February Meeting

Tuesday • February 20th, 2018

Old Market Pub • 6959 SW Multnomah Blvd, Portland, OR

6 PM — Social

6:45 PM — Dinner

(Salad & Pizza)

7:30 PM — Presentation

Reservations by 4 PM the Friday before. Dinner $25*, Students FREE with RSVP ($5 if no RSVP). *There is a $2 surcharge for those who do not reserve by the deadline

Reservations can be made here: RSVP by 4pm Friday February 16th

Large dextral motion on the Gales Creek fault, NW Oregon

Guest Speaker: Dr. Ray Wells

New geologic mapping, gravity and magnetic surveys, and laser terrain mapping (Lidar) reveal the Gales Creek Fault (GCF) to be a major right lateral fault in northwest Oregon. The northwest-trending GCF forms the boundary between the Cost Range and the Tualatin and northern Willamette basins. A steep gravity gradient up to 110 mgal marks the fault along the western margin of the Tualatin basin, which is 5 km deep based on gravity inversions. Sharp magnetic boundaries suggest 10-15 km dextral offset of Eocene magnetic basement. The fault consists of two sub parallel strands near Gales Creek. The eastern strand, previously mapped as the GCF by Schlicker and Deacon, follows the Gales Creek valley north at least to the Highway 6 bridge, but it has no obvious physiographic expression in the valley. The western strand crosses east-flowing Gales Creek tributaries that all exhibit kilometers of dextral offset where they cross the fault. An anticline cored by unusual, subaerial Siletz River Volcanics (49 Ma) intruded by Eocene diabase provides a piercing point across the western strand and indicates 12 km of post-Eocene right lateral offset north of Hagg Lake, consistent with the offset of aeromagnetic anomalies. On LiDAR imagery of the western strand, uphill-facing scarps, offset streams and shutter ridges can be traced intermittently along the mapped bedrock fault for 20 km between Hagg Lake and Highway 6. Southwest of Forest Grove, the fault splays into en echelon strands which pass between the Dundee Hills and Chehalem Mountain. Paleoseismic trenching of the fault by the Bureau of Reclamation near Scoggins Dam in 2017 indicates late Quaternary, and possibly Holocene displacement of bedrock, loess and flood plain deposits. Additional work is planned in 2018.



Dr. Wells was a research geologist with the USGS for 40 years, where he used field geology, paleomagnetism, and GPS to understand the tectonic evolution and seismic hazards of active continental margins. He has studied subduction zones around the world to better understand the controls on great megathrust earthquakes and has applied that understanding to the Cascadia convergent margin. Ray is particularly interested in how the oblique component of convergence is partitioned into permanent deformation of the forearc, producing faults, earthquakes, and tectonic rotation of the upper plate. Dr. Wells is the recipient of the Distinguished Service Award of the Department of the Interior and the 2017 recipient of the Geological Society of America’s Geologic Mapping Award in honor of Florence Bascom. Recently retired, Ray is a Scientist Emeritus stationed at the USGS Oregon Water Science Center and is a Research Associate with the Geology Department at Portland State University.