Gravity Measurements in Alaska

Outlines development and present status of observation program carried out by the authors since 1950. Field procedure consisted of establishment of base stations and a calibration standard for all gravity meters used, and setting up subsidiary base stations. Observations were made at 5- to 10-mile intervals along the highways, the Alaska Railroad between Fairbanks and Seward, and on an 800-mile trip down the Lewes and Yukon Rivers from Whitehorse to Circle. Free Air and Bouguer anomalies were determined, the latter being computed for selected densities to obtain maximum geological value. "Reasonably accurate regional anomaly maps" are presented for Naval Petroleum Reserve No. 4 and for southeastern Alaska; a generalized geologic map of Alaska with Bouguer anomalies (density=2.67) superimposed, indicates a great gravity low near Cook Inlet, and positive anomalies over the Aleutians, on the Yukon-Kuskokwim delta, and near Nenana. Available basic geodetic data are indicated; need for additional observations is stressed

Magnetic Surveys Over the Hawaiian Islands and Their Geologic Implications

A geophysical and geological analysis is made of a total field magnetic survey of the major islands of Hawaii. It is established that the regional distortion of the earth's normal magnetic field due to the topographic mass of the Hawaiian Ridge rising in places to over 30,000 ft above the ocean floor seldom exceeds 150 gammas. On each island, local magnetic anomalies having the form of lenticular and circular dipoles are found. The lenticular dipole anomalies appear to be related to crustal rifts that have been invaded by magmatic material of mantle origin , and the circular dipole anomalies are associated with primary areas of volcanic eruption. Although the inferred crustal rifts have surface geologic expression in some areas, such as the Koolau Mountains on Oahu, for the most part they do not. Furthermore, offshore magnetic data indicate that these features extend beyond the islands and out into the adjacent , deep-water , oceanic area where they can be traced for 100 miles or more. The most pronounced of these features is associated with the ocean floor Molokai Fracture Zone, which magnetically extends across the Hawaiian Ridge without interruption for an unknown distance to the west. The circular dipole anomalies appear to represent the effect of intrusions in volcanic pipes or vents rising from these crustal rifts which strike essentially east-west on the islands of Hawaii, Lanai, Maui, and Molokai, and west northwest-east southeast on Oahu, Kauai, and Niihau. With two exceptions, all of the anomalies indicate normal polarization conformable with the earth's present field

Crustal and Upper Mantle Structure of the Solomon Islands as Revealed by Seismic Refraction Survey of November-December 1966

A seismic refraction survey was carried out in the waters around the Solomon Islands during November and December 1966. Three ships were involved in the survey: two, stationed at the end points of the traverses, acted as recording ships; the third steamed along the traverses and dropped explosives. Reflection profiling and magnetic surveys were simultaneously carried out with the refraction survey. The results show that (a) on the Ontong Java Plateau to the northwest of the islands the crust is about 25 km thick with subnormal crustal velocities; (b) southwest of the New Georgia Islands the crust is thinner than normal and is underlain by a mantle with low velocity; (c) southwest of Bougainville Island the crust is generally of normal oceanic structure underlain by a mantle with low velocity; and (d) mantle material in the Slot is found at a depth of 14 km