O
evidence is that lateral variation in density or
elastic properties can exist and the reasonably sure
conclusion drawn from the post glacial uplift data
indicate that some flow is most probable.
A brief discussion on the nature of these upper
regions will be useful for the subsequent discussion.
Extending to a variable depth ranging between
40 and 100 km is a layer generally called the litho-
sphere. This layer includes the crust of the earth
and extends to some depth beyond it.
Its characteristic is that it can be considered as
sufficiently rigid to be able to support limited stresses
and strains without immediate deformations.
Generally the lithosphere tends to be thinner and
cooler when underlying the oceans than for the
continental regions and in consequence there is a
slight density difference between oceanic litho
sphere and continental lithosphere. Underneath
this rigid layer, and extending to a depth of be
tween 700 and 900 km is a region of relatively low
and variable viscosity and which can only support
density anomalies by flow. Referred to as the asthe-
nosphere, it is the layer that deforms when the
lithosphere is subjected to large loads caused by,
for example, glacial ice.
If we rule out the lower mantle as the source for
the gravity anomalies we are left with two alter
native possibilities: They arise from density varia
tions in the lithosphere or in the asthenosphere. In
the first instance they must be maintained by the
finite strength of this layer and in the second in
stance they must be maintained by flow. For the
gravity anomalies we are considering here, of
wavelengths of 1000 km or more, it can be shown
that the first hypothesis is insufficient. McKenzie [9]
for example, has shown that the shear stresses that
would exist if the anomalies were supported by the
lithosphere are very much greater than can actually
be supported without immediate fracture. For
example, if we consider a feature of 1500 km extend
with a peak gravity anomaly of 20 mgal, we require
a shear stress for the lithosphere of 800 bars
whereas laboratory work and earthquakes indicate
that fracture occurs at about 200 bars or less for
crustal materials at low temperatures.
The two layered horizontal model is complicated
180°
120°
150°
240°
300°
60°
90°
180°
330°
30°
80°-
60°-
60
O
40°-
- 40
- 20°
O
0°-
--40°
-40°-
-80°
120°
90°
150°
180°
270°
60°
180°
240°
300°
330°
30°
210°
Fig. 5. Gravity anomalies with respect to a reference figure containing all gravity terms up to and including 4,4. Contour
interval is 20 mgal.
ngt 72
49
