HR: 1340h
AN: GP33D-1603    [Abstracts]
TI: Mapping dikes and faults in the Oregon forearc using high-resolution aeromagnetic data
AU: * Aboud, E
EM: eaboud@gmail.com
AF: National Research Institute of Astronomy and Geophysics, NRIAG, El-Marsad St.,Helwan, Cairo, 11722, Egypt
AU: Wells, R
EM: rwells@usgs.gov
AF: US Geological Survey, 345 Middlefield Road, Menlo Park, CA 94025, United States
AU: Blakely, R
EM: blakely@usgs.gov
AF: US Geological Survey, 345 Middlefield Road, Menlo Park, CA 94025, United States
AB: The diversity of volcanic rocks in the Cascadia forearc is well displayed in high-resolution aeromagnetic data recently acquired over the Oregon Coast Range. The aeromagnetic survey extends from the Western Cascade Range to the continental shelf including the cities of Eugene and Florence, Oregon. Magnetic anomalies illuminate various geologic domains: (1) Numerous linear west-northwest-striking magnetic anomalies correspond in some places with mapped Oligocene and Eocene dikes and sills that intruded middle Eocene marine sedimentary rocks of the Tyee Formation. This pervasive linear pattern suggests that many similar dikes are concealed at shallow depth. (2) Arcuate, long-wavelength anomalies indicate folds within Eocene Siletz River Volcanics (SRV) lying beneath the Tyee Formation. (3) A pattern of short-wavelength anomalies is caused by Oligocene volcanic rocks of the Western Cascade Range. Superimposed on these anomaly patterns are linear, northeast-trending lineaments caused by various faults, including the Corvallis fault that juxtaposes SRV against Tyee Formation. To help differentiate these various magnetic signals, we have implemented a three-stage analysis. First, matched filters were designed to emphasize shallow magnetic sources. Second, a tilt-derivative (TDR) filter was applied to the shallow-source anomalies to distinguish noise from signal and map geologic contacts. Finally, an Euler analysis was applied using various structural indices to distinguish between dike and fault anomalies. This approach allows us to map both dikes and faults in detail, which is important since exposures are largely concealed in the heavily forested Coast Range. Modeling experiments indicate that we can detect dikes as thin as 25-75 m if they are separated by 1.5-2.5 km or more. In one case, a pair of WNW-striking magnetic anomalies overlies two separate mapped dikes, each about 25-75 m in thickness, 15 km long, and separated by 1.5 km. The magnetic doublet indicates that these two parallel dikes extend at least ~40-50 km and are offset approximately 1.5-2.0 km in two places by NNE-striking faults. We can resolve at least 40 sub- parallel, WNW-striking magnetic anomalies throughout the aeromagnetic survey. Assuming all of the WNW- striking linear anomalies are caused by dikes, and ignoring the possible presence of dikes thinner than our limits of resolution, we estimate that this episode of dike injection was accompanied by about 1 percent crustal extension. Taking into account 50° of clockwise rotation, the azimuth of maximum horizontal extension was 330°, approximately parallel to the Eocene convergent plate boundary.
DE: 0900 EXPLORATION GEOPHYSICS
DE: 0925 Magnetic and electrical methods (5109)
DE: 5109 Magnetic and electrical properties (0925)
SC: Geomagnetism and Paleomagnetism [GP]
MN: 2007 Fall Meeting