Resistive (dry?) lower crust in an active orogen, Nanga Parbat, northern Pakistan

Conductivity models beneath the Nanga Parbat Haramosh Massif (NPHM) derived from magnetotelluric soundings reveal that there is no widespread, interconnected, conductive aqueous fluid to minimum depths of 40 km below sea level. Given the continuing tectonic denudation, young granitic and migmatitic bodies indicating partial melt at shallow crustal depths, and active seismicity, this result is surprising in light of similar studies in active tectonic regimes elsewhere. Away from the NPHM, models reveal the usual conductive lower crust. We propose that deep magmatic and metamorphic fluids are produced beneath NPHM in isolated zones but that the active deformation permits their escape through the brittle-ductile transition. A magnetotelluric survey in an area as complicated as Nanga Parbat required the development of methodologies for utilizing standard two-dimensional inversions in a three-dimensional environment. We show here how to identify which parts of the magnetotelluric responses are adequately represented with two-dimensional approximations. Unlike previous efforts, we do not attempt to create a set of generic rules that may be applicable to all geologic environments, Instead, a procedure is outlined that can be tailored to each interpretation. One important result of this work is that magnetotelluric data along a profile can be used to constrain structure off the ends of the profile. (C) 2000 Elsevier Science B.V. All rights reserved.