Gravimetric expression of graben faulting in Santa Fe country and the Espanola Basin, New Mexico
— Lindrith Cordell

The Espanola basin generally is considered to be one in a chain of graben along the axis of the Rio Grande rift, and contains the type locality of the upper Cenozoic, graben-filling Santa Fe Group (Galusha and Blick, 1971; Manley, 1979). Disagreement exists, however, on the position, and even the existence, of the graben-border faults. This paper concerns the delineation of faults by gravity data, based on the premise that the sandy, poorly consolidated Santa Fe Group has low density relative to the graben basement.

A long history of structural interpretation of the Espanola basin is cited and evaluated in three recent papers. Kelley (1979) characterizes the basin as fault-bounded. Manley (1979) emphasizes a narrow axial graben (the Velarde graben) within the basin, whereas Baltz (1978) characterizes the basin as essentially a syncline. The question of whether the basin is fundamentally a graben or a syncline is important, because the geometry of a keystone-shaped graben requires crustal extension and "rifting," whereas a synclinal fold does not. Actually, Baltz too describes major zones of normal faulting, although he maps displacement along the easternmost basin edge in a sense opposite to that of some previous workers.

Such differing interpretations suggest that scientific reproducibility is not being realized, and therefore, that the limit of resolution of geologic mapping has been reached. I have suggested previously (Cordell, 1970, 1976, 1979) that through out the Rio Grande rift, the pattern of exposed faults commonly provides few and unreliable clues to the configuration of the graben-bounding master faults to 1 to 5 km depths, as delineated by gravity data. Possible factors in this are: (1) stratigraphic throw on growth faults increases unpredictably with depth; (2) some exposed faults are secondary slump features; and (3) most of the master faults are covered by an alluvial veneer and cannot be seen at the surface.

In this spirit, I show here gravity data from the Espanola basin, including previously unpublished data, with the aim of delineating buried master faults. In a nutshell: the gravity data do not establish the existence of faults, although this is the preferred interpretation. If faults are present, the gravity data show their approximate positions.