The Lithosphere
There term lithosphere is in a variety of ways.
The most general use is as: The lithosphere is the upper region of
the crust and mantle that behaves more or less rigidly, transmitting
stress on a global scale. Earth’s tectonic plates are contiguous
regions of lithosphere that move relative to each other and deform
primarily at their boundaries.
Because we don’t have a unified theory for what the lithosphere
actually is, it is common to talk about the lithosphere as defined by
particularly properties. We thus have:
The thermal lithosphere
• Earth’s upper thermal boundary layer
The mechanical lithosphere
• The portion of the thermal lithophere that behave elastically
The seismic lithosphere, or lid
• The high-velociy region overlying the asthernospheric LVZ
These are mostly used for oceanic lithosphere. Continental lithosphere
is quite different, so
Continental lithosphere
• Often described as a “chemical boundary layer”
Thermal lithosphere
• Earth’s upper thermal boundary layer
• temperature-dependent density makes the lithosphere sink
• thermal subsidence of oceanic lithosphere with age is the primary
constraint on lithospheric thermal structure
• heat flow measurements also provide some constraint
• the two main models for how temperature evolves with time in
the lithosphere are the half-space and plate-cooling models
• these models predict both thermal subsidence and heat flow as
a function of lithospheric age
• the most common explanation for why the plate model seems more
appropriate than the half-space cooling is secondary convection
Heat Flow
Why we care
• Internal heating due to radioactive decay plus cooling due to conductive
heat flow drives mantle convection, plate tectonics, and enables life
• physical properties of rocks are temperature-dependent, e.g. (T), (T)
Quantities
• Heat: like the total “vibrational” energy of all elements in a system
• Temperature: like the average “vibrational” energy
Conductive heat flow
• heat “flows” from hot regions to cold regions
• rate of heat flow is proportional to the thermal gradient
The Heat Equation
Specific heat: CP = is the amount of heat necessary to raise 1kg of material 1°C
Using CPV to convert heat to temperature,
• Solve this equation with different boundary conditions to predict T(x,t) of the lithosphere
• With this solution, knowledge of (T) enables prediction of h(t) = seafloor depth with age
Isostasy
• Isostasy is the concept that adjacent "columns" of mass,
extending from some reference level at the Earth's surface to
some depth of compensation, should be equal.
• Isostasy follows from the assumption that Earth’s mantle
behaves like a fluid at long time scales, and so the pressure
within the fluid must be equal at a given depth.
Predictions of the Half-space cooling model
1D Solution:
Thermal expansion:
(T) = (1 - VT)
V = volumetric coefficient of thermal expansion
T = (T - T0)
Bathymetry:
Plate cooling model, early observations
L = 125 ± 10 km
TM = 1333°C ± 274°C
V = (3.28 ± 1.19) x 10 -5 °C-1
L = 128 ± 10 km
TM = 1365°C ± 276°C
V = (3.1 ± 1.11) x 10 -5 °C-1
Parsons and Sclater, 1977
Stein and Stein, 1992
Stein and Stein, 1992
Ritzwoller et al., 2004
The mechanical lithosphere
• The lithosphere is plate like in that it behaves rigidly
- there are very few intraplate earthquakes
- plates transmit stress; little deformation except at the edges
• The lithosphere behaves elastically over geologic timescales,
whereas the underlying mantle behaves like a viscous fluid
• Temperature and volatiles seem to control the lithosphere’s
mechanical properties
Two main types of observations are used to infer the mechanical
properties of the lithosphere (still just oceanic lithosphere).
• The maximum depth of intraplate earthquakes
- intraplate earthquakes at inferred temperatures greater than
600°C are rare, and so the region above this isotherm is commonly
referred to as the mechanical lithosphere or boundary layer.
• “Elastic” plate thickness, as determined from plate flexure in response
to discrete loads, provides information on relative mechanical
properties, though it’s not clear what this thickness really means.
MBL
TBL
Some of these EQs are from
near islands and so involve
reheated lithosphere and
magmatic activity.
Some are from transforms and
involve serpentinization.
The most useful ones are from
trenches, and so these may
also be serpentinized.
This distribution probably can’t
be made sense of.
The deepest earthquakes
probably mark something like a
brittle/ductile transition.
Weins and Stein, 1983
Elastic plate thickness
• The lithosphere is observed to flex under applied loads
• This flexure resembles that of a thin elastic plate
• Equations exist that relate plate flexure to elastic thickness
• When these equations are applied to observed lithospheric flexure,
it is hard to make sense of the results
The seismic lid
• Seismic velocity is a material physical property
• There are two classes of seismic waves, body waves and surface
waves
• There two types of body wave, compressional and shear waves,
which propagate at different speeds:
VP = [ (K + 4/3)/ ]1/2 , VS = (  / )1/2
• Surface waves travel along the surface, and the two most important
type of surface waves are Rayleigh and Love waves. Their seismic
velocities are complicated.
• The best constraints on lithospheric seismic velocity come from
surface waves, which provide good constraint on path-averaged
absolute velocity as a function of depth.
2007
The seismic lid
Three key points
1) The high velocity of the lid can not be explained by temperature
alone. This suggests a compositional component to the lithosphere
and associated properties. Likely sources of compositional
differences wrt the deeper mantle are:
- depletion in volatiles
- existence of an eclogite phase
2) The LVZ is the primary evidence for a low-viscosity channel, referred
to as the asthenosphere. The low velocities are too low to be
explained solely by temperature, suggesting the possible presence
of melt.
3) There is some suggesting that LID structure does not vary with age,
suggesting that lithospheric mechanical properties do not evolve
as the lithosphere cools, I.e. that something other than temperature
controls mechanical properties.