Horizontal Deformation Rates Measured by Ten Years of GPS Observations on the Socorro Magma Body, New Mexico, USA

The Socorro magma body (SMB), located near Socorro, New Mexico, USA within the central Rio Grande Rift (RGR), has been identified by seismic studies as a subhorizontal reflector at a depth of 19 km, interpreted as a mid-crustal igneous intrusive body extending ~70 km N-S and ~45 km E-W. Geodetic leveling and InSAR measurements have identified an unevenly distributed pattern of uplift with averaged rates of ~2-4 mm/yr centered in the northern portion of the SMB, and decreasing to near zero in the southern portion. Trilateration studies in 1980 and 1985 over the southern half of the SMB failed to identify horizontal deformation in excess of the ~ 1 mm/yr E-W extension rate inferred by geological studies of the RGR. We measure horizontal deformation rates using continuous and survey-mode GPS observations beginning in 2002 with the installation of nine bedrock monuments in two approximately E-W transects across the central and northern SMB. In 2004, a transect of three additional bedrock monuments was added across the southern SMB, and two continuous GPS stations were installed along the central SMB transect in 2005. We estimate horizontal and vertical deformation using observations from campaign GPS surveys undertaken in 2002, 2003, 2004, 2006, and 2011, and available continuous GPS observations from the SMB, IGS, NGS CORS, and EarthScope-PBO and PBO-RGR networks for 2002 to 2012. We use the GAMIT/GLOBK processing software and estimate deformation relative to the Stable North American Reference Frame (SNARF) realized by the PBO network. We observe significant horizontal velocities of ~1-1.5 mm/yr and vertical velocities of ~2 mm/yr in a radial dilatational pattern, located in the region of maximum uplift identified by previous studies. These results are consistent with an inflationary magmatic source and the increased resolution of horizontal velocities will, when combined with vertical velocity data, allow us to develop a more complete model of the actively deforming SMB, and address whether a shallower magma body exists in addition to the seismically identified SMB and if there are temporal variations in the rate of deformation of the SMB, as have been suggested by previous studies.