First Advisor

Ansel G. Johnson

Term of Graduation

Summer 1990

Date of Publication

7-6-1990

Document Type

Thesis

Degree Name

Master of Science (M.S.) in Geology

Department

Geology

Language

English

Subjects

Faults (Geology) -- Oregon -- Portland Metropolitan Area, Gravity -- Oregon -- Portland Metropolitan Area, Geology -- Oregon -- Portland Metropolitan Area, Structural geology

DOI

10.15760/etd.5868

Physical Description

1 online resource (3, viii, 79 pages)

Abstract

Growing concern over earthquakes in the Pacific Northwest has prompted the mapping and location of near surface faults in the Portland area, Oregon. Visible evidence of faults is poor, requiring the use of geophysical methods to assist in mapping and defining structures in the basin. Gravity maps and models may help in addressing this problem.

Two free-air gravity models were produced. The first model constructed from existing data crosses the basin from Petes Mountain northeast to the Columbia River. The second model is from a gravity survey along Forsythe Road near Clackamas Heights.

The line crossing the basin confirms previous models which located a 320 meter, down-to-the-east offset of the Columbia River Basalt Group. This model tested and confirmed the hypothesis that the gravity high near Oak Grove, Oregon was caused by an Eocene basalt high rather than an intrusive related to the Boring Lavas. Mt. Scott was modeled as a 2.87 g/cm3 basalt high with a sediment filled channel along the southwest flank. The east side of the basin is modeled as faulted, confirming previous work.

The Clackamas Heights line was designed to locate the Portland Hills fault but due to the depth to the fault and lack of subsurface control of the Waverly Heights basalt, the position of the fault could not be determined.

Complete Bouguer, free-air and residual Bouguer anomaly maps were produced from 1600 data stations compiled from previous gravity surveys. These maps are consistent with the state maps produced by Berg and Thiruvathukal (1967) but show more detail. The prominent features on the maps are a gravity low centered over the Tualatin Basin and a gravity high near Oak Grove, Oregon. The west edge of the maps show the high gravity north-south contours caused by the Coast Range. The east edge of the map exhibits north-south low gravity contours caused by the Cascade Mountain Range.

The Portland Basin has been called a pull-apart basin associated with wrench tectonics. This investigation supports the idea that the Portland and the Tualatin basins are related to the strike-slip motion and are formed by that motion.

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Comments

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Persistent Identifier

https://archives.pdx.edu/ds/psu/22674

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