Gravity and aeromagnetic anomalies of southeastern Arizona
— Carlos L. V. Aiken

Southeastern Arizona is in the Mexican Highlands physiographic sub-province of the Basin and Range province (fig. 1). The highest elevations between the Pacific Ocean and the Great Plains at the latitude of southeastern Arizona occur in southeastern Arizona, with the axis of the elevation high at the Arizona-New Mexico border.

 

Drewes (1978) has pointed out the significance of south-eastern Arizona in the regional tectonic framework of North America. It lies in an area with strong northwest trends in Precambrian and younger rocks. Major Precambrian and Mesozoic tectonic boundaries have been defined in the area. Drewes (1978) also suggested that the so-called Texas lineament was a consequence of these boundaries and not produced by a strike- slip feature.

In a topographic sense half of the area is made up of valleys. However, wide pediments in these valleys, covered by alluvium up to 1000 m thick, may be structurally related to the mountain ranges. The grabens between these pediments constitute only a small fraction of the total valley surface area. This north-south structure reflects late Cenozoic Basin and Range tectonism.

 

Information on the depth and composition of the basins are dependent on 76 holes drilled for hydrocarbons and 14 holes drilled for stratigraphic information in southeastern Arizona. Only about 25 are bottomed in what is considered to be non- Cenozoic consolidated material and these are located over pediments (Aiken and Sumner, 1974). A deep test hole drilled by Exxon in Picacho basin northwest of Tucson extends to the Precambrian basement at 3103 m but is outside the area of this study (Conley, 1974).

 

Considerable gravity and magnetic data have been gathered in southeastern Arizona as part of larger programs or for specific projects in the area. Results are summarized in several maps: Bouguer gravity (West and Sumner, 1973), residual Bouguer gravity (Aiken, 1975), free-air gravity (Aiken and others, 1976) and residual aeromagnetic (Sauck and Sumner, 1970) anomaly maps of Arizona. General analyses of these basic data sets for southeastern Arizona have been made by Sauck (1972), West (1972), Aiken (1976), Sumner and others (1976) and Aiken (1978); for the area in particular by Aiken and Sumner (1974); and for selected areas by Eaton (1972), Spangler (1969), Bittson (1976), Lynch (1972) and Robinson (1975). In this discussion the results of Aiken and Sumner (1974) and Aiken (1976) will be used extensively.
Geophysical data other than gravity and aeromagnetic cov-erage is very sparse. Ongoing studies of heat flow (M. Reiter, pers. commun.), electrical surveys (J. Hermance, pers. commun.) and crustal seismic surveys (G. R. Keller, pers. commun.) will soon provide the needed information to better analyze the crust and mantle of southeastern Arizona. Existing data can aid in indicating the general character of the crust and mantle in the region.

 

Interpretation of some of the existing geophysical data appears contradictory. Crustal thickness is estimated by Pakiser and Zietz (1965) to increase from 35 to 40 km west to east across the area. Warren (1969), however, interpreted ambiguous changes in thickness of the crust. A possible change in Pn velocity from 7.8 to 8.1 km/sec west to east (Herrin and Tagger, 1962) also occurs and could contribute to errors generated in the computation of crustal thicknesses.
Hermance and Pedersen (1976) fit the geomagnetic variation data of Schmucker (1964) with vertical prisms of varying resistivities instead of horizontal layering. In this model south-eastern Arizona and southwestern New Mexico have higher resistivities than west of Tucson and in the Rio Grande rift.

 

Heat-flow data are sparse, and completely corrected data even more rare. At present there is only one heat-flow value between Tucson and New Mexico, in the Dragoon Mountains, a surprisingly low 1.5 HFU (Warren and others, 1969). Just east of the New Mexico line, values of 1.2 to 2.2 HFU are juxtaposed (Reiter and others, 1973). If the 2.2 HFU value adjacent to the 1.2 HFU values is really aberrant, then south-eastern Arizona and southwestern New Mexico have relatively low heat flow. To the south, in northern Chihuahua and Sonora, heat-flow values are also very low (1.2 HFU, fully corrected; D. Smith, pers. commun.).

 

Gravity and aeromagnetic data provide a basis for studying the shallow and deep structure of southeastern Arizona as reflected in the lithologic densities and magnetic susceptibilities. The two data sets will be used to analyze the details of the Basin and Range structure of the area.