GEO 5/6690 Geodynamics 21 Nov 2014 © A.R. Lowry 2014 Read for Mon 1 Dec: T&S Last Time: The Lithosphere Revisited There are several different processes and associated observations that researchers may actually mean when they refer to “ the lithosphere ”: Thermal Boundary Layer is the uppermost region where heat transfer is dominated by conduction with some advection Seismic Lid lies above a mantle negative velocity gradient that is sharp enough to generate wave conversions, called by seismologists “lithosphere-asthenosphere boundary” ( LAB ) Original Lithosphere is the layer that supports stress on long timescales; often modeled as an elastic plate but really a rheological definition involving dynamically maintained stress Seismogenic layer is the brittle-field portion of the dynamical lithosphere

Next Journal Article Reading: For Monday Nov 24: Mierdel et al. (2007) Water solubility in aluminous orthopyroxene and the origin of Earth’s asthenosphere, Science (I will lead)

Rigid Plate ~Rigid Mesosphere Velocity u = u 0 Assume “no-slip” boundary conditions (fluid velocity at the boundary with the plate = plate velocity u 0 ; fluid velocity at mesosphere boundary = 0) and constant viscosity, then z = 0 z = h constant viscosity  u = 0 h Lithosphere = Plate in Plate Tectonics?

What is a Plate? “Plate tectonics” postulates that rigid “plates” move around on the Earth’s surface… A theory that is more geometrical than physical. Reality is that the Earth’s near-surface has higher viscosity so resists flow & deformation. Rigid Fluid

General expectation based on observational data (e.g., postglacial rebound, Earth tidal response, flow modeling of the geoid + deep seismic anomalies, & mineral physics) that viscosity is high near the surface, high again in the lower mantle, with low velocity zones in the upper mantle and in the thermal boundary layer at the base of the mantle Pa s

A simplified model of motion of the plates relative to the deep mantle is given by Couette flow in a channel: Rigid Plate Rigid Mesosphere Velocity u = u 0 Assume “no-slip” boundary conditions (fluid velocity at the boundary with the plate = plate velocity u 0 ; fluid velocity at mesosphere boundary = 0) and constant viscosity, then z = 0 z = h constant viscosity  u = 0 h

Colorado Plateau NOAM 2.5 cm/yr One implication of this would be that only the uppermost mantle material travels with the plate on long timescales: E.g., for the current viscosity structure of the Colorado Plateau, uppermost <100 km!

Problem however: Seismologists image velocity structure that they and geologists have related to Precambrian tectonic events inferred from surface geology… to depths of 200+ km! Karlstrom et al GSA-Today Mar 2002

Further complication: Asthenospheric flow driven by deep density anomalies should be even faster than Couette flow driven by plate motions! Behn et al, EPSL, 2004

Major Element Chemistry: The Tectosphere & Cratonization Note ocean bathymetry departs from half-space cooling at ~ 60 Myr…  “plate cooling”, i.e. a limit imposed by small-scale convection. Huang et al, JGR, 2005

Karlstrom et al GSA-Today Mar 2002 Problem however: If cooling never evolves beyond ~60 Myr, the thermal boundary layer (“conductive lithosphere”) can never grow beyond ~ km depth… And 60 Ma lithosphere should be equally strong as 2.5 Ga lithosphere. Not what we observe here...

Greater depth of the thermal boundary layer under ancient (cratonic) lithosphere is also inferred from high shear wave velocity to ~200 km depths; T e > 100 km Cratonization (old, thick, strong lithosphere)  ???

Tom Jordan [1975, 1981] examined chemistry, density, seismic velocity of mantle xenoliths and found that “fertile” mantle lithosphere (garnet lherzolite) is more dense, has lower seismic velocity than residuum mantle after melting (peridotite). Expressed density in terms of a molar ratio of Fe to Mg, R, and molar fraction Al 2 O 3 ( X Al ) as: Using measurements of xenoliths (at surface P, T ), he empirically found the partial derivative terms to be –0.70 and 0.32, respectively.

Melt relations (e.g. compilation of McKenzie & Bickle, J. Petrol., 1988; subsequent incarnations such as MELTS, pMELTS) suggest X Al drops, X Fe stays ~constant, X Mg increases with increasing degree of melting: solidus liquidus (in melt) (in melt) (in melt) (GPa)

With result that the density of peridotite is much less than fertile mantle that still contains pyroxene & garnet Compositional difference greater than that due to a 500°C temperature difference!!! (BUT, more recent analyses would suggest smaller partial derivatives at realistic P, T )

Note also that depletion of basalt may increase the P-wave velocity of mantle rock (possible sensitivity in V S also, but thought to be much less so). Hence, high-velocity “rolls” north and south of the Snake River plain were hypothesized to be residue of melting in the center, which is low density because of both temperature and composition… Humphreys et al., GSA-Today, 2000 partial melt residuum