Geoscience Reference
In-Depth Information
10.1.2 Geophysical characteristics of continents
The continental crust averages 38 km thick. We have already used this value in
calculations of isostasy in Section 5.5 and in the calculation of the thickness of the
oceanic crust (Section 9.2.1). Although a 'normal' or 'standard' oceanic crustal
structure can be defined, it is much more difficult to give a standard continental
crustal structure. The variability of the structure of the continental crust is, like
all the other properties of the continents, a direct result of the diverse processes
involved in their formation and the long time over which they have formed.
Figure 10.2(a) shows a global crustal-thickness map. The large variations in
crustal thickness are very clear. The thickest crust is found beneath the Tibetan
plateau, the Andes and Finland. This global map has a 5
◦
×
5
◦
cell size, which
means that features narrower than 200 km, such as mountain belts, are not accu-
rately depicted. Generally, the crust is thick beneath young mountain ranges, mod-
erately thick beneath the ancient shield regions and thin beneath young basins and
rifts such as the North Sea and Rhine Graben in Europe, the East African Rift and
the Basin and Range Province in the U.S.A. as well as beneath the continental mar-
gins, passive margins and active fore-arcs. The global average thickness of conti-
nental crust is 38 km, but the thickness typically ranges between 30 and 45 km.
The seismic-velocity structure of the crust is determined from long seismic-
refraction lines. The advent of deep reflection lines has delineated the fine struc-
ture of the crust very well, but such data usually cannot yield accurate velocity
estimates (see the discussion of stacking velocities in Section 4.5.3). The offset
between the source and the further receiver must be increased considerably to
obtain better deep velocity information from reflection profiling.
Teleseismic earthquake recordings can be used to confirm gross crustal and
upper mantle interfaces through the use of P to S mode conversions. The technique
is referred to as the 'receiver function' method since interfaces are identified for
each 'receiver' or seismograph location. When there are several receivers in a
study area it is possible to establish a gross crustal, or lithosphere, thickness map.
The direct wave which travels in the crystalline, continental basement, beneath
surface soil and sedimentary cover, termed Pg, normally travels with a velocity
of about 5.9-6.2 km s
−
1
. The velocity of the upper 10 km of the crust is usually in
the range 6.0-6.3 km s
−
1
; beneath that, in the middle crust, the velocity generally
exceeds 6.5 km s
−
1
.Atsome locations there is another, lower crustal layer with
velocity greater than 7 km s
−
1
.For many years, a major discontinuity at the base
of the upper crust (the
Conrad discontinuity
)was thought to be a universal feature
of continental crust and to be underlain by a basaltic layer having a velocity of
6.5 km s
−
1
. This is no longer thought to be the case. Continental crustal structure
is complex and, while some regions have a well-developed discontinuity at the
base of the upper crust, not all do. Low-velocity zones at various locations at
all depths in the continental crust have been described and represent the com-
plexity of the history of the crust. The continental crust does not have a standard
structure: Fig. 10.2(b) shows the general variation. In the middle crust velocities
