by discontinuities in the lithosphere. The upper
layer consists in fact of a number of slabs that can
move relative to each other and which are bounded
by oceanic ridges, trench or island-arc systems and
transform faults. Only the first two, however, play
a role in this discussion.
The ridges are recognized as areas of tension; where
adjacent lithospheric slabs are moving apart. Either
in consequence of this separation or as the cause of
it, hot basaltic type materials rise up close to the
surface and sometimes break through the surface
as volcanos. The ridges therefore tend to grow and
are often referred to as areas of crustal generation.
The trenches, of which the circum-pacific belt are
the outstanding examples, are also referred to as
island arcs since arcs of islands often occur on the
continental side of the trench. The trenches are
areas of compression where adjacent lithospheric
slabs push against each other.
The consequence of this collision is that the cooler
and thinner oceanic slab is pushed under its con
tinental counterpart and back into the astheno-
sphere. The trenches or island arcs are therefore
also referred to as areas of crustal distruction. The
Alpide belt running from the western Mediterra
nean to the Himalayas are also compressional fea
tures but in this case the action is between two thick
continental slabs and buckling rather than down
thrusting tends to occur. A continental equivalent
to the oceanic ridges is the rift valley of East Africa,
but the surface characteristics are again distorted
by the thick crust. These models are supported
very strongly by seismic data [23] and the rates of
motion have been estimated from marine magnetic
anomalies of as much as 10 cm/year [24, 25].
Figure 6 gives a schematic diagram of areas of ex
tension and compression and figure 7 gives the
sinuous path of the two types of features. Also
shown in this figure are deep ocean basins below
depths greater than 5000 meters.
Comparing figures 1, 4 or 5 with the tectonic fea
tures we find an immediate and consistent correla
tion between the earth's gravity field and the oceanic
trenches. In each case the anomalies are strongly
positive and there is a faint suggestion that the
region of maximum anomaly occurs just to the
continental side of the trench. When we look at the
Alpine belt we find the correlation positive in the
western part but turning negative in its eastern part.
The oceanic ridges also show quite a good positive
correlation with the gravity field although the val
ues of the anomalies are generally smaller and
sometimes the correlation tends to turn negative.
Ocean basins tend to give a negative correlation
with gravity.
Kaula [26] has attempted to interpret these corre
lations in terms of the rather complex interactions
between the flow patterns in the asthenosphere
and the lower lithospheric boundary. The positive
anomalies just behind the island arcs would be a
consequence of the cool oceanic slab being pushed
under the continental slab and some of the dis
placed viscous asthenospheric material would tend
to push the lithosphere upwards.
At the ridges, above the up-welling limbs of the
convection pattern we would expect negative gravity
anomalies unless the lithosphere is being pushed
upwards as well. In this latter case the anomalies
would be positive if the mass pushed upwards out
weighs the decrease in density. Recent results by
McKenzie [27] are relevant to this problem.
McKenzie carried out extensive calculations of
CRUST
THO -
SPHERE
ASTHENOSPHERE
CRUST
Fig. 6. Schematic cross section of ocean ridges and island arcs.
50
ngt 72
