GEO 5/6690 Geodynamics 21 Nov 2014

© A.R. Lowry 2014Read for Mon 1 Dec: T&S 410-427

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 315 364-368.(I will lead)

Rigid Plate

~Rigid Mesosphere

Velocity

u = u0

Assume “no-slip”boundary conditions(fluid velocity at theboundary with the plate = plate velocityu0; fluid velocity atmesosphere boundary= 0) and constantviscosity, then

z = 0

z = h

constantviscosity

€

u = u0 1−z

h

⎛

⎝ ⎜

⎞

⎠ ⎟

u = 0

h

Lithosphere = Plate in Plate Tectonics?

What is a Plate?

“Plate tectonics” postulates that rigid“plates” move around onthe Earth’s surface…

A theory that is more geometrical than physical.

Reality is that the Earth’snear-surface has higherviscosity so resists flow &deformation.

Rigid

Fluid

General expectation based on observational data (e.g.,postglacial rebound, Earth tidal response, flow modelingof the geoid + deep seismic anomalies, & mineral physics)

that viscosity is highnear the surface, highagain in the lower mantle, with low velocity zones in the upper mantle and in the thermal boundary layer at the base of the mantle.

10231021 Pa s

A simplified model of motion of the plates relative to thedeep mantle is given by Couette flow in a channel:

Rigid Plate

Rigid Mesosphere

Velocity

u = u0

Assume “no-slip”boundary conditions(fluid velocity at theboundary with the plate = plate velocityu0; fluid velocity atmesosphere boundary= 0) and constantviscosity, then

z = 0

z = h

constantviscosity

€

u = u0 1−z

h

⎛

⎝ ⎜

⎞

⎠ ⎟

u = 0

h

Colorado PlateauNOAM2.5 cm/yr

One implication of this would be that only the uppermostmantle materialtravels with the plateon long timescales:E.g., for the currentviscosity structure ofthe Colorado Plateau,uppermost <100 km!

Problem however: Seismologistsimage velocity structure that theyand geologists have related toPrecambrian tectonic events inferred from surface geology…to depths of 200+ km!

Karlstrom et al GSA-Today Mar 2002

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are needed to see this picture.

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 bathymetrydeparts from half-spacecooling 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 neverevolves beyond ~60 Myr, the thermal boundary layer (“conductive lithosphere”)can never grow beyond ~80-100 kmdepth… And 60 Ma lithosphere should beequally 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 wavevelocity to ~200 km depths; Te > 100 kmCratonization (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 residuummantle after melting (peridotite).Expressed density in terms of amolar ratio of Fe to Mg, R,and molar fraction Al2O3 (XAl) as:

Using measurements of xenoliths(at surface P,T), he empiricallyfound the partial derivative termsto be –0.70 and 0.32,respectively.

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QuickTime™ and aTIFF (Uncompressed) decompressor

are needed to see this picture.

€

R =XFe

XFe + XMg

€

ρ =ρ P,T( ) 1−∂ lnρ 0

∂XAl

ΔXAl +∂ lnρ 0

∂RΔR

⎡

⎣ ⎢

⎤

⎦ ⎥

Melt relations (e.g. compilation of McKenzie & Bickle, J. Petrol.,1988; subsequent incarnations such as MELTS, pMELTS)suggestXAl drops,XFe stays~constant,XMg

increaseswithincreasingdegree ofmelting:

solid

us

liqui

dus

(inmelt)

(inmelt)

(inmelt)

(GPa)

With result that thedensity of peridotiteis much less thanfertile mantle that still contains pyroxene & garnet

Compositional difference greaterthan that due to a500°C temperaturedifference!!!

(BUT, more recent analyses would suggest smaller partial derivatives atrealistic P,T)

Note also that depletionof basalt may increasethe P-wave velocityof mantle rock (possiblesensitivity in VS also, butthought to be much lessso).

Hence, high-velocity“rolls” north and south ofthe Snake River plain werehypothesized to be residueof melting in the center,which is low density because of bothtemperature andcomposition…

Humphreys et al., GSA-Today, 2000

partialmelt

residuum