Abstract This volume will synthesize and describe the geology of the eastern northern Pacific Ocean Basin, a particularly large, variable, and dynamic region that is far less studied than most continental areas. Interpretation of the geology and geophysics of oceanic regions has been remarkably useful for elucidating the geometry and history of large-scale tectonics, especially lithospheric plate tectonics. Because it is accomplished by remote sensing techniques that generalize sea-floor structures and features, the structural complexities that are observed onshore at road-cut scales are smoothed out in marine studies. Thus, in many ways the success of marine studies is that they observe the forest without the confusion of looking at each tree, while on land the geologist must try to interpret the forest by the more difficult task of generalizing the study of features with the relative size of the trees. In the past few years the scales of onshore and offshore geologic studies have been converging. Continental geologists are using more geophysics and remote sensing, while marine geologists have tried to conduct higher-resolution studies. This volume will strive to provide a comprehensive description of our present state-of-knowledge of marine geology. At the same time, we will emphasize two general objectives in our field, as follows:

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.