Potential Fields SIG: Potential-field Anomalies Illuminate Cascadia Tectonic History from ... - Sep 17th

Complete Title: Potential-field Anomalies Illuminate Cascadia Tectonic History from Regional  to Outcrop Scale

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Speaker: Richard J. Blakely, US Geological Survey at Moffett Field, California 

A two-decade program of airborne data acquisition by the USGS has yielded 150,300 km2 of high-resolution magnetic data over the Cascadia forearc and backarc of Oregon and Washington. These data provide a rich tapestry of information about subsurface geology and tectonic history when interpreted in combination with modern geologic mapping, LiDAR topography, and other geophysical methods. Magnetic anomalies and geologic mapping suggest that active faults in the forearc are kinematically linked to structures in the backarc. The Southern Whidbey Island fault (SWIF) in the forearc, for example, aligns with faults of the Yakima folds in the backarc. Forearc-backarc linkage would imply that documented dextral shear on the SWIF continues into the backarc, as supported by observations of late Quaternary right-lateral slip seen in recent paleoseismic studies in the Yakima folds, including on a strand of the Wallula fault zone (WFZ). The WFZ offsets 8.5 Ma Ice Harbor dikes, mostly concealed to view but spectacular displayed in magnetic anomalies. We used a variety of potential-field techniques to “tease out” dike or dike-swarm anomalies that are correlative across the WFZ; these techniques include spectral analysis to design filters matched to specific source depths, tilt derivatives to estimate source depth, and ground-magnetic traverses to resolve individual dikes. A single Ice Harbor dike (or swarm) is confidently identified with 1,980 m of right-lateral offset across one strand of the WFZ, indicating an average slip rate of 0.23 mm/y over the last 8.5 Ma. This rate should be considered minimum, given that the WFZ has multiple strands. We offer this study as an example of how analysis of high-resolution magnetic data can illuminate tectonic history at both regional and outcrop scales.

Speaker Biography: Richard J. Blakely, US Geological Survey at Moffett Field, California
Richard Blakely is a Research Geophysicist Emeritus at the U.S. Geological Survey at Moffett Field, California. He received his BS degree from Oregon State University in 1968, and MS and PhD degrees from Stanford University in 1971 and 1972, respectively. He served as Assistant Professor at Oregon State University before joining the USGS in 1975.

Blakely uses potential-fields and other geophysical methods to address a variety of geologic problems, including the mitigation of adverse effects of earthquake and volcanic hazards around the Circum Pacific, assessment of mineral potential in the Western United States, and evaluation of ground-water resources in the arid Southwest United States. He is sole author of a textbook, Potential Theory in Gravity and Magnetic Applications, and author or coauthor of 196 publications in refereed journals and refereed government reports.

Blakely is a Fellow of the American Geophysical Union (AGU), a Fellow of the Geological Society of America, and recipient of the Department of Interior Meritorious Service Award. He served as President of the Geomagnetism and Paleomagnetism Section of AGU, member of the AGU Council, and Associate Editor of the Journal of Geophysical Research and Reviews of Geophysics and Space Physics. He also is a member of the Geological Society of America and the Society of Exploration Geophysicists.


9/17/2020 4:00 PM - 5:00 PM
Central Standard Time

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