Magnetotelluric results opposing magma origin of crustal conductors in the rio grande rift

Abstract

Although conductive intracrustal layers appear to be characteristic of continental rift zones, they also have been detected in stable crustal environments. The low-resistivity layers are often explained by free water or crustal magma; however, alternative explanations are possible. Purely geometrical effects can be mistaken as conductive layers at depth in the interpretation of magnetotelluric or geomagnetic depth soundings. Certain minerals such as graphite and hydrated minerals e.g. serpentine, may be important. Thermally activated processes associated with ductile flow mechanisms may also enhance conductivity. Two-dimensional analysis of a portion of 25 new magnetotelluric stations in the central Rio Grande rift supports the presence of a conductive zone at 10 km depth or less. However, very significant differences are apparent in the crustal sections at two close localities. Mid-crustal magma lenses, well-defined by seismic data, cannot explain these differences. In fact, the crust is modeled to be more conductive by at least an order of magnitude where magma is not detected compared to where it occurs at shallow and intermediate levels. Only the possible effects of regional three-dimensional current channeling could conceivably alter this conclusion. To explain the unexpected result, it is suggested that a conductive horizon occurs where an impermeable, ductile cap traps pore fluids beneath. The cap may be nearly an order of magnitude more conductive (~100's Ωm) than the dry, brittle crust above; the zone of trapped pore fluids much more conductive by more than an order of magnitude (~10 Ωm). Where active magma injection destroys the integrity of the ductile cap, trapped fluids may escape and produce an overall decrease in conductivity. The final electrical signature with this dynamic concept depends on the thermal gradient, the relative impermeability of the cap, the extent of pore fluids beneath, and the amount (and frequency) of magma intrusion. The temporal and spatial distribution of earthquake foci in our study area supports the existence of a ductile layer at about 10 km depth and the hypothesis of magma injection.