Geoscience Reference
In-Depth Information
Figure 10.56.
A cross
section of crust and
upper-mantle structure
across the Rio Grande
Rift. Velocities are based
on refraction and
surface-wave results.
(From Sinno
et al
.(1986).)
W E
Basin and
Range
Rio Grande
Rift
Great Plains
0
5.9
5.8
Upper crust
6.0
6.2
6.1
20
6.6
6.5
6.7
Lower crust
7.7
8.0
7.1
40
Upper mantle
8.2
100 km
60
Figure 10.57.
The Rio
Grande Rift. (a) The
Bouguer gravity anomaly
on a profile at
approximately 33
◦
N.
(After Cordell (1978).)
(b) Interpretation of the
Bouguer gravity anomaly
after corrections for the
shallow structure have
been made. (After
Ramberg (1978).)
(a)
−
100
−
200
RIFT
−
300
−
400
−
200
0
200
400
(b)
2900
0
2740
3050
2940
3200
3100
100
3300
−
100
0
100
200
Distance (km)
been modelled with synthetic seismogram programs. Provided that the upper few
kilometres of the lower crustal layer have a low S-wave velocity, the amplitudes
of the synthetic seismograms are in agreement with the data.
Figure 10.57 shows the gravity anomaly along a profile crossing the rift.
A small positive Bouguer gravity anomaly is superimposed on a wide, low
(
200 mgal) anomaly. The interpretation of these gravity data is ambiguous,
but, like the East African Rift, the broad, low anomaly appears to be caused
−
