In order to understand the structural styles and tectonic development of plate margins and interiors, it is necessary to consider the nature and magnitude of all the forces that act on plates. Forsyth & Uyeda (1975) solved the inverse problem of determining the relative magnitude of plate forces from the observed motions and geometries of plates. Since the present velocities of plates appear to be constant, each plate must be in dynamic equilibrium, with the driving forces being balanced by inhibiting forces. Forsyth & Uyeda (1975) used the corollary of this, that the sum of the torques on each plate must be zero, to determine the relative size of the forces on the 12 plates which they assumed make up the Earth's surface. The asthenosphere's role in this scenario was considered to be essentially passive. A similar set of computations based on a similar method, and providing similar results, was made by Chapple & Tullis (1977). The following description of forces is based on the extensions of the work of Forsyth & Uyeda (1975) made by Bott (1982).
At ocean ridges the ridge push force FRP (Fig. 12.7) acts on the edges of the separating plates. This derives from the buoyancy of the hot inflowing material causing the elevation of the ridge and hence an additional hydrostatic head at shallow depths which acts on the thinner lithosphere at the ridge crest. It may also arise from the cooling and thickening of the oceanic lithosphere away from the ridge (Section 6.4), which exerts a pull on the ridge region. Hence, it is basically a gravitational force. The ridge-push force may be two or three times greater if a mantle plume (Section 5.5) underlies the ridge (Bott, 1993), because of the increased pressure in the asthenosphere at the ridge crest. The separation of plates at ocean ridges is opposed by a minor ridge resistance Rr that originates in the brittle upper crust and whose existence is demonstrated by earthquake activity at ridge crests. The resisting forces are small so that the net effect is the presence of a driving force.
Beneath plate interiors a mantle drag force acts on the base of both the oceanic and continental lithospheres if the velocity of the underlying asthenosphere differs from that of the plate. If the asthenosphere velocity exceeds that of the plate, mantle drag enhances the plate motion (FbO, Fbc), but if the asthenosphere velocity is lower, as shown in Fig. 12.7, the mantle drag tends to resist plate movement (RbO, Rbc). Mantle drag beneath continents is about eight times the drag beneath oceans; this may be due to the increased thickness of the subcontinental lithosphere beneath cratonic areas (Sleep, 2003).
At subduction zones the major force acting on plates results from the negative buoyancy (FNB) of the cold, dense slab of descending lithosphere. Part of this vertical force is transmitted to the plate as the slab-pull force FSP . The density contrast, and hence FNB, is greatly enhanced at depths of 300-400 km where the olivine-spinel transition occurs in the slab. FSP is opposed by a slab resistance (RS), which mainly acts on the leading edge of the descending plate where it is five to eight times greater than the viscous drag on its upper and lower surfaces. Underthrusting involves a downward flexure of the lithosphere in response to FNB, and since it behaves in an elastic manner in the top few tens of kilometers flexure is opposed by a bending resistance (Rb). A further resistance to motion at subduction zones is the friction between the two plates. This overriding plate resistance (RO) is expressed in the intense earthquake and tectonic activity observed at shallow depths at destructive plate margins. The downgoing slab achieves a terminal velocity when FSP is nearly balanced by RS + RO. If FSP exceeds RS + RO, the slab descends at greater than the terminal velocity and throws the slab into tension at shallow depths. If FSP is less than RS + RO the slab is thrown into compression. The balance between driving and resistive forces may thus control the distribution of stress types, as revealed by earthquake focal mechanism solutions, within downgoing slabs (Section 9.4).
In the region on the landward side of subduction zones the overriding lithosphere is thrown into tension by the trench suction force (FSU). There are several possible causes of this force (Fig. 12.8):
1 It may arise because the angle of subduction becomes progressively greater with depth (Fig. 12.8a). Tension would then arise as the overriding plate collapses toward the trench.
2 The tension could result from the "roll-back" of the underthrusting plate (Fig. 12.8b). That is, the downgoing slab retreats from the overriding plate.
3 Tension could be generated by secondary convective flow in the region overlying the downgoing slab (Fig. 12.8c). This would require
a relatively high geothermal gradient giving rise to a relatively low viscosity in the asthenosphere (Section 12.5.2).
4 Tension may arise from any of several mechanisms proposed for the formation of backarc basins on the landward side of subduction zones (Fig. 12.8d) as described in Section 9.10. However, once backarc spreading commences the landward plate becomes decoupled from the trench system (Fig. 12.8e).
When two plates of continental lithosphere are brought into contact after the complete consumption of an intervening ocean at a subduction zone, the resistance to any further motion is known as collision resistance. The mechanism of this resistance is complex because it takes place both at the suture between the plates and within the overriding plate (Sections 10.4.3, 10.4.6). Finally, transform fault resistance affects conservative plate margins in both continental and oceanic areas. The resistance acts parallel to the faults and gives rise to earthquakes with a strike-slip mechanism (Section 2.1.5) confined to a shallow depth. More complex resistance is encountered where the fault trend is sinuous so that motion is not purely strike-slip (Section 8.2).
The relative magnitude of the forces acting on plates and their relevance to the driving mechanism of plate tectonics will be discussed in Section 12.7.
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