CAUSES OF HIGH CRUSTAL CONDUCTIVITY BENEATH THE IAPETUS SUTURE ZONE IN GREAT-BRITAIN

Two magnetic variation anomalies in southern Scotland and northern Eng land 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 observ ations. Groom-Bailey decomposition of the impedance and hypothetical e vent analysis of the magnetic variation data agree in defining differe nt strike directions for the two halves of the MT profile: N50 degrees E for the northern part over the Southern Uplands, and N90 degrees E for the southern part over the Northumberland Trough. TE and TM mode a pparent resistivity and phase data at representative frequencies have been inverted for the two sub-profiles separately using the smooth inv erse of Smith & Booker (1990). The uppermost layer has a low resistivi ty, is variable in thickness, and correlates well with the known posit ion and thickness of Carboniferous sedimentary rocks. The second layer has a very high resistivity (thousands of Omega m), and reaches the s urface where the Lower Palaeozoic metamorphic rocks of the Southern Up lands crop out. A relatively rapid transition to low resistivities occ urs at depths of between 8 and 16 km. The conducting 'layer' appears t o be quasi-continuous, but where the profile crosses the anomalies ide ntified by magnetic variation measurements, the conductance increases, and the upper surface is shallower. The spatial coverage of the magne tic variation data has enabled us to extrapolate conductive features a way from the line of section and project the electrical image onto the NEC vertical-incidence seismic reflection profile in the North Sea. T here is excellent agreement between a number of features in the acoust ical and electrical images. The shallowing of the low-resistivity laye r to form a narrow wedge-like feature corresponds to the offshore posi tion 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 t he Northumberland Trough spans two zones which were differentiated on the NEC profile in terms of their reflectivity. The shape of the condu ctor's upper surface in the vicinity of the Stublick fault agrees well with the model of Chadwick & Holliday (1991), who proposed that the I apetus 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) t han the presence of fluids. There is also strong support for interpret ing the northern conductor in the same way. Its upper surface is relat ively flat and occupies the position predicted as the horizontal detac hment surface, over which wedges of the Southern Uplands rocks were th rust. Its northeastward extension is sampled by a group of Carbonifero us vents from which xenolith suites of crust and upper mantle origin h ave been obtained. The middle to upper crust is believed to be represe nted by quartzo-feldspathic gneisses that contain abundant graphite.