Geology Reference
In-Depth Information
Figure 10.11 Inclined, Overturned, and Recumbent Folds
Inclined
Overturned
Axis
Axial plane
Axial plane
20
75
25
75
25
55
b Overturned folds. Both fold limbs dip in the same direction,
but one limb is inverted. Notice the special strike-and-dip
symbol to indicate overturned beds.
a An inclined fold. The axial plane is not vertical, and the fold
limbs dip at different angles.
Recumbent
Oldest
rocks
Youngest
rocks
Axial
plane
c Recumbent folds.
d An overturned fold in Switzerland.
Recall our discussion of strike and dip of rock
layers. Fault planes are also inclined planes and they,
too, are characterized by strike and dip (Figure 10.15a).
In fact, the two basic varieties of faults are defi ned by whether
the blocks on opposite sides of the fault plane moved parallel
to the direction of dip (dip-slip faults) or along the direction
of strike (strike-slip faults) (see Geo-inSight on pages 262
and 263).
example, the hanging wall block moved down relative to the
footwall block, giving rise to a normal fault . In contrast,
in a reverse fault , the hanging wall block moves up relative
to the footwall block (Figure 10.16b). In Figure 10.16c, the
hanging wall block also moved up relative to the footwall
block, but the fault has a dip of less than 45 degrees and is a
special variety of reverse fault known as a thrust fault .
Figure 10.3b shows that normal faults result from
tension. Numerous normal faults are present along one
or both sides of mountain ranges in the Basin and Range
Province of the western United States where the crust is
being stretched and thinned. The Sierra Nevada at the
Dip-Slip Faults All movement on dip-slip faults takes
place parallel with the fault's dip; that is, movement is verti-
cal, either up or down the fault plane. In
Figure 10.16, for
 
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