PALEOMAGNETISM OF THE MIOCENE INTRUSIVE SUITE OF KIDD CREEK - TIMING OF DEFORMATION IN THE CASCADE ARC, SOUTHERN WASHINGTON

Paleomagnetic study of the intrusive suite of Kidd Creek in the southe rn Washington Cascades (23 sites in dikes and sills) was undertaken to help determine if these rocks are comagmatic and whether they postdat e regional folding of the volcanic are. Fission track and Ar-40-Ar-39 age determinations indicate an age of similar to 12.7 Ma (middle Mioce ne) for these rocks. The similarity of normal-polarity characteristic directions for most samples corroborate the available geochemical data indicating that these rocks are most likely comagmatic. Reversed-pola rity directions for samples from four sites, however, show that emplac ement of Kidd Creek intrusions spanned at least one reversal of the ge omagnetic field. The paleomagnetic directions for the dikes and sills fail a fold test at the 99% confidence level indicating that the Kidd Creek rocks postdate regional folding. The mean in situ direction also indicates that the Kidd Creek and older rocks have been rotated 22 de grees +/-6 degrees clockwise about a vertical or near-vertical axis fr om the expected Miocene direction. Compression and regional folding of the Cascade are in southern Washington therefore had ended by similar to 12 Ma prior to the onset of deformation resulting in rotation of t hese rocks.