An alternative early opening scenario for the Central Atlantic Ocean

Abstract The opening of the Central Atlantic Ocean basin that separated North America from northwest Africa is well documented and assumed to have started during the Late Jurassic. However, the early evolution and the initial breakup history of Pangaea are still debated: most of the existing models are based on one or multiple ridge jumps at the Middle Jurassic leaving the oldest crust on the American side, between the East Coast Magnetic Anomaly (ECMA) and the Blake Spur Magnetic Anomaly (BSMA). According to these hypotheses, the BSMA represents the limit of the initial basin and the footprint subsequent to the ridge jump. Consequently, the evolution of the northwest African margin is widely different from the northeast American margin. However, this setting is in contradiction with the existing observations. In this paper, we propose an alternative scenario for the continental breakup and the Mesozoic spreading history of the Central Atlantic Ocean. The new model is based on an analysis of geophysical data (including new seismic lines, an interpretation of the newly compiled magnetic data, and satellite derived gravimetry) and recently published results which demonstrate that the opening of the Central Atlantic Ocean started already during the Late Sinemurian (190 Ma), based on a new identification of the African conjugate to the ECMA and on the extent of salt provinces off Morocco and Nova Scotia. The identification of an African conjugate magnetic anomaly to BSMA, the African Blake Spur Magnetic Anomaly (ABSMA), together with the significant change in basement topography, are in good agreement with that initial reconstruction. The early opening history for the Central Atlantic Ocean is described in four distinct phases. During the first 20 Myr after the initial breakup (190–170 Ma, from Late Sinemurian to early Bajocian), oceanic accretion was extremely slow (∼ 0.8 cm/y). At the time of Blake Spur (170 Ma, early Bajocian), a drastic change occurred both in the relative plate motion direction (from NNW–SSE to NW–SE) and in the spreading rate (an increase to ∼ 1.7 cm/y). After a small increase between Chron M25 (∼ 154 Ma, Kimmeridgian) and Chron M22 (∼ 150 Ma, Tithonian), the spreading rate slowed down to about 1.3 cm/y and remained fairly constant until Chron M0 (125 Ma, Barremian–Aptian boundary). In addition, kinematic reconstructions illustrate a significant spreading asymmetry during the early history of the Central Atlantic Ocean; the accretion rates were higher on the American side and led to the formation of more oceanic crust on this plate. We infer that this asymmetry could be related to the fact that the thermal anomaly responsible for the significant magmatism of the Central Atlantic Magmatic Province (CAMP) was preferentially located below the African plate.

Providing a balance between principles and practice, this state-of-the-art overview of geophysical methods takes readers from the basic physical phenomena, through the acquisition and processing of data, to the creation of geological models of the subsurface and data interpretation to find hidden mineral deposits. Detailed descriptions of all the commonly used geophysical methods are given, including gravity, magnetic, radiometric, electrical, electromagnetic and seismic methods. Each technique is described in a consistent way and without complex mathematics. Emphasising extraction of maximum geological information from geophysical data, the book also explains petrophysics, data modelling and common interpretation pitfalls. Packed with full-colour figures, also available online, the text is supported by selected examples from around the world, including all the major deposit types. Designed for advanced undergraduate and graduate courses in minerals geoscience, this is also a valuable reference for professionals in the mining industry wishing to make greater use of geophysical methods. In 2015, Dentith and Mudge won the ASEG Lindsay Ingall Memorial Award for their combined effort in promoting geophysics to the wider community with the publication of this title.

Geophysics for the Mineral Exploration Geoscientist

Ice-sheet dynamics and ice streaming along the coastal parts of northern Norway

A new generation of aeromagnetic data documents the post-Caledonide rift evolution of the southwestern Barents Sea (SWBS) from the Norwegian mainland up to the continent-ocean transition. We propose a geological and tectonic scenario of the SWBS in which the Caledonian nappes and thrust sheets, well-constrained onshore, swing from a NE-SW trend onshore Norway to NW-SE/NNW-SSE across the SWBS platform area. On the Finnmark and Bjarmeland platforms, the dominant inherited magnetic basement pattern may also reflect the regional and post-Caledonian development of the late Paleozoic basins. Farther west, the pre-breakup rift system is characterized by the Loppa and Stappen Highs, which are interpreted as a series of rigid continental blocks (ribbons) poorly thinned as compared to the adjacent grabens and sag basins. As part of the complex western rift system, the Bjornoya Basin is interpreted as a propagating system of highly thinned crust, which aborted in late Mesozoic time. This thick Cretaceous sag basin is underlain by a deep-seated high-density body, interpreted as exhumed high-grade metamorphic lower crust. The abortion of this propagating basin coincides with a migration and complete reorganization of the crustal extension toward a second necking zone defined at the level of the western volcanic sheared margin and proto-breakup axis. The abortion of the Bjornoya Basin may be partly explained by its trend oblique to the regional, inherited, structural grain, revealed by the new aeromagnetic compilation, and by the onset of further weakening later sustained by the onset of magmatism to the west.

Crustal and basin evolution of the southwestern Barents Sea: From Caledonian orogeny to continental breakup

We describe a new technique that can be used to level data collected along regular and irregular line patterns with or without tie-line control. The technique incorporates a moving differential median filter to minimize line-level errors, to level survey-line data, and to microlevel data with no tie-line control. This overcomes the problem of standard leveling methods that lose their effectiveness with irregular flight patterns. To validate the method, we use it to level very-low-frequency (VLF) electromagnetic (EM) data from a helicopter survey where flight lines are parallel. Leveling is also performed on a set of vintage aeromagnetic data from the North Sea, gathered from nonparallel flight lines. Results show that the differential median filter leveling technique is superior to the standard leveling method because it results in fewer line errors and less distortion of high-wavenumber anomalies when processing irregular survey lines, making the method suitable for a wide variety of data sets.

Leveling aerogeophysical data using a moving differential median filter

Aeromagnetic data collected in areas with severe diurnal magnetic variations (auroral zones) are difficult to level. This paper describes levelling of an aeromagnetic survey where such conditions prevail, and where sophisticated levelling techniques are needed. Corrections based on piecewise low-order polynomial functions are often used to minimize mis-ties in aeromagnetic data. We review this technique and describe similar mis-tie fitting methods based on low-pass filter levelling, tensioned B-spline levelling and median levelling. It is demonstrated that polynomial levelling, low-pass filter levelling and tensioned B-spline levelling depend on the careful editing of outlying mis-ties to avoid the introduction of false anomalies. These three techniques are equally efficient at removing level errors. Median levelling also removes level errors efficiently, but it is more robust in the sense that mis-tie editing is not required. This is due to the inherent noise-removal capabilities of the median filter. After mis-tie editing, the total field anomalies of the other three techniques closely resemble the unedited median-levelled total field anomaly.

A comparison of aeromagnetic levelling techniques with an introduction to median levelling

Summary: The airborne gamma-ray spectrometric method is primarily used for the detection of ore bodies, fallout from nuclear accidents and as a lithological mapping tool. Modern gamma-ray spectrometers can record at least 256 channels of data in the energy range 0-3 MeV. After applying conventional processing, there is still noise in the spectra. Several approaches are suggested to remove this remaining noise. These methods include noise-adjusted singular value decomposition (NASVD), with or without clustering, and maximum noise fraction (MNF). The methods are applied to raw spectra data prior to integrating counts in the K, U and Th windows and proceeding to conventional processing. The three methods are applied to a survey data set for comparison. All methods are shown to reduce noise in the original data set.

https://www.ngu.no/upload/Publikasjoner/Rapporter/2005/2005_014.pdf

Eirik Mauring

Papers

Leveling aerogeophysical data using a moving differential median filter

A comparison of aeromagnetic levelling techniques with an introduction to median levelling

Reducing noise in radiometric multi-channel data using noise-adjusted singular value decomposition (NASVD) and maximum noise fraction (MNF).