Deep structure of the Rio Grande Rift: A magnetotelluric interpretation

Abstract

The interpretation of long‐period magnetotelluric data (T≤104 s) from several sites in the Rio Grande rift indicates a thin zone of anomalously high conductivity (S = 2200 S) at a depth of 10–17 km beneath the north central rift (Santa Fe) and a similar zone (S = 1600 S) at a depth of 21–28 km beneath the southern rift (El Paso). We suggest that these zones are contiguous with the tabular body previously delineated seismically over a restricted area (1700 km2) beneath Socorro, New Mexico, by Sanford and his colleagues and probably have a similar generic cause. Although the electrical conductivity itself can be affected by (1) electrolytic conduction in hydrothermal pore fluids, (2) conduction in the bulk silicate material itself, or (3) solid conduction through hydrated phases, such as layered silicates, we find the suggestion offered by Sanford and his colleagues for the Socorro structure to be the most attractive: the intracrustal zone represents a generic class of phenomena associated with the accumulation of basaltic melt derived from sources at greater depth. If this is the case, we may be seeing the effects of a thin zone (1 km) comprised totally of melt or a thicker zone containing as little as a 15–35% melt fraction. The periodic (T≤5×104 years) emplacement of such a magma accumulation and the subsequent perturbation and partial remelting of the surrounding country rock may in fact account for episodic silicic volcanism along the rift such as that which is seen, for example, in the Jemez Mountains.