Summary We report herein on a finite element algorithm for 2‐D magnetotelluric modelling which solves directly for secondary variations in the field parallel to strike, plus the subsequent vertical and transverse auxiliary fields, for both transverse electric and transverse magnetic modes. The governing Helmholtz equations for the secondary fields along strike are the same as those for total field algorithms with the addition of source terms involving the primary fields and the conductivity difference between the body and the host. Our approach has overcome a difficulty with numerical accuracy at low frequencies observed in total field solutions with 32‐bit arithmetic for the transverse magnetic mode especially, but also for the transverse electric mode. Matrix ill‐conditioning, which affects total field solutions, increases with the number of element rows with the square of the maximum element aspect ratio and with the inverse of frequency. In the secondary formulation, the field along strike and the auxiliary fields do not need to be extracted in the face of an approximately computed primary field which increasingly dominates the total field solution towards low frequencies. In addition to low‐frequency stability, the absolute accuracy of our algorithm is verified by comparison with the TM and the TE mode analytic responses of a segmented overburden model.

To illuminate rifting processes across the Transition Zone between the extensional Great Basin and stable Colorado Plateau interior, we collected an east‐west profile of 117 wideband and 30 long‐period magnetotelluric (MT) soundings along latitude 38.5°N from southeastern Nevada across Utah to the Colorado border. Regularized two‐dimensional inversion shows a strong lower crustal conductor below the Great Basin and its Transition Zone in the 15–35 km depth range interpreted as reflecting modern basaltic underplating, hybridization, and hydrothermal fluid release. This structure explains most of the geomagnetic variation anomaly in the region first measured in the late 1960s. Hence, the Transition Zone, while historically included with the Colorado Plateau physiographically, possesses a deep thermal regime and tectonic activity like that of the Great Basin. The deep crustal conductor is consistent with a rheological profile of a brittle upper crust over a weak lower crust, in turn on a stronger upper mantle (jelly sandwich model). Under the incipiently faulted Transition Zone, the conductor implies a vertically nonuniform mode of extension resembling early stages of continental margin formation. Colorado Plateau lithosphere begins sharply below the western boundary of Capitol Reef National Park as a resistive keel in the deep crust and upper mantle, with only a thin and weak Moho‐level crustal conductor near 45 km depth. Several narrow, steep conductors connect conductive lower crust with major surface faulting, some including modern geothermal systems, and in the context of other Great Basin MT surveying suggest connections between deep magma‐sourced fluids and the upper crustal meteoric regime. The MT data also suggest anisotropically interconnected melt over a broad zone in the upper mantle of the eastern Great Basin which has supplied magma to the lower crust, consistent with extensional mantle melting models and local shear wave splitting observations. We support a hypothesis that the Transition Zone location and geometry ultimately reflect the middle Proterozoic suturing between the stronger Yavapai lithosphere to the east and the somewhat weaker Mojave terrane to the west. We conclude that strength heterogeneity is the primary control on locus of deformation across the Transition Zone, with modulating force components.

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S U M M A R YTen tensor magnetotelluric (MT) soundings have been acquired in a 54 km long profile across the South Pole area, East Antarctica. The MT transect was offset from the South Pole station ∼5 km and oriented 210 grid north, approximately normal to the Trans-Antarctic Mountains. Surveying around South Pole station was pursued for four main reasons. First, we sought to illuminate first-order structure and physico-chemical state (temperatures, fluids, melts) of the crust and upper mantle of this part of East Antarctica. Secondly, conditions around the South Pole differ from those of previous MT experience at central West Antarctica, so that the project would help to define MT surveying feasibility over the entire continent. Thirdly, the results would provide a crustal response baseline for possible long-term MT monitoring to deep upper mantle depths at the South Pole. Fourthly, because Antarctic logistics are difficult, support facilities at the South Pole enable relatively efficient survey procedures. In making the MT measurements, the high electrical contact impedance at the electrode-firn interface was overcome using a custom-design electrode pre-amplifier at the electrode with low output impedance to the remainder of the recording electronics. Non-plane-wave effects in the data were suppressed using a robust jackknife procedure that emphasized outlier removal from the vertical magnetic field records. Good quality data were obtained, but the rate of collection was hampered by low geomagnetic activity and wind-generated, electrostatic noise induced in the ice. Profile data were inverted using a 2-D algorithm that damps model departures from an a priori structure, in this case a smooth 1-D profile obtained from inversion of an integral of the TM mode impedance along the profile. Inverse models show clear evidence for a pronounced (∼1 km thickness), conductive section below the ice tentatively correlated with porous sediments of the Beacon Supergroup. Substantial variations in sedimentary conductance are inferred, which may translate into commensurate variations in sediment thickness. Low resistivities below ∼30 km suggest thermal activity in the lower crust and upper mantle, and mantle support for this region of elevated East Antarctica. This contrasts with resistivity structure imaged previously in central West Antarctica, where resistivity remains high into the upper mantle consistent with a fossil state of extensional activity there.

The Transantarctic Mountains (TAM) are the world’s longest rift shoulder but the source of their high elevation is enigmatic. To discriminate the importance of mechanical vs. thermal sources of support, a 550 km-long transect of magnetotelluric geophysical soundings spanning the central TAM was acquired. These data reveal a lithosphere of high electrical resistivity to at least 150 km depth, implying a cold stable state well into the upper mantle. Here we find that the central TAM most likely are elevated by a non-thermal, flexural cantilever mechanism which is perhaps the most clearly expressed example anywhere. West Antarctica in this region exhibits a low resistivity, moderately hydrated asthenosphere, and concentrated extension (rift necking) near the central TAM range front but with negligible thermal encroachment into the TAM. Broader scale heat flow of east-central West Antarctica appears moderate, on the order of 60–70 mW m−2, lower than that of the U.S. Great Basin.