HR: 1340h
AN: T33C-1392    [Abstracts]
TI: A Three-Dimensional View of the Tualatin and Northern Willamette Basins, Oregon, from Inversion of New Gravity Data
AU: * Langenheim, V E
EM: zulanger@usgs.gov
AF: U.S. Geological Survey, 345 Middlefield Road, Menlo Park, CA 94025 United States
AU: McPhee, D K
EM: dmcphee@usgs.gov
AF: U.S. Geological Survey, 345 Middlefield Road, Menlo Park, CA 94025 United States
AU: Morin, R L
EM: morin@usgs.gov
AF: U.S. Geological Survey, 345 Middlefield Road, Menlo Park, CA 94025 United States
AU: Blakely, R J
EM: blakely@usgs.gov
AF: U.S. Geological Survey, 345 Middlefield Road, Menlo Park, CA 94025 United States
AU: Wells, R E
EM: rwells@usgs.gov
AF: U.S. Geological Survey, 345 Middlefield Road, Menlo Park, CA 94025 United States
AB: A regional gravity study was initiated during the summer of 2004 over the Tualatin and northern Willamette basins in support of ongoing seismic hazard and ground-water studies. More than 1200 new gravity measurements were acquired over the basins and surrounding areas, increasing the average spacing of 1 station per 25 km$^{2}$ to 1 station per 5 km$^{2}$ over much of the lowlands. To first order, the gravity data reflect the substantial density contrast (400 to 600 kg/m$^{3}$) between dense Eocene oceanic basement rocks (e.g. Tillamook Volcanics and Siletz River Volcanics), exposed extensively in the Coast Range west of the valleys, and overlying Eocene to Miocene marine sedimentary rocks and Quaternary deposits. Isostatic gravity values as high as +70 mGal occur over Eocene basement rocks west of the valleys. Relatively high gravity values of $\sim$ +20 mGal coincide with a few scattered outcrops of the Tillamook Formation and Basalt of Waverly Heights (considered Eocene volcanic basement) on the eastern margin of the Tualatin basin, suggesting that Eocene basement extends across the Tualatin basin. However, the lower values on the eastern margin may indicate lower-density facies within the volcanic basement (i.e. breccias rather than flows) and/or a deeper oceanic slab. Both the Tualatin basin and the Willamette basin to the south are marked by gravity lows. The lowest gravity values (minus 45 mGal) of the entire region occur in the central part of the Tualatin basin, north of the intrabasinal, east-striking Beaverton Fault, whereas the gravity low over the northern Willamette basin reaches only minus 20 mGal. The two basins are separated by a broad, northeast-trending gravity high along the upthrown side of the Sherwood fault. The western margin of the Tualatin gravity low coincides with the Gales Creek fault and is marked by a steep gravity gradient, which broadens south of the Beaverton Fault. The southward continuation of the Gales Creek fault into the northern Willamette Valley, and the Mt. Angel fault farther southward, have little or no gravity expression, likely due to small vertical offsets. Gravity data filtered to enhance shallow sources (depths $<$ 2 km) show strands of the Portland Hills fault (along the northeast margin of the Tualatin basin) extending 15-20 km northwest of their mapped traces. The newly acquired gravity measurements were inverted to provide a preliminary three-dimensional view of Eocene basement and related tectonic structure. The Tualatin basin is characterized by a deep sub-basin surrounded by steep ($\sim$40$\deg$) margins. The southern margin of the Tualatin sub-basin corresponds to the concealed, east-striking Beaverton fault, where Eocene basement deepens abruptly by more than 2 km, consistent with a north-verging reverse fault. The northern Willamette basin is also characterized by a sub-basin, although smaller in horizontal and vertical dimensions. In contrast to the east-striking Beaverton fault, northwest- and northeast-trending strike-slip faults, like the Mt. Angel fault, show only modest vertical offsets. Pertinent future research should focus on obtaining detailed gravity profiles across the Beaverton, Canby-Molalla, Sherwood, Gales Creek, Sylvan, and Portland Hills faults for joint gravity and magnetic modeling and for extending regional coverage over the northern margin of the Tualatin basin and the eastern margin of the northern Willamette basin. A worthy goal is to develop a three-dimensional geologic model of the two basins, including Eocene basement, Columbia River basalt, interbasalt sedimentary units, and regional fault systems.
DE: 8105 Continental margins and sedimentary basins
DE: 8107 Continental neotectonics
DE: 8110 Continental tectonics--general (0905)
DE: 1219 Local gravity anomalies and crustal structure
DE: 1234 Regional and global gravity anomalies and Earth structure
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting