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During the summers of 1982 and 1983, a U.S.G.S. crew led by Dr. Frank Frischknecht acquired 29 magnetotelluric soundings along a profile traversing the transition zone between the Great Basin and Colorado Plateau. The profile was centered along the 36°30' parallel, north of the Lake Mead region dividing Northwest Arizona and Southeast Nevada. The transition zone between the Great Basin and Colorado Plateau has long been of interest because it represents active encroachment of a continental rift system into a more stable intracontinental plate. This margin is characterized geologically by rapid Cenozoic uplift, high heat flow, high conductivity and low Pn velocities. There were two stages of processing and analysis involved in this project. After discarding unuseable stations, primarily due to equipment problems in the first summer, we were left with 18 good stations. One dimensional inversions of three or more layers were derived from these remaining stations. Secondly, the one dimensional inversions were used as starting models for two dimensional inversion and modelling. We used the data from thirteen of the stations to construct the traverse for two dimensional modelling. This paper discusses results of inversions of the magnetotelluric soundings and their inferred relationship with the geology of the transition zone. INTRODUCTION The Colorado Plateau is characterized as having undergone rapid late Cenozoic uplift creating deeply incised canyons and monoclinal folding. The crust underlying the Colorado Plateau is intermediate in thickness between the stable interior plains and Basin and Range at about 35-45 km (Braile and others, 1974). General isostatic equilibrium of the Colorado Plateau yields near zero gravitational anomalies. The Basin and Range, in contrast, represents one of the world's largest areas of intracontinental extension. The Great Basin, the northern half of the Basin and Range, is characterized by high heat flow, low Pn velocities, Quaternary and Tertiary volcanism, crust of 15-25 km thickness and thin lithosphere (Thompson and Burke, 1974) . The topography is dominated by north-south trending horst-graben sequences bounded by steeply dipping normal faults. The transition zone between the Great Basin and Colorado Plateau is an approximately 50km wide area west of the Grand Wash Cliffs. This zone is called the Grand Wash Trough in analogy to the Wasatch Trough in Utah. The Grand Wash Trough is dominated by late Cenozoic normal faulting (Pearthree and others, 1983), high heat flow and conductivity, (Gough, 1983). The survey the U.S.G.S. undertook extends from east to west from the Uinkaret Plateau in the Central part of the Colorado Plateau across the Hurricane Cliffs, the Shivwits Plateau and Grand Wash Cliffs through the St. Thomas Gap separating the North and South Virgin Mountains and finally onto the Mormon Mesa. This is shown in Fig. 1. A Bouguer gravity anomaly map, at a slightly different scale, is presented as Fig. 2 and shows the location of the magnetotelluric soundings. Soundings used in the two dimensional inversion are indicated as solid circles. GEOLOGY Regional Geology and Geophysics As recently as the early 1970's, the consensus of earth scientists was that the uniformly uplifted Colorado Plateau and the complexly extended Basin and Range Province were thought to be essentially independent. Looking at Fig. 2 for instance, one can see a substantial reduction in the density of gravity contours as you proceed into the Colorado Plateau. The dominant hypothesis was that a relatively isolated mantle diapir was heating the lithosphere below the Basin and Range causing it to wrench apart, this was known as the pure shear hypothesis. Another related opinion held the belief that the intrusion of large volumes of igneous rocks in vertical dikes wedged the Basin and Range apart, the wedging hypothesis. Evidence has been presented to question these hypotheses and has caused earth scientists to rethink the tectonic processes involved in this area. The chief argument against the pure shear and the wedging hypothesis, bases on palinspastic reconstructions, are that such processes could not adequately account for the large amounts of extension present in the Basin and Range (from 60-100% or 140km,(Weraicke and others, 1982) ) without a much thicker lithosphere than is geophysically evident (40 km). An explanation that could account for these observations is the recognition of the importance of low angle normal faults or detachment faults as the dominant tectonic mechanism present in the area at depth. This recognition relegates brittle, high angle normal faults to a role as antecedents of the ductile low angle normal faulting (Wernicke, 1981) . One important consequence of the low angle normal faulting hypothesis is the definition of a discrepant zone. Wernicke (1983) defines the discrepant zone as the area where there is a deficiency in crustal thickness based on fault uplift calculations. This discrepancy is due to the thin crust created by the low angle normal faulting consuming the crust of the more stable plate. Pearthree and others (1983) showed that based on Cenozoic normal faulting, the Great Basin appears to be encroaching upon and thinning the lithosphere under the Colorado Plateau, precisely matching the definition of a discrepant zone. Survey Specific Geology The Uinkaret and Shivwits Plateaus which the profile traverses (Fig. 1) are comprised primarily of Paleozoic and Mesozoic sedimentary rocks. Cenozoic volcanic rocks and pediment surfaces are also present in this area. The Grand Wash Cliffs and Hurricane Cliffs are formed by steep normal faults because of rapid Cenozoic uplift and resistant limestones preventing alluvial fan development. The Northern Virgin Mountains are a faulted parallel plunging anticline and syncline metamorphic sequence of very high relief (Seager, 1970). The Northern Virgin Mountains and the Beaver Dam Mountains to the north represent breakaway mountain ranges (Wernicke, 1983). The profile continues onto the Mormon Mesa which is a flat surface of Quaternary Alluvium. THE MAGNETOTELLURIC SURVEY Magnetotelluric soundings depend on ionospheric-generated currents propagating through the earth. These "telluric" currents have periods from milliseconds upward. For the lowest frequencies, with periods of greater than a day for instance, measurements must be taken from fixed geomagnetic observatories whereas the magnetotelluric method utilizes frequencies high enough so that field measurements are possible and practical. These frequencies are typically from .001 Hz to 100 Hz. The higher the frequency the less the penetration into the surface and the less volume of material that is sampled by the current. Thus the lowest frequencies of propagation are influenced by the deepest structure. Near the surface, the flow of telluric currents is primarily controlled by electrolytic conduction through the pore fluids, as the surrounding rock matrix is essentially an insulator. Therefore, resistivity is dependent on porosity, abundance of fractures, and the salinity of the pore fluids. At depth, these pores and fractures close resulting in many fewer conduction paths and a subsequent increase in resistivity. Throughout much of the crust and often into the upper mantle, the closure of pores dominates the resistivity profile resulting in high values. Continuing deeper, dependent on the geothermal gradient, increased ionic conduction of semiconductor metallic ions due to higher temperatures dominates the resistivity profiles resulting in lower resistivities. Keeping in mind the inverse relationship between frequency and depth of investgation, an idealized magnetotelluric sounding would detect a bell-shaped curve with lower apparent resistivities at high and low frequencies and high apparent resistivities at intermediate frequencies. Because of high geothermal gradients in the transition zone and Great Basin, magnetotelluric soundings were thought capable of helping to unravel the complex geology in the transition zone. Field measurements of magnetotelluric data require measurement of impedance, which is actually a complex tensor Z(u), related to the resistivity-depth function. Assuming layered geometry and a horizontal surface, Z(u) is a function of the horizontal electrical (Ex , Ey) and magnetic fields (Hx,Hy). The third component of the magnetic field (Hz ) is often measured as a reference channel to detect the presence of non-layered geometry. The data are normally reduced to a complex scalar approximation of the impedance tensor comprised of an amplitude component, apparent resistivity ^>0 , and an imaginary component, phase (Vozoff, 1972). Data Acquisition and Reduction The U.S.G.S. has developed a portable, minicomputer based field system to measure and process magnetotelluric data real-time in the field. (Stanley and Tinkler, 1979, Stanley and Frederick, 1979). This field acquistion and processing system was used to collect the soundings over the two field sessions. The system uses two sets of copper sulfate electrodes separated by 150 meters to measure the electric field components (Ex ,Ey ). A low noise magnetometer is used to measure the magnetic field components (Hx ,Hy , and Hz ). Coherency, a measure of data quality, is calculated using various combinations of the above components. During the first season of field work, a cryogenic magnetometer was used but proved to be unworthy for the rugged field conditions encountered. Thus stations 1-13 had very poor coherencies due to the erratic recorded magnetic field and were unuseable. An induction coil magnetometer was used for the second field session, summer 1983, and provided much more coherent data. While the data are collected, gains are applied and it is bandpass filtered. The data is then decimated, Fourier-transformed from the time to the frequency domain and stored for future processing all in real time. The

One and two-dimensional inversion of magnetotelluric data across the basin-range-Colorado Plateau, Virgin Mountains area, NW Arizona and SW Nevada