On the Alaskan continental shelf between the Shumagin Islands and Prince William Sound, ships of the U. S. Coast and Geodetic Survey have made about 12 traverses while recording gravity, sparker, magnetic, and bathymetry data, and about 12 other traverses while recording magnetic and bathymetry data alone. These marine measurements have been combined with gravity and geologic data obtained by the U. S. Geological Survey on adjacent shorelines to make a reconnaissance gravity map which provides new information on the structure of the continental end of the Aleutian arc.

The gravity anomalies associated with the oceanic part of the arc do not extend very far onto the continental shelf. A gravity high over the Aleutian Islands diminishes gradually near the continental margin, and negative Bouguer anomalies are present among the volcanoes of the eastern Aleutian Range and the southern Alaska Range; the gravity low associated with the inside edge of the oceanic trench is replaced by a gravity high that extends along the entire northern edge of the continental-margin trench. Between the eastern Aleutian gravity low and the continental-shelf gravity high is a series of elongate anomalies that parallel the tectonic trend and may be correlated with sedimentary and volcanic rock units.

Small gravity depressions in areas of positive gravity anomalies on the continental shelf indicate the presence of Cenozoic sedimentary deposits east of Kodiak and west of Middleton Island. Sparker data show that these deposits thicken northwest of a shelf-edge anticline where the free-air anomalies are greatest. However, a much larger decrease of 125-200 mgal. occurs at the southern edge of the Chugach Mountains geosyncline, which lies north of a coastal belt of lower Cenozoic submarine volcanic rocks (largely non-magnetic). These rocks cause local highs that are especially well developed in Prince William Sound and account for the steep gravity gradient between the continental shelf and Chugach Mountains. This 125-200-mgal. gravity change nearly coincides also with the line separati g the emergence and subsidence areas of the 1964 Alaska earthquake. That earthquake increased the positive continental-shelf anomalies south of the gradient. At the northern edge of the Chugach Mountains geosyncline, another gravity high coincides with a belt of lower Mesozoic submarine volcanic rocks (largely magnetic); this high separates the Chugach Mountains low from a low caused by sedimentary rocks in Cook Inlet and Shelikof Straights. Although the gravity data indicate the presence of several thick sedimentary bodies, the large gradients associated with the volcanic and tectonic arcs make estimation of the thickness of the sedimentary column difficult.

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