Interpretation of aeromagnetic, magnetic, and gravity data from the San Francisco Mountains vicinity, southwestern Utah

Abstract

Regional and local geologic structures in the San Francisco Mountains vicinity, southwestern Utah, are investigated using magnetic-field intensity and gravimetric data.

Aeromagnetic data are interpreted to indicate a buried Tertiary pluton whose northern boundary runs from east to west across the 980 square mile study area, and which extends beyond the coverage of the aeromagnetic map to the east, south, and west. Local cupolas, some of which outcrop at the surface, extend upwards from the main igneous body. A three-dimensional model which generates a magnetic field similar to that which is observed is developed, using a digital computer, for the distribution of the magnetic intrusive rocks in the area. These model studies indicate that the pluton is distinctively tabular, and about five miles thick, and has a near-vertical and linear northern edge which may have resulted from the structural control exerted by an east-west trending fault zone.

The Bouguer gravity anomalies in the San Francisco Mountains district reflect horst and graben structures typical of the Basin and Range Province, and indicate that an approximate density contrast with the surrounding consolidated sedimentary rocks of +.10 grams per co. is associated with the local intrusive cupolas. Three interpretive east-west geologic cross-sections reproduce the observed residual gravity anomalies. The cross-sections are obtained using a computer program for the evaluation of the two-dimensional gravity anomalies on the ground surface.

The regional gravity data are interpreted to show the effect of at least three regional structures which are distinct from the local near-surface structures:

1) An underlying intrusive body.

2) An east-west fault zone bounding the intrusive body on the north.

3) A continuation od Basin and Range structure at depths exceeding two miles.

Vertical-field ground magnetic data were obtained during three and one-half months of fieldwork in selected areas of the Star Range, located in the southeast quadrant of the thesis area. The igneous structure compromising the local Milford Flat intrusive body is found to terminate abruptly to the northwest, presumably as a result of post-intrusive faulting, while to the east the depth of burial increases gradually. The intrusive rock near the Wild Bill mine plunges steeply to the east and more gently to the south. An anomaly in the south-central portion of the Wild Bill magnetic grid is interpreted as representing an upward protrusion of the local quartz monzonite body. The contact zone along the southern edge of the Shauntis intrusive body, southeast of the old townsite of Shamite, is mapped on the basis of magnetic evidence, and four zones of magnetite mineralization between 8 and 25 feet thick are defined.