GEOPHYSICAL MODELING ACROSS THE TUALATIN BASIN, OREGON, AND IMPLICATIONS ON SEISMIC HAZARDS AND WATER RESOURCES

Two-dimensional (2D) modeling of gravity and aeromagnetic data across the Tualatin basin, Oregon, reveal basin and fault geometry and the distribution of Miocene Columbia River basalt (CRB) deposits that may be significant to seismic hazard and ground-water studies. The location and thickness of CRB deposits are needed for assessing ground-water resources, particularly because CRB aquifers are becoming increasingly important sources of water in the area. These studies are being used to develop a three-dimensional (3D) model of the Tualatin basin, including Eocene basement, CRB, interbasalt sedimentary units, and regional fault systems.

New gravity data collected since 2004 in the Tualatin basin and northern Willamette Valley show that the Tualatin basin is marked by a large gravity low (~ -40 mGal) reflecting the less dense sedimentary rocks and bedded CRB flows, although the CRB may be as dense as the Eocene basement in some cases. 3D inversion of gravity data shows that the basin is deep (~ 8 km) and asymmetric, with a steeper western margin marked by the Gales Creek fault. Matched filtering of high resolution aeromagnetic data across the basin allows us to distinguish anomalies arising from shallow (< 1 km) sources, that likely reflect CRB deposits, from deeper sources that are likely caused by underlying basement rocks. Magnetic data reveal both normal and reversed magnetization of CRB flows. In addition, magnetic data show linear anomalies associated with previously mapped faults, such as the Gales Creek, Beaverton, Oatfield and Portland Hills faults, some of which have magnetic signatures that extend beyond their mapped locations.

We present 2D forward models across the Tualatin basin that are computed from gravity and aeromagnetic data and constrained by surface geology and deep wells. The 3D basement surface derived from the inversion of gravity data is used to constrain the geometry of the basin floor. Although up to 300 m thick, the CRB is a minor component of the basin fill, whose maximum depth is ~ 12 km along one 50 km-long profile. The maximum depth of the CRB is about 500 m, and the Beaverton, Sylvan-Oatfield, and Portland Hills faults show vertical offsets of the CRB up to 200 m.