Faults, Folds, and Landscapes
Objectives
• Introduce basic terminology used in describing
rock structure
• Distinguish between types of fault movements
and the landforms they produce
• Discuss the folding of rocks and relate it to the
landforms produced
• Call attention to occurrence of regional
deformation of the crust, in addition to more
local faults and folds
Terminology of Structure
• Strike
– Line representing the
intersection of a planar
feature with a
horizontal plane
• Dip
– Angle of a tilted bed
relative to a horizontal
plane
Fault Terminology
• Fault
– Fracture in crustal rock
involving displacement of
one side of the facture with
respect to the other
• Joint
– Fracture in the earth’s crust,
but with no displacement of
rock bodies
An aerial view of the San Andreas fault in the Carrizo Plain,
Central California. © USGS
http://pubs.usgs.gov/gip/earthq1/how.html
Fault Terminology
• Hanging Wall
– Block of rocks above
the fault line
• Footwall
– Block of rocks below
the fault line
Figure. Image of hanging wall and
footwall. © USGS NPS
Compressional Faults
• Reverse fault
• Hanging wall moves up relative
to the footwall
• Fault plane angle >45°
Compressional Faults
• Thrust fault
• Hanging
wall moves
up relative
to footwall
• Fault plane
angle <45°
Tensional Fault
• Normal fault
– Hanging wall moves down
relative to footwall
– Produced by extensional
forces
Tensional Fault
Tensional Fault Landscapes
• Basin and Range
– A series of mountains
(horsts) and valleys
(grabens)
• Rift Valleys
– Long, narrow band of
downdropped block
caused by plates
being pulled apart
(tension) and
Transverse Fault
• Strike-slip fault
– Blocks of rock slide past
one another
– Horizontal displacement
Fold Formation
• Folding
– Compressional force is applied to rocks
– Rocks respond by bending before fracturing
Fold Structures
• Anticlines
– Archlike fold with limbs
dipping away
[Insert 34.12 - anticline]
Fold Structures
• Synclines
– Troughlike
fold with
limbs
dipping
toward the
axes
Anticlines
• Erosion removes upper portion of anticline
• Limbs dip toward the center of fold
• Older rocks are near center and becomes
progressively younger with distance
Synclines
• Erosion removes
upper portion of
anticline
• Limbs dip toward
the center of fold
• Older rocks are
near center and
becomes
progressively
younger with
distance
Plunging Folds and Associated Landscapes
• Anticlines and synclines often plunge; axes dip