Hunting the Saddle Mountain Fault Zone in the Olympic Peninsula With Airplane and Canoe

Two strands of the Saddle Mountain fault zone in the southeastern Olympic Peninsula of Washington, first recognized in the early 1970s, are now well mapped in the Saddle Mountain and Price Lake area on the basis of lidar surveys, aerial photography, and trench excavations. The east-side-up Saddle Mountain East and Saddle Mountain West strands trend north-northeast, cutting across Price Lake and Saddle Mountain for a total distance of 4 km. Drowned trees and trench excavations demonstrate Holocene deformation on the Saddle Mountain fault zone approximately contemporaneous with the Seattle fault earthquake 1100 years ago. Saddle Mountain and surrounding areas are underlain by highly magnetic basalts of the upper member of the Eocene Crescent Formation, and thus the Saddle Mountain fault zone should produce distinctive aeromagnetic anomalies. Aeromagnetic anomalies are obscured, however, by the proximity of two strong magnetic gradients: to the east, the eastern boundary of the Crescent Formation; and to the west, the transition to weakly magnetic, lower member rocks of the Crescent Formation. After processing aeromagnetic data to emphasize shallow crustal contacts, however, aeromagnetic anomalies clearly reflect the Saddle Mountain fault zone as an east-side-up structure and permit mapping the zone at least 18 km from Lake Cushman to the Hamma Hamma River. North-northeast strike and oblique reverse slip in the Saddle Mountain fault zone may mark a transition between two regional strain fields: north-directed compression in the Puget Lowland and northeast-directed compression (parallel to the plate convergence vector) in the Olympic Peninsula. A ``marine-magnetic survey'' of Price Lake shows the magnetic expression of the Saddle Mountain West strand in detail. We acquired 26 track lines over the entire lake, a total line-distance of 9 km and an average track-line spacing of 40 m, using a nonmagnetic canoe, GPS navigation, and cesium-vapor magnetometer system. Fifteen track lines crossed the lakeward projection of the Saddle Mountain West lidar scarp; the Saddle Mountain East scarp was unreachable by canoe. The canoe-magnetic survey illuminated a linear, north-northeast-trending magnetic trough, approximately 150 m wide and 1000 nT in amplitude, connecting lidar scarps north and south of the lake. Trial-and-error, two-dimensional models based on these data are consistent with two east-side-up reverse faults in Crescent Formation at >30 m depth. These offsets at 30-m depth project upward to lidar scarps but have significantly larger displacements (>20 m) than implied by either topography (scarps as high as 8 m) or trench excavations (dip slip as much as 3.5 m). Lastly, we acquired 22 km of ground-magnetic transects on logging roads north and south of Price Lake and found pronounced ground-magnetic anomalies at all lidar-scarp crossings; other prominent anomalies with north-northeast trend observed away from scarps may reflect concealed faults not expressed in topography.