Fracture zone

A fracture zone is a linear oceanic feature—often hundreds, even thousands of kilometers long—resulting from the action of offset mid-ocean ridge axis segments. They are a consequence of plate tectonics. Lithospheric plates on either side of an active transform fault move in opposite directions; here, strike-slip activity occurs. Fracture zones extend past the transform faults, away from the ridge axis; seismically inactive (because both plate segments are moving in the same direction), they display evidence of past transform fault activity, primarily in the different ages of the crust on opposite sides of the zone.

In actual usage, many transform faults aligned with fracture zones are often loosely referred to as "fracture zones" although technically, they are not.

Mid-ocean ridges are divergent boundaries that push apart two plates. As the plates on either side of a mid-ocean ridge move, transform faults are created due to variances in plate motion. As the plates continue to move over time, these faults offset the mid-ocean ridge. These offsets cause fractures in the ocean floor that can extend for hundreds of kilometers out from a mid-ocean ridge.

Fracture zones and the transform faults that form them are separate features. Transform faults are plate boundaries, meaning that on either side of the fault is a different plate. In contrast, on either side of a fracture zone, the crust belongs to the same plate.

As many areas of the ocean floor, particularly the Atlantic Ocean, are currently inactive, it can be difficult to determine past plate motion. However, by observing the fracture zones, one can determine both the direction and rate of past plate motion. This is found by observing the patterns of magnetic striping on the ocean floor (a result of the reversals of Earth's magnetic field over time). By measuring the offset in the magnetic striping, one can then determine the rate of past plate motions. In a similar method, one can use the relative ages of the seafloor on either side of a fracture zone to determine the rate of past plate motions. By comparing how offset similarly aged seafloor is, one can determine how quickly the plate has moved.

The Blanco Fracture Zone is a fracture zone running between the Juan de Fuca Ridge and the Gorda Ridge. The dominating feature of the fracture zone is the 150 km long Blanco Ridge, which is a high-angle, right-lateral strike slip fault with some component of dip-slip faulting.

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