Thermal Structure Of The Downgoing Slab

The strength and high negative buoyancy of subducting oceanic lithosphere and its capacity for sudden failure in the generation of earthquakes are consequences of its relatively low temperature with respect to normal mantle material at these depths. The subducting

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).

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 mechanical identity to a considerable depth until sufficient heat has been transferred to it from the mantle to increase its temperature to that of its surroundings.

The variation of temperature within the sinking slab can be calculated from heat conduction equations provided that its thermal properties and boundary states are specified. The factors controlling the temperature distribution are:

1 the rate of subduction: the more rapid the descent the less time there is for absorption of heat from the surrounding mantle by conduction;

2 the age and hence thickness of the descending slab: the thicker the slab the greater the time taken for it to equilibrate thermally with the surrounding asthenosphere;

3 frictional heating of the upper and lower surfaces of the slab as the descent of the slab is resisted by the asthenosphere;

4 the conduction of heat into the slab from the asthenosphere;

Figure 9.16 Location map and cross-sections across the Aleutian arc showing earthquake foci (redrawn from Jacob et al., 1977, by permission of the American Geophysical Union. Copyright © 1977 American Geophysical Union).

5 the adiabatic heating associated with compression of the slab as the pressure increases with depth;

6 the heat derived from radioactive decay of minerals in the oceanic lithosphere, likely to be small as oceanic plates are largely barren of radioactive minerals;

7 the latent heat associated with phase transitions of minerals to denser crystalline structures with depth: the principal phase changes experienced by the slab are the olivine-spinel transition at about 400 km depth which is exothermic, and the spinel-oxides transition at about 670 km, which is endothermic (Section 2.8.5).

O Aleutians a Tonga - Kermadec

□ New Zealand x Central America

■ South America + Lesser Antiles

1400

1200

2000 4000 6000 8000 10000 12000 Rate x age (km)

Figure 9.17 Relationship between length of Benioff zone and the product of convergence rate and age. Approximate uncertainties given by error bars in upper left corner (redrawn from Molnar et al., 1979, with permission from Blackwell Publishing).

Different solutions for the temperature distribution have been derived by various workers, depending on the assumptions made concerning the relative contributions of the above phenomena. Two models derived by Peacock & Wang (1999) and representing relatively cool and warm subducting lithosphere are shown in Plate 9.3 (between pp. 244 and 245). Although differing in detail, all such models indicate that the downgoing slab maintains its thermal identity to great depths and that, exceptionally, temperature contrasts up to 1000°C may exist between the core of the slab and normal mantle at a depth of 700 km.

As noted in Section 9.4, the length of the Benioff zone depends on the depth to which the subducting oceanic lithosphere maintains a relatively cold central core. Molnar et al. (1979) deduced that the downward deflection of isotherms, and hence the length of the seismic zone, is proportional to both the rate of subduction and the square of the thickness of the lithosphere. Lithosphere thickness is proportional to the square root of its age (Turcotte & Schubert, 2002) so that the length of seismic zones should be proportional to the product of convergence rate and age. That this is generally so is illustrated by Fig. 9.17, and although there is considerable scatter the data appear to fit the relationship length (km) = rate (mm a-1) X age (Ma)/10.

Boating Secrets Uncovered

Boating Secrets Uncovered

If you're wanting to learn about boating. Then this may be the most important letter you'll ever read! You Are Going To Get An In-Depth Look At One Of The Most Remarkable Boating Guides There Is Available On The Market Today. It doesn't matter if you are just for the first time looking into going boating, this boating guide will get you on the right track to a fun filled experience.

Get My Free Ebook


Post a comment