Geoscience Reference
In-Depth Information
transect a sharp, apparently continuous reflection from the Moho was observed
at about 2.5 s beneath the top of the oceanic basement. Eleven expanding-spread
(wide-angle) profiles were also recorded along this transect; Fig. 9.7(a) shows
the record section for one of these profiles. Modelling of these travel times and
amplitudes yielded a detailed structure of the oceanic crust at this location, which
is some 7.5 km in thickness. Study of the amplitudes of those seismograms shows
that there may be a low-velocity zone in the lower crust, but it is not necessarily
needed to satisfy the data. A reduced record section from a seismic refraction
line shot over 140-Ma-old oceanic crust in the western central Atlantic Ocean
is shown in Fig. 9.7(b). The main features of these typical seismograms are the
large-amplitude mantle reflections P
m
P, the weak mantle refraction P
n
and the
clear shear waves.
Thus, several lithologies have been proposed for layer 3, and we will not
be able to determine definitively the composition of layer 3 and its subdivision
until a hole (better yet, several holes) is drilled completely through the oceanic
crust.
Ophiolites
(a sequence of rocks characterized by basal ultramafics overlain by
extensive thicknesses of basaltic and gabbroic material: gabbro, dykes, lavas and
deep-sea sediments) are often regarded as examples of oceanic crust. However,
because they are now tectonically emplaced on land, they are atypical and might
not represent normal oceanic crust. Ophiolites are probably samples of young
oceanic crust produced in back-arc basins, or fore-arc basins, associated with
subduction zones (Section 9.6). It is more likely that such crust, rather than typical
oceanic crust from a section of old cold lithosphere, could become emplaced on
land. Figure 9.8 shows the principal ophiolite belts of the world. One well-studied
ophiolite is the Semail ophiolite in the Sultanate of Oman, which is believed to
have been formed some 95 Ma ago at a spreading centre in the Tethyan Sea and
later emplaced on the Arabian Plate as Tethys closed when Africa and Arabia
moved northwards and collided with Asia (see Section 3.3.3). The results of the
extensive geological and seismic-velocity studies of this classical ophiolite are
summarized in Fig. 9.9.Itisapparent that the seismic-velocity structure is similar
to that of the oceanic crust and upper mantle, with gradients throughout layer 2,
a rapid increase in velocity at the layer-2-layer-3 boundary and a relatively high-
velocity layer 3B at the base of the crust. The transition from crust to mantle is
sharp. Similar results have been determined for the Blow Me Down Massif of
the Bay of Islands ophiolite in Newfoundland, Canada. However, it is unwise to
make a direct comparison between the seismic velocities measured on ophiolite
rock samples at ultrasonic frequencies in a laboratory and the seismic velocities
determined from reflection and refraction experiments using very-low-frequency
signals; the frequency scale and the length scale of the velocity determinations
differ.
The oceanic crust has more or less the same thickness and velocity structure
throughout the world's oceans, on all plates. This observation can be explained
