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Geophysical data are presented and discussed for an area approximately 290 km long and 220 km wide in the Aleutian trench south of Kodiak Island. Variations in total magnetic intensity (residual) of more than 600 -• were found in the center of the trench and more than 1100 -• on the southern flank. The free-air gravity values varied by as much as 150 mgal. The residual magnetic and gravity anomalies suggest that a fissure zone, filled with basic igneous material, is located in the center of the trench. Free-air anomaly calculations indicate that the top of the mantle is 4 km below the trench floor. The magnetic anomaly trend o.n the southern flank of the trench is interpreted as a fracture zone. Both magnetic and gravity data suggest the existence of a ridge of igneous rock material underlying the northern trench slope. It is concluded that the geophysical results in this area are comparable to those obtained in many other continental margins of the earth. This conclusion supports the theory that continental margins and trenches are affected by tensional forces in the earth's crust. INTRODUCTION As part of the National Oceanographic Program, the U.S. Coast and Geodetic Survey in 1961 initiated a comprehensive survey of the area between the Aleutian and Hawaiian islands.The survey includes oceanographic, hydrographic, and geophysical measurements along N-S survey lines spaced 20 km apart and along widely spaced E--W lines. Navigation was controlled by Loran C over most of the area.The geophysical instruments used were a Varian V-4931 towed marine magnetometer (proton free precession) and the LaCoste and Romberg air-sea gravity meter, number S-11. Soundings were obtained with a precision depth recorder (PDR). The accuracy of the magnetic data is limited by the diurnal and shorter-period variations in the magnetic field. Records from the Sitka and Honolulu magnetic observatories were studied, and small parts of the data which were recorded during the times of large magnetic bays were excluded. The largest part of line 13 north of latitude 49øN (Figure 1) was excluded because of the magnetic storm of June 20 and 21, 1961.The average daily variation in total intensity was 5 7 at Sitka and 25 7 at Honolulu. Since these daily variations were small and their period much longer than the period of anomaly measurements made as the ship towed the magnetometer sensor over underlying magnetic rocks, no corrections for daily variations were made.The magnetometer sensor was towed 140 m astern, so that the influence of the ship's magnetic field on the data was insignificant.Accuracy of gravity data is dependent upon accurate knowledge of the ship's speed, course, and position and the period and magnitude of the ship's accelerations. Data were omitted when the horizontal accelerations were larger than 500 mgal. It was assumed that the data for the two E-W lines (16 and 17 in Figure 1) are accurate within -----5 mgal because the encountered accelerations were smaller than 100 mgal. (The major part of line 17 crosses the N-S lines s...

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