Structure and thermal regime beneath the South Pole region, East Antarctica, from magnetotelluric measurements



Ten 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.