Download The link between the Moho depth of the NE Atlantic margin and

SEG workshop “Mapping the Moho”
The link between the Moho depth of the NE Atlantic margin and magnetic
and gravity fields and satellite gravity gradients
Jörg Ebbing (Geological Survey of Norway, Trondheim & Department of Petroleum
Technology and Applied Geophysics, Norwegian University of Science and Technology,
Trondheim)
The Moho depth is an important horizon, especially in combination with the top basement, to
characterize the distribution of basement domains and to understand the tectonic setting of
passive margin systems. Here, the link between the gravity and magnetic anomalies (Figure 1)
and the depth to the Moho (Figure 2a) for the passive margin system of the NE Atlantic
margin is discussed. Both the potential fields as the Moho depth compilations reflect the
different characteristics and segmentation of the margin and between the oceanic and
continental domains.
For example the Moho boundary generally deepens from the centre of the Fennoscandian
Shield towards the continent–ocean transition. Typical values at the coastline are 30–35 km
and the depth decreases to values around 10–15 km at the continental edge. Over the stable
shelf of the Barents Sea the crustal base is for most of the area located at a depth larger than
30 km. While on the mid-Norwegian margin and the North Sea thick sedimentary sequences
correlate with a shallow Moho as expected for classical extensional, pure shear, rift basins;
the deep basins of the Barents Sea do not reveal a similar expression in the Moho geometry.
Moho depth and crustal thickness
Figure 2a shows the Moho depth for onshore Norway and the continental shelf. The Moho
depth is based on the integration of different geophysical methods, especially the integration
of OBS profiles and gravity modeling.
In general, the Moho boundary, is defined as a jump in seismic velocities to >7.9 km/s, and to
densities of >3300 kg/m3. For the resulting Moho depth compilation Grad et al. (2009)
estimates that Moho depth is described with an accuracy of the order ±3–6 km. The
uncertainties are different for different seismic techniques, even different for the same
techniques in different experiments and areas, and if seismic and gravity data are used.
In addition, a further complication is given by the presence of a distinct lower crustal body
(LCB) on the mid-Norwegian margin, which is expressed in OBS data as a high-velocity layer
with p-wave velocities from 7.1-7.9 km/s, which relates to densities in the range of 3100-3200
kg/m3. This LCB has been mapped to have a thickness of more than 6 km, and its overall
structural trend follows the general strike of the margin as indicated by seismically mapped
faults. But due to the small velocity and density contrast to the upper mantle, the distinction
between LCB and upper mantle, and hence identification of the Moho, is often ambiguous.
The LCB is interpreted to consist of a combination of magmatic underplated material,
partially eclogitized rocks or serpentinized upper mantle (e.g. Mjelde et al. 2009, Reynisson et
al. 2010).
The error in the estimates of the thickness and distribution of the LCB is related to the Moho
depth estimates, as the top LCB can be relatively well constrained. For crustal thickness
estimates the definition of the LCB and the Moho boundary is critical. The crustal thickness
in Figure 2b represents the entire crustal column from top basement to the Moho boundary.
But in the areas with extremely thin crust on the mid-Norwegian margin, the LCB is located.
If the LCB is not regarded as part of the crystalline crust, below parts of the margin almost no
crystalline crust exists and extreme stretching factors should be used for basin modeling
Figure 1 (a) Total magnetic field anomaly, and (b) Bouguer anomaly (Olesen et al. 2010)
Magnetic field and crustal structure
Often the Moho depth is also defined as the bottom of the magnetic sources, which would
imply a correlation with the Curie isotherm. This simplification can be justified to limit
magnetic sources to the crust for regional magnetic modeling, However, for the NE Atlantic
margin comparison with a 3D model based on the interpretation of potential field and seismic
reflection data and thermal modelling shows that the depth extent of the magnetic sources is
merely controlled by the overall geometry of the crystalline crust and not the temperature
distribution (Ebbing et al. 2009). The bottom of magnetic source from spectral estimates
correlates with the geometry of the basement and specifically between upper and lower crust.
3D modeling of the crustal structure of the mid-Norwegian margin reveals basement domains
with different petrophysical properties, which can be connected with lithological basement
units of onshore Norway. Due to the thermal gradient between the oceanic plate and the stable
continent, the magnetic field is only of limited use to analyze the deep structure of the margin,
while it clearly helps to study crustal domains and top basement.
Gravity and GOCE satellite data
The thermal gradient between the oceanic and continental plate is also observable as a long
wavelength trend in the gravity field, which reflects the change in lithospheric thickness. This
long-wavelength trend is often removed before interpretation or addressed by placing a small
density contrast at the lithosphere-asthenosphere boundary. However, the choice of the trend
is also affecting Moho depth estimates. An increase in lithospheric thickness is often
associated with an increase in crustal thickness, which leads to gravity effects of comparable
wavelength.
Figure 2 (a) Continental Moho depth for the NE Atlantic, and (b) crystalline crustal
thickness. The black lines in (a) present regional wide-angle seismic profiles
One way to overcome this is to use gravity gradients from the GOCE satellite mission. GOCE
especially measures the gravity field in the wavelength range critical for crustal thickness
estimates. Only based on GOCE gravity data, it would be feasible to provide a gravity field
with 80 km resolution (Bouman et al. 2011). At the same time, the second and more
revolutionary novelty is that GOCE measures gravity gradients. Gravity gradient data are
generally sensitive to shallower structures and less sensitive to regional trends, like for
example from sub-crustal domains. Sensitivity tests show that the satellite gradients are only
sensitive to the changes in crustal thickness and intra-crustal sources, but not to the density
distribution in the upper mantle. This makes the satellite gradients them an ideal tool to
evaluate seismic crustal thickness estimates. The different tensor components have
furthermore a different sensitivity to the density contrast at the Moho interface. This allows to
discuss the composition of the lower crust as reflected by a lateral changing density contrast.
References
Bouman, J., Ebbing, J., Fuchs, M. Schmidt, M., Bosch, W. & Schwatke, C., Abdul Fattah, R.,
Meekes, S., Abbink, O. & Schavemaker, Y. 2011. Heterogeneous gravity data combination
for Earth interior and geophysical exploration research. Proc. 4th International GOCE User
Workshop, ESA SP-696, Munich.
Ebbing, J. & Olesen, O. 2010: New compilation of top basement and basement thickness for
the Norwegian continental shelf reveals segmentation of the passive margin system.
Petroleum Geology Conference series; v. 7; p. 885-897.
Ebbing, J., Gernigon, L., Pascal, C., Olesen, O. and Osmundsen, P.T., 2009. A discussion of
structural and thermal control of magnetic anomalies on the mid-Norwegian margin.
Geophysical Prospecting, 57, 665-681doi: 10.1111/j.1365-2478.2009.00800.x.
Grad, M., Tiira, T.&ESC Working Group 2009. The Moho depth map of the European Plate.
Geophysical Journal International, 176, 279–292.
Mjelde, R., Faleide, J. I., Breivik, A. J. & Raum, T. 2009. Lower crustal composition and
crustal lineaments on the Vøring Margin, NE Atlantic: a review. Tectonophysics, 472, 183–
193.
Olesen, O., Brönner, M., Ebbing, J., Gellein, J., Gernigon, L., Koziel, J., Lauritsen, T.,
Myklebust, R., Sand, M., Solheim, D. and Usov, S., 2010. New aeromagnetic and gravity
compilations from Norway and adjacent areas – methods and applications. Petroleum
Geology Conference series; v. 7; p. 559-586.
Reynisson, R.F., Ebbing, J. Lundin, E., Osmundsen, P.T., 2010. Properties and distribution of
lower crustal bodies on the mid-Norwegian margin. Petroleum Geology Conference series;
v. 7; p. 843-854.