LAUSR

Abstract We present magnetotelluric (MT) data from the northern Fennoscandian shield, which show extreme behavior due to the complex geoelectric setting left behind by the tectonic reworking of the Archaean bedrock during the Palaeoproterozoic. The core of the MT data set from the central Finnish Lapland were acquired in 2014 in the context of the MaSca-project. Extreme data are defined by the condition where the determinant of the real and/or imaginary part of the impedance tensor becomes negative. This is related to, but is a more anomalous condition than out-of-quadrant off-diagonal impedance phases. Physically, these are associated to reversal of e.g. the electric field as compared to its direction for one-dimensional resistivity structure. So far, only few examples of extreme data have been explicitly reported in geophysical literature. Data previously classified as erroneous might belong to the class of extreme data instead and data quality analysis should take this into account. A resistivity contrast of at least ca. 1:1000 is required to observe extreme data, as indicated by synthetic modelling studies. Extreme data are challenging from a modelling point of view and thus also drive the need for new developments in forward and inverse codes for MT. In this study, behavior of the phase tensor parameterization in case of extreme data is clarified. The quadrants of the principal value phases, a piece of information inherently missing from the phase tensor, can be recovered if the parameters are followed as a function of period. The behavior of the determinant (average) of the impedance tensor is investigated, since out-of-quadrant determinant phases are sometimes observed in conjunction with extreme data. A special condition is demonstrated, which links out-of-quadrant determinant phases and extreme data to negative determinant of the galvanic distortion tensor.