Page 275 - Global Tectonics

260 CHAPTER 9

Figure 9.15 Summary of the distribution of down dip stresses in Benioff zones. Open circles, events with
compressional axis parallel to dip of zone; solid circles, events with tensional axis parallel to dip of zone; crosses,
neither P- nor T-axis parallel to zone; solid lines, approximate form of seismic zone (redrawn from Isacks & Molnar,
1971, by permission of the American Geophysical Union. Copyright © 1971 American Geophysical Union).

lithosphere can retain its separate thermal and mechan- of heat from the surrounding mantle by
ical identity to a considerable depth until suffi cient heat conduction;
has been transferred to it from the mantle to increase 2 the age and hence thickness of the descending
its temperature to that of its surroundings. slab: the thicker the slab the greater the time
The variation of temperature within the sinking slab taken for it to equilibrate thermally with the
can be calculated from heat conduction equations pro- surrounding asthenosphere;
vided that its thermal properties and boundary states
3 frictional heating of the upper and lower

are specified. The factors controlling the temperature
distribution are: surfaces of the slab as the descent of the slab
is resisted by the asthenosphere;
1 the rate of subduction: the more rapid the 4 the conduction of heat into the slab from the
descent the less time there is for absorption asthenosphere;