D. H. Matthews

Crustal structure across the Caledonides from the ‘WINCH’ seismic reflection profile: influences on the evolution of the Midland Valley of Scotland

The Western Isles–North Channel (‘WINCH’) deep seismic reflection profile runs through the North Channel across the extension of the Midland Valley into the Firth of Clyde. A variety of shallow Carboniferous and younger sedimentary basins dominates the upper crustal structure. The deep structure includes a reflective lower crust, bounded downwards by the Moho, and dipping ‘thrusts’. No margins to the Midland Valley are observed equivalent to the bounding faults on land. Thrusts below both the Highlands and the Southern Uplands have a variety of dip directions indicative of zig-zag crustal shortening of 100 km in the Highlands and 60 km in the Southern Uplands. The Moho varies only a little in depth, despite the gross crustal deformation in the Caledonides. Isostatic recovery of the Moho on unlocking of collided crustal blocks may explain the formation of some sedimentary basins.

Extensional structures on the western UK continental shelf: a review of evidence from deep seismic profiling

Summary BIRPS deep seismic reflection data collected on the western United Kingdom continental shelf show the existence of a wide variety of sedimentary basins, most of which originated during Palaeozoic-Mesozoic crustal extension. Symmetrical interior-fracture basins are numerous, but more complex basins are also common and show the importance of fault reactivation and the influence of pre-existing structures on basin development. All of the major basement-penetrating faults are interpreted as having been reactivated. They do not visibly cut through the entire crust and into the upper mantle. The crystalline crust thins dramatically beneath the basins indicating local crustal extensions of up to 60%, although regional extension is less than 30%. The typical BIRPS profile shows a highly reflective lower crust sandwiched between an unreflective upper crust and upper mantle. This pattern of reflectivity appears to be characteristic of deep seismic data collected from within extended regions. The relatively small amount of extension which has affected this region suggests that the highly reflective lower crust is more likely to be due to lithological variation formed by mafic igneous intrusion and underplating during crustal extension than to-extensional ductile fabrics within the lower crust.

Seismic reflections from the lower crust around Britain

Summary A cartoon is presented which represents a time-section typical of the first 3000 km of BIRPS profiling over the continental shelf north, west and south of Britain. It shows a Mesozoic sedimentary basin in the hanging wall of a low-angle normal fault zone. The dipping fault reflection merges with layering in the lower crust which appears as several strong but short (c. 4 km) horizontal reflections between about 6 s two-way travel time and the base of the crust at about 10 s. The uppermost mantle is devoid of reflections except where dipping events extend from the layered lower crust towards the base of the section at 15 s two-way time. The cartoon is justified by reference to examples from profiles. Discussion states present hypotheses that are being tested. The layering in the lower crust is real and due to lenses of flat-lying foliation about 4 km across and tens of metres in thickness in which the seismic velocity has been reduced by the presence of fluids. Brine has been suggested to account for the electrical conductivity of the lower crust, and carbon dioxide may be associated with granulites. The geometry implies that the lower crust was ductile at the time of faulting, and that the foliation became horizontal during Mesozoic stretching of West Europe.

Iapetus suture located beneath the North Sea by BIRPS deep seismic reflection profiling

The Iapetus suture, one of the more fundamental geologic boundaries of the British Isles, has been located beneath the North Sea by deep seismic reflection profiling carried out by the British Institutions Reflection Profiling Syndicate (BIRPS). Reflection profiling and magnetotelluric sounding have previously been used to infer the position of the Iapetus suture beneath the Irish Sea and the Southern Uplands, but the present study provides the first direct evidence for the location of the Iapetus suture east of Britain. A comparison of the seismic reflection profiles over the Iapetus suture east and west of Britain shows variations in the dip of the suture, which steepens to the northeast beneath the North Sea. The comparison also reveals unexpected changes in lower crustal reflectivity along the strike of the suture and suggests that there may be no straightforward relation in this area between the crustal reflectivity and the crustal age.

A discussion concerning the floor of the northwest Indian Ocean - The Owen fracture zone and the northern end of the Carlsberg Ridge

Examination of the shape of the earthquake epicentre belt near Socotra led to the suggestion that a major fracture displaces the axis of the mid-ocean ridge in that area (Matthews 1963). Subsequent surveys have confirmed the existence of a fracture zone which extends 1500 miles from the coast near Karachi southwestwards to the middle of the Somali Basin. Linear ridges and troughs in the zone are associated with negative gravity anomalies but not with magnetic anomalies. Where the fracture zone crosses the line of the Carlsberg Ridge a sinuous trough is developed: south of this feature a characteristic pattern of magnetic anomalies is associated with the volcanic structures of the mid-ocean ridge, north of it a line of large non-magnetic seamounts has been found. It is concluded that the structure underlying the Owen fracture zone is a system of parallel transcurrent faults affecting the ocean floor only, at which the axis of the mid-ocean ridge suffers a net right lateral displacement of 170 mi.

Three non-magnetic seamounts off the Iberian coast

The seamounts of the deep oceans are strongly magnetized and are made up of basic volcanic rocks with or without a capping of limestone. Some seamounts of the continental margins differ from these by being weakly magnetized. Three such seamounts are described. Magnetic surveys over them strongly suggest that they do not have cores of basaltic rocks. There is good evidence that pellet limestones are exposed in the flanks of one of them (Vigo Seamount). These rocks resemble pellet limestones of Jurassic age which occur in Portugal. The results of dredging and photography on the top of Vigo Seamount and on Galicia Bank suggest that detrital limestones, porcellanous limestones, and soft foraminiferal limestones, of Cretaceous to Eocene age, are exposed there. The facies of these calcareous rocks resembles that of the Mesozoic limestones of Portugal and southern France. The remainder of the dredged rocks are believed to be ice-rafted erratics from farther north in Europe or America. The results of seismic refraction work on Galicia Bank are consistent with a structure in which 0.4 km of uncemented limestones overlie 4.2 km of hard limestones that rest on a crystalline basement. The mode of origin tentatively suggested for the seamounts involves the formation of limestones during Mesozoic time on a gradually subsiding platform of continental rocks. As there is no substantial gravity anomaly, subsidence must have been accompanied by density changes or by removal of the lower part of the continental crust near the present position of the continental margin.

GEOPHYSICAL STUDIES OF THE SEYCHELLES BANK

Geological studies of the islands on the Seychelles Bank, and the results of seismic refraction experiments made on the bank, are reviewed. They show that the crust is of continental type under the centre of the bank. Gravity measurements confirm that the thick crust extends to the northern edge of the bank and show that the Mohorovicic discontinuity slopes upward at an angle of about 19° under the peripheral cliff. Large narrow magnetic anomalies occurring in the central area of the bank are ascribed to minor intrusions of dolerite found in the Precambrian granites, and it is suggested that the edge of this area may mark the limit of the granite mass. Magnetic anomaly profiles of the Mascarene Ridge are similar to those over the Seychelles Bank and could result from a similar structure.

Altered Basalts from Swallow Bank, an Abyssal Hill in the NE Atlantic, and from a Nearby Seamount

Three hundred and fifty-seven specimens dredged from Swallow Bank in 1958 are vesicular hypocrystalline porphyritic basalts. Chemical, thin section and X-ray studies were made. The least altered specimens gave an analysis close to the tholeiitic ocean floor basalt type. The glassy selvage of pillows altered sequentially to palagonite, fibro-palagonite and montmorillonite. Within the flows the mesostasis and pyroxene phenocrysts have been replaced by chlorophaeite, fibro-chlorophaeite and obscure chlorites, the calcic cores of plagioclase phenocrysts by a mesh of orthoclase plates and their sodic rims by (?) montmorillonite. Similar, but more extensive, mineralogical changes have affected the lavas from the seamount. Chemically, at Swallow Bank, the alteration of the flow interior involved increases in K2O (from 1.0 to 3.5g/100g), in H2O + (from 1.2 to 5.2 %) and in oxidation ratio, Fe2O3:Fe20 3 + FeO (from 71 to 98%) with concomitant loss in CaO (from 11.1 to 2.3 %) and MgO (from 4.7 to 1.8%). In these rocks radioactivity increased, seismic velocity decreased and intensity of magnetization remained substantially unchanged. Similar altered rocks could be widespread in layer 2 beneath the ocean.

Aspects of the Geology of the Scotia Arc

The Scotia arc, originally called the Southern Antilles, is the elongated loop of islands and shallow banks that extends 1,500 miles eastward in the south Atlantic from Tierra del Fuego and Graham Land peninsula. The known geology of the islands is reviewed, and a comparative stratigraphic table compiled. The presence of thick geosynclinal deposits and granite intrusions near South Georgia and a highly metamorphosed basement complex in the South Orkneys are among the features that distinguish the islands from other oceanic islands. It is concluded that they probably represent areas with a continental-type crustal structure.

Progress in BIRPS deep Seismic reflection profiling around the British Isles

Abstract The results of 4000 km of new BIRPS profiles are briefly outlined as an introduction to other BIRPS papers in this volume. Highlights of these profiles include the tracing for 900 km of a reflection pattern in the lower crust associated with the lapetus suture and the observation of more than 20 separate reflections from the mantle, while progress in solving the problem of deep-water multiples has enabled the joining of the continental and oceanic Moho reflections off SW England. Developments in old interpretational problems are reviewed and the new problem of dating reflections is mentioned.