New Paleomagnetic Data from the Central Aleutian Arc: Evidence and Implications for Block Rotations

Abstract

Early paleomagnetic measurements (late 1960s and early 1970s) from the central Aleutian Island Arc produced both scattered and internally consistent results based solely on alternating field demagnetization. Archived core samples collected from Amchitka and Adak islands for these previous paleomagnetic studies were reanalyzed for this study using modem thermal demagnetization techniques. The new measurements from Amchitka Island indicate significant clockwise rotation (R = 54.7 ± 8.0°) between 14 Ma and the present. Adak Island showed no rotation for volcanic rocks of Quaternary age associated with the present volcanic arc, which is consistent with the lack of rotation seen in other young volcanic rocks (<2 Ma) sampled at localities from Unalaska Island in the east to Kanaga Island in the west. The Eocene Andrew Lake Formation shows a clear magnetic overprint that completely masks any information about possible rotations. Other published studies show rotations similar to those seen for Amchitka Island on Umnak, Amlia, Bering, and Medny islands. Models predicting clockwise rotations involve breaking the forearc into discrete blocks and slivers that rotate and/or travel westward along the curve of the arc in response to the arc-parallel component of the Pacific Plate motion. These models have difficulty explaining the large rotations seen in the easternmost islands sampled, where the relative motion of the Pacific Plate is more or less perpendicular to the arc. The data are insufficient to resolve the mechanism involved in rotations, but it seems likely that any or all of the possible mechanisms could have played a role.