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Contribution 1101, Hawaii Institute of Geophysics

An isopach map of total sediment thickness of the Nazca plate north of 30°S latitude shows deviations from the the obvious sediment trends that are governed by crustal age, sea-floor depth, and surface biologic productivity. Sediment on the eastern flank of the East Pacific Rise has accumulated at half the rate of that on the western rise flank; this difference is attributed in part to asymmetric spreading. Broad areas of thin sediment occur over the crest of the fossil Galapagos Rise and in a 300-km-wide zone on the eastern plate margin parallel to the Peru-Chile Trench. Whereas the thin sediment over the fossil rise crest is the natural result of relatively young crust, the thin sediment paralleling the trench is characterized by postdepositional volcanic rocks and is related to, but cannot be totally explained by, the rupturing of the upper oceanic plate prior to subduction.

Seismic reflection records reveal that volcanism and basement structure are responsible for the initiation of submarine valley erosion near the Carnegie Ridge and the confining of sediments on the axis of the Nazca Ridge. Reflection records also indicate that two periods of tectonic activity have occurred in the Bauer Basin: the first when the basin crust was generated at the Galapagos Rise and left seamounts that are now covered with sediment; the second when spreading activity migrated westward to the East Pacific Rise and left younger, sediment-free seamounts.

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