Geology Reference
In-Depth Information
length of
L
0
= 38.3 km, and the straight line distance across the top basement in the graben
of
L
1
= 40.7 km. The Moho is at a depth of 27 km directly below the graben and is 31 km
deep outside it (Fig. 11.64b). The
value for the graben is 1.29 from Eq. 11.10 (
e
L
= 29%),
assuming the original thickness of the crust in the Rhine graben was 31 km and the final
thickness is 24 km (27 km minus 3 km of post-Permian sediments in the graben). The drama-
tic difference in extension calculated from the two different equations is the result of the
specific pure-shear model applied. The large value is for a vertical-sided graben and the
small value is the expected result near the top of a normal-fault-bounded graben. Because
the Rhine graben has normal-fault boundaries, it is reasonable to conclude that the model
of Fig. 11.63 applies to the graben and that the small strain magnitude at the top of the base-
ment does not imply a major discrepancy in the extension, but rather is the expected value.
β
11.10
Exercises
11.10.1
Cross-Section Validation and Interpretation 1
Validate (or invalidate) the cross section in Fig. 11.65. Is it length balanced? Is it area
balanced? Apply the area-depth relationship to find the best-fitting lower detachment,
displacement, and the strain in each layer.
11.10.2
Cross-Section Validation and Interpretation 2
Validate (or invalidate) the cross section in Fig. 11.66. Is it length balanced? Is it area
balanced? Apply the area-depth relationship to find the best-fitting lower detachment,
displacement, and the strain in each layer.
Fig. 11.65.
Cross section of an anticline
