Geoscience Reference
In-Depth Information
Many seismic-refraction lines have been shot in the Alps. The results have
been used in the preparation of the Moho-depth map in Fig. 10.2: The crust
thickens from 25 km beneath the Rhine graben to over 55 km in the Central Alps
and then thins again on the southern side of the Alps. Figure 10.24(a) shows a
seismic-refraction record section from the Jura region, north of the Alps. There
the crust is 27 km thick and has a complex structure with two distinct low-velocity
zones, one in the upper crust and the second immediately above the Moho. These
low-velocity zones have been detected because of the offset in the travel times
between the wave which travelled in the overlying high-velocity material and the
wave reflected from the base of the low-velocity zone (see Section 4.4.3). One
prominent feature of this record section is the large amplitude of the P
n
phase (the
Moho headwave), which indicates that there is a strong positive velocity gradient
in the upper mantle. Figure 10.24(b), data from the southern Swiss/Italian Alps,
shows evidence of a low-velocity zone in the upper crust but not in the lower crust.
The large amplitude of the wide-angle reflection from the Moho, P
m
P, indicates
that there is a large velocity contrast at the Moho beneath the southern Alps.
The crust reaches a maximum thickness of 56 km beneath the Alps and is
underlain by the southward-dipping European mantle. Just to the north of the
Insubric Line there is a very sudden step offset in the Moho. To the south the Moho
is much shallower and dips northwards. Beneath the Southern Alps the Moho
is at 33 km depth. The bases of the European and the Adriatic crusts are imaged
on the reflection line shown in Fig. 10.22(d). The depth extent of the European
Moho and details of the European mantle lithosphere as a continuous interface
are not clear. The Insubric Line itself is well imaged on the reflection profile and
can be traced to a depth of 17 km.
The crust beneath the Alps is complex, both vertically and laterally along
the length of the Alps, but it can be broadly described as upper crust and lower
crust. Lower-crustal P-wave velocities are greater than 6.5 km s
−
1
,whereas upper-
crustal P-wave velocities are less than 6.2 km s
−
1
(Fig. 10.22(c)). The upper crust
is characterized by having a complex geometry and thin layers, but in part this is
due to the greater resolution possible in the upper crust (Section 4.4.4). There is a
pronounced low-velocity, 5.7 km s
−
1
, zone in the upper crust beneath the Southern
Alps - this may be part of the southward-oriented thrust sheets (Fig. 10.21(d)).
Beneath the Penninic nappes a thin high-velocity, 6.5 km s
−
1
,layerat 10 km
depth can be matched to normal-incidence reflections from an interface within
the Penninic nappes. The second high-velocity layer at
20 km depth beneath
the Penninic nappes is also clearly identifiable on the normal-incidence-reflection
profile. This reflection horizon may be the top of the Adriatic lower crust.
The Alps are not particularly active seismically. Earthquakes do occur but
not frequently, and, although sometimes damaging, they are usually of lower
magnitude than Himalayan events. To the north of the Alps and beneath the
Southern Alps, earthquakes occur throughout the crust. Beneath the central Alps,
however, earthquake activity is restricted to the upper 15-20 km of the crust. This
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