8. Fracture Zones and Transform Faults

Fracture zones are major discontinuities disrupting the linearity of spreading ridge segments. They form depressions separating two regions of plates that slip in opposite directions. The motion of the plates has gashed the lithosphere down to more than 4000 m deep, often exposing deep mantle material. Fracture zones vary in length from a few tens to hundreds of kilometers and are narrow, no more than a few tens of kilometers wide. The seismically and tectonically active portion of a fracture zone is also called a “transform fault”. The area of the Equatorial Atlantic between the African and South American continents located between 5°N and 5°S corresponds to the highest concentration of closely spaced fracture zones in the World’s ocean. That is where the St. Peter and St. Paul’s Rocks Fracture Zone crosses the entire Atlantic Ocean from the Amazon basin at the west through the coast of Liberia to the east. Most transforms are regions of intense tectonic events but are deprived of volcanic activity, except for a few transform faults associated with the fast spreading ridge systems in the Pacific Ocean where recent volcanic activity has been detected.