Geological and Geophysical Observations in an Abyssal Hill Area Using a Deeply Towed Instrument Package

In order to study the smaller scale geological and geophysical phenomena of the deep ocean floor near-bottom observations have been made over two north-south trending abyssal hills in the northeast Pacific. The study area is located in a region of magnetic lineations 130 kilometers south of the Murray Fracture Zone (vicinity 32 deg 25'N, 125 deg 45'W). The hill on the west is an elongate shield volcano, while the eastern hill is a block fault (horst) structure. The hills have a variable sediment cover that is thickest in the valley between them. The horizontal variation in sediment thickness is affected primarily by continuous down-slope transport of sediments resuspended by burrowers; and further, by covering of the sedimentary section by lava flows, trapping of sediments in local depressions and base levels, non-deposition or erosion due to spatially varying bottom currents, and mass movement. Measurements of the relative sedimentation rate show that it is presently up to twice as high in the valley as on the adjacent highs. The magnetic anomalies measured at depth are lineated parallel to the bottom topography and the anomalies at the ocean surface. Magnetic model studies suggest that they are caused by a magnetization within the second layer that varies smoothly in a direction perpendicular to the lineations. Empirical evidence from other authors suggests that paleofield fluctuations, frozen into the crust as it is created at the crests of oceanic rises, are the most likely cause. (Author).