Geodetic measurement of horizontal deformation across the Rio Grande rift near Socorro, New Mexico.

Trilateration surveys of a geodetic network across the Rio Grande rift near Socorro, New Mexico, in 1972, 73, 76 and 79 have failed to detect any significant strain accumulation. The surveys place an upper bound (95% confidence limit) of 1 mm/a (a = years) on EW spreading across the rift in 1972-79. There is marginal evidence from triangulation for an episode of EW spreading across the rift within the interval 1954-72. The trilateration network lies on the S flank of an uplift caused by magma intrusion into a midcrustal sill during this century according to Reilinger and Oliver. The horizontal deformation induced by sill inflation is sufficiently small that continued uplift during 1972-79 cannot be excluded by the observed absence of significant horizontal deformation.-Authors