Page 64 - Global Tectonics

THE INTERIOR OF THE EARTH 51

imposed on the crust and expressed in the topography. demonstrated, the type of deformation experienced
McKenzie (2003) goes so far as to suggest that, if one depends upon the duration of the applied loads. Over
makes allowance for buried loads, the elastic thickness periods of a few thousand years, most of the region
of the lithosphere is probably less than 25 km in both exhibiting power-law creep does not deform signifi -
oceanic and continental areas. By contrast, Perez- cantly and consequently is included within the elastic
Gussinge & Watts (2005) maintain that T e is greater than lithosphere. Long term loading, however, occurring
60 km for continental lithosphere greater than 1.5 Ga in over periods of a few million years, permits power-law
age and less than 30 km for continental areas less than deformation to occur so that this region then belongs
1.5 Ga in age. They suggest that this is a result of the to the asthenosphere.

change in thickness, geothermal gradient, and composi- The lithosphere can, therefore, be defined in a
tion of continental lithosphere with time due to a number of different ways that provide different esti-
decrease in mantle temperatures and volatile content mates of its thickness. This must be borne in mind
(Section 11.3.3). Under tectonically active areas, such as throughout any consideration of plate tectonic
the Basin and Range Province, the elastic thickness may processes.
be as small as 4 km (Bechtel et al., 1990). Such very thin The asthenosphere is believed to extend to a depth
elastic thicknesses are undoubtedly due to very high of about 700 km. The properties of the underlying
geothermal gradients. region are only poorly known. Seismic waves that cross
Yet another aspect of the lithosphere is the maximum this region do not suffer great attenuation (Section 9.4),
depth to which the foci of earthquakes occur within it. and so it is generally accepted that this is a layer of
This so-called seismogenic thickness is typically less than higher strength, termed the mesosphere. The composi-
25 km, that is, similar to or somewhat less than the tional and rheological layering of the Earth are com-
elastic thickness in most areas (Watts & Burov, 2003). pared in Fig. 2.39.
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 T e, particularly in Precambrian shield 2.13 TERRESTRIAL
areas because of the subdued topography and the pres-
ence of buried loads. However, there are alternative
explanations that invoke the role of the ductile layer in HEAT FLOW
the lower continental crust in decoupling the elastic
upper layer from the lower lithosphere, the role of
increased overburden pressure in inhibiting frictional The study of thermal processes within the Earth is
sliding, and the fact that there is some evidence for somewhat speculative because the interpretation of the
earthquakes and faulting in the lower crust and upper distribution of heat sources and the mechanisms of heat
mantle. It is thought that the latter may occur in the transfer are based on measurements made at or near
relatively rare instances where the lower crust and/or the surface. Such a study is important, however, as the
upper mantle are hydrated (Watts & Burov, 2003). process of heat escape from the Earth’s interior is the
Thus, the concept of the lithosphere as a layer of direct or indirect cause of most tectonic and igneous
uniformly high strength is seen to be over-simplistic activity.
when the rheological layering is considered. The upper The vast majority of the heat affecting the Earth’s
20–40 km of the lithosphere are brittle and respond to surface comes from the Sun, which accounts for some
stress below the yield point by elastic deformation 99.98% of the Earth’s surface energy budget. Most of
accompanied by transient creep. Beneath the brittle this thermal energy, however, is reradiated into space,

zone is a layer that deforms by plastic flow above a yield while the rest penetrates only a few hundred meters
point of about 100 MPa. The lowest part, which is con- below the surface. Solar energy consequently has a neg-
tinuous with the asthenosphere, deforms by power-law ligible effect on thermal processes occurring in the inte-

creep and is defined as the region where the tempera- rior of the Earth. The geothermal energy loss from heat
ture increases with depth from 0.55 T m to 0.85 T m . The sources within the Earth constitutes about 0.022% of its
lithosphere is best thought of as a viscoelastic rather surface energy budget. Other sources of energy include
than an elastic layer (Walcott, 1970) for, as Walcott the energy generated by the gradual deceleration of the