Hypotheses to explain the origin and location of Upper Palaeozoic sedimentary basins are presented. Group A basins were those Old Red Sandstone basins whose location and subsidence were controlled by Caledonide features, including active and remnant fore-arc basins (Midland Valley) and major transcurrent movements (Orcadian basin). Newer Granite plutonic magmas in the Southern Uplands subduction complex post-dated subduction and resulted from post-collision cratonization processes as yet little understood. Group B basins were those whose initial positions in space were controlled by palaeo-plate factors, in particular the location of granitoid plutons, but whose subsidence was controlled by active Hercynian (neoplate) processes. Some Group A basins were regenerated by this process (Midland Valley). A new hypothesis is presented to account for the origin of a Carboniferous rift-to-sag trend similar to that proven in modern continental margin basins such as the North Sea. The hypothesis is an adaptation of McKenzie9s lithospheric stretching model, some of whose major features are well shown in the Carboniferous history of the north and central British Isles. Group B processes were controlled by Ligurian/Bretonic plate consumption in the S Brittany–Massif Central–Vosges areas. Group C basins in the Rheno-Hercynian zone also owe their location and origin to back-arc rifting and continental crustal ‘oceanization’ processes caused by Liguro-Bretonic subduction.

Upper Palaeozoic basins of the British Isles—Caledonide inheritance versus Hercynian plate margin processes

Shear wave polarization anisotropy in the wedge portion of the upper mantle between a subducting plate and the earth's surface is investigated using three-component seismograms of intermediate depth and deep earthquakes recorded at 14 local stations in Honshu, Japan Eighty nine high-quality seismograms were selected from a period of 3 years The data used in this study are restricted such that incidence angles are smaller than the critical angle of 30° to the earth's surface in order to avoid phase shifts in the shear wave train To find directions of the maximum and minimum velocities in split shear waves, where shear waves are resolved into two phases with the maximum time separation, each set of the two horizontal component seismograms is rotated in the horizontal plane The split shear waves thus obtained are again recombined after the correction of anisotropy, and the anisotropy-corrected particle motion is compared with the focal mechanism for a cross-check of the observed anisotropy Directions of the maximum axes are plotted on azimuthincidence angle stereograms at each station The stereograms and the cross sections of seismic ray paths show that (1) the anisotropic material is distributed at intermediate location between earthquake sources and receiving stations, and (2) the anisotropic region is separated into two parts: one in the north of the present study area with the polarization of the maximum velocity shear wave trending 0° to 30° from the north (north anisotropy) and the other in the south with it trending 90° to 120° (south anisotropy) The maximum time delays between the two shear waves along a vertical seismic ray is about l s for both the anisotropic regions The horizontal extent of the anisotropic area in the north is 50 km at depths of 50 to 150 km perhaps prevalent in west Honshu However, the depth of anisotropic material is not well constrained because of insufficient coverage of seismic ray paths and angles If we assume the vertical extent of the anisotropic material to be 100 km, the maximum velocity contrast would be 4% If we adopt a crack alignment model, for the observed anisotropy, cracks are inferred to be distributed vertically trending 0° to 30° in the north and 90° to 120° in the south If we assume an olivine alignment model, olivine crystals are inferred to be aligned in the north with the a axis of the slip direction trending 0° to 30° and the slip plane normal to the b axis being vertical and in the south anisotropy with the axis trending 90° to 120° and the slip plane being horizontal

Shear wave polarization anisotropy in the upper mantle beneath Honshu, Japan

Summary. This paper presents those results from the 1974 Lithospheric Seismic Profile in Britain (LISPB) which relate to the compressional velocity structure of the crust and uppermost mantle beneath Northern Britain. A combination of interpretation techniques suitable for modelling laterally inhomogeneous media, including two-dimensional ray-tracing and time-term analysis, has resulted in a detailed seismic cross-section across the Caledonian orogenic belt. The main features of this section are a possible horizontal discontinuity in the Pre-Caledonian’basement, a change in the relationship between the lower crust and the uppermost mantle from north to south and a considerable thickening of the crust beneath the Caledonian fold belt. These results place considerable constraints upon tectonic models for the evolution of the Caledonides in particular in their implication of differing crustal structures north and south of the Southern Uplands and their indication of the primary significance of the Southern Uplands Fault.

/pdf/lispb-iv-crustal-structure-of-northern-britain-4v379r0gtw.pdf

LISPB — IV. Crustal structure of Northern Britain

Summary. Earlier interpretations of Pn travel-times from the extensive quarry-blast observation scheme in western Germany — now supplemented by explosion data from the 1972 Rhinegraben experiment — have been checked and enhanced using the new MOZAIC time-term method. The large data set (762 travel times) continues to require a considerable anisotropy of upper-mantle P velocity. The resulting estimates of the overall velocity variation — probably 0.50–0.60 km/s about a mean value of 8.05 km/s, that is, 6 to 7 per cent anisotropy — and of the direction of the maximum velocity (close to 20° E of north) are reasonably reliable. However, the detailed form of the anisotropy is obscured by various limitations of the data.



These results allow a realistic assessment of the resolving power of refraction-based studies of velocity anisotropy in the lithosphere. It is concluded that though such studies are probably adequate if the measurement of in situ anisotropy is required within the context of a generalized discussion of lithospheric dynamics they are not appropriate if a detailed specification of the anisotropy is desired.

Pn velocity anisotropy in a continental upper mantle

Summary. At many localities in the world there have been coincident comprehensive electromagnetic (EM) studies and seismic reflection profiles conducted. Unfortunately, over many more regions the seismic reflection images are interpreted without the constraints afforded by electrical conductivity information. This paper is an attempt to convince the reader that a collocated magnetotelluric (MT) study should, in almost every case, be made wherever a seismic reflection survey is undertaken. Examples are shown from six studies in which the EM results aided the geological/tectonic interpretations of the seismic sections.



Also, difficulties with the MT technique are discussed, and the interpretations of conducting zones within the lower crust are examined. Finally, a generalised model is proposed for the continental crust that may account for both the reflectivity and conductivity of the zone at the top of the lower crust.

MT and reflection: an essential combination

Summary

The planning, execution and preliminary results of a major Anglo-German explosion seismic project are presented in this, paper I of a series. This Lithospheric Seismic Profile in Britain (LISPB) was planned as a reversed 1000 km line between two major sea-shot points off Cape Wrath in Scotland and one in the English Channel; additional sea-shots and intermediate land-shots were fired to give reversed and overlapping crustal coverage (to 180–400 km distance) along the line. In all, 29 shots were fired and 60 mobile magnetic tape stations recorded three-components of ground‘motion. The resulting 14 crustal and three long-range profiles have observations at intervals of typically 2–4 km. Recordings have been digitized and four examples of filtered, computer-plotted record sections are presented to illustrate data quality. In a preliminary analysis, phase correlations are discussed and some models presented; the latter especially are more relevant to future interpretations than to geological or tectonic problems. However, significant variations in crustal thickness and in the nature of the crust-mantle transition do seem to occur beneath the British Isles.

/pdf/a-lithospheric-seismic-profile-in-britain-i-preliminary-1mpemm1i4e.pdf

A Lithospheric Seismic Profile in Britain—I Preliminary Results*

Summary

The time-term approach to the interpretation of refraction travel-time data is extended to deal with observation systems which are not, at first sight, suitable for time-term analysis. This so-called MOZAIC method is based on the premise that there exist areas of equal delay time to any refractor and that the distribution of such areas can be intelligently estimated using ancillary information such as geological or gravity maps. It is then possible to construct, from an essentially heterogeneous observation scheme, a MOZAIC network to which more or less conventional time-term analysis can be applied.



The MOZAIC method is applied to Pn, data from the quarry-blast recording network in western Germany and is shown to offer several real advantages over the Delay-Time-Function Method. In particular the improved interpretation strengthens the hypothesis that the data requires a considerable anisotropy of upper mantle P wave velocity.



Future uses of the MOZAIC method are discussed.

/pdf/mozaic-time-term-analysis-1i9kwyq9t4.pdf

D. Bamford

Papers

A Lithospheric Seismic Profile in Britain—I Preliminary Results*

MOZAIC Time-Term Analysis