Tectonics of the Nazca-Pacific divergent plate boundary

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Abstract
Five regions along the Nazca-Pacific plate boundary are the sites of detailed surveys completed during the Nazca Plate Project, four in relatively typical axial regions and one in the area of the tectonically complex Easter plate. Deep tow surveys have also been conducted on the spreading center and in a fracture zone. In addition to the survey data, many tracklines of a reconnaissance nature cross the spreading center. Sea-floor spreading occurs at about 160 mm/yr (whole rate) along this part of the East Pacific Rise. South of the Garret Fracture Zone at 13.5°S spreading is asymmetrical, being faster to the east than to the west. The lithosphere near fast-spreading rises is quite thin and therefore more susceptible to deformation than along slow-spreading ridges. The effects of a weak lithosphere take several forms: a long oblique ridge in the active portion of the Wilkes Fracture Zone at 9°S; twinned spreading centers surrounding the small Easter plate; and small, 10 to 15 km, offsets of the spreading center that can be formed or healed rapidly. These offsets are all formed within a time span of 0.5 m.y. or less and may involve segments of the rise axis up to 200 km long. Apparently they are the result of small, discrete axis jumps facilitated by the unusually thin lithosphere. Reconnaissance data between the 13.5°S and 4.5°S fracture zones are adequate to decipher the history of formation of the East Pacific Rise in this region. Spreading activity shifted 600 to 850 km westward from the old Galapagos Rise, now in the center of the Nazca plate, to its present location by three large jumps. Each of these jumps resulted in the formation of a fracture zone-bound section of the new rise; the jumping process covered about 2.5 m.y., from 8.2 to 5.7 m.y. ago.

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