The Socorro magma body is one of the largest known midcrustal magma bodies on Earth: it is a sill-like feature, about 3,400 square kilometers in area, about 150 meters thick, and lies at about 19 km depth under the central Rio Grande rift in the Socorro, New Mexico area The Socorro magma body causes surface uplift with a maximum rate of about 2.5-3 mm/yr centered over the northern part of the body. Models of surface uplift due to an elastic crustal response to magmatic inflation at 19 km depth agree well with InSAR interferograms. The age of the magma body and its related surface uplift are controversial. In order to resolve this controversy, the New Mexico Tech research team will carry out a pilot study of terrace remnants to determine the age of onset of surface uplift. They will map the terrace remnants with the aid of Digital Elevation Models and correlate them using soil characteristics (clay and carbonate content), geomorphic characteristics (surface textures, degree of desert varnish, location within flights of terraces), sedimentological characteristics of terrace deposits (clast types, sedimentary structures, grain size), and 36Cl ages. Well-preserved and unaltered surfaces will be dated using 36Cl cosmogenic isotope profile dating. Lateral changes in terrace elevations and vertical terrace spacing will, in turn, allow identification of those terraces affected or unaffected by Socorro magma body inflation. Terrace ages will bracket onset of Socorro magma body -related uplift and constrain long-term rates of uplift and magma-body inflation.

The Socorro magma body is one of the largest known midcrustal magma bodies on Earth. Leveling and InSAR studies indicate that the magma body is causing surface uplift at rates of about 2.5 mm/yr (in the central zone of fastest uplift). The Socorro Seismic Anomaly has the highest rate of upper crustal seismicity in New Mexico and lies above the magma body. Microseismicity presumably is triggered by deformation and ascending hydrous fluids above the Socorro magma body. This project seeks to constrain the age of the Socorro magma body, which is relevant to understanding crustal rheology, evolution of the brittle-ductile transition, crustal growth in rifts, magma genesis during rifting, stress evolution during rifting, and volcanic hazard.

Project Report

The Socorro Magma Body (SMB) is a sheet-like intrusion of partially molten rock at about 19 km depth under Socorro, NM. It is about 100-150 m thick and underlies about 3500 square kilometers so has a volume of at least 350 cubic kilometers, making it the second largest known magma body on Earth. For comparison, a typical stratovolcano such as Mount St. Helens or Pinatubo has magma body volume of less than a few cubic kilometers. The land surface above the SMB is uplifting in a bulls-eye pattern with maximum uplift rates at the center approaching 3 mm per year, which is actually quite a high rate for steady vertical motions in the Earth (as opposed to sudden motions such as earthquakes). This was first measured in the 1980s by releveling surveys of railroad lines and has been measured over the past ~20 years using satellite geodesy (Interferometric Synthetic Aperture Radar, or InSAR). This project used river terraces formed in the past 800,000 years by the Rio Grande and its tributatries, especially the Rio Salado, to determine the age of long-term uplift caused by the SMB. Simple thermal models of the SMB indicate that it should cool and solidify in 500-1000 years if there is no ongoing supply of magma to replenish the heat and molten rock. Our work was focused along the RIo Salado, which passes over the north flank of the uplift "bulls eye," so abandoned river terraces should be arched if the SMB has a long-term history. Our results suggest that there were two episodes of uplift, one since ~60,000 years ago but before the modern uplift event started, and one before ~122,000 years ago. Understanding the history of SMB-related uplift is relvant to evaluating volcanic hazards in central New Mexico, to understanding evolution of magma bodies and sheet-like plutons, to evolution and modification of the continental crust in rift environments, and to geothermal processes. In addition, the Socorro Seismic Anomaly, a region that contains by far the largest amount of seismic activity in New Mexico, is underlain by the SMB. Most of the historical earthquakes in the seismic anomaly are small (generally magnitude < 3, some 4's) and occur between 3 and 12 kilometers depth (some continue down to ~15 kilometers). However, the destructuve earthquake that occurred near Socorro in 1906 was considerably larger (magnitude ~6?) and probably should be considered a part of the seismic anomaly. It is known that elevated fluid pressure can lead to siesmic fault slip and the heat from the SMB probably drives fluid motions in the seismic anomaly that, in turn, cause seismicity.

National Science Foundation (NSF)
Division of Earth Sciences (EAR)
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David Fountain
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New Mexico Institute of Mining and Technology
United States
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