Implications for the lithospheric geometry of the Iapetus suture beneath Ireland based on electrical resistivity models from deep-probing magnetotellurics

Abstract

S U M M A R YBroad-band and long period magnetotelluric (MT) data were acquired at 39 stations along five NNW-SSE profiles crossing the Iapetus Suture Zone (ISZ) in Ireland. Regional strike analyses indicate that the vast majority of the MT data is consistent with an assumption of a 2-D geoelectric strike direction. Strike is N52 • E for the three easternmost profiles and N75 • E for the two westernmost profiles; these directions correlate well with the observed predominant geological strike of the study region. 2-D inversions of the galvanic distortion-corrected TE and TM mode data from each profile are shown and discussed. As mapped geological variations between the neighbouring profiles suggest a heterogeneous subsurface, it is important to verify the robustness of the presence and geometries of prominent conductivity anomalies by employing 3-D forward and inverse modelling. A high conductivity layer (resistivity of 1-10 m), found at middle to lower crustal depths and presumed to be indicative of metamorphosed graphitic sediments rich in sulphides deposited during the convergence of the Laurentian and Avalonian continents, essentially constitutes the electrical signature of the ISZ. Shallow conductors observed are probably due to black shales that were widely deposited within the sedimentary accretionary wedge during Ordovician time. We interpret the moderately low resistivity at shallow depths from west to east across Ireland as indicative of an increase in maturity of the black shales in the easterly direction. From our conductivity models the southern extent of the ISZ is inferred to lie between the Navan Silvermines Fault and the Navan Tipperary Line, and shows clear resistivity contrast along all the profiles at the southern MT stations. The change in resistivity deduced from the 2-D models is spatially related to the composition of Lower Palaeozoic Ordovician, Silurian, Devonian and Carboniferous rocks. At upper mantle depths of about 60 km, a high conductivity block below the central MT stations is found to lie within the accretionary wedge of the Iapetus suture, and the location of the conductive anomaly corroborates reasonably well with the inferred spreading head of the putative Iceland plume-related magmatic intrusion. The low resistivity upper crust beneath the ISZ is indeed rich in Ordovician rocks with black shale content in the eastern as well as the central part; the western part is largely underlain by a highly resistive block of volcanic and metamorphosed rocks forming crystalline basement.

The advantages of complementing MT profiles in 3-D environments with geomagnetic transfer function and interstation horizontal magnetic transfer function data: results from a synthetic case study

“…Interstation horizontal magnetic transfer function (H) and the geomagnetic transfer function (T) data, which are not affected by electric field effects caused by galvanic distortion from charges on conductivity boundaries or gradients on small-scale structures, have been used to improve MT results and constrain the subsurface Meqbel et al 2014, Z, T and Z + T of field data, Rao et al 2014, Z and Z + T of field data; Yang et al 2015, Z and Z + T of field data; Varentsov 2015a,b, performing 2-D, 2-D+ and quasi 3-D inversion using Z, T and H with theoretical and field data), but no-one has yet undertaken an examination of the advantages of including H and T, simultaneously, when performing 3-D inversion of Z data (Z + H + T in 3-D). It should be noted that the magnetic effects caused by galvanic distortion charges do affect Z, H and T (e.g.…”

Integrating Electromagnetic Data with Other Geophysical Observations for Enhanced Imaging of the Earth: A Tutorial and Review

“…on average the observations are fitted within the estimated error. In practice however, real data can rarely be fit to that level (e.g., Rao et al 2014;Peacock et al 2015;Yang et al 2015). There can be a variety of reasons for this including overly optimistic error estimates or the inability to include sufficiently small surface structures in the inversion.…”

Two-Dimensional Magnetotelluric Modelling of Ore Deposits: Improvements in Model Constraints by Inclusion of Borehole Measurements

“…If vertical electrical and/or magnetic transfer functions are the only available borehole data, the procedure is condensed to a two-step procedure where the first step is the same as in the three-step procedure. Note that similar stepwise inversion strategies are typically applied in the inversion of surface impedance and VMTF data (e.g., Weckmann et al 2007;Rao et al 2014). When the number of complementary data sets entering into the inversion is increased, such as in our three-step procedure, it can be expected that the acceptable model space is reduced and the model becomes better constrained by the data.…”