Interpretation of geophysical observations between the Orkney and Shetland Islands

Bott, M. H. P.; Browitt, C. W. A.

doi:10.1144/gsjgs.131.4.0353

Cited by 21 publications

(6 citation statements)

References 19 publications

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“…Time-term analysis has been a key factor in interpreting the data, the Shetland and Orkney stations yielding P, and P* time terms and stations on the Scottish mainland yielding the P,, time terms with excellent internal consistency. P, time terms were only observed for the easternmost six shots, but upper crustal structure along the line is known from gravity and short refraction lines (Bott & Watts 1970;Browitt 1972;Bott & Browitt 1975). It is convenient to discuss the time terms ( Fig.…”

Section: Interpretation Of Line B (Shelf Section)mentioning

confidence: 99%

“…The present project is the first major crustal seismic investigation of the North Scottish shelf, although some short refraction lines have been shot (Browitt 1972;Bott & Browitt 1975) and are used for local control in the present work. Nearly all the shots other than those of line E (Moray Firth) were fired on the Caledonian foreland, with a few shots to the east of the boundary with the Caledonian belt but probably over foreland crust at depth.…”

Section: Introductionmentioning

confidence: 99%

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Structure of the Crust Beneath the Caledonian Foreland and Caledonian Belt of the North Scottish Shelf Region

Smith

Bott

1975

Geophysical Journal International

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Summary This paper presents those results from the sea‐to‐land North Atlantic Seismic Project of 1972 which apply to the North Scottish shelf region. Time‐term analysis based on first arrivals yielded Pg of 6.10 ± 0.15 km s−1P* of 6.48 ± 0.06 km s−1 and Pn of 7.99 ± 0.02 km s−1. The presence of a P* refractor yielding first arrivals distinguishes the northern British crust from that south of the Caledonian belt. Beneath the Caledonian foreland, the P* refractor varies in estimated depth between 2 and 16 km and its depth correlates with the Bouguer anomalies; it most closely approaches the surface beneath a 90 mgal Bouguer anomaly high west of the Shetland Islands, and is interpreted in terms of widespread occurrence of Lewisian granulites beneath the region. The Moho beneath the foreland occurs at a remarkably uniform estimated depth of 26 ± 2 km. The P* refractor has been identified beneath the western part of the Caledonian fold belt and beneath the Moray Firth, and may occur more extensively. The crust beneath the fold belt is of the order of 20 per cent thicker than beneath the foreland, except beneath Moray Firth where the crustal thinning to the present estimate of about 23 km may have occurred during the post‐Caledonian formation of the Moray Firth basin.

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Section: Interpretation Of Line B (Shelf Section)mentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Structure of the Crust Beneath the Caledonian Foreland and Caledonian Belt of the North Scottish Shelf Region

Smith

Bott

1975

Geophysical Journal International

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“…The WBF has been continued offshore south of Shetland on the basis of a series of colinear bathymetric depressions [Flinn, 1961], magnetic [Flinn, 1969], gravity, and sparker reflection data [Bott and Browitt, 1975], and mapping from proprietary industry reflection data [British Geological Survey (BGS), 1984;; GECO Exploration Services/Alastair Beach Associates (GECO), 1989] to a point just southeast of Orkney (Figure 1). At this point, the fault is laterally displaced by a distance of-•35 km east of the northward projection of the Great Glen fault.…”

Section: Regional Geologic Settingmentioning

confidence: 87%

Structure of a continental strike‐slip fault from deep seismic reflection: Walls Boundary fault, northern British Caledonides

McBride¹

1994

J. Geophys. Res.

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Reprocessing of four offshore deep reflection profiles across the Walls Boundary strike‐slip fault, postulated as the northward continuation of the Great Glen fault over the Shetland platform, provides useful constraints on the lithospheric structure of a major continental transform and some insight into the manner in which strike‐slip faults deform. The reprocessing was aimed at delineating complex structure and reducing noise contamination and consisted of analyses of diffractions and amplitude variations and experimentation with seismic migration. The results demonstrate a highly variable crustal and uppermost mantle structure for the Walls Boundary fault along more than 150 km of its length. Six key observations can be made from this study: (1) as middle and lower crustal reflections approach the fault, they are truncated by, and do not continue across, the downward projection of the fault; (2) trends in reflection structure of the crust and/or uppermost mantle differ markedly from one side of the fault to the other; (3) diffractions typically are concentrated at the level of the Moho discontinuity directly beneath the fault; (4) after seismic migration, steeply dipping reflectors are imaged in places directly beneath the fault in the lower crust; (5) in one location, the Moho reflection is deflected across the fault into a narrow synform or “keel” structure which is exactly bisected by the downward projection of the fault; and (6) in a separate location, a prominent upper mantle reflector is truncated across the fault. A vertical or steeply dipping structure is deduced for the fault in much of the crust and perhaps parts of the uppermost mantle; however, more diverse reflecting structures are interpreted for the lower crust, such as the Moho deflection and steeply dipping reflectors directly beneath the fault that may have been related to a localized component of transpression and resultant crustal thickening. The presence of diffractions at the level of the Moho discontinuity where the downward projection of the fault intersects it suggests points of structural “roughness” that may be related to deformation of the Moho by late motion along the fault. Integrating the interpretation of the reflection data with previous geological studies implies that the age of the Moho deflection is post Early Cretaceous but that the dipping structure adjacent to and cut off by the fault is probably Caledonian (Silurian‐Early Devonian). Although the disparate structure of the crust on opposite sides of the fault supports geologic interpretations of large‐scale displacement, a major step in the Moho discontinuity directly beneath the Walls Boundary fault cannot be substantiated from the seismic data.

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“…On the other hand, the Permo-Trias is commonly developed as coarse, often conglomeratic sands, but is confined to faulted basins such as the Fitful basin (Bott and Browitt, 1975) Graben systems, and it seems unlikely that it was ever widespread over the Platform. The mineralogy of these sands on and around the Platform (Table 3 ) is extremely variable, which is probably largely a function of their derivation from local sources.…”

Section: Sand Provenancementioning

confidence: 99%

The Provenance and Distribution of the Palaeocene Sands of the Central North Sea

Morton

1979

Journal of Petroleum Geology

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A study of the detrital heavy minerals from Palaeocene sandstones of the central North Sea basins has revealed that variations exist which cannot be explained in terms of varying conditions either during or after deposition, and therefore reflect the influence of distinct source areas. Four sand bodies have been defined on a mineralogical basis, two of which occur in the Central Graben and southern Viking Graben, derived from the E. Shetland Platform, the other two being developed in the Moray Firth area, derived from the Scottish landmass. These sand bodies, when related to the existing lithostratigraphic framework, define four phases of basin subsidence, with the sand depocentre alternating between the Central Graben and the Moray Firth basin. This oscillation in basin subsidence, the association with volcanogenic sediments and the apparent basement control on distribution of sands in the Moray Firth indicate a degree of tectonic control hitherto undescribed for the Palaeocene of the area. Studies of sandstones in or around the inferred source areas have revealed that the metamorphic basement was the major contributor to the sands of the Moray Firth, rather than the pre‐existing sandstones of Devonian, Permo‐Trias or Jurassic ages which occur on the margins of the Firth. Likewise the Old Red Sandstone or Permo‐Trias sandstones of the East Shetland Platform are not suitable source rocks for the sands of the Central Graben and in this case it has been necessary to postulate the existence, at least until the end of Palaeocene times, of Carboniferous sands on the Platform. Such sands in the central North Sea have been shown to possess a mineralogy not dissimilar from that of the Palaeocene sands in question.

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Interpretation of geophysical observations between the Orkney and Shetland Islands

Cited by 21 publications

References 19 publications

Structure of the Crust Beneath the Caledonian Foreland and Caledonian Belt of the North Scottish Shelf Region

Structure of the Crust Beneath the Caledonian Foreland and Caledonian Belt of the North Scottish Shelf Region

Structure of a continental strike‐slip fault from deep seismic reflection: Walls Boundary fault, northern British Caledonides

The Provenance and Distribution of the Palaeocene Sands of the Central North Sea

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