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Chapter 7
DEFORMATION AND METAMORPHISM
Michaelangelo’s David is
composed of marble, a
metamorphic rock that
forms when its
protolith, or parent
rock, limestone, is
recrystallizes owing to
increased pressure and
heat.
What geologic processes
likely caused the increased P
& T?
Why did Michaelangelo use
marble instead of limestone?
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FIGURE 7.1 (a) An outcrop of originally horizontal rock
layers bent into folds by compressive tectonic forces.
[Phil Dombrowski.]
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FIGURE 7.1 (b) An outcrop of once-continuous rock
layers displaced along small faults by tensional tectonic
forces. [Tom Bean.]
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Fig.
7.3
FIGURE 7.2 Dipping limestone and shale beds on the
coast of Somerset, England. Children are walking along
the strike of beds that dip to the left at an angle of about
15°. [Chris Pellant.]
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The strike and dip of a rock layer define its orientation at a
particular place. Strike is the compass direction of a rock layer as
it intersects with a horizontal surface; dip is the angle of steepest
descent
of the rock layer from the horizontal, measured at right
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angles to the strike. Here strike is east-west & dip is 45° to the S.
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Fig. 7.5: Lab
experiment
on marble
shows brittle
vs. ductile
deformation.
A geologic map and cross section make up
a two dimensional representation of a
three-dimensional geologic structure.
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FIGURE 7.6 View of the San
Andreas fault, showing the
northwestward movement of the
Pacific Plate with respect to the
North American Plate. The map
shows a formation of volcanic rocks
23 million years old that has been
displaced by 315 km. The fault runs
from top to bottom (dashed line)
near the middle of the photograph.
Note the offset of the stream
(Wallace Creek) by 130 m as it
crosses the fault. [John S. Shelton.]
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FIGURE 7.7 The
orientation of
tectonic forces
determines the
style of faulting.
Can we make any calculations
with the above data? If so, what,
and how?
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FIGURE 7.8 The Red Canyon fault scarp cuts across a road in Hebgen Lake,
Montana. [I. J. Witkind/USGS.]
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FIGURE 7.9 Small-scale folds in sedimentary beds of anhydrite (light) and
shale
(dark), West Texas. [John Grotzinger/ Ramón Rivera-Moret/Harvard
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Mineralogical Museum.]
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FIGURE 7.10 Rock folding is described by the direction of
folding and by the orientation of the fold axis and the axial
plane.
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FIGURE 7.11 The geometry of plunging folds. Note
the converging pattern of the layers where they
intersect the land surface, which you can also see in
Figure 7.4.
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FIGURE 7.13
Intersecting sets of
joints have made this
rock vulnerable to
weathering. (The backpack is
shown for scale.) [Courtesy of Peter Kaufman.]
FIGURE 7.12 As deformation increases, folds are
pushed into asymmetrical shapes.
[(left) courtesy of Cleet Carlton/Golden Gate Photo; (center)
courtesy
of Mark
McNaught;
John
Grotzinger.]
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Earth 1e
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by W. H. Freeman and Company
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FIGURE 7.14 The
orientation of
tectonic forces
determines the
style of continental
deformation. [After
John Suppe, Principles
of Structural Geology. Upper Saddle
River, N.J.: Prentice Hall, 1985.]
FIGURE 7.15 A rift valley results from tensional forces that
produce normal faulting. The African Plate, on which Egypt rides,
and the Arabian Plate, bearing Saudi Arabia, are drifting apart.
The tensional forces have created a rift valley, filled by the Red
Sea. The diagram shows parallel normal faults bounding the rift
valley in the crust beneath the sea. [NASA/TSADO/Tom Stack.]
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FIGURE 7.16 The Keystone thrust
fault of southern Nevada is a largescale overthrust sheet of a kind found
in California and southern Nevada.
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Compressive forces have detached a
sheet of rock layers (D, C, B) and
thrust it a great distance horizontally
over a section of the same rock
layers (D, C, B, A).
FIGURE 7.18 Temperatures, pressures, and depths at which low- and highgrade metamorphic rocks form. The dark band shows the rates at which
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Earthpressure
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Freeman and Company
temperature
and
increase
with depth over much of the continents.
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FIGURE 7.20 Directed
pressure on rocks
containing platy
minerals causes
foliation.
FIGURE 7.17
Pressure and
temperature increase
with depth in all
regions, as shown in
these cross sections
of a volcanic arc, a
continental region,
and a region of
ancient stable
continental
lithosphere.
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FIGURE 7.19 Different types of
metamorphism occur in different
plate tectonic settings.
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FIGURE 7.21 Foliated rocks are classified based on
metamorphic grade, crystal size, type of foliation, and
banding.
[slate, phyllite, schist, gneiss: John Grotzinger/Ramón Rivera-Moret/ Harvard Mineralogical Museum; migmatite: Kip Hodges.]
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FIGURE 7.22 Granoblastic
(nonfoliated) metamorphic rocks
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FIGURE 7.22 Granoblastic
(nonfoliated) metamorphic
rocks:
FIGURE 7.23 Garnet
porphyroblasts in a
schist matrix. The
minerals in the matrix
are continuously
recrystallized as
pressure and
temperature change
and therefore grow to
only a small size. In
contrast, porphyroblasts
grow to large size
because they are stable
over a broad range of
pressures and
temperatures. [Chip
Clark.]
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FIGURE 7.25 Metamorphic facies correspond to particular combinations of
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temperature
and
correspond
to particular plate tectonic
settings.
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FIGURE 7.24 Index minerals define the
different metamorphic zones within a belt of
regional metamorphism. (a) Map of New
England, showing mineral zones based on index
minerals found in rocks metamorphosed from slate.
(b) Rocks produced by the metamorphism of slate at
different temperatures and pressures. [slate, phyllite,
schist, gneiss: John Grotzinger/Ramón RiveraMoret/Harvard Mineralogical Museum; blueschist:
courtesy of Mark Cloos; migmatite: Kip Hodges.] (c)
Changes in the mineral composition of metamorphic
rock
derived
from slate define its metamorphic facies
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and are used to indicate its metamorphic grade.
FIGURE 7.26 Changes in the mineral composition of mafic rock
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during
metamorphism
its metamorphic grade.
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FIGURE 7.27 P-T paths indicate the trajectory of rocks during
metamorphism at (a) an ocean-continent convergence and (b) a
continent-continent convergence. The different P-T paths of
rocks formed in these different plate tectonic settings indicate
differences in geothermal gradients. Rocks transported to similar
depths—and pressures—beneath mountain belts become much
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hotter
than rocks at an equivalent depth in subduction zones.
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