New paleomagnetic data from the Aleutian Islands: Implications for terrane migration and deposition of the zodiac fan

Abstract

Sedimentary rocks of late Eocene to early Oligocene age from the Aleutian Islands, have been paleomagnetically studied to determine the paleolatitude of this island arc. Samples were collected from gently dipping, thin‐bedded, laminated siltstone, mudstone, and very fine grained sandstone of marine origin. After demagnetization, samples from each locality showed both polarities of a remanent magnetization that thermal, isothermal remanent magnetization, and Js‐T experiments suggest is carried by magnetite. After tectonic correction, the α95 decreases in both localities, suggesting magnetization was acquired before folding of the beds. The characteristic magnetization shows that no significant poleward motion of the Aleutian arc has occurred with respect to the North American plate since the early Oligocene (Amlia, F =−4 °, ΔF=5 °; Umnak, F = 0.8 °, ΔF=3.5 °), but the declinations are rotated significantly clockwise (Amlia, R=70 °, ΔR=23 °; Umnak, R=36.7 °, ΔR=11.2 ° ). Using these paleomagnetic data, relevant Euler poles, and hotspot locations, we propose a new model for the formation of the Zodiac fan. In this model, the start of Zodiac fan deposition corresponds to accretionary, igneous, and erosional events in the Pacific Northwest and fragmentation of the Farallon plate along the margin of the North American plate into small plates. Fan deposition on the Pacific plate was controlled by the geometry of the Bowie and Cobb seamount chains, which prevented sediment from being dispersed over the bathymetrically lower oceanic crust to the west, and by the bathymetry of the Pacific plate, which focused sedimentation in the magnetic bight region of the plate. Our model suggests that the present‐day Zodiac fan is only a remnant of a much larger fan system.