Circular 91—Gravity survey in central Socorro County, New Mexico

By A. R. Sanford, 1968, 14 pp., 1 table, 9 figs.

Detailed gravity survey covering parts of Rio Grande depression and adjacent areas, presented by regional and residual Bouguer anomaly maps and cross sections showing Rio Grande valley to be a series of linked structural depressions asymmetrical in cross section and other similar significant structural depressions that exist in the region. Principal results are that (1) the Rio Grande valley is a series of linked structural depressions that can be quite asymmetrical in cross section, (2) other structural lows as significant as the Rio Grande depression exist in the region, and (3) recent earthquake activity near Socorro does not originate along the major normal faults bordering the depressions.

About one thousand gravity observations have been made in west-central New Mexico. A majority of the gravity stations lie within or adjacent to the Rio Grande structural depression in the vicinity of Socorro and San Antonio, New Mexico. Quadrangles covered in considerable detail by the gravity data include Magdalena, Socorro, San Antonio, and Carthage. This circular describes the survey and computational procedures, gives regional and residual Bouguer gravity anomaly maps, and presents a structural interpretation of the results. Included also is a brief discussion of the relation between structures as revealed by the gravity data and seismic activity.

The base station for the entire gravity survey was U.S. Geological Survey Bench Mark R 222 on the NMIMT campus. The bench mark monument, originally located at the northwest corner of the Research and Development Building, was destroyed during construction of an addition to the building in 1961. However, the outside sidewalk on the north side of the building opposite Room 24 is at the same elevation and is within a few feet horizontally of the original monument.

Absolute gravity at the base station is 979185.34 milligals. This value was obtained by making two consecutive runs between the NMIMT station and Woolward's gravity base station at the old Albuquerque Airport. Two Worden gravity meters with instrument constants of 0.0886 and 0.9385 milligal/division were used. About half the survey was done with each instrument.