimum Ra appears related to strain rate; high outlier values are associated with magmatism… Implications for mechanism of transfer?