decreases rapidly away from the ridge until, in a mature oceanic lithosphere with a thickness approaching 100 km, the thermal gradient is close to 13°C km-1. The increases in temperature with depth within a mature section of oceanic lithosphere is shown in Figure 3.23a. The upper 25 km, which initially represented the original elastic layer, shows the depth to which the rock is essentially brittle. We have argued and adduced evidence that the uppermost 15 km of this elastic layer will be cut by thrusts, while the lower level may be cut by strike-slip faults. From a depth of 25-45 km, the behaviour of the rock material will change from brittle and become progressively more plastic (i.e. ductile) and weaker.
Let us now consider this section of oceanic lithosphere to be covered by a 10 km thick sheet of sediments (Figure 3.23b). The geothermal gradient in such a pile of sediments will, in part, depend upon the type of sediments it contains. For example, a 1.0 km thick layer of clays and muds may exhibit a temperature difference between the bottom and top of as much as 100°C. Here we shall be conservative and assume that the sediments have attained thermal equilibrium with an average gradient, throughout the 10 km of sediments, of an extremely modest 15°C km-1. Hence, the temperature at the interface between a 10 km thick pile of sediments and the oceanic lithosphere will be 150°C. In our model, this results in the specific temperature levels in the oceanic lithosphere being transposed upwards. The elastic layer is significantly reduced in thickness from 30 km to 16.5 km. With such a reduction in thickness of the elastic layer, the maximum compressive fibre stresses, for the thickness of basin sediments of 10 km and 12 km, are respectively 4.1 kb and 5.0 kb.
In addition, it is necessary to note that the upper, brittle portion of the strong zone is transposed to contain the zone in which it is likely that thrusts have already developed, and that the 15 km thick section, which comprises the 'brittle' and 'plastic' zones in Figure 3.23a, are reduced to a mere 1.65 km in Figure 3.23b.
Moreover, the vertical (confining) pressure (Figure 3.23b), at any specific depth, is significantly lower that shown in Figure 3.23 a, so that re-shearing along existing thrusts will be enhanced.
Let us now see how the various changes induced by the sedimentary blanket over the oceanic lithosphere will influence the latter's behaviour.
The magnitude of the least principal stress in the fractured zone down to the neutral surface (NS) is given by line AB in Figure 3.24. The re-shear conditions on existing thrusts, represented by line CD, will be sustained by the gravity-glide mechanism set out earlier in this chapter. The loading of the oceanic lithosphere by a thick layer of sediments can cause downward bending of the oceanic lithosphere, which gives rise to compressive fibre stresses. If the thickness of the sedimentary pile is 10 km, the maximum notional fibre stress is 4 kb (line DE). These notional fibre stresses in the upper layer of the oceanic lithosphere will be dissipated by causing, or enhancing, re-shear, thereby extending existing thrust-fractures.
At these high levels in the oceanic lithosphere, the re-sheared thrusts can disrupt the sediment-lithospheric interface, thereby permitting water from wet sediments to penetrate downward into the erstwhile dry basalts. The development of clay surfaces on the thrust zones will greatly reduce the differential stress required to induce further shear movement. Consequently, the uppermost regions of the oceanic lithosphere will become significantly weaker, thereby reducing the effective thickness of the downward-flexing beam formed by the 16.5 km thick strong-zone, from depths of 10-26.5 km.
The thinner the bending beam, for a given bending moment, the greater the degree of flexure. Consequently, the outer fibre-stresses will tend to cause the onset of an accelerating process, in which the neutral surface migrates deeper into the oceanic lithosphere. One or more thrust planes develop and extend, by re-shearing, until the strong layer is completely cut through by one or more such fractures.
The initial resistance of pushing a wedge of relatively cold oceanic lithosphere into hot asthenosphere will be considerably smaller than the head-to-head confrontation of the near irresistible force of the oceanic lithosphere and the near-immovable continental unit. This will result in a significant reduction of the horizontal stresses in the oceanic plate. This, in turn, releases stored strain-energy, which permits elastic recovery, which will extend the length of the oceanic lithosphere by an amount which could be several tens of kilometres; thereby, for a short period, enhancing the rate of development of the subduction zone. Thus, if we take the elastic recovery to be 30-40 km, the subduction process will have been initiated, coupled with the early development of the 'down-slab' pull. This process will have taken place with no apparent change in length of the oceanic lithosphere, between spreading-ridge and subduction trench, as determined at the surface, so that there will be no noticeable change in the rate of plate motion.
The model we have discussed above is complex and, therefore, difficult to quantify precisely. However, we believe we have shown that the gravitational gliding stresses can certainly fracture the upper zone of the lithosphere, but may not be able to generate sufficient energy to cause a through-going fracture to develop in the oceanic lithosphere. However, where continental and oceanic lithospheres abut, a thick blanket of sediments will cause a downward flexure to develop in the oceanic lithosphere. If the layer of sediments approaches a thickness of about 10 km, the weight of the sediments brings about a change in geometry of the oceanic lithosphere, coupled with a change in geothermal gradient, such that the changes in the environment so weakens the strong layer of the oceanic lithosphere that failure of the whole lithosphere takes place and a subduction zone is initiated. These conditions are met only infrequently.
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