Paleomagnetic Evidence and Implications for Structural Block Rotation

By Clare Tochilin

Whitman College, Walla Walla WA


The Aleutian Islands are a volcanic arc that formed as a result of the subduction of the Pacific plate beneath the North American plate.  The obliquity of the plate convergence has caused separation of the arc crust into five structural blocks that have undergone varying amounts of clockwise rotation.  The amount of block rotation varies longitudinally due to the increasing obliquity of convergence moving west along the arc. This study addresses whether there has been rotation in the eastern portion of the arc during the last two million years.  For this study, paleomagnetic core samples from young Quaternary and older Tertiary flows on Unalaska Island were analyzed using alternating field and thermal demagnetization techniques to determine whether rotation has occurred.  Bulk geochemical analysis and mineralogical analysis showed that the presence of pyrrhotite, titanomagnetite, and titanohematite and the degrees of oxidation of these minerals have a significant effect on the magnetic signatures in rocks of the Tertiary Unalaska Formation.  The study determined that there has been no rotation on the island since the emplacement of the Lava Ramp flows (~54 ka).