The Rio Grande rift: new electromagnetic constraints on the Socorro magma body

Abstract

The Socorro ‘magma body’ in central New Mexico is a prime example of an intrusive magma body associated with continental extension in one of the major active rift zones of the world: the Rio Grande rift. To study this feature further, in particular its relation to large-scale features delineated elsewhere beneath the rift, in 1988 Brown University occupied a sequence of densely spaced (5–10 km), wide-band (10–4000 s) remote referenced, three-component magnetic variation sites along an east-west line which transected the southern margin of the magma body. Our profile was deployed so as to cross precisely above the center of what has been the most seismically active portion of the structure southwest of Socorro. Complex magnetic transfer coefficients at up to 15 periods from 21 sites along our profile have been inverted to determine two-dimensional conductivity models for vertical cross-sections transverse to the strike of the rift. Two independent generalized inverse algorithms are used; an overdetermined block inversion and a mixed-determined ‘flat’ continuous resistivity inversion. Both inversions require the presence of a conducting zone beneath the rift at intermediate depths (15–20 km) in the crust. This feature has properties which are similar to those of the intracrustal conductor seen elsewhere along the rift. In addition to this conducting zone in the deeper crust, the data require a more conductive core at shallow depth (10–15 km) which appears to be associated with the base of a zone of increased microseismicity described by Sanford and his colleagues. This latter feature might be a zone of diffuse magmatic intrusions, enhanced hydrothermal circulation, or a combination of both. Finally, our two inversions suggest that the resistivity structure beneath the western end of our profile, which is physiographically adjacent to the Colorado Plateau, has properties which are similar to those reported for interior regions of the Plateau.