Significance of geothermal and gravity studies in the Las Cruces area
— E. R. Decker, F. A. Cook, I. B. Ramberg, and S. B. Smithson

The Las Cruces area in the middle of the southern Rio Grande rift (Fig. 1), as defined by Chapin (1971), exhibits several important geophysical features attributed to late Ceno-zoic rifting. These features are: 1) a geothermal high between Orogrande and Cookes Peak (Decker and Smithson, 1975), 2) large negative gravity anomalies over the Cenozoic basins of the rift, 3) gravity isoanomaly trends oblique to the rift (Ramberg and Smithson, 1975) and 4) a positive gravity anomaly that occurs along the axis of the rift just east of Las Cruces. This report will summarize description and interpretation of the heat flow and gravity data.

 

In such a brief account as this, discussion will be restricted to outstanding features and implications of the geophysical data. Emphasis is therefore placed on syntheses of results. For example, combined heat flow and radioactivity data are summarized on a map and two profiles, while gravity anomalies based on contours determined from measurements at about 4,000 stations are illustrated by a regional map and a single profile. The University of Wyoming's geothermal and gravity data are also discussed in two recent reports (Decker and Smithson, 1975; Ramberg and Smithson, 1975) and readers are referred to those papers for details concerning reduction and reliability of these measurements.

 

The areas of investigation are shown in Figure 1. The boundaries of the Basin and Range province, the High Plains, and the Colorado Plateau are taken from the map of Fenneman (1946). The outline of the Rio Grande rift follows the definition given by Chapin (1971) rather than that given by Kelley (1952, 1956).

 

Throughout the text the units of density (p) are gm/cm, the acceleration due to the gravity is in mgal., and the units of heat flow are abbreviated to HFU, where 1.0 HFU = 1.0 x 10 cal/cm sec. The term "unreduced" is associated with the heat flow obtained after corrections for steady-state terrain and, if needed, complex geologic structure. The "reduced" heat flow refers to the flux obtained after the unreduced value was reduced for the heat from local bedrock radioactivity according to procedures given by Roy and others (1972) and Decker and Smithson (1975). Also, ranges of unreduced and reduced flux are shown for those sites at which corrections for complexities in local geology were applied.