- Introduction
Chapin and Cather, 1997. Interpreted Euler pole of rotation of the opening of the Rio Grande rift. Abbreviations refer to individual rift and other continental basins. Miocene and Neogene sediments are shaded.
Balch et al., 1997. Figure showing stations of the Socorro seismic network, the rough outline of the Socorro magma body, and the earthquakes used to define the extent of the magma body. Inset shows a map of New Mexico and the Rio Grande rift (RGR).
The Rio Grande rift is an actively extending intracontinental structure characterized by asymmetric basins trending NNE separated by accommodation zones. The rifting, which commenced 25-30 mya, was accompanied by normal faulting and magmatism (Chapin and Cather, 1994). The Socorro magma body is a magmatically active feature of the rift, and is generally characterized by a 18.75 km deep sill surrounding the town of Socorro, New Mexico (Sanford and Long, 1965, Sanford et al., 1977, Brown et al., 1979, Ake and Sanford, 1988, Balch et al., 1997). Reflected shear waves (SzP and SzS) from local earthquakes provide evidence for a liquid-solid interface at depth. The injection of magma may not be directly causing these earthquakes, which generally happen above 10 km depth. Possible mechanisms include increased pore pressure due to active magma injection, percolating fluids related to the magma body, and triggering from distant earthquakes (Stankova et al., 2008, Ruhl et al., 2010). Earthquakes may be individual events or swarms of events happening over the time scale of a month. The magma body is also rising at a constant rate of 2-3 mm/yr (Fialko and Simons, 2001, Van Wijk et al., 2008, Pearse and Fialko, 2010). This has been measured by means of leveling in the past, although today it is measured by Interferometric Synthetic Aperture Radar (InSAR). The bottom of the magma body is not well-defined, although synthetic seismograms have been used to investigate a tapering geometry at the bottom of the sill (Schlue et al., 1996).
The magma body is of tectonic interest because it is one of the few examples of active magma injection in a continental setting. The investigation of the magma body provides a unique opportunity to discover how magmatic injection occurs in an active sill. The flat surface may indicate buoyancy controls on the sill or lateral transport of fluids. It has been suggested that the swarms have a volumetric component and are a result of active injection resulting in extension of the crust above the magma body (Balch et al., 1995).
An interesting problem arises from the discrepancy between the uplift rate and thickness (100 m) and the time scale for magma solidification. The latter time scale is much shorter than the estimated age of the magma body (~50 ka), which precludes active magma injection. It has been suggested that the crustal rheology above the magma body is viscoelastic and not strictly elastic, which allows for higher magma injection rates (Fialko and Simons, 2001). Another possibility is that the magma body is a few hundred years old (Van Wijk et al., 2008).