A proposed outline of the extent of the magma body relative to the State of New Mexico. This area also roughly outlines the extent of most of the shallow earthquake activity within New Mexico. In the lower right is a depiction oif one possible arrangment of the magam body. Conceptually, dikes periodically ascending from the mantle source stall at the mid-crustal boundary. Flexure of the overlying brittle crust gives rise to the abundant seismicity. .
The SMB lies at the southern end of the Albuquerque basin part of the Rio Grande rift (see image above). It is strictly defined as a mid-crustal anomaly identified by geophysical observations. These include the presence of unusual crustal properties, defined by analysis of the way seismic energy is reflected from the surface of the proposed magma body and transmitted through the crust from distant (teleseismic) earthquakes, and by the concentrated distribution of earthquakes. These observations define the presence of a crustal anomaly, but it is the close spatial correlation with the Socorro seismic anomaly (SSA) (Sanford et al., 1977), one of the most seismically active parts of the rift, that implies volume moments associated with active magma movement.
The SSA is a roughly 5000 km2 area of intense micoearthquake activity that coincides with the greater than 3400 km2 area (Balch, 1992; Balch et al., 1997) of the SMB defined from reflection data. Most of the microearthquakes in the SSA are located between a depth of 4 to 14 km (Jiracek et al., 1987). This is within the brittle zone lying above the approximate 20 km depth of the SMB mid-crustal anomaly. That the seismicity is anomalous is apparent when one considers that 45 percent of the total seismicity above magnitude 2.5 in New Mexico occurs within this area, yet it occupies only 2 percent of the area of the state (Balch, 1992). Hypotheses for the origin of the microearthquake activity include fractures caused by bending stresses in the brittle upper crust as the magma body inflates and fractures are induced during small vertical diking events. The inset in the lower right corner of the state map above and the drawing below is a simplified depiction of a possible configuration of the SMB and its feeder dikes. The salient characteristic is the great width relative to the thickness. Geophysical work to date suggests that the SMB is on the order of 0.1 to 0.5 km thick and contains a volume of 102 to 103 km3 (Ake and Sanford, 1987).
Drawing depicting the geometry of the proposed Socorro magma body. The main body occurs as a sheet roughly 100 m thick spread out at the interface between the upper and lower crust.
The volume of actual melt is not well constrained. Current methods do not resolve whether the region is comprised of a series of sheets separated by layers of solid or whether the entire layer consists of some large fraction of melt. It is likely that the layer is supplied by frequent arrivals of melt via dikes from the mantle source region. Occasionally, some dikes may move upward above the magma body to shallower (a few km) depths, based on the apparent presence of shallow bodies in the vicinity of Socorro (Sanford, 1979; Roach, 1982).
Some background references
Ake, J.P., and Sanford, A.R. , 1987, Internal Structure of the Socorro Magma Body Deduced from Microearthquake P-phase Reflections. Geophysics Open-file Report 55, Geophysical Research Center, New Mexico Institute of Mining and Technology.
Aldrich, M. J., Chapin, C. E., and Laughlin, A. W., 1986, Stress history and tectonic development of the Rio Grande rift, New Mexico: Journal of Geophysical Research, v. 91, p. 6199-6211.Balch, R. S., 1992, New constraints on the Socorro Magma Body based on improved hypocenter estimates: Geophysics Open-file Report 63, New Mexico Tech, Socorro, 120 pp.
Arnet, F., Kahle, H-G. , Klingelé, E., Smith, R. B., Meertens, C. M., and Dzurisin, D., 1997, Temporal gravity and height changes of the Yellowstone caldera, 1977-1994. Geophysical Research Letters, v. 24, p. 2741-2744.
Balch, R. S., 1992, New constraints on the Socorro Magma Body based on improved hypocenter estimates; Geophysics Open-file Report_63, Geophysical Research Center, New Mexico Institute of Mining and Technology, 120 pp.
Balch, R. S.,Hartse, H. E., Sanford, A. R., and Lin, K., 1997, A new map of the geographic extent of the Socorro magma body, Bulletin of the Seismological Society of America, v. 87, p.174-182.
Baldridge, W. S., 1979, Mafic and ultramafic inclusion suites from the Rio Grande rift (New Mexico) and their bearing on composition and thermal state of the lithosphere: Journal of Volcanoloy and Geothermal Research, v. 6, p. 319-351.
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Reilinger, R., and Oliver, J., 1976, Modern uplift associated with the proposed magma body in the vicinity of Socorro, New Mexico: Geology, v. 4, p. 573-586.
Reilinger, R., Oliver, J., and Brown, L., 1979, New measurements of crustal doming over the Socorro magma body, New Mexico. Geology, v. 8, p., 291-295.
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Roach, J. M., 1982, The mapping of shallow magma bodies near Socorro, New Mexico by the use of seismic attenuation. Geophysics Open-file Report 42, Geophysical Research Center, New Mexico Institute of Mining and Technology.
Sanford, A. R., Mott, R. P., Shuleski, P. J., Rinehart, E. J., Caravella, F. J., Ward, R. M., and Wallace, T. C., 1977, Geophysical evidence for a magma body in the vicinity of Socorro, New Mexico; in Heacock, J. G., ed, The Earth's Crust: Its Nature and Physical Properties: Geophysical Monograph Series, 20, American Geophysical Union, Washington, D.C., p. 385-404.
Sanford, A. R., 1979, Characteristics of the Rio Grande rift in the vicinity of Socorro, New Mexico, from geophysical studies; Guidebook to the Rio Grande Rift in New Mexico and Colorado, Circular 116, p. 116-119.
Sanford, A. R., Mott, R. P., Shuleski, P. J., Rhinehart, E. J., Caravella, F. J., Ward, R. M., and Wallace, T. C., 1977, Geophysical evidence for the existence of a magma body in the vicinity of Socorro, New Mexico; in Heacock, J. G., ed, The Earth's Crust: Its Nature and Physical properties. Geophysical Monograph Series, 20, p. 385-404.
Savage, J. C., Lisowski, M., Prescott, W. H., and Sanford, A. R., 1980, Geodetic measurement of horizontal deformation across the Rio Grande rift near Socorro, New Mexico: Journal of Geophysical Research, v. 85, p. 7215-7220.
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View Socorro Active Magma Body in a larger map