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					         Structural and thermal control on the depth to
         bottom of magnetic sources – A case study
         from the mid-Norwegian margin
                                         Jörg Ebbing (1,2), Laurent Gernigon (1), Christophe Pascal (1), Claudia Haase (3)
                 1
                ( ) Geological Survey of Norway, Trondheim, Norway (2) Department of Petroleum Engineering and Applied
                                   Geophysics, NTNU Trondheim, Norway (3) Institute of Geosciences, CAU Kiel, Germany




Summary
We discuss the correlation between the depth extent of magnetic sources, the Curie temperature depth
and crustal structures on the mid-Norwegian margin. Our results show that classical spectral methods
can be used to estimate the depth extent of magnetic sources, even if the bottom is located in the lower
crust, and only with limited resolution since large analyzing windows (> 200 km) have to be used to
resolve the deep sources. The depth extent of the magnetic sources is controlled by the crustal
geometry and not the temperature distribution in the crust. Comparison with thermal modeling allows
us to argue that the observed magnetic field on the mid-Norwegian margin does not reflect the Curie
temperature depth as previously assumed, but the geometry of the basement and lower crust.


Introduction
The dynamics of sedimentary basins are often related to the structure of the underlying basement.
Common interpretation techniques of gravity and magnetic data can help to identify the top basement
and interpretation of gravity, magnetic and electromagnetics allow further to identify different intra-
basement domains, if expressed by changes in petrophysical properties. On the mid-Norwegian margin
the basement consists of Precambrian and Caledonian basement units with different densities and
magnetic properties.
Also the thermal state of the lithosphere is controlled by, amongst other parameters, basement
composition (i.e. relative content of heat-producing elements). Therefore, it is crucial to estimate the
heat production associated to the different basement units in order to perform accurate basin modelling
and to identify potential hydrocarbon sources. The magnetic field shows also a temperature
dependency since rocks loose their magnetic properties with temperatures higher than the Curie
temperature. If it would be possible to identify the depth to the Curie temperature from magnetic data,
it would be possible to give an independent constraint for the present-day thermal state. Here we
discuss the implications of thermal and structural control on the depth of the magnetic sources.
We present a combination of forward and inverse modeling techniques to discuss the respective roles
of structural and thermal controls on the bottom of magnetic sources on the mid-Norwegian margin
(Figure 1). In this area, previous studies argued that an abrupt change in the Curie temperature depth
explains the change from large magnetic amplitude in the innermost margin to low-amplitude
anomalies at the outer margin (e.g. Fichler et al. 1999; Figure 1). But also the top basement is
deepening from depths <9 km below the Trøndelag Platform to depths between 11 and 15 km below
the Vøring Basin, pointing towards a correlation between the internal basement structure and the
observed magnetic anomalies (Ebbing et al. 2006).


                                 EGM 2007 International Workshop
             Innovation in EM, Grav and Mag Methods:a new Perspective for Exploration
                                   Capri, Italy, 16 - 18 April 2007
Methodology
Gravity and magnetic field anomalies generally contain different wavelengths due to the different
dependency on the distance to the source. The Bouguer gravity field is mainly caused by the density
distribution in the lithosphere, while the magnetic field is mainly dependent on upper crustal sources.
The stronger dependency on upper crustal sources is due to the stronger dependency on the source
distance and the temperature dependency of magnetic properties.
Magnetic data will only provide information for the part of the crust that presently resides below the
Curie temperature. Rocks at higher temperatures will not generate a discernible magnetic signal.
Magnetite, with a Curie temperature of 580 ºC (Hunt et al. 1995), is regarded to be the dominant
magnetic mineral of in crustal rocks. The depth to the Curie temperature and therefore also between
magnetic and non-magnetic material generally runs through the lower continental and oceanic crust.
Nevertheless, the anomalies on an aeromagnetic map are caused by an ensemble of sources. On
passive margins the main sources of the magnetic field are the basement and intra-sedimentary
volcanic rocks. Sediments are relatively nonmagnetic, while the underlying basement as well as
intrusive rocks has a high magnetic susceptibility. Common methods to estimate the source depth are
Euler deconvolution, power spectrum methods or Werner deconvolution. These methods have been
proven to give reliable estimates on the top basement, especially if partially constrained by seismic
data (e.g. Ebbing et al. 2006).




Figure 1 a) Gravity anomaly (Offshore: free-air; onshore: Bouguer), and b) magnetic anomaly of the
mid-Norwegian margin. The yellow line shows the location of the transect in Figure 2.For more
details on the potential field data see Ebbing et al. (2006).


For the Norwegian margin, the gravity field does not include a prominent signal of the top basement
because only a small density contrast exists between the sediments and the top basement. This is due
to the fact that in 10 km depth, a typical depth range for the top basement on the mid-Norwegian


                                 EGM 2007 International Workshop
             Innovation in EM, Grav and Mag Methods:a new Perspective for Exploration
                                   Capri, Italy, 16 - 18 April 2007
margin, the increase with pressure leads to a compaction of the sediments. Hence, only magnetic
interpretation allows for determining the top basement geometry.
Okubo et al. (1989) suggested after Spector & Grant (1970) that a single prism can represent an entire
set of different scattered magnetic sources, and the bottom of the prism could indicate the depth to the
Curie temperature. The properties (zt,zb) of a statistical prism are a function of the grid sample (or
moving windows) sizes and the windows must cover in the correct way the main anomalies of the
magnetic field. In practical terms this implies that the shape of the prism is depending on the window
size and a deep sources can only be detected with a sufficient large analysis window. Sensitivity tests
show that at least a window size of 200 km x 200 km is required to study sources in a depth of 20 km.




Figure 2 Transect through the oceanic and continental domain of the mid-Norwegian margin and
comparison between the geometry of a 3D model (after Ebbing et al. 2006) and the depth solutions
from the spectral approach (Zt: top magnetic source, Zb: base magnetic source). The 550ºC isoline is
estimated from thermal modeling (Gernigon et al. 2006).


The estimates for the bottom of the sources are largely dependent on the window size. With a window
size of 200 km x 200 km the bottom of the magnetic sources can be located in the lower crust.
However, only rough estimates for very large regions can be made with this approach.
But how reliable are these results, and can they be associated with the Curie depth E.g. we know that
the Curie depth in large depth is not a sharp boundary, but a transition zone (Curie Window).
For the transect the depth estimates for the bottom of the magnetic sources are around 10 km in the
oceanic domain and 18 km depth in the continental domain (Fig. 2). To test the geological meaning we
compare the Curie depth estimates with the border to the lower crust as estimated from seismic data

                                 EGM 2007 International Workshop
             Innovation in EM, Grav and Mag Methods:a new Perspective for Exploration
                                   Capri, Italy, 16 - 18 April 2007
and density modeling. 3D density modeling based on seismic data allows modeling the crustal and
upper mantle density distribution (e.g. Ebbing et al. 2006).
The depth estimates for the mid-Norwegian margin are located in the lower crust and appear to
correlate with the border between the Precambrian basement and the underlying lower crust. On the
oceanic domain the depth solution are closer to the Moho depth. The 550 ºC isoline is in the
continental and most of the oceanic domain located below the estimated bottom of magnetic sources.
Therefore, the bottom of the magnetic sources seems to be controlled by the structure of the margin
and not its thermal state.
Conclusions
The combined interpretation of gravity and magnetic allows to identify the crustal structure on the
Norwegian passive margin. Potential field models can be used to identify the top of the basement,
even in large depths, as well as the deep crustal configuration.
The bottom of magnetic sources is on the Norwegian passive margins primary controlled by the
structure and not the thermal state of the crust. These results clearly show that the Curie temperature
depth cannot be easily derived from an aeromagnetic map. Clearly, in areas with a large depth to the
top basement, the dependency on large-window sizes in estimating the bottom of magnetic sources,
make the spectral method further not a valid stand-alone approach.
Structural 3D modeling of petrophysical properties based on seismic data is a more useful approach to
investigate continental shelfs and the structures underlying deep sedimentary basins.
Acknowledgements
The study is done as part of the project "Continental Crust and Heat Generation in 3D" in cooperation
with Statoil. We thank Christine Fichler (Statoil) & Odleiv Olesen (NGU) for helpful
discussions.
References
Ebbing, J., Lundin, E., Olesen, O. and Hansen, E.K, 2006. The mid-Norwegian margin: A discussion
of crustal lineaments, mafic intrusions, and remnants of the Caledonian root by 3D density modelling
and structural interpretation. Journal of the Geological Society, London, 163, 47-60.
Fichler, C., Rundhovde, E., Olesen, O., Sæther, B.M., Rueslåtten, H., Lundin, E. and Doré, A.G.,
1999. Regional tectonic interpretation of image enhanced gravity and magnetic data covering the Mid-
Norwegian shelf and adjacent mainland. Tectonophysics 306, 183-197.
Gernigon, L., Lucazeau, F., Brigaud, F., Ringenbach, J.C., S., P. & Le Gall, B. 2006a: A moderate
melting model for the Vøring margin (Norway) based on structural observations and a thermo-
kinematical modeling: Implication for the meaning of the lower crustal bodies. Tectonophysics 412,
255-278.
Hunt, C., Moskowitz, B.M. & Banerje, S.K. 1995: Magnetic properties of rocks and minerals. In:
Rock Physics and Phase Relations. A Handbook of Physical Constraints. AGU Reference Shelf 3, 189-
204.
Okubo Y., Tsu H. and Ogawa K., 1989, Estimation of Curie point temperature and geothermal
structure of island arcs of Japan.; Thermal aspects of tectonics, magmatism and metamorphism:
Tectonophysics 159, p. 279-290.
Spector, A. & Grant, F.S. 1970: Application of high sensitivity aeromagnetic surveying to offshore
petroleum exploration. Geophysical Prospecting 18, 474-475.

                                   EGM 2007 International Workshop
               Innovation in EM, Grav and Mag Methods:a new Perspective for Exploration
                                     Capri, Italy, 16 - 18 April 2007

				
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posted:10/3/2012
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