PALEOMAGNETIC AND STRUCTURAL INVESTIGATION OF THE BASALTS OF SUMMIT CREEK, WASHINGTON STATE

The Summit Creek Basalts are a sequence of steeply dipping subaerial late Eocene basaltic flows located southeast of Washington’s Mount Rainier. Despite previous petrologic and paleomagnetic investigations, the origins of these basalts are poorly understood. It is uncertain whether they erupted in situ or were transported to their present location by tectonic processes. It is possible that these lavas were derived from the same magma source as a sequence of flows in the Crescent Formation on the Olympic Peninsula, as both erupted between 45 and 50 million years ago and have similar chemical compositions. New paleomagnetic analyses of the orientation of the magnetic field recorded in the Summit Creek Basalts were conducted to provide evidence for any rotation or north-south motion that has occurred since the flows erupted. Over a dozen flows were sampled along Washington State Highway 12 and Carleton Ridge in May and June of 2014. The basalt cores were analyzed with a Molspin spinner magnetometer using standard alternating field demagnetization and thermal demagnetization. These methods determined the thermal remnant magnetization of the flows and clues to the types of magnetic minerals in the basalt. Many of the samples behaved well paleomagnetically, but a few exhibited magnetization too weak to provide useful information. This discrepancy is most likely due to alteration of the primary magnetic minerals. Similar alteration has been observed in the basalts of the Crescent Formation. Structural features were also mapped to provide further insight into the stresses that shaped the unit. These studies provide insight into the origin and subsequent deformation of these enigmatic Eocene basalt flows.