38. Linking the Yakima Fold Belt to Its Forearc Analogs Through the Cascade Arc

Clockwise rotation of Oregon drives shallow crustal faulting and earthquakes in the Columbia embayment of Oregon and Washington. GPS measurements document northward motion of Oregon at about 8 mm/yr, squeezing Washington against slower-moving Canada. The Yakima fold belt (YFB) accommodates this north-south shortening, largely between the Olympic-Wallowa lineament (OWL) and the Columbia River. We interpret the YFB to extend westward across the arc and into the forearc, where it includes E-W folds and faults of the Seattle, Tacoma, Doty and lower Columbia River fault systems. Holocene earthquakes in the forearc are well documented and linked to long term shortening rates and rotations determined from geologic mapping and paleomagnetism. However, the YFB structures crossing the Cascade arc are less well known. Pre-35 Ma tectonic activity along the OWL and along the Straight Creek fault is well documented, but Quaternary deformation across the arc is not well understood. GPS data, when combined with diffuse seismicity, regional structural mapping, and kinematic models suggest the greater YFB is active from the Tri-Cities to the Pacific Coast, and from the OWL south to at least the latitude of Maupin, OR -- the source of a continuing swarm of earthquakes. Geologic rates of deformation are needed in the greater YFB to compare with predicted long-term rates from GPS and localized paleoseismic studies near Hanford. Without geologic constraints, the strain budget for shallow crustal faulting is highly dependent on the model used for Cascadia megathrust behavior. Defining kinematic linkages in the Columbia embayment will help us model the potential size of earthquakes on YFB structures and more easily quantify deformation in an area of major federal infrastructure, including numerous dams, power stations, electrical grids, irrigation systems, and nuclear facilities at Hanford.

Research under this Opportunity will contribute valuable constraints to a geologic model for the distribution of permanent strain and earthquake hazard in the upper plate of the Cascadia convergent margin. We need to understand the relative importance of shallow faulting driven by convergence-related compression versus deformation superimposed on the convergent margin by Pacific-North American shear and the marginal effects of Basin-Range spreading. The postdoctoral research will identify major structures that accommodate present-day margin-parallel shortening in the Columbia embayment and document their rate and style of deformation. The researcher will also examine how the YFB traverses the Cascade arc to link with similar structures in the forearc. Areas of current interest include: 1) the OWL, where the White River Fault and other structures pass through a large gap in the Quaternary arc and where the USGS and its cooperators are acquiring high resolution aeromagnetic and LiDAR surveys; and 2) the Columbia River Gorge, where the river follows YFB structures across the Cascade arc, and new U.S. Geological Survey (USGS) geologic mapping, LiDAR, and potential field surveys are underway. The researcher will be working collaboratively with geologists, geophysicists, seismologists, and paleoseismologists. He or she may produce new or revised geologic, structural, or neotectonic maps of the study area; analyze new and existing data from geologic mapping, potential fields, paleomagnetism, seismology, and crustal deformation (GPS); and develop a framework for the structural accommodation of stress associated with subduction and its interaction with shear-driven blocks.

Proposed Duty Station: Menlo Park, CA

Areas of Ph.D.: Geology, geophysics

Qualifications: Applicants must meet one of the following qualifications: Research Geologist, Research Geophysicist

(This type of research is performed by those who have backgrounds for the occupations stated above. However, other titles may be applicable depending on the applicant's background, education, and research proposal. The final classification of the position will be made by the Human Resources specialist.)

Research Advisor(s): Ray Wells, (650) 329-4933, rwells@usgs.gov ; Richard Blakely, (650) 329-5316, blakely@usgs.gov; Russell Evarts, (650) 329-4929, revarts@usgs.gov; Brian Sherrod, (206) 553-0153, bsherrod@usgs.gov; Ralph Haugerud, (206) 553-5542, rhaugerud@usgs.gov; Craig Weaver, (206) 553-0627, craig@usgs.gov; Steve Reidel (Washington State University), 509-372-7397, sreidel@verizon.net

Human Resources Office contact: Candace Azevedo, (916) 278-9393, caazevedo@usgs.gov

Summary of Opportunities