A NEW MAP OF THE GEOGRAPHIC EXTENT OF THE SOCORRO MIDCRUSTAL MAGMA BODY

For 35 years, strong reflected phases from the sill-like, mid-crustal Socorro magma body (SMB) have been observed on microearthquake seismog rams recorded in the central Rio Grande rift, New Mexico. In 1979, the lateral extent of the SMB was estimated at 1700 km(2) by mapping refl ection points for 220 observed SzS arrivals. We have remapped the magm a body using 697 PzP, 2169 SzP, and 2589 SzS reflections observed on s eismograms recorded between 1975 and 1995 by New Mexico Tech seismic n etworks. These data indicate that the SMB covers an area greater than 3400 km(2). By comparing the distribution of observed reflection point s with the distribution of all possible reflection points, limits can be set for the northern and southern boundaries of the SMB. The simila r to 80 km north-south extent of the SMB falls just inside a similar t o 100-km north-south range of enhanced seismicity near Socorro. This a rea, the Socorro seismic anomaly (SSA), covers similar to 5000 km(2) a nd has the same elliptical shape as the outline of the SMB based on ob served reflection points. The close spatial relation between the SSA a nd the mid-crustal magma body along their northern and southern bounda ries suggests that the observed seismicity can be used to place limits on the eastern and western boundaries of the magma body. Using the di stribution of earthquakes within the SSA over the last 34 years, the m aximum east-west extent of the SMB is <60 km, our reflection based val ue is similar to 50 km. Our data indicates that the upper surface of t he magma body displays no regional dip. Considering timing errors, max imum possible relief on the magma body surface is +/-0.50 km.