Causes of high crustal conductivity beneath the Iapetus suture zone in Great Britain

Abstract

SUMMARY Two magnetic variation anomalies in southern Scotland and northern England have been linked to the position of the Iapetus suture. Previous magnetotelluric measurements along a 140 km profile that crosses both anomalies have been supplemented by new high-quality broad-band observations. Groom-Bailey decomposition of the impedance and hypothetical event analysis of the magnetic variation data agree in defining different strike directions for the two halves of the MT profile: N50°E for the northern part over the Southern Uplands, and N90°E for the southern part over the Northumberland Trough. TE and TM mode apparent resistivity and phase data at representative frequencies have been inverted for the two sub-profiles separately using the smooth inverse of Smith & Booker (1990). The uppermost layer has a low resistivity, is variable in thickness, and correlates well with the known position and thickness of Carboniferous sedimentary rocks. The second layer has a very high resistivity (thousands of Ω m), and reaches the surface where the Lower Palaeozoic metamorphic rocks of the Southern Uplands crop out. A relatively rapid transition to low resistivities occurs at depths of between 8 and 16 km. The conducting ‘layer’ appears to be quasi-continuous, but where the profile crosses the anomalies identified by magnetic variation measurements, the conductance increases, and the upper surface is shallower. The spatial coverage of the magnetic variation data has enabled us to extrapolate conductive features away from the line of section and project the electrical image onto the NEC vertical-incidence seismic reflection profile in the North Sea. There is excellent agreement between a number of features in the acoustical and electrical images. The shallowing of the low-resistivity layer to form a narrow wedge-like feature corresponds to the offshore position of the Stublick fault, while, to the north of the fault, the top of the layer coincides with a south-dipping reflector thought to be a thrust. However, the zones of high conductance and high lower-crustal reflectivity do not in general correlate. The good conductor beneath (he Northumberland Trough spans two zones which were differentiated on ihe NEC profile in terms of their reflectivity. The shape of the conductor's upper surface in the vicinity of the Stublick fault agrees well with the model of Chadwick & Holliday (1991), who proposed that the Iapetus suture was a whole-crustal shear with a gently dipping central ramp. The coincidence between the (present-day) low-resistivity layer and a surface of weakness that was active 300–400 Myr ago is much more readily explained in terms of mineralogy (the presence of graphite) than the presence of fluids. There is also strong support for interpreting the northern conductor in the same way. Its upper surface is relatively flat and occupies the position predicted as the horizontal detachment surface, over which wedges of the Southern Uplands rocks were thrust. Its northeastward extension is sampled by a group of Carboniferous vents from which xenolith suites of crust and upper mantle origin have been obtained. The middle to upper crust is believed to be represented by quartzo-feldspathic gneisses that contain abundant graphite.