1. Transform Plate Boundaries
Chapter 20
Dynamic Earth
Eric H Christiansen

2. 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.

3. 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.

4. 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.

5. Transform Plate Boundaries
San Andreas fault in northern California connects two plate boundaries.

6. 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.
The major transform plate boundaries and associated oceanic fracture zones are related to spreading at midocean ridges. Also, most curve because they are parallel to small circles around the poles of plate rotation. Other transform boundaries are related to convergent margins in regions of complex plate movement.

7. Transform Plate Boundaries
Transform boundaries are strike- slip 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

8. Types of Transform Plate 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.
Connect other boundaries
Ridge-Ridge boundaries
Ridge-Trench boundaries
Trench-Trench boundaries
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.
a. Ridge-ridge transform fault.
b. Ridge-trench transform fault.
c. Trench-trench transform fault.

9. Oceanic Transform Plate Boundaries and Fracture Zones
Figure 20.04: Various topographic expressions result from juxtaposition of rock bodies with different temperatures, ages, and internal structure.
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.

10. 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.

11. Oceanic Transform Boundaries
This map of the flanks of the mid-Atlantic ridge shows all of the hallmarks of a transform fault.
(Courtesy of D. Blackman)
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 boundaries.
The abyssal hills bend to make J-shaped curves as they reach the transform.
Linear valleys, depressions, and ridges are all aligned along the fault.
Courtesy of D. Blackman

12. The Romanche Fracture Zone
Courtesy of D. T. Sandwell and W. H. F. Smith, Scripps Institution of Oceanography, University of California at San Diego
The Romanche fracture zone extends across most of the Atlantic Ocean, forming a huge ridge and trough system some 5000 km long and almost 100 km wide. The active transform boundary lies between the offset ridge axis. (Courtesy of D. T. Sandwell and W. H. F. Smith, Scripps Institution of Oceanography, University of California at San Diego)
Figure 20.05: The Romanche fracture zone extends across most of the Atlantic Ocean.
Extends over the entire width of the Atlantic Ocean
Fault system is 10’s of km wide
Separates the African and S. American plates. Active transform is ~ 600 km long
A small portion rises above sea level

13. 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.
The Clipperton transform fault cuts the East Pacific Rise, in the northern part of this map. It is 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. Note the height of the plate immediately north of the East Pacific Rise and the rugged relief in the fault zone. The fracture system where no shear occurs extends beyond the active transform fault. Blue is low elevation and light pink is highest elevation on seafloor. (Courtesy of K.C. Mcdonald, University of California at Santa Barbara)
© 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.

14. 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.

15. 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 extensional structures are produced by bends or offsets in the transform fault system.
Small fold belts mark zones of transpression (left) and pull-apart basins mark transtensional bends (right).

16. 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.
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.
The thermal structure of a transform boundary is related to differences in age and temperature of the lithosphere across the fault. The cross section along A–A′ is parallel to a ridge segment and shows the thermal structure of a ridge-transform boundary. The older, cooler lithosphere creates a “cold wall” that inhibits magmatic processes and concentrates deformation into a narrow zone. The younger, hotter lithosphere stands higher than the older cooler lithosphere. Thus, the scarp alternates from one side of the fracture zone to the other. In addition, a hot bulge forms on the older lithosphere that is adjacent to the hot ridge.
Figure 20.09: The thermal structure of a transform boundary is related to differences in age and temperature of the lithosphere across the fault.

17. 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.
Figure 20.10B: A large-offset on a transform fault (or one that has a slow-shearing rate) has a narrow zone of deformation.
A large-offset on a transform fault (or one that has a slow-shearing rate) has a narrow zone of deformation.
a. The transform fault is marked by a deep linear valley. Long, narrow, linear ridges commonly parallel the faults. Also, note how the spreading ridge bends into the fault zone. Red is high elevation and deep blue is low elevation. (Courtesy of K. C. Macdonald University of California, Santa Barbara)
b. The schematic block diagram shows the pronounced contrast in thickness of the lithosphere from the accreting ridge where the lithosphere consists only of hot, new oceanic crust to the much older, colder, and thicker lithosphere on the opposite side.
© K. C. Macdonald/Science Photo Library

18. 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.
Figure 20.11B: This cross section shows that there is little contrast in lithospheric thickness across the transform zone.
A small offset transform fault (or one that has a high-shearing rate) has a wide zone of deformation.
a. The transform fault zone may be tens of kilometers wide. Several shear zones within the transform system form elongate ridges and valleys. They are linked together by extensional pull-apart basins or segments of spreading centers that trend obliquely across the shear zone. Volcanism occurs along these short ridge segments. Red is high elevation and deep blue is low elevation.
(Courtesy of K. C. Macdonald University of California, Santa Barbara)
b. The block diagram shows that there is little contrast in lithospheric thickness across the transform zone. This relative uniformity permits a wide belt of deformation.
© K. C. Macdonald/Science Photo Library

19. 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

20. Continental Transform Faults
Figure 20.12B: The San Andreas Fault slices through California, marking the transform boundary between moving tectonic plates.
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.

21. 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.
Continental transform faults produce very distinctive landforms.
a. Transform  fault zones include strike-slip faults, fault scarps, linear ridges and valleys, offset drainages, and local elongate lakes and ponds.
b. The San Andreas Fault slices through California, marking the transform boundary between moving tectonic plates. The great scar along the fault line is marked by linear valleys, sharp contrast in landforms, and displaced drainage.

22. 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
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

23. 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.

24. The San Andreas Fault
Courtesy of Mike Poland/USGS

25. Movement Along the San Andreas Fault
The arrows are vectors showing the speed and direction of movement of each surveyed spot. Parts of coastal California are “quickly” sliding to the northwest at a rate of about 5 cm/yr. The rapidly moving region is on the western side of the San Andreas fault system. These velocity estimates are very similar to those found by measuring offset features on the fault and constitute a powerful affirmation of the role played by moving plates over millions of years.
Data from: R. A. Bennett, J. L. Davis, and B.P. Wernicke

26. The Dead Sea Transform System
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.
The Dead Sea Transform System
Connects the Red Sea spreading ridge with the Alpine convergent belt.
Llong, deep, narrow pull- apart 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.
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. (Base map by Ken Perry, Chalk Butte, Inc.)
Base map by Ken Perry, Chalk Butte, Inc.

27. 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). These form three deep basins along the floor of the Gulf of Aqaba and the Dead Sea basin that lies below sea level. (Courtesy of NASA)
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

28. The Alpine Transform Fault
Figure 20.16A: The transform system of the Alpine Fault, New Zealand, connects Tonga-Kermadec 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.
Data from the New Zealand GeoNet Project

29. 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.
(Photograph by New Zealand Institute of Geological and Nuclear Sciences)
Figure 20.17: The Alpine Fault, New Zealand, is a strike-slip fault that connects two plate boundaries.
© GNS Science Photo Library

30. 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.
Shallow earthquakes on a midocean ridge are more frequent on the transform faults, where the crust is cooler, thicker, and more brittle, than on the ridge crest itself. The region directly beneath the ridge is too hot and ductile to produce many earthquakes. The map shows strike-slip earthquakes (red) on transform faults and extensional earthquakes on normal faults (blue) on the rift valley along a portion of the Mid-Atlantic Ridge. <FS>(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.
Courtesy of D. T. Sandwell and W. H. F. Smith, Scripps Institution of Oceanography, University of California at San Diego

31. 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.
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.
(Modified from A. Robinson)
Data from: A. Robinson

32. Metamorphism and Magmatism at Transform Plate Boundaries
Figure 06.09A: Strongly foliated schist with aligned grains of chlorite that grew in a differential stress field during contraction.
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.

33. 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.
Base map by Ken Perry, Chalk Butte, Inc.
Magmatism at Transform Plate Boundaries
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

34. 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.

35. 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.