Analysis of repeated levelings in and near a part of the Rio Grande rift in New Mexico reveals a zone of anomalous relative uplift in the vicinity of Socorro. Maximum uplift relative to nearby bench marks was about 20 cm from 1911 to 1951, corresponding to an average relative velocity of 5 mm/yr. This feature is one of the most clear-cut and striking anomalies of this scale ever detected by leveling in the United States. The principal data come from a north-south profile extending from El Paso, Texas, to Albuquerque, New Mexico. The primary anomaly extends approximately 70 km north from a point about 15 km south of Socorro; the maximum observed uplift occurs approximately 25 km north of Socorro. This zone of anomalous relative uplift overlies and closely correlates spatially with the unusual crustal discontinuity that was detected by Sanford and others and which, on the basis of a variety of evidence, was tentatively identified by them as the upper boundary of a magma body at a depth of about 18 km beneath Socorro. A simple elastic model for surface deformation associated with subsurface magmatic activity indicates that the observed movement in the Socorro area could result from the expansion of a magma chamber at a depth approximately equal to the depth of the discontinuity reported by Sanford and others, but the model is nonunique, and an elastic model may not be realistic.

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