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 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

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 transform faults Similar in structure Seismically active Penetrate entire lithosphere Distinct linear features Examples: San Andreas, Dead Sea systems

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

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? Subduction of oceanic lithosphere Continental collision Continental rifting Transform faulting

The Alpine Fault, New Zealand

Earthquakes Where are earthquakes more common? On ocean ridges Along continental margins 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

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

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.jpg

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.