Geoscience Reference
In-Depth Information
on transform faults are harder to evaluate. Earthquakes on transform faults are
usually shallow, even though the plates can be perhaps 80 km in thickness (Section
9.5.3). It is probable that their total resistive contribution is of the same magnitude
as the ridge-push driving force, or smaller. Estimates of the resistive force acting
on thrusts at the convergent plate boundaries, as indicated by earthquakes, give
values of 10 12 Nm 1 .Again, this is less than the mantle-drag force.
To summarize, the main driving force is slab-pull, and the main resistive forces
occur as drag along the base of the plate and on the descending slab.
Does mantle convection control plate tectonics?
Whether convection in the mantle drags the plates around or whether the forces
acting at the edges of the plates drive the plates, which in turn drag the mantle, is
a complicated 'chicken or the egg' type of question. From analysis of the driving
and resistive forces, it is clear that the pull of the descending slab is a factor in
determining the form of mantle flow. If the only locations for ridges were above
the rising limbs of convection cells, then in simple schemes (e.g., Fig. 8.14(a))
each plate should have one edge along a ridge and the other along a subduction
zone. Clearly this is not the case; for example, the Antarctic and African plates
are bounded almost entirely by ridges. Where could the return flow go? In these
instances it seems reasonable to assume that the ridges form where the lithosphere
is weakest and that mantle material rises from below to fill the gap. Plates with
subducting edges move with higher velocities than do those without (see Figs. 2.2
and 2.20), in agreement with the earlier estimate of the importance of the slab-
pull driving force. To first order, ridge-push and continental collisional forces
control the stress regimes in the plate interiors (Fig. 2.21).
Analysis of the stress within the North American continent permits analysis
of the forces which drive and deform the continental part of that plate. The main
driving force is the ridge-push from the Mid-Atlantic Ridge. Since the resistive
forces amount to only about a quarter of the driving forces, the continent is being
compressed against the Pacific plate to the west. The implication of the low values
for resistive forces is that the 'root' beneath the North American continent which
extends down to
300 km is moving as one with the underlying asthenosphere.
Thus, in conclusion, though there is still much that is not understood about flow
in the mantle and the motion of lithospheric plates, the pull of the descending plate
at convergent boundaries due to its decrease in temperature seems to be a major
factor both in the thermal modelling of the mantle flow and in the mechanical
models of the forces involved.
Did plate tectonics operate during the Archaean?
A force-balancing model can be used to investigate the possibility of plate tec-
tonics operating during the Archaean and to estimate probable plate velocities.
The Earth was probably much hotter then than it is now, with temperatures at the
top of the asthenosphere of about 1700 C compared with 1300-1400 C today.
Search WWH ::




Custom Search