Transcript Chapter 20 Transform Plate Boundaries PPT

Transform Plate
Boundaries
Chapter 20
Dynamic Earth
Eric H Christiansen
Major Concepts
• At transform plate boundaries plates move horizontally past each other
on strike-slip faults. Lithosphere is neither created nor destroyed.
• The three major types of transform boundaries are: (a) ridge-ridge
transforms, (b) ridge-trench transforms, and (c) trench-trench
transforms.
• Parallel ridges and valleys, pull-apart basins, and belts of folds form.
Compression and extension develop in only small areas.
• Oceanic fracture zones 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 age difference across the fracture zone.
Major Concepts
• 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.
Characteristics of Transform Plate Boundaries
• Transform plate boundaries are zones
of shearing, where two plates slide
horizontally past each other.
• Rocks in the shear zone are strongly
deformed, but no new lithosphere is
created and none is consumed.
• Transform boundaries in ocean basins
and on the continents are expressed
by steep, linear ridges and valleys.
• The major types of transform plate
boundaries are ridge-ridge
transforms, ridge-trench transforms,
and trench-trench transforms.
Transform Plate Boundaries
• San Andreas fault in northern California connects two plate boundaries.
Transform Plate Boundaries
Figure 20.01: Map of transform plate boundaries and associated oceanic fracture zones shows most are
related to spreading at mid-ocean ridges.
Transform Plate Boundaries
• Transform boundaries are strikeslip faults
• Faults are nearly vertical and
parallel to movement
• Plates move laterally past one
another
• No lithosphere is created or
consumed
• Most associated with divergent
margins
Types of Transform Plate Boundaries
Connect other boundaries
• Ridge-Ridge boundaries
• Ridge-Trench boundaries
• Trench-Trench boundaries
Figure 20.02: Transform faults can connect convergent and divergent
plate boundaries in various combinations. Note that relative motion
occurs only along the boundary between the plates, shown in red. In
all cases, the trend of a transform fault is parallel to the direction of
relative motion between plates. This characteristic is helpful in
determining the direction of plate motion.
Oceanic Transform Plate Boundaries and
Fracture Zones
• Prominent linear features that are
perpendicular to the midocean
ridges.
• Short, active parts of fracture
zones that may be several
kilometers wide and thousands of
kilometers long.
• The characteristics of oceanic
fracture zones depend on the age
difference between the lithosphere
on either side of the fault zone.
Figure 20.04: Various topographic expressions result
from juxtaposition of rock bodies with different
temperatures, ages, and internal structure.
Fracture Zones
• Oceanic transform boundaries are part of
fracture zones
• Large features up to 10,000 km long
• Generally very narrow, 10’s of km at most,
but contain numerous faults
• Include faults that offset oceanic ridges
• Transform boundary is a small portion of
fracture zone
• Active displacement only occurs between
ridge crests
• Only part of fracture zone with opposing
plate motion directions
• Remainder of fracture zone is inactive
Figure 02.12: Transform fault zones include strike-slip faults,
fault scarps, linear ridges, valleys, offset drainages, local
elongate lakes and ponds.
Oceanic Transform Boundaries
Figure 20.03: This map of the flanks of the mid-Atlantic ridge shows all of the hallmarks of a transform fault. Intense
shearing occurs at transform plate boundaries.
• Intense shearing occurs at transform plate • The abyssal hills bend to make J-shaped
boundaries.
curves as they reach the transform.
• Linear valleys, depressions, and ridges are
all aligned along the fault.
Courtesy of D. Blackman
Courtesy of D. T. Sandwell and W. H. F. Smith, Scripps Institution
of Oceanography, University of California at San Diego
The Romanche Fracture Zone
Figure 20.05: The Romanche fracture zone extends across most of the Atlantic Ocean.
• Extends over the entire width of the Atlantic Ocean
• Separates the African and S. American plates. Active
transform is ~ 600 km long
• Fault system is 10’s of km wide
• A small portion rises above sea level
The Clipperton Fracture Zone
• Cuts the East Pacific Rise and just
west of the Central American coast.
• The transform fault forms a series
of ridges and troughs connecting
two segments of the oceanic ridge.
• The offset is about 85 km.
• The plate north of the East Pacific
Rise is higher
• A fracture zone where no shear
occurs extends beyond the active
transform fault.
© K. C. Macdonald/Science Photo Library
Figure 20.06: The Clipperton transform fault cuts the
East Pacific Rise, in the northern part of this map. The
transform fault forms a series of ridges and troughs
connecting two segments of the oceanic ridge.
Structures in a Transform Shear Zone
• The fault zone consists of
innumerable vertical faults and is
marked by complex breccias made
of fragments from basaltic dikes.
• Talus breccias are interlayered with
sediments and basaltic lava flows,
which include pillow basalts that
erupted at the transform boundary
• The large mass of serpentine
intruded the fracture zone.
• Interpreted from exposures in the
Troodos ophiolite complex on
Cyprus.
Figure 20.07: The structure of a transform shear as
interpreted from exposures in the Troodos ophiolite
complex on Cyprus.
Compression and Extension Along Strike-Slip Faults
Figure 20.08A: Secondary compressional and extensional structures are produced by bends or offsets in the transform fault
system. Small fold belts mark zones of transpression (A) and pull-apart basins mark transtensional bends (B).
• Secondary compressional and
• Small fold belts mark zones of
extensional structures are produced by transpression (left) and pull-apart
bends or offsets in the transform fault
basins mark transtensional bends
system.
(right).
Thermal Structure of a Transform Fault
• The topography and thermal structure of a
transform boundary are related to differences
in age and temperature across the fault.
• The cross section along A–A′ is parallel to a
ridge segment and shows the topography and
temperature a ridge-transform boundary.
• The older, cooler lithosphere creates a “cold
wall” that inhibits magmatic processes and
concentrates deformation into a narrow zone.
Figure 20.09: The thermal structure of a transform
boundary is related to differences in age and temperature
of the lithosphere across the fault.
• The younger, hotter lithosphere stands higher
than the older cooler lithosphere.
• A hot bulge forms on the older lithosphere
that is adjacent to the hot ridge.
Large Offset on Transform Fault
Figure 20.10A: The transform fault is marked by a deep
linear valley. Long, narrow, linear ridges commonly
parallel the fault.
© K. C. Macdonald/Science Photo Library
Figure 20.10B: A large-offset on a transform
fault (or one that has a slow-shearing rate)
has a narrow zone of deformation.
Small Offset on Transform Fault
Figure 20.11A: Transform fault zone may be tens of
kilometers wide. Several shear zones within the
transform system form ridges and valleys.
© K. C. Macdonald/Science Photo Library
Figure 20.11B: This cross section shows
that there is little contrast in lithospheric
thickness across the transform zone.
Processes at Oceanic Transforms
 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
• Diapiric intrusion of the
hydrated serpentinites create
ridges
• Intense shearing along strike slip
faults
Continental Transform Faults
• Continental transform faults are
similar to oceanic transform faults
but not as common.
• They are seismically active and
penetrate entire lithosphere.
• Fault scarps, linear ridges and
troughs, and displaced stream
channels formed by strike-slip
faulting.
• Pull-apart basins and fold belts
develop along bends in the faults.
Figure 20.12B: The San Andreas Fault slices through
California, marking the transform boundary between
moving tectonic plates.
Continental Transform Faults
Figure 20.12A: Transform fault zones include strike-slip
faults, fault scarps, linear ridges, valleys, offset
drainages, local elongate lakes and ponds.
Figure 20.12B: The San Andreas Fault slices through
California, marking the transform boundary between
moving tectonic plates.
The San Andreas Fault
• The San Andreas–Gulf of California transform
system extends from northern California to
just beyond the tip of Baja California.
• Connects the Mendocino fracture zone, the
Cascade trench, and the East Pacific Rise.
• Forms a series of strike-slip faults with
intervening pull-apart basins and
compressional ridges in California.
• In the Gulf of California, where the transform
system involves oceanic crust, the fault zone
consists of a series of long transform faults
connecting short spreading ridge segments.
• 30 my old with ~ 300 km of offset
Figure 20.13: The San Andreas–Gulf of California
transform system extends from northern California
to just beyond the end of Baja California.
The San Andreas Fault
Figure 02.13: The San Andreas Fault system in CA is part of a long transform plate boundary separating North
America plate from Pacific plate.
Base map by Ken Perry, Chalk Butte, Inc.
The San Andreas Fault
Courtesy of Mike Poland/USGS
Movement Along the San Andreas Fault
Data from: R. A. Bennett, J. L. Davis, and B.P. Wernicke
The Dead Sea
Transform System
• Connects the Red Sea
spreading ridge with the
Alpine convergent belt.
• Llong, deep, narrow pullapart basins of the Gulf of
Aqaba and the Dead Sea
• The transpressional folds
formed in the Palmyra
Mountains of Lebanon and
Syria.
• Small eruptions of basalt
occurred near the pull-apart
basins.
Base map by Ken Perry, Chalk Butte, Inc.
Figure 20.14: The Dead Sea transform system connects the Red Sea spreading
ridge with the Alpine convergent belt. The movement along the transform
zone has produced the long, deep, narrow pull-apart basins of the Gulf of
Aqaba and the Dead Sea as well as the contractional folds of the northern
Sinai and the Palmyra Mountains of Lebanon and Syria. Small eruptions of
basalt occurred near the pull-apart basins.
Figure 20.15: Pull-apart basins in the Gulf of Aqaba and
the Dead Sea dominate this photograph taken by
astronauts aboard the Space Shuttle. Such basins are
caused by movement on strike-slip faults that have sharp
bends and offsets of the major faults (inset).
Courtesy of NASA
Data from the New Zealand GeoNet Project
The Alpine Transform Fault
Figure 20.16A: The transform system of the
Alpine Fault, New Zealand, connects TongaKermadec subduction zone to Macquarie
subduction zone.
Figure 20.16B: The transform system of the Alpine
Fault, New Zealand, connects Tonga-Kermadec
subduction zone to Macquarie subduction zone.
The Alpine Fault
• The Alpine Fault, New Zealand
connects two convergent plate
boundaries
• The valley was created by
differential erosion along the
long linear fault.
• Deformation along the fault
created the high Alps of the
southern islands.
© GNS Science Photo Library
Figure 20.17: The Alpine Fault, New Zealand, is a
strike-slip fault that connects two plate boundaries.
Earthquakes at Transform Boundaries
• Earthquakes at transform
plate boundaries are
especially abundant.
• The seismicity is shallow
and shows strike-slip
characteristics.
• Shallow earthquakes on
a midocean ridge are
more frequent on the
transform faults.
Courtesy of D. T. Sandwell and W. H. F. Smith, Scripps Institution
of Oceanography, University of California at San Diego
Figure 20.18: Shallow earthquakes on a mid-ocean ridge are more
frequent on the transform faults.
Earthquakes on the San Andreas Fault
Figure 20.19: Earthquakes on the San Andreas continental
transform system are concentrated along the fault and its
branching subsidiaries.
Data from: A. Robinson
• Earthquakes are concentrated
along the fault and its
branching subsidiaries.
• Almost all of the earthquakes
occur at depths less than 15
km.
• The southern part of the fault
has had many historic
earthquakes while the
northern segment has not.
• Perhaps strain is building
toward a large earthquake on
the northern strand.
Metamorphism and Magmatism at Transform
Plate Boundaries
• Metamorphism along transform
fault zones creates deformation
fabrics, seafloor metamorphism,
and serpentinite.
• Volcanoes rarely develop on
transform faults, but small
volumes of basalt may erupt in
pull-apart basins.
Figure 06.09A: Strongly foliated schist with
aligned grains of chlorite that grew in a
differential stress field during contraction.
Base map by Ken Perry, Chalk Butte, Inc.
Magmatism at Transform
Plate Boundaries
Figure 20.14: The Dead Sea transform system connects the Red Sea
spreading ridge with the Alpine convergent belt. The movement along the
transform zone has produced the long, deep, narrow pull-apart basins of the
Gulf of Aqaba and the Dead Sea as well as the contractional folds of the
northern Sinai and the Palmyra Mountains of Lebanon and Syria. Small
eruptions of basalt occurred near the pull-apart basins.
• Magmatism is rare on
transforms—continental or
oceanic
• Midocean ridge magmatism
declines toward transforms
• Leaky transforms produce small
amounts of basaltic magma in
both oceanic and continental
environments
• Pull apart may initiate partial
melting
Summary of the Major Concepts
• At transform plate boundaries plates move horizontally past each other
on strike-slip faults. Lithosphere is neither created nor destroyed.
• The three major types of transform boundaries are: (a) ridge-ridge
transforms, (b) ridge-trench transforms, and (c) trench-trench transforms.
• Parallel ridges and valleys, pull-apart basins, and belts of folds form.
Compression and extension develop in only small areas.
• Oceanic fracture zones 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
age difference across the fracture zone.
Summary of the Major Concepts
• 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.