Implications of magnetotelluric modeling for the deep crustal environment in the Rio Grande rift

Block forward modeling and smooth Backus-Gilbert linearized inversion have independently been used to two-dimensionally model two east-west magnetotelluric profiles in the central Rio Grande rift. A crustal conductive zone of at least 1500 S has been detected at about 10 km depth along the northern line. However, along a line 40 km to the south where shallow, active magma is inferred and microseismicity is highest, no high conductivity zone is found. The conductive zone beneath the northern line is hypothesized to occur where water is trapped beneath an undisturbed ductile cap. Under the southern line, magma injection through the cap may release the water, thereby resulting in a relatively more resistive crust with no conductive, midcrustal layer. The resulting paradox is that where there is active magma injection into the upper crust of the Rio Grande rift, the crust is more resistive rather than more conductive.