IMPLICATIONS OF SIGNIFICANT DISTANCES BETWEEN MAJOR FAULTS AND LARGE VERTICAL DISPLACEMENTS BASED ON GEOPHYSICAL EVIDENCE, CENTRAL RIO GRANDE RIFT, NEW MEXICO

Faults that juxtapose materials of different densities and/or magnetic properties produce characteristic gravity and/or magnetic anomalies, respectively. The anomaly shape and amplitude provide information on the depth and thickness of the juxtaposed materials and the strength of the physical-property contrast. In the Albuquerque basin in the central Rio Grande rift, most major mapped faults generally correspond to geophysical anomalies that signify large vertical displacement. For example, the surface trace of the western basin-bounding Sand Hill fault generally follows magnetic and gravity anomalies that indicate major vertical displacement. In contrast, the Hubbell Spring fault, which is considered part of the active eastern basin margin, is located about 1 km east of where gravity and regional magnetic anomalies mark the largest offset (>1 km) of the eastern basin margin. No geologic or aeromagnetic evidence of shallow faulting corresponds to the geophysical anomalies indicating large vertical displacement. Aeromagnetic anomalies associated with the Hubbell Spring fault indicate moderate (200-500 m) displacement at shallow depths. The large distance separating evidence of a buried basin margin at depth from one having moderate displacement near the surface suggests that active margin faulting has shifted outward from the basin. On the other hand, an apparent eastern splay of the Hubbell Spring fault system even further outboard from the basin is only evident in high-resolution aeromagnetic data, indicating it has been inactive for some time.

Geophysical evidence for major vertical displacement is also separated from the down-to-the-west, intrabasin West Paradise fault. A prominent aeromagnetic anomaly is located about 500 m east of the fault where it is exposed in an arroyo northwest of Albuquerque, New Mexico. Although subtle magnetic anomalies and contrasts in magnetic properties are measured across the fault exposure, the prominent aeromagnetic anomaly is separated from these contrasts. The significant distances between the locations of major faults at the surface and the locations of large vertical displacements at depth, evident only in geophysical data, suggests that a number of faults in the central Rio Grande rift have ceased activity or shifted location with time.