Paleomagnetism of an east-west transect across the Cascade arc in southernWashington: Implications for regional tectonism

Paleomagnetic data from a transect across the Cascade are in southern Washi ngton were collected to test models of vertical axis rotations for this par t of the Pacific Northwest and to provide information on the tectonic histo ry of the St. Helens seismic zone. The 75 site-mean directions are divided into three groups based on isotopic, fission track, and stratigraphic ages. Group 1 consists of samples from rocks deposited between 39 and 30 Ma and shows a mean clockwise vertical axis rotation of 34 degrees +/- 13 degrees, group 2 consists of samples from rocks deposited between 30 and 24 Ma and shows a mean clockwise rotation of 17 degrees +/- 11 degrees, a,nd group 3 consists of samples from rocks between 24 and 16 Ma and shows a mean clockw ise rotation of 20 degrees +/- 12 degrees. Although these three values of r otation are statistically indistinguishable at the 95% confidence level, we interpret them in combination with other data to indicate differential rot ation across the St. Helens seismic zone (SHZ). The available paleomagnetic data for Eocene-Oligocene rocks west of the seismic zone show clockwise ve rtical axis rotations of 30 degrees +/- 8 degrees and 35 degrees +/- 9 degr ees consistent with the value for group I. The rotational values for groups 2 and 3 and the intrusive suite of Kidd Creek (13 Ma), despite their diffe rent ages, have similar values east of the SHZ. Comparing these groups of d ata east and west of the seismic zone indicates a differential rotation of 10 degrees +/- 3 degrees across it. In addition, the rates of rotation in s outhern. Washington are similar to those for rocks in the western Cascades of Oregon and indicate that rotational deformation of Miocene age extends n orthward into southern Washington and eastward into the axis of the arc.