No description available.

No description available.

No description available.

No description available.

No description available.

Detailed mapping of the rocks to the north and south of Plymouth reveals a sedimentary sequence deformed by a series of folds and thrusts. Two structural zones exist each with a different style of deformation. In the Lower to Upper Devonian rocks of the southern zone, slip vectors, vergence and facing of D 1 folds indicate that the transport direction is to the north west. The northern zone, comprising Upper Devonian and lower Carboniferous strata, extends into central Devon and east Cornw~ll and the geometry and facing of the early folds in these rocks indicate a transport direction to the south or south east. Dl folds generally verge north but are downward facing towards the south. The two structural zones confront each other at an E-W trending line which passes through Cargreen to the north of Plymouth. The confrontation is interpreted as a northerly dipping backthrust produced by underthrusting of the Carboniferous foreland basin flysch deposits which become inverted and backthrust towards the south. The above sequence of events is dependent on a thin-skinned tectonic model and can be incorporated into the well established Early Carboniferous plate tectonic setting. Parameters indicating strike slip movement can also be incorporated within the envisaged thrust regime. These are thought to have been generated by differential movement related to the shape of the Variscan Front. The oblique trend of the Variscan Front to the regional transport direction observed in Southern Ireland, Wales and England, where it trends WNWESE, is thought to be a function of lateral buttressing against the Irish and LondonlBrabant Massifs during NW transport of thrust sheets. Re-mapping in the Plymouth area has also led to modifications of the Devonian stratigraphy and sedimentology of the Lower Devonian. The previously named Dartmouth Beds/Slates have been re-named the Dartmouth Group and comprise the Renney Rocks, Wembury, Yealm and Warren Formations. The latter pass upwards, by interdigitation, into the Meadfoot Group which in turn consists of the Bovisand and Staddon Grit Formations. Overlying the Meadfoot Group is the Plymouth Group which is divided into the Jennycliff Slate Formation, Plymouth Limestone Formation, Compton Slate Formation and the Saltram Slate Formation. The division of the previously named Plympton Formation into the two latter formations has allowed more control on structural mapping in the area north of Plymouth. The recognition of repeated stratigraphy and its further subdivision indicates that, due to thrusting, the sequences are much thinner than previously thought. The sedimentological character of the Dartmouth Group implies deposition in an alluvial setting and sub-environments include channels, sheetflow, overbank flow and lake deposition. Debris flows indicate that the area was periodically unstable. It is envisaged that deposition took place on a wet alluvial plain with rare drying out. The deposits probably represent a distal setting to the more proximal Old Red Sandstone alluvial deposits of South Wales. Higher up the sequence the Meadfoot Group records a major marine transgression across the area (Bovisand Formation) with a minor regressive pulse represented by the Staddon Grit Formation. The boundary conditions of these two formations has been examined and the sedimentology of this part of the sequence indicates the presence of a series of offshore bar forms. They are thought to represent mouth bar sequences related to the overlying deltaic sequence of the Staddon Grit Formation. Analysis of the onshore New Red Sandstone indicates the presence of a topographic high in the Start area. This local palaeogeographic detail can be related to the regional offshore Permian Basin form.

No description available.

The field determinations of crustal electrical conductivity/depth profiles show anomalously high conductivities in the lower crust. This has yet to be explained by a consistent theory and interpretation of field results is difficult due to the lack of laboratory conductivity measurements of saturated rocks at lower crustal temperatures. A cell was designed to measure the electrical conductivity of saturated rocks up to the lower crustal conditions of; confining pressures of 1 GPa, pore-fluid pressures of 1 GPa and temperatures of 900°C. This complex exercise required the use of a metal sleeve and the use of guard-ring techniques to remove leakage currents induced by the sleeve. The development of the cell involved several years work and is a breakthrough in measurement techniques as it has enabled the measurement of saturated rock conductivities at lower crustal temperatures and high pressures for the first time. The conductivity of 14 samples of acidic and metabasic rocks was measured at a variety of confining pressures (<0.2 GPa), porefluid pressures (<0.2 GPa), temperatures (<900°C) and saturation fluids. The pressure variations showed that the basic rocks had a conductivity too high to be explained by conduction through saturating electrolyte alone. The temperature variation showed a dramatic difference between acidic and basic rocks. The acidic rocks showed large decreases in conductivity above 350°C after initial rapid increases in conductivity. The basic rocks showed no such reduction in conductivity indicating that a conduction mechanism in addition to pore-fluid conduction was present. Results indicate that saturated -rocks at high temperatures have 7/ conductivities sufficient to explain the high conductivity layers in the upper lower crust whereas acidic rocks do not. The results also demonstrate that graphite may be an important additional conduction mechanism possibly accounting for the moderately high conductivities in the lower lower crust even in the absence of electrolyte conduction

No description available.

To date, most shear velocity heterogeneity models in the lower mantle have been derived using long period data. Comparatively little use has been made of the vast ISC database of shear wave arrival times, which covers the years 1964 to 1991. The aim of this study is to use this database to construct global models of shear wave heterogeneity in the lower mantle and compare it with existing P models using similar period data in order to evaluate the hypothesis that P and S heterogeneity are proportional. In order to reproduce the resolution operator inherent in these existing models, the same parameterization has been employed; lateral dependence is in terms of spherical harmonics up to degree and order 6, and radial dependence in terms of a quartic polynomial. The inversion uses data from over 4500 events selected according to criteria which minimize the bias introduced by uneven source receiver coverage. Despite having approximately one sixth of the amount of data the model correlates well with a P wave model computed using data from the same events, and this in turn is almost identical to models computed by other authors using similar inversion methods. Assuming proportionality of P and S heterogeneity, a joint inversion of the P and S data has been performed. The resultant model correlates extremely well with the P model of this study, and similar variance reductions are obtained for both the P and S datasets as were found in the original inversions. Several model parameterizations have been used to try to investigate the dependence of the model expansion on the results. Consequently, taking into account results from synthetic experiments and from experiments into trade-offs with other inversion parameters, the average ratio of relative S to P heterogeneity ^{d ln v_s}andfrasl;_{d ln v_p} throughout the lower mantle is constrained to be in the interval (1.8,2.1) with emphasis on higher values within these bounds. This value is in agreement with other studies using similar period data, and the upper bound is close to the optimum value of 2.27 obtained by Li et al., 1991 from normal mode data, suggesting that the ratio is at least only weakly dependent on frequency. In addition evidence is presented that the ratio is increasing with depth although attempts to extract specific gradients have failed because of the very poor distribution of S rays in the deepest mantle.