Mendocino Fracture Zone

▪ fracture zone, Pacific Ocean

       submarine fracture zone in the eastern Pacific Ocean, defined by one of the major transform faults dissecting the spreading centre of the Gorda Ridges. The Mendocino Fracture Zone extends west from immediately offshore of Cape Mendocino, California, for at least 2,500 miles (4,000 km). Topographically, over much of its length, the Mendocino Fault forms a south-facing scarp some 5,000 to 10,000 feet (1,500 to 3,000 metres) high; farther west it is about 8,400 feet (2,600 metres). Regional bathymetric depths north of the fracture zone are consistently 2,600 to 3,900 feet (800 to 1,200 metres) shallower than to the south.

      Magnetic intensities of seafloor rocks indicate an apparent lateral offset of 700 miles along the fracture zone; rocks north of the fracture zone are 23 to 27 million years younger than rocks to the south. This apparent displacement is the scar of transform faulting that accompanied seafloor spreading, a process still going on along the Gorda Ridges north of the fracture zone. This spreading, estimated to proceed at a rate of 1.1 inches (2.9 cm) per year on either side of the Gorda Ridges, results in earthquakes along the fracture zone, from the crest of the ridge eastward to the coast.

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