Page 297 - Laboratory Manual in Physical Geology

Constructing Geologic Cross Sections Normal Faults
Geologic maps contain evidence of the surface locations Normal faults are caused by tension (rock lengthening).
and orientations of formations and the structures into As tensional stress pulls the rocks apart, gravity pulls down
which they have been deformed. To help visualize the the hanging wall block. Therefore, normal faulting gets its
geologic structures, geologists convert this surface name because it is a normal response to gravity. You can
information into vertical geologic cross sections like the recognize normal faults by recognizing the motion of the
sides of the block diagram in FIGURE 10.2 . hanging wall block relative to the footwall block. First,

Geologic cross sections are often drawn perpendicular imagine that the footwall block is stable (has not moved).
to strike, so you can see the dip of the rocks more If the hanging wall block has moved downward in relation
exactly. Most are drawn beneath a topographic profile to the footwall block, then the fault is a normal fault.

( FIGURE 10.6 ), so you can see the topographic expression

of the formations and geologic structures. However, Reverse Faults
some geologic cross sections are just rectangular cross Reverse faults are caused by compression (rock
sections that do not show the topography. Once you have shortening). As compressional stress pushes the rocks
constructed a topographic profile (or drawn a rectangular together, one block of rock gets pushed atop another. You
space) for the map line segment of the cross section can recognize reverse faults by recognizing the motion of

( FIGURE 10.6 ), then follow the directions in FIGURE 10.6 to the hanging wall block relative to the footwall block. First,

add the geologic information. You will need to use a pencil imagine that the footwall block is stable (has not moved).
(with a good eraser), protractor, ruler, and colored pencils If the hanging wall block has moved upward in relation
and be very neat and exact in your work. to the footwall block, then the fault is a reverse fault.
Thrust faults are reverse faults that develop at a very low
Fractures and Faults angle and may be very difficult to recognize ( FIGURE 10.7 ).

Reverse faults and thrust faults generally place older strata
Brittle deformation is said to occur when rocks fracture on top of younger strata.
(crack) or fault (slide in opposite directions along a crack
in the rock). Motion and scraping of brittle rocks along Strike–Slip Faults
the fault surfaces causes development of slickensides ,
polished surfaces with lineations and steplike linear ridges Strike–slip faults (lateral faults) are caused by shear and

that indicate the direction of movement along the fault involve horizontal motions of rocks ( FIGURE 10.7 ). If you

( FIGURE 10.7 ). If you gently rub the palm of your hand stand on one side of a strike–slip fault and look across it,
back and forth along the slickensides, then one direction then the rocks on the opposite side of the fault will appear
will seem smoother (down the step like ridges) than the to have slipped to the right or left. Along a right-lateral
other. That is the relative direction of the side of the fault (strike–slip) fault , the rocks on the opposite side of the
represented by your hand. fault appear to have moved to the right. Along a left-lateral
Faults form when brittle rocks experience one of these (strike–slip) fault , the rocks on the opposite side of the
three kinds of directed pressure (stress): tension (pulling fault appear to have moved to the left.
apart or lengthening), compression (pushing together,
compacting, and shortening), or shear (smearing or Folded Structures
tearing). The three kinds of stress produce three different
kinds of faults: normal, reverse/thrust, and strike-slip Folds are upward, downward, or sideways bends of rock
( FIGURES 10.1 , 10.7 ). layers. Synclines are “downfolds” or “concave folds,” with

Normal and reverse/thrust faults both involve the youngest rocks in the middle ( FIGURE 10.8A ). Anticlines
vertical motions of rocks. These faults are named by are “upfolds” or “convex folds” with the oldest rocks in the
middle ( FIGURE 10.8B ).
noting the sense of motion of the top surface of the fault
(top block) relative to the bottom surface (bottom In a fold, each stratum (rock layer) is bent around an
block), regardless of which one actually has moved. imaginary axis, like the crease in a piece of folded paper.
The top surface of the fault is called the hanging This is the fold axis (or hinge line ). For all strata in a
wall and is the base of the hanging wall (top) block fold, the fold axes lie within the axial plane of the fold

of rock. The bottom surface of the fault is called the ( FIGURE 10.8A – D ). The axial plane divides the fold into two

footwall and forms the top of the footwall block . limbs (sides, FIGURE 10.8B ). For symmetric anticlines and
Whenever you see a fault in a vertical cross section, synclines, the fold axis is vertical, but most anticlines and
just imagine yourself walking on the fault surface. synclines are asymmetric. The axial plane of asymmetric
The surface that your feet would touch is the footwall. folds is leaning to one side or the other, so one limb is
steeper and shorter than the other.

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