PALEOMAGNETISM INDICATES COUNTERCLOCKWISE TECTONIC ROTATION FOR EOCENE-OLIGOCENE ROCKS NORTH OF THE OLYMPIC MOUNTAINS, WASHINGTON

Paleomagnetic studies of the early Cenozoic rocks of the Coast Ranges of Oregon and Washington have consistently shown a post-Oligocene clockwise tectonic rotation. Early paleomagnetic studies of Eocene and Oligocene rocks north of the core of the Olympic Mountains have suggested a slight counterclockwise rotation, but the data were inconclusive. A recent resampling of three key units in the region has consistently produced counterclockwise rotations in all three regions. The late Oligocene (Chron C6Cr) Sooke Formation near Victoria, souther Vancouver Island, British Columbia, is entirely reversed in polarity and rotated 55 +/- 12 degrees counterclockwise. The upper Eocene-lower Oligocene Quimper and Marrowstone Formations on the northeast corner of the Olympic Peninsula 62 +/- 8 degrees counterclockwise, passes a reversal test, and correlates with Chron C13r-C16r. The upper Eocene-lower Oligocene Makah and Hoko River Formations on the northwest corner of the Olympic Peninsula is rotated 50 +/- 3 degrees counterclockwise, passes a reversal test, and correlates with Chrons C12r-C15r. All of these results are inconsistent with the clockwise block rotation models of the rest of the Coast Ranges, but consistent with the oroclinal bending model of the Olympics, where the core Olympics push eastward and rotate the rocks to their south clockwise, and the rocks to their north counterclockwise.