1. Prepared by Eric H. Christiansen Brigham Young University

2. 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

3. 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.

5. Major Plate Boundaries

6. Transform Boundary Types
Transform boundaries connect other boundaries
Ridge-Ridge boundaries
Ridge-Trench boundaries
Trench-Trench boundaries
Compensate for differential movement

7. 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

8. 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

9. 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

10. Romanche Fracture Zone
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

11. Structure of a transform fault

12. 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

13. Large Offset Transform

14. Small Offset Transform

15. 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

16. Trench-Trench Transforms
Least common type Connects two trench systems
Direction of subduction may change from one trench to the other

17. Continental Transform Faults
Not as common as oceanic transform faults
Similar in structure
Seismically active
Penetrate entire lithosphere
Distinct linear features
Examples: San Andreas, Dead Sea systems

18. Continental Transform Faults
Not as common as oceanic transform faults
Similar in structure
Seismically active
Penetrate entire lithosphere
Distinct linear features
Examples: San Andreas, Dead Sea systems

19. 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

20. Continental Transform Faults

21. Landforms along the San Andreas Transform

22. Transform Boundary Processes
Contraction & Extension
Braided system of strike-slip faults
Compression = uplift and folding
Extension = basins

23. Transform Boundary Processes
Contraction & Extension
Braided system of strike-slip faults
Compression = uplift and folding
Extension = basins

24. Transform Boundary Processes
Contraction & Extension
Braided system of strike-slip faults
Compression = uplift and folding
Extension = basins

25. Transform Boundary Processes
Contraction & Extension
Braided system of strike-slip faults
Compression = uplift and folding
Extension = basins

26. 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.

28. Dead Sea Transform

30. The Alpine Fault, New Zealand
What causes all of the seismicity in northern New Zealand?
Subduction of oceanic lithosphere
Continental collision
Continental rifting
Transform faulting

31. The Alpine Fault, New Zealand

32. Earthquakes Where are earthquakes more common? On ocean ridges
Along continental margins
Along transform faults

33. 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

34. Magmatism Magmatism is decreased along oceanic transforms
Where was the Sermon on the Mount?
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 C D

35. 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

36. 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.