ANALYSIS OF DEFORMATION IN PILLOW BASALTS OF THE CRESCENT FORMATION, OLYMPIC MOUNTAINS, WASHINGTON, USA

The Olympic Mountains of northwestern Washington are an accretionary complex from the Cascadian Subduction Zone that has been subsequently uplifted. The Crescent Formation is of geologic interest because it is one of the deepest exposures of Siletzia, a large igneous accreted terrane. Understanding the deformational history of the Crescent Formation could have implications for what is currently occurring at depth near the Cascadian Subduction Zone. A gradient in metamorphism has been detected in the formation which increases from east to west, however the processes that created this are not fully understood (Hirsch & Babcock 2009). The western portion is the focus of this study which is composed primarily of pillow basalts. Detailed outcrop mapping and micro-structure analysis are used in this study to reconstruct the deformational history of the Crescent Formation. Three main structural features have been documented: conjugate fractures with orientations that correlate to structures found in the eastern meta-sedimentary core (Tabor & Cady 1978), several generations of veins, and zones of deformation, which are of particular interest. These zones are marked by interconnecting sedimentary layers that were deposited during the time of pillow basalt formation and therefore reflect an orientation of initial bedding. Within the zones, there is evidence for shear as well as micro-faults and folds. Along the zones, the pillow rims show evidence of alteration that differs from pillows not near the zones. This suggests that the deformation localizes in the weaker sedimentary layers. Further understanding of these structures may provide insight to the processes that occurred prior to the formation's exhumation.