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The ocean–continent boundary off the western continental margin of Iberia—I. Crustal structure at 40°30'N (1990)

Whitmarsh, R. B. ; Miles, P. R. ; Mauffret, A.

Oxford, UK : Blackwell Publishing Ltd

Geophysical journal international 103 (1990), S. 0

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ISSN: 1365-246X

Source: Blackwell Publishing Journal Backfiles 1879-2005

Topics: Geosciences

Notes: The western continental margin of the Iberian peninsula has the characteristics of a rifted non-volcanic margin with half-graben and tilted fault blocks seen in several places. The ocean–continent boundary (OCB) is therefore expected to be where thinned continental crust and oceanic crust are juxtaposed, as at many similar margins worldwide. It is particularly useful to locate the OCB off western Iberia in order to constrain the pre-rift fit of North America to Iberia and, by implication, the shape of the proto-Bay of Biscay. The fit is only marginally constrained by sea-floor spreading magnetic anomalies because anomaly 34 is believed to be far to the west of the OCB and it is even possible that all older oceanic crust was created during the Cretaceous constant polarity interval. The best way to distinguish oceanic crust from thinned continental crust appears to be the crustal seismic velocity structure. Therefore in 1986 a series of seismic refraction lines was shot parallel to, and normal to, the continental margin. These lines enabled us to bracket the location of the OCB. A further constraint on the location was obtained by modelling an east–west magnetic profile which included the enigmatic J-anomaly. This anomaly can be explained as either just pre-anomaly MO or as part of the Cretaceous constant polarity interval, depending on whether spreading began about 127 or after 118 Myr ago, respectively. The evidence favours the former explanation. Lastly the depth to acoustic basement was contoured from a compilation of seismic reflection profiles. This indicated a new fracture zone at 41°15'N which offsets the OCB. A few key reflection profiles also suggest that the OCB can be identified by an abrupt landward step-down in acoustic basement. We conclude that the OCB in the eastern Iberia Abyssal Plain lies between 12°10’ and 12°30'W and has a trend just east of north. This westerly location is consistent with recent estimates of the location of the OCB off the Grand Banks but brings into question the proposed location at about 11°W of the OCB in the Tagus Abyssal Plain.

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1111/j.1365-246X.1990.tb01788.x

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The seismic velocity structure of some NE Atlantic continental rise sediments; a lithification index? (1990)

Whitmarsh, R. B. ; Miles, P. R. ; Pinheiro, L. M.

Oxford, UK : Blackwell Publishing Ltd

Geophysical journal international 101 (1990), S. 0

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ISSN: 1365-246X

Source: Blackwell Publishing Journal Backfiles 1879-2005

Topics: Geosciences

Notes: Two separate sets of experiments with digital ocean-bottom seismographs (DOBS) and airguns, on continental rise areas off Madeira and west of Portugal, produced en echelon second arrivals from the sediment layer on record sections. Traveltime and synthetic seismograrn modelling indicate that the arrivals represent multiply-reflected refracted phases which have undergone reflection within the sediment layer itself. Further, although the P-wave contrast at the intrasediment reflecting horizon is relatively small, the modelling indicates a large downward increase in S-wave velocity from 100–250 m s−1 (Poisson's ratio of at least 0.42) to about 1200 m s−1 (Poisson's ratio of about 0.25). A reflection event can usually be found on reflection profiles along the refraction lines at almost exactly the same ‘depth’ as the intrasediment reflector. In one case such an event can be traced to a nearby Deep Sea Drilling Project (DSDP) borehole where it is associated with the transition from ooze to chalk. This, and other circumstantial evidence, suggests that the intrasediment reflector marks an important increase in lithification within the sediment layer. If so it means that, in future, straightforward OBS experiments may be used to measure the depth of this increase without resorting to the drill.

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1111/j.1365-246X.1990.tb06575.x

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3

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The ocean–continent boundary off the western continental margin of Iberia—II. Crustal structure in the Tagus Abyssal Plain (1992)

Pinheiro, L. M. ; Whitmarsh, R. B. ; Miles, P. R.

Oxford, UK : Blackwell Publishing Ltd

Geophysical journal international 109 (1992), S. 0

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ISSN: 1365-246X

Source: Blackwell Publishing Journal Backfiles 1879-2005

Topics: Geosciences

Notes: An 80 km long reversed seismic refraction line (Line 5) was shot over the Tagus Abyssal Plain off Portugal. The main P-wave reflected and refracted phases were modelled both for traveltime and amplitude. The resulting P-wave velocity/depth model has the following features: (a) an extremely thin crust of about 2 km; (b) the absence of oceanic layer 3; and (c) very low upper mantle velocities between 7.6 and 7.9 km s-1. This very unusual seismic velocity crustal structure is quite unlike thinned continental crust but is remarkably similar to the seismic crustal structures found at Atlantic fracture zones, and in particular to the structures found in profiles shot along the transform valley and near ridge-transform intersections. A magnetic anomaly chart seems to allow the possibility of several fracture zones one of which could intersect the centre of Line 5.As an alternative to the fracture zone hypothesis we show that if the ocean–continent transition in the Tagus Abyssal Plain is located at about 11°30'W, in a symmetric position with respect to the ocean–continent transition in the conjugate South Newfoundland Basin, then magnetic anomalies can be modelled simply by assuming sea-floor spreading west of 11°45'W at 10 mm yr-1 beginning at M11 time (133 Myr BP), and blocks of rifted continental crust to the east. The location of the proposed ocean–continent transition in the Tagus Abyssal Plain is marked by a well-defined N–S linear magnetic anomaly which is adjacent to the oldest sea-floor spreading block. East of the proposed ocean–continent transition there is an increase in the depth to basement similar to that found east of the ocean–continent transition in the Iberia Abyssal Plain and elsewhere. This model also allows us to explain why Purdy's (1975) seismic refraction line A–AR in the Tagus Abyssal Plain cannot be interpreted as a conventional reversed pair because most of Line A was shot over the ocean–continent transition zone and most of Line AR over thinned continental crust.Remarkably similar velocity/depth structures to that under Line 5 are found close to the ocean–continent transition zone off the whole of western Iberia, in areas which show no clear evidence of fracture zones. Therefore it appears more likely that the seismic structure of Line 5 is due to its proximity to the ocean–continent transition than to a local association with a fracture zone and further, that its structure is typical of this transition off the western margin of Iberia. We also suspect that the low upper mantle velocities associated with the ocean–continent transition indicate the widespread occurrence of serpentinized peridotite.

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1111/j.1365-246X.1992.tb00082.x

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4

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The fault pattern of a slow-spreading ridge near a fracture zone (1975)

WHITMARSH, R. B. ; LAUGHTON, A. S.

[s.l.] : Nature Publishing Group

Nature 258 (1975), S. 509-510

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ISSN: 1476-4687

Source: Nature Archives 1869 - 2009

Topics: Biology , Chemistry and Pharmacology , Medicine , Natural Sciences in General , Physics

Notes: [Auszug] On the mosaic, numerous sub-parallel linear echoes, oriented at about 020, and therefore parallel to the median valley, can be followed continuously for tens of kilometres. Narrow beam echo-sounder profiles, roughly at right angles to GLORIA'S track, have established that these echoes come from ...

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1038/258509a0

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5

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A technique for conducting seismic refraction experiments on the ocean bed using bottom shots (1991)

Kirk, R. E. ; Robertson, K. ; Whitmarsh, R. B. ; [et al.]

Springer

Marine geophysical researches 13 (1991), S. 153-160

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ISSN: 1573-0581

Keywords: seismic source ; sea bed ; explosive ; bottom shot

Source: Springer Online Journal Archives 1860-2000

Topics: Geosciences , Physics

Notes: Abstract A technique has been devised for firing arrays of bottom shots on the ocean bed in depths upto 4000 m or more. Ten kilogram explosive charges are dropped from the surface while the shooting ship is navigated acoustically. They are detonated at preset times by an electronic timer which initiates an electrical detonator, detonating cord and cast PETN/TNT explosive. Ranges to ocean bottom seismographs, and the shot instants, can be calculated from the arrival-time differences of the direct and surface-reflected water waves. The accuracy, which is dependent on water-depth and range, was better than 22 m in range and 14 msec in shot instant for our experiments.

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1007/BF00286286

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6

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Pop-up bottom seismic recorder (PUBS) of the Institute of Oceanographic Sciences, U.K. (1977)

Langford, J. J. ; Whitmarsh, R. B.

Springer

Marine geophysical researches 3 (1977), S. 43-63

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ISSN: 1573-0581

Source: Springer Online Journal Archives 1860-2000

Topics: Geosciences , Physics

Notes: Abstract A pop-up bottom seismic recorder designed for seismic refraction experiments was built by the Institute of Oceanographic Sciences in 1968. The device is housed within a 71 cm diameter sphere weighing 270 kg when launched. signals picked up by a hydrophone are recorded in analogue form on magnetic tape in the band 2–100 Hz. The total continuous recording period is 12 hr but the lifetime of the system can be effectively extended by cycling the tape-recorders to allow shooting to go on for up to 3 days. Ballast release is by acoustic command or by pre-set clock. The instruments have been used in water depths from 150 to 4820 m making a total of 63 deployments with a 95% recovery rate. A new version with three-component geophones is being built.

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1007/BF00309793

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