ALBERT

Notes: Total field magnetic anomaly maps can be reduced to the pole by wavenumber filtering operations which allow both for the orientation of the Earth's field vector and for the direction of magnetization of the bodies producing the anomalies. If the same bodies have a density contrast relative to the surrounding rock, the corresponding gravity field can be converted into a pseudo-magnetic map by another wavenumber filter. Cross-spectral analysis between this and the magnetic map at the pole yields a response function and coherence which depend on the assumptions made about the direction of magnetization. Tests with synthetic data show that the correct direction is the one which maximizes the coherence and at the same time generates zero imaginary response. the corresponding value of the real response gives the correct ratio of magnetization to density contrast for the body. the direction of the total magnetization vector can be combined with the measured directions of remanent magnetization of rocks of different ages from the region to isolate the remanent component and estimate its age, hence ‘dating’ concealed magnetized rocks in the crust. the benefits and difficulties of the method are illustrated by applying it to gravity and magnetic maps of the English Lake District.

Notes: 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.

Notes: Abstract. The specificity of the major protease secreted by Aeromonas salmonicida has been explored using a number of proteins and p-nitroanilides as substrates. The 70kDa protease was found to hydrolyse two p-nitroanilides which have been reported to be specific substrates for thrombin. Kinetic parameters (kcat, and Km) were compared for the 70kDa protease and for thrombin as were the effects of a number of inhibitors. The 70kDa protease is able to degrade proteins which have a relatively open structure, for example, caseins or denatured bovine scrum albumin, to small fragments mostly of Mr〈2500. However, proteins with a more compact structure are more resistant to the protease. It was concluded that the 70kDa protease shows some of the specificity features of thrombin, although it is less discriminating in its choice of both low and high Mr substrates than thrombin. In preliminary experiments, the 70 kDa protease was found, like thrombin, to decrease the clotting time of rainbow trout blood. The possible physiological significance of these results is discussed.

Notes: A pre-requisite for more precise estimates of the electromagnetic response of the Earth is a better knowledge of the origin and spatial structure of geomagnetic time variations with periods of a few hours to several years. The period range from four hours to four days is particularly crucial for constraining upper mantle conductivity models, but is difficult to analyse because of overlapping contributions from current systems with different spatial scales.A period band centred on two days has been isolated from two years of hourly mean values recorded at up to 130 observatories. Plots of the coherence between pairs of records of the same component of the field at different observatories, as a function of the station separation, prove to be a valuable diagnostic tool. They show that two groups of sources contribute to this part of the spectrum. One group has a spatial scale in excess of 10 000 km, dominates the geomagnetic north (Xm) component, but produces no east (Ym) component. It can be satisfactorily modelled by a P01 spherical harmonic, and must be generated by fluctuations in the symmetric magnetospheric ring current. The spatial coherence of the second group falls to zero in a distance of 5000–7000 km, and can be predicted by a stochastic source model for which the autocovariance scale length is 7000 km.The spatial structure of the two groups is further investigated using the principal component technique of Egbert & Booker (1989). The spectrum is dominated by the largest eigenvalue, and the eigenfunctions corresponding to this mode show the expected P01 source behaviour. The modes which correspond to the next largest eigenvalues are organized into foci approximately 10 000 km across. The vertical fields associated with these modes are of low amplitude at mid-latitudes, but increase steeply beyond 50d̀. Estimates of the P01 response based upon the horizontal and vertical fields recorded at a single station will be biased by the contribution from the higher order modes. The magnetic observatory network is incapable of describing them properly on a global basis, and only the European subarray is sufficiently dense to form locally valid response estimates.