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Published mechanisms for rift tip propagation at spreading centers include extensional deformation and an initial period of slow spreading. We investigate whether the gravity signal and inferred crustal structure at pseudofaults formed in medium to superfast spreading environments resemble the gravity signal at fracture zones or nontransform discontinuities formed in slow spreading environments. We find that altimetry-based gravity anomalies on the Mathematician, Bauer, Easter, Juan Fernandez, and northern Chile Ridge pseudofaults, located in 75–150 mm/yr (full rate) seafloor spreading environments, are similar in amplitude and form to Atlantic fracture zones with 20–30 mm/yr spreading rates. A 5–15 mGal positive mantle Bouguer anomaly is observed on the pseudofault bounding the eastern Juan Fernandez microplate, comparable to those at some similar age-offset nontransform discontinuities in slow spreading environments. Our results suggest that the deeps associated with active propagating rift tips result from both a dynamic mantle component and anomalous crust, the latter of which remains frozen at pseudofaults. We predict that any pseudofaults with age offsets more than ∼1 m.y. and not coincident with hotspot volcanism will be associated with thin (and possibly unusually dense) crust, even in superfast seafloor spreading environments.

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Journal of Geophysical Research-Solid Earth, v. 105, issue, B12, p. 28399-28410.

© Copyright 2000, American Geophysical Union.

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