GEOS 260
Lithospheric Environments I


OUTLINE OF TOPICS COVERED

1. Introduction:
    Define the lithosphere
    Discuss its significance

2. Isostasy and flexure

3. Lithospheric boundaries

4. Lithospheric evolution


 What is the Lithosphere?
When was the term introduced?

Lithosphere = “rock” sphere
Having the properties of rock
The layer with strength
(cf. Asthenosphere = weak sphere)

Defined by Joseph Barell in a series of papers all under the same title, "The Strength of the Earth's Crust," and published over a period of a year and a half in 1914-15:
Journal of Geology, vol. 22, 1914,
pp. 28-48, 145-165, 209-236, 289-314, 441-468, 537-555, 655-683, 729-741.
Journal of Geology, vol. 23, 1915,
pp. 27-44, 425-443, 499-515.

Quotations from Barell (my emphasis):

"The zone of compensation, being competent to sustain stresses imposed by the topography and its isostatic compensation must obey the laws pertaining to the elasticity of the solid state and is to be regarded therefore as of the nature of rock.  Consequently there may be extended to all of it the name of lithosphere, even though it included from time to time molten bodies, the constituents of the pyrosphere."

The theory of isostasy shows that below the lithosphere there exists in contradistinction a thick earth-shell marked by a capacity to yield readily to long-enduring strains of limited magnitude. … It may be the generating zone of the pyrosphere; it may be a sphere of unstable state, but this to a larger extent is hypothesis and the reason for choosing a name rests upon the definite part it seems to play in crustal dynamics.  Its comparative weakness is in that connection its distinctive feature.  It may then be called the sphere of weakness –the asthenosphere, …

The terms lithosphere and asthenosphere predate plate tectonics by about 50 years, and they were ready-made for their application to plate tectonics.


 Evolving Lithospheres:

Although Barell had very definite purposes for his terms when he defined them, their adoption into plate tectonics has resulted in a number of definitions of the lithosphere:

1. Original Definition – Layer with strength – difficult to measure directly.

2. Elastic lithosphere – thickness of an equivalent elastic layer Te – can be measured, but does not directly correspond to any specific structure in the lithosphere.

3. Seismic lithosphere – base at top of low velocity layer (LVZ), presumably where mantle is close to the solidus temperature – not found under shields.

4. Thermal lithosphere – layer in which heat transfer is dominated by conduction over convecting asthenosphere – thickness must be extrapolated from surface heat flow data or from xenolith pressure and temperature data.

5. Mechanical lithosphere – layer strong enough to resist small-scale convection – based on concept that there is a relatively thin layer of convection cells at the base of the mantle, but these have never been directly observed.

6. Petrologic lithosphere – layer that maintains a distinct chemical/isotopic character over time.  However, as lithosphere is heated it may become more like the asthenosphere in its physical properties, but maintain its "lithosphere" chemical and isotopic characteristics.

7. Plate tectonic lithosphere – lithosphere = plate – rigid layer that moves as a unit in plate tectonics.
 

Figure 1

Significant differences exist among the different definitions of the lithosphere, as shown in the plot to the left of elastic and thermal lithosphere thickness as a function of time for cooling oceanic lithosphere.  [From Angevine et al., 1990]


 Significance of the Lithosphere

1. Without a “rigid” outer layer, there would be no geology.  Geology requires a rigid solid surface to "record" geologic events.

2. Without a “rigid” outer layer there would be no topography.  Mountains would flow away and the sides of basins would flow into their centres.

3. The lithosphere allows local and global predictive models; e.g., structure and plate tectonics.  As the lithosphere behaves as a rigid block, we may have some confidence in extrapolating observed features, such as faults to depth or gross global plate movements.

4. Prior to 3.9 Ga no lithosphere?  No geology.  We have no direct record of the first 600 Ma of Earth's geological history either because there was no lithosphere to record the geology, or because this lithosphere was destroyed.

5. Lithospheres exist on all terrestrial planets, the Moon, and satellites of the Outer Planets – one-plate planets.  We can use geological principles on these bodies because they have lithospheres to "record" events, although they do not have plate tectonics.


 Isostasy





What do you understand by isostasy?

Isostasy is the balance between surface topographic loads and subsurface compensation.  The diagram to the right shows the two traditional interpretations of isostasy, the Pratt model in which compensation is achieved by lateral density variations in the crust, and the Airy model in which compensation is achieved by thickness variations in the crust.  There are examples of both types of compensation in nature, but compensation can also occur below the crust in the mantle lithosphere.  [From Angevine,1990]

Requirements of Isostasy

Barell's original definition of isostasy recognized that isostasy did not work perfectly for every mountain, but that the topographic loads are spread out by strength in the layer on which they rest.  In addition wherever the compensation is, it must also be in a layer that is strong enough to prevent the compensation density contrasts from flowing sideways.  These requirements are the basis for Barell's definition of the lithosphere.  Barell also recognized that for isostasy to be dynamic, i.e., to remain valid during mountain building and erosion, the "rigid" lithosphere must "float" on a "fluid" layer that can flow to allowed the lithosphere to adjust in response to changed in the topographic load.  This layer he called the weak layer, or asthenosphere.

Warning:  These terms are all relative to geologic time and “rigid”, “floating” and “fluid” must be understood in terms of rheology.  In terms of a conceptual model, the asthenosphere behaves like glacial ice, brittle and solid on a short time scale, but flowing over time, compared with the lithosphere which is like rock at the surface, essentially solid and strong over all time scales.

Demonstration of the spreading of a topographic load by the lithosphere.  In terms of the same load that must be supported, the total area of the root in D is equal to the single block root in B.  [From Angevine et al., 1990]

Example of lithospheric flexure. Loading by overthrusting causes downwarp of the lower plate and flexure of the lower plate.  Depending on the flexural rigidity of the lower plate this will form either a broad, shallow wedge-shape basin on the lower plate for high rigidity, or a narrow, deep basin for low rigidity.  Generally old, cold lithosphere has high rigidity, young, hot lithosphere has low rigidity.  [From Angevine et al., 1990]

Implications

Surface loads (+ve and –ve) are balanced by equal, but opposite subsurface compensating masses (Archimedes’ Principle).  A visible example of this principle is the floating of icebergs.  Ice has a density of 0.9 g cm-3.  Water has a density of 1.0 g cm-3.  The compensation for the mass of ice above water level is achieved by the replacement of water with a density of 1.0 with ice with a density of 0.9, and thus the compensation has a density of (0.9 - 1.0) = -0.1g cm-3.  Thus we are compensating a topographic mass with a density of 0.9 (ice) with a compensation mass with a density of -0.1 (ice - water), so to give an equal mass, the compensation must have a volume of 0.9/0.1, or 9 times the volume of ice above water level.

Using a similar example with a continental crustal root, if topography has a density of 2.7 g cm-3, and the lower crust has a density of 2.8 g cm-3 extending down into a mantle with a density of 3.3 g cm-3, then the root will have a volume that is 2.7/(2.8 - 3.3), or 5.4 times bigger than the volume of the topography (the negative sign is dropped because it simple signifies compensation for a +ve topographic mass).

The situation becomes more complicated when we include the Pratt model of isostasy and the interaction between the mantle lithosphere and the asthenosphere.  The asthenosphere is less dense than the mantle lithosphere, and therefor a root of mantle lithosphere extending down into the asthenosphere causes a positive load (more dense lithosphere replacing less dense asthenosphere) and adds to the topographic load rather than compensating it.  However, a topographic load may be supported by a thinning of the mantle lithosphere, as under a mid-ocean ridge.


 Simple “Global” Formulation of Isostasy

    The following formulation of isostasy may be adapted to lithospheric columns of any complexity.  It is based on the concept that if the the asthenosphere behaves as a fluid over long time period, then pressures at depth in the asthenosphere below the deepest lithospheric root must be equal at any particular depth.  If pressures are unequal, then the asthenosphere will flow from high to low pressure until the pressures are equalized.  As the asthenosphere flows, then the overlying lithosphere sinks or rises as the local volume of underlying asthenosphere changes.

1. Choose a depth equal to or exceeding the maximum depth of isostatic compensation (the depth below which there are no more lateral density contrasts) – somewhere in the asthenosphere.

2. Calculate pressure P in reference column at that depth:

            P = d1gL1 + d2gL2 + d3gL3 + ...

where d1 is the density of the first layer and L1 is its thickness, d2 is the density of the second layer and L2 is its thickness, etc., and g is the acceleration due to gravity.

3. Pressure at same depth beneath all columns = P.  Solve for length of columns (elevation) layer thickness, etc.

As pressures of different columns are compared, units are unimportant, as long as they are consistent among columns.  Also g occurs in every term and so may be canceled when columns are compared and is usually ignored in the calculations.  See examples below:

Example of two lithospheric columns in isostatic equilibrium to a compensation depth of 120 km.  Note the inclusion of water in the right-hand column.  [From Angevine et al., 1990]


Complications in Isostasy

The general formulation of isostasy given above is always valid, but other changes may need to be taken into account.  The effects of flexure spread the region over which isostasy must be applied.  If the flexural wavelength if 20 km (hot, thin lithosphere), then topography must be considered in areas of at least 20 km on a side.  If the flexural wavelength is 800 km, then areas 800 km on a side must be considered.

Additionally levels or layer densities may change with time.  These changes are particularly important in sedimentation where sediments may be laid down at different levels with respect to sea level and seal level may change during sedimentation.  Some to these effects are illustrated below.

If no compaction of the sediment is assumed, and all sedimentation is assumed to occur at sea level, the the sedimentary record directly records the basin depth through time.  [From Angevine et al., 1990]


 
 

Studies of diagenesis (conversion of sediment into sedimentary rock) indicate that porosity decreases with depth with an accompanying decrease in volume and increase in density.  We may assume that compacted sediments at depth were deposited as high-porosity, low density sediments at the surface.  [From Angevine et al., 1990]

When the effects of sediments compaction are taken into account, the evolution of the basin in terms of its depth through time during sedimentation is different from the record indicated directly by sediment thicknesses if compaction is not taken into account.  [From Angevine et al., 1990]


 
 

Sediments are commonly assumed to be deposited at or close to sea level,  However, some sediments may indicate deposition in deeper waters (or sea level may be indicated to change), and these effects must be included in calculations of basin evolution and changes in basin depth through time during sedimentation.  [From Angevine et al., 1990]


Reference Cited

Angevine, C. L., P. L. Heller and C. Paola, Quantative Sedimentary Basin Modeling, Continuing Education Course Note Series #32, American Association of Petroleum Geologists, Tulsa, 133 pp., 1990.



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