Essentials of Geology, 8e
Frederick K. Lutgens & Edward J. Tarbuck
Structural Geology and Mountain Building
Chapter 17
Essentials of Geology, 8e
Stan Hatfield and Ken Pinzke
Southwestern Illinois College
Deformation
• Deformation refers to all changes in the original form and/or
size of a rock body
• Most crustal deformation occurs along plate margins
• How rocks deform
• Rocks subjected to stresses begin to deform by folding, flowing,
or fracturing
• Deformation is strain and occurs in response to stress:
– Compressional stress (Appalachians)
– Tensional stress (Basin and Range)
– Shear stress (San Andreas Fault)
• How rocks deform
• General characteristics of rock deformation
– Elastic deformation – the rock returns to nearly its original size and shape when the
stress is removed
– Once the elastic limit (strength) of a rock is surpassed, it either flows (ductile or plastic
deformation) or fractures (brittle deformation)
– Rock strength is influenced by:
• Temperature
• Confining pressure
• Rock type
• Time
Folds
• During crustal deformation rocks are often bent into a series
of wave-like undulations called folds
• Most folds result from compressional stresses which shorten and
thicken the crust
• Parts of a fold
– Limbs – refers to the two sides of a fold
– Axis – a line drawn down the points of maximum curvature of each
layer
– Axial plane – an imaginary surface that divides a fold symmetrically
• Common types of folds
• Anticline – upfolded or arched rock layers (oldest rocks in
middle)
• Syncline – downfolds or troughs of rock layers (youngest rocks
in middle)
• Depending on their orientation, anticlines and synclines can be
described as
– Symmetrical, asymmetrical, recumbent (overturned fold), or plunging
Strike and Dip
• Provides a way of describing the orientation of rock beds,
fault planes, or joints
• Strike is the intersection of a horizontal plane with the bed (a
horizontal line measured as a compass direction)
• Dip is the direction straight down the slope and is normal to
the strike (measured from horizontal)
Folds
• Common types of folds
• Monoclines – large, step-like folds in otherwise horizontal
sedimentary strata
• Other types of folds
• Dome
– Upwarped displacement of rocks
– Circular or slightly elongated structure
– Oldest rocks in center, younger rocks on the flanks
Monoclines May Result From Buried Faults
A Domal Structure (Black Hills of S. Dak.)
• Other types of folds
• Basin
– Circular or slightly elongated structure
– Downwarped displacement of rocks
– Youngest rocks are found near the center, oldest rocks on the flanks
Faults
• Faults are fractures in rocks along which appreciable
displacement has taken place
• Sudden movements (failures) along faults are the cause of
most earthquakes
• Classified by their movement relative to the break (fault
plane), and can be parallel to
• Strike-slip, dip-slip, or oblique-slip
• Types of faults
• Dip-slip faults
– Movement is mainly parallel to the dip of the fault surface
– May produce long, low cliffs called fault scarps
– Parts of a dip-slip fault include the hanging wall (rock surface above the
fault) and the footwall (rock surface below the fault)
• Types of dip-slip faults
– Normal fault
• Hanging wall block moves down relative to the footwall block
• Accommodate lengthening or extension of the crust
• Most are small with displacements of a meter or so
• Larger scale normal faults are associated with structures called
fault-block mountains
• Types of dip-slip faults
– Reverse and thrust faults
• Hanging wall block moves up relative to the footwall block
• Reverse faults have dips greater than 45o and thrust faults have dips
less then 45o
• Accommodate shortening of the crust
• Strong compressional forces
• Strike-slip fault
• Dominant displacement is horizontal and parallel to the strike of
the fault
• Types of strike-slip faults
– Right-lateral – as you face the fault, the block on the opposite side of the
fault moves to the right
– Left-lateral – as you face the fault, the block on the opposite side of the
fault moves to the left
• Strike-slip fault
• Transform fault
– Large strike-slip fault that cuts through the lithosphere
– Accommodates motion between two large crustal plates
Joints
•
•
•
•
Joints are among the most common rock structure
A joint is a fracture with no offset
Most occur in roughly parallel groups
Significance of joints
• Groundwater movement is often controlled by joint systems
• A joint system can influence the direction that a stream course follows
• Chemical weathering tends to be concentrated along joints
Mountain belts
• Orogenesis – the processes that col-lectively produce a
mountain belt
• Includes folding, thrust faulting, meta-morphism, and igneous
activity
• Mountain building has occurred during the recent geologic
past
• Alpine-Himalayan chain
• American Cordillera
• Mountainous terrains of the western Pacific
• Older Paleozoic- and Precambrian-age mountains
• Appalachians
• Urals in Russia
• Several hypotheses have been proposed for the formations of
Earth’s mountain belts
Mountain building at
convergent boundaries
• Plate tectonics provides a model for orogenesis
• Mountain building occurs at convergent plate boundaries
• Of particular interest are active subduction zones
– Volcanic arcs are typified by the Aleutian Islands and the Andean arc
of western South America
• Aleutian-type mountain building
• Where two ocean plates converge and one is subducted beneath the other
• Volcanic island arcs result from the steady subduction of oceanic
lithosphere
– Most are found in the Pacific (the Ring of Fire)
– Continued development may produce mountainous topography consisting of igneous
and metamorphic rocks
– Active island arcs include the Mariana, New Hebrides, Tonga, Japan, and Aleutian
arcs
• Andean-type mountain building
• Mountain building along continental margins
– Involves the convergence of an oceanic plate and a plate whose leading
edge contains continental crust
– Exemplified by the Andes Mountains
• Stages of development - passive margin
– First stage
– Continental margin is part of the same plate as the adjoining oceanic
crust
– Deposition of sediment on the continental shelf produces a thick wedge
of shallow-water sediments
– Turbidity currents are deposited sediment on the continental rise and
slope
• Stages of development – active continental margins
– Subduction zone forms
– Deformation process begins
– Convergence of the continental block and the subducting oceanic plate
leads to deformation and metamorphism of the continental margin
– Continental volcanic arc develops
• Stages of development – active continental margins
– Accretionary wedge may form
• Chaotic accumulation of sedimentary rocks and metamorphic rocks
with occasional scraps of ocean crust
• Can become large enough to stand above sea level (California Coast
Ranges)
• Composed of roughly two parallel zones
– Volcanic arc
• Develops on the continental block
• Consists of large intrusive bodies intermixed with high-temperature
metamorphic rocks
– Accretionary wedge
• Seaward segment
• Consists of folded, faulted, and metamorphosed sediments and
volcanic debris
• Andean-type mountain building
• Sierra Nevada and Coast Ranges
– One of the best examples of an active Andean-type orogenic belt
– Subduction of the Pacific Basin under the western edge of the North
American plate
– Sierra Nevada batholith formed as a volcanic arc
– Coast Ranges formed as acrretionary wedge
• Continental collisions
• Two lithospheric plates, both carrying continental crust
• The Himalayan Mountains are a youthful mountain range formed from the
collision of India with the Eurasian plate about 45 million years ago
• The Himalayan Mountains
– Spreading center that propelled India northward is still active
– Similar but older collision occurred when the European continent
collided with the Asian continent to produce the Ural mountains
• Continental accretion and mountain building
• A third mechanism of orogenesis
• Small crustal fragments collide and merge with continental
margins
• Responsible for many of the mountainous regions rimming the
Pacific
• Accreted crustal blocks are called terranes
• Continental accretion and mountain building
• Terranes consist of any crustal fragments whose geologic history
is distinct from that of the adjoining terranes
• As oceanic plates move, they carry embedded oceanic plateaus,
volcanic island arcs and microcontinents to an Andean-type
subduction zone
• In addition to the horizontal movements of lithospheric
plates, vertical movement also occurs along plate margins as
well as the interiors of continents far from plate boundaries
Vertical movements of the crust
• Isostatic adjustment
• Less dense crust floats on top of the denser and deformable
rocks of the mantle
• Concept of floating crust in gravitational balance is called
isostasy
• If weight is added or removed from the crust, isostatic
adjustment will take place as the crust subsides or rebounds
End of Chapter 17