Definition of continental ocean boundaries (COB) and relationship to:

The COB is commonly estimated from gravity gradients/bathymetry (e.g. shelf edges) and magnetic anomalies in concert with seismic studies (crustal thickness and seaward dipping reflectors; SDR).  Novel methods (such as those being developed by affiliated researcher Professor Kusznir, where crustal thickness is estimated from gravity inversion coupled to thermal corrections; Fig. 1) are showing promise.

SDRs are subaerial basalts normally located landward of the oldest magnetic anomaly, and probably erupted close to sea level.    The COB is often picked within the lateral bounds of the SDR sequence.  However, the COB in reality is probably better described as a mappable transitional zone perhaps some tens of kilometres wide.  Properly establishing the location of the COB is critical to plate-reconstruction-derived estimates of pre-drift extension.  Correct identification of the COB and hence the nature of the crust beneath the sedimentary substrate (continental vs. oceanic crust) is also of importance for estimating heat flow, which in turn has direct implications hydrocarbon maturation and reservoir quality.

Magnetic anomalies in, for example, the NE Atlantic (Fig. 2) are far better defined than those of the South Atlantic because of better sampling (numerous industry-supported high resolution surveys), but partly also because much of the early South Atlantic oceanic crust was generated during the Cretaceous Normal Superchron between ~ 118 and 84 Ma (i.e. no magnetic reversals occurred).  However, even in the NE Atlantic there is room for improved picking of magnetic anomalies and the COB as demonstrated by a recent  high-resolution aeromagnetic survey flown in 2003 by NGU in the Lofoten area.

The quality of a mapped COB may be tested by restoring it to its geographic position at the time of continental break-up and observing the amount of overlap (i.e. pre-break extension) or underlap its reconstructed geometry implies.  Thus, well-defined COBs and well-constrained reconstruction parameters are essential to understanding the geological evolution and petroleum potential of divergent margins.

Reconstructing gridded data (e.g. magnetics in Fig. 3) from conjugate margins, made possible by modern computing power, inserts a new tool into an old game.  At NGU we have developed powerful numerical tools with which to rotate and analyze gridded data.  Rotation backwards in time of entire portions of the gridded magnetic data themselves will allow pattern assessment of the fit of the actual data themselves, rather than of lines interpreted from their present day position.

 

FIGURE 1 Crustal thickness (in meters) derived from satellite gravity data for the North Atlantic (50-70o N).  Satellite derived gravity anomaly data and bathymetry data were used to derive the mantle residual gravity anomaly which then was inverted to give Moho depth.  The gravity inversion includes a correction for the large negative residual thermal gravity anomaly from oceanic lithosphere and stretched continental margin lithosphere (Kusznir et al. 2003).

FIGURE 2  Magnetic anomaly map of Norway and adjacent ocean areas overlain with identified magnetic anomalies, fracture zones and spreading axes.  The Continental-Ocean-Boundary (COB) is shown as a bold black line.  The main structural features of the Mid-Norway offshore region are indicated: HH, Helland-Hansen Arch; HT, Halten Terrace; IB, innermost passive margin boundary fault; MB, Møre Basin; TP, Trøndelag Platform; VB, Vøring Basin; VG, Viking Graben. Orange indicates inversion features (Mosar, Lewis & Torsvik 2002).

FIGURE 3 Reconstructed magnetic grids from the North Atlantic at 48 Ma (Lundin et al. 2002)