Transcript EDS_20_Transform_sm

Prepared by Eric H. Christiansen
Brigham Young University
Characteristics
 Transform boundaries
are strike-slip faults
 Faults are nearly
vertical & parallel to
movement
 Plates move laterally
past one another
 No lithosphere is
created or consumed
 Most associated with
divergent margins
Major Concepts




Transform plate boundaries are unique in that
the plates move horizontally past each other
on strike-slip faults. Lithosphere is neither
created nor destroyed.
The three major types of transform boundaries
are: ridge-ridge transforms, ridge-trench
transforms, and trench-trench transforms.
Transform plate boundaries are shear zones.
During shearing, secondary features are
created, including parallel ridges and valleys,
pull-apart basins, and belts of folds.
Compression and extension develop in only
small areas.
Oceanic fracture zones are prominent linear
features that trend perpendicular to the
oceanic ridge. They may be several kilometers
wide and thousands of kilometers long. The
structure and topography of oceanic fracture
zones depend largely on the temperature (or
age) difference across the fracture and on the
spreading rate of the oceanic ridge.
 Continental transform fault zones
are similar to oceanic transforms,
but they lack fracture zone
extensions.
 Shallow earthquakes are common
along transform plate boundaries;
they are especially destructive on
the continents.
 Volcanism is rare along transform
plate boundaries, but small
amounts of basalt erupt locally from
leaky transform faults.
 Metamorphism in transform fault
zones creates rocks with strongly
sheared fabrics, as well as hydrated
crustal and even mantle rocks.
Major Plate Boundaries
Transform Boundary Types
 Transform boundaries
connect other boundaries
 Ridge-Ridge boundaries
 Ridge-Trench boundaries
 Trench-Trench boundaries
 Compensate for
differential movement
Oceanic Transform Boundaries
 Active displacement
only occurs between
ridge crests
 Only region of fracture
zone with opposite
plate motion
 Remainder of fracture
zone is inactive
 Vertical relief, ridge &
trough, due to age of
crust on opposite sides
of boundary
Ridge-Ridge Transforms
 Most abundant type
 Active displacement
only occurs between
ridge segments
 Plate movement is in
opposite directions
between ridge crests
 Considerable shearing
creates mylonite
 Numerous shallow
earthquakes
 No significant amount of
lithosphere is created or
destroyed along boundary
 Shearing and
deformation are
considerable
 Little or no igneous
activity
 Considerable movement
occurs, allowing
adjustment of rigid
plates
Fracture Zones
 Oceanic transform
boundaries are part of
fracture zones
 Large scale features up to
10,000 km long
 Generally very narrow, 10’s
of km at most, but contain
numerous faults
 Appear as faults offsetting
oceanic ridges
 Transform boundary is a
small portion of fracture
zone
Romanche Fracture Zone
 Extends over the entire width of the Atlantic
Ocean
 Active transform is ~ 600 km long
 Separates the African and S. American
plates
 A small portion rises above sea level
 Fault system is 10’s of km wide
Structure of a transform fault
Transform Boundary Processes
 Ridge offset controls
Temperature contrast
 Increased T contrast
tends to narrow the fault
zone
 The cold wall tends to
slow volcanism, thinning
the crust beneath the
ridge
 Seawater penetrating the
thin crust alters mantle
peridotite to serpentinite
Large Offset
Transform
Small
Offset
Transform
Ridge-Trench Transforms
 Less common, but
important connection
between divergent and
convergent boundaries
 Longest are of this type
 Ridge systems may be
connected to either side
of a trench system
 Overriding plate or
subducting plate
Trench-Trench Transforms
 Least common type
Connects two trench systems
 Direction of subduction may change from one trench to the other
Continental Transform Faults
 Not as common as oceanic
 Penetrate entire lithosphere
transform faults
 Similar in structure
 Distinct linear features
 Seismically active
 Examples: San Andreas, Dead Sea
systems
Continental Transform Faults
 Not as common as oceanic
 Penetrate entire lithosphere
transform faults
 Similar in structure
 Distinct linear features
 Seismically active
 Examples: San Andreas, Dead Sea
systems
San Andreas System
 Ridge-ridge system
extending ~ 3000 km
 System is composed of
numerous faults
 Accommodate motions
of Pacific and N.
American plates
 Earthquakes are
shallow
 30 my old with ~ 300
km of offset
Continental Transform Faults
Landforms along the San Andreas Transform
Transform Boundary Processes
 Contraction & Extension
 Braided system of strike-slip faults
 Compression = uplift and folding
 Extension = basins
Transform Boundary Processes
 Contraction & Extension
 Braided system of strike-slip faults
 Compression = uplift and folding
 Extension = basins
Transform Boundary Processes
 Contraction & Extension
 Braided system of strike-slip faults
 Compression = uplift and folding
 Extension = basins
Transform Boundary Processes
 Contraction & Extension
 Braided system of strike-slip faults
 Compression = uplift and folding
 Extension = basins
70 % Chance That Large Earthquake Will Strike
San Francisco By 2030
(Oct. 14, 1999) -- There is
a 70 percent probability
that one or more damaging
earthquakes of magnitude
6.7 or larger will strike the
San Francisco Bay area
during the next 30 years,
according to the U.S.
Geological Survey. A
magnitude 6.7 earthquake
is equivalent to the 1994
Northridge earthquake
which killed 57 people and
caused $20 billion in
damage.
Dead Sea
Transform
The Alpine Fault, New Zealand
 What causes all of the
seismicity in northern
New Zealand?
A. Subduction of oceanic
lithosphere
B. Continental collision
C. Continental rifting
D. Transform faulting
The Alpine Fault, New Zealand
Earthquakes
Where are
earthquakes
more common?
A. On ocean
ridges
B. Along
continental
margins
C. Along
transform
faults
Earthquakes at Transform Faults
 Especially abundant
along transforms
 More abundant in
oceanic systems than
ridge related quakes
 Results of colder, more
brittle crust
 Typically shallow
 Relatively low intensity
 Quakes are more intense
along continental
transforms
Where was the Sermon on the Mount?
Magmatism
 Magmatism is
decreased along
oceanic transforms
 Leaky transforms
produce small amounts
of basaltic magma in
both oceanic and
continental
environments
 Pull apart may initiate
partial melting
A
B
D
C
Metamorphism
 Horizontal shearing
creates most
metamorphism
 Creates fault breccias
and mylonites
 Ductile deformation
occurs at higher T
 Contact
metamorphism occurs
opposite ridge crests
 Aided by influx of
seawater
http://ic.ucsc.edu/~casey/eart150/Lectures/Foliations&Lineations/Mylonite.mod.jp
Major Concepts




Transform plate boundaries are unique in that
the plates move horizontally past each other
on strike-slip faults. Lithosphere is neither
created nor destroyed.
The three major types of transform boundaries
are: ridge-ridge transforms, ridge-trench
transforms, and trench-trench transforms.
Transform plate boundaries are shear zones.
During shearing, secondary features are
created, including parallel ridges and valleys,
pull-apart basins, and belts of folds.
Compression and extension develop in only
small areas.
Oceanic fracture zones are prominent linear
features that trend perpendicular to the
oceanic ridge. They may be several kilometers
wide and thousands of kilometers long. The
structure and topography of oceanic fracture
zones depend largely on the temperature (or
age) difference across the fracture and on the
spreading rate of the oceanic ridge.
 Continental transform fault zones
are similar to oceanic transforms,
but they lack fracture zone
extensions.
 Shallow earthquakes are common
along transform plate boundaries;
they are especially destructive on
the continents.
 Volcanism is rare along transform
plate boundaries, but small
amounts of basalt erupt locally from
leaky transform faults.
 Metamorphism in transform fault
zones creates rocks with strongly
sheared fabrics, as well as hydrated
crustal and even mantle rocks.