Lithosphere And Asthenosphere

It has long been recognized that for large-scale structures to attain isostatic equilibrium, the outermost shell of the Earth must be underlain by a weak layer

Figure 2.35 Section from San Francisco, California to Lamar, Colorado based on seismic refraction data (redrawn from Pakiser, 1963, by permission of the American Geophysical Union. Copyright © 1963 American Geophysical Union).

that deforms by flow. This concept has assumed fundamental importance since it was realized that the subdivisions of the Earth controlling plate tectonic movements must be based on rheology, rather than composition.

The lithosphere is defined as the strong, outermost layer of the Earth that deforms in an essentially elastic manner. It is made up of the crust and uppermost mantle. The lithosphere is underlain by the astheno-sphere, which is a much weaker layer and reacts to stress in a fluid manner. The lithosphere is divided into plates, of which the crustal component can be oceanic and/or continental, and the relative movements of plates take place upon the asthenosphere.

However, having made these relatively simple definitions, examination of the several properties that might be expected to characterize these layers reveals that they lead to different ideas of their thickness. The properties considered are thermal, seismic, elastic, seismogenic, and temporal.

Temperature is believed to be the main phenomenon that controls the strength of subsurface material. Hydrostatic pressure increases with depth in an almost linear manner, and so the melting point of rocks also increases with depth. Melting will occur when the temperature curve intersects the melting curve (solidus) for the material present at depth (Fig. 2.36). The asthenosphere is believed to represent the location in the mantle where the melting point is most closely approached. This layer is certainly not completely molten, as it transmits S waves, but it is possible that a small amount of melt is present. The depth at which the asthenosphere occurs depends upon the geothermal gradient and the melting temperature of the mantle materials (Le Pichon

Figure 2.36 Variation of temperature with depth beneath continental and oceanic regions. A, ocean ridge; B, ocean basin; C, continental platform; D, Archean Shield (redrawn from Condie, 2005b, with permission from Elsevier Academic Press).

Depth (km)

Figure 2.36 Variation of temperature with depth beneath continental and oceanic regions. A, ocean ridge; B, ocean basin; C, continental platform; D, Archean Shield (redrawn from Condie, 2005b, with permission from Elsevier Academic Press).

et al., 1973). Beneath ocean ridges, where temperature gradients are high, the asthenosphere must occur at shallow depth. Indeed, since it is actually created in the crestal region (Section 6.10), the lithosphere there is particularly thin. The gradient decreases towards the deep ocean basins, and the lithosphere thickens in this direction, the increase correlating with the depth of water as the lithosphere subsides as a result of contraction on cooling (Section 6.4). The mean lithosphere thickness on this basis beneath oceans is probably 6070 km. Beneath continents a substantial portion of the observed heat flow is produced within the crust (Section

Asthenosphere Flow
Figure 2.37 Shear wave model of the thickening of oceanic lithosphere with age. Velocities in km s-(redrawn from Forsyth, 1975, with permission from Blackwell Publishing). The 150 km transition may be somewhat deeper.

2.13), so the temperature gradient in the sub-crustal lithosphere must be considerably lower than in oceanic areas. It is probable that the mantle solidus is not approached until a significantly greater depth, so that the continental lithosphere has a thickness of 100250 km, being at a maximum beneath cratonic areas (Section 11.3.1).

The depth of the Low Velocity Zone (LVZ) for seismic waves (Section 2.2) agrees quite well with the temperature model of lithosphere and asthenosphere. Beneath oceanic lithosphere, for example, it progressively increases away from the crests of mid-ocean ridges, reaching a depth of approximately 80 km beneath crust 80 Ma in age (Forsyth, 1975) (Fig.2.37). Beneath continents it occurs at greater depths consistent with the lower geothermal gradients (Fig. 2.36). Within the LVZ attenuation of seismic energy, particularly shear wave energy, is very high. Both the low seismic velocities and high attenuation are consistent with the presence of a relatively weak layer at this level. As would be expected for a temperature-controlled boundary, the lithosphere-asthenosphere interface is not sharply defined, and occupies a zone several kilometers thick.

When the Earth's surface is loaded, the lithosphere reacts by downward flexure (Section 2.11.4). Examples include the loading of continental areas by ice sheets or large glacial lakes, the loading of oceanic lithosphere by seamounts, and the loading of the margins of both, at the ocean-continent transition, by large river deltas. The amount of flexure depends on the magnitude of

Lithosphere Surface
Figure 2.38 Comparison of short-term "seismic" thickness and long-term "elastic" thickness for oceanic lithosphere of different ages (redrawn from Watts et al., 1980, by permission of the American Geophysical Union. Copyright © 1980 American Geophysical Union).

the load and the flexural rigidity of the lithosphere. The latter, in turn, is dependent on the effective elastic thickness of the lithosphere, Te (Section 2.11.4). Thus, if the magnitude of the load can be calculated and the amount of flexure determined, Te may be deduced. However as indicated above (Section 2.11.6), Te may be determined more generally from the spectral analysis of gravity and topographic data. Results obtained by applying this technique to oceanic areas are very consistent. They reveal that the elastic thickness of oceanic lithosphere is invariably less than 40 km and decreases systematically towards oceanic ridges (Watts, 2001) (Fig. 2.38). By contrast, the results obtained for continental areas vary from 5 to 110 km, the highest values being obtained for the oldest areas - the Precambrian cratons. However, McKenzie (2003) maintains that if there are sub-surface density contrasts that have no topographic expression, so-called buried or hidden loads, the technique yields an overestimate of the elastic thickness. Such loads are thought to be more common in continental areas, particularly in the cratons, because of their thick and rigid lithosphere. In oceanic areas loads are typically super imposed on the crust and expressed in the topography. McKenzie (2003) goes so far as to suggest that, if one makes allowance for buried loads, the elastic thickness of the lithosphere is probably less than 25 km in both oceanic and continental areas. By contrast, Perez-Gussinge & Watts (2005) maintain that Te is greater than 60 km for continental lithosphere greater than 1.5 Ga in age and less than 30 km for continental areas less than 1.5 Ga in age. They suggest that this is a result of the change in thickness, geothermal gradient, and composition of continental lithosphere with time due to a decrease in mantle temperatures and volatile content (Section 11.3.3). Under tectonically active areas, such as the Basin and Range Province, the elastic thickness may be as small as 4 km (Bechtel et al., 1990). Such very thin elastic thicknesses are undoubtedly due to very high geothermal gradients.

Yet another aspect ofthe lithosphere is the maximum depth to which the foci of earthquakes occur within it. This so-called seismogenic thickness is typically less than 25 km, that is, similar to or somewhat less than the elastic thickness in most areas (Watts & Burov, 2003). On the face of it this appears to lend support to the conclusion of McKenzie (2003) that the spectral analysis of topography and gravity anomalies systematically overestimates Te, particularly in Precambrian shield areas because of the subdued topography and the presence of buried loads. However, there are alternative explanations that invoke the role of the ductile layer in the lower continental crust in decoupling the elastic upper layer from the lower lithosphere, the role of increased overburden pressure in inhibiting frictional sliding, and the fact that there is some evidence for earthquakes and faulting in the lower crust and upper mantle. It is thought that the latter may occur in the relatively rare instances where the lower crust and/or upper mantle are hydrated (Watts & Burov, 2003).

Thus, the concept of the lithosphere as a layer of uniformly high strength is seen to be over-simplistic when the rheological layering is considered. The upper 20-40 km of the lithosphere are brittle and respond to stress below the yield point by elastic deformation accompanied by transient creep. Beneath the brittle zone is a layer that deforms by plastic flow above a yield point of about 100 MPa. The lowest part, which is continuous with the asthenosphere, deforms by power-law creep and is defined as the region where the temperature increases with depth from 0.55 Tm to 0.85 Tm. The lithosphere is best thought of as a viscoelastic rather than an elastic layer (Walcott, 1970) for, as Walcott demonstrated, the type of deformation experienced depends upon the duration of the applied loads. Over periods of a few thousand years, most of the region exhibiting power-law creep does not deform significantly and consequently is included within the elastic lithosphere. Long term loading, however, occurring over periods of a few million years, permits power-law deformation to occur so that this region then belongs to the asthenosphere.

The lithosphere can, therefore, be defined in a number of different ways that provide different estimates of its thickness. This must be borne in mind throughout any consideration of plate tectonic processes.

The asthenosphere is believed to extend to a depth of about 700 km. The properties of the underlying region are only poorly known. Seismic waves that cross this region do not suffer great attenuation (Section 9.4), and so it is generally accepted that this is a layer of higher strength, termed the mesosphere. The compositional and rheological layering of the Earth are compared in Fig. 2.39.

Was this article helpful?

0 0
How To Have A Perfect Boating Experience

How To Have A Perfect Boating Experience

Lets start by identifying what exactly certain boats are. Sometimes the terminology can get lost on beginners, so well look at some of the most common boats and what theyre called. These boats are exactly what the name implies. They are meant to be used for fishing. Most fishing boats are powered by outboard motors, and many also have a trolling motor mounted on the bow. Bass boats can be made of aluminium or fibreglass.

Get My Free Ebook


Responses

  • sm bolger
    What is the asthenosphere thickness?
    2 years ago
  • gabriele
    Which layer of lithosphere developed in fossil?
    1 year ago
  • tom weissmuller
    Do seismic waves reveal the composition of the asthenosphere?
    7 months ago
  • Hayley
    What happens to the asthenosphere during a earthquake?
    4 months ago
  • juan
    What is the purpose of the asthenosphere during the formation of an earthquake?
    4 months ago
  • Lete
    What layer the asthenosphere belong?
    1 month ago

Post a comment