Revised Eburnean geodynamic evolution of the ... - Tectonique.net

22 S. Perrouty et al. / Precambrian Research 204– 205 (2012) 12– 39
Fig. 4. Image of the gamma-ray data draped over a shaded digital elevation model. This image shows the relative proportion of potassium, thorium and uranium as an RGB
image, and their correlation with the topography. Pixel size is 100 m. U and Th rich areas with high topography correspond to Sefwi Group metavolcanics as around Cape
Three Points and proximal to the Ashanti Fault. K-rich granitoids are well defined in the eastern part of the area.
gravity data can only be used to broadly differentiate regional
rock packages and image the continuity of the major structures at
depth.
3.2.4. ALOS PALSAR data
Advanced Land Observing Satellite; Phased Array type L-band
Synthetic Aperture Radar high-resolution data (ALOS PALSAR data)
collected in 2009 were used to image structure within lithological
packages. Two polarisation channels (HH and HV) (available from
http://www.eorc.jaxa.jp/ALOS/en/) were gridded with a resolution
of 50 m. A ternary RGB image was then produced (Fig. 6), where
red, green, blue colours correspond respectively to the HH, HV and
(HH + HV/2) polarisations. These radar data are able to image structural
details within sedimentary units such as bedding, with better
resolution than the combined radiometric and digital elevation
model.
3.3. Petrophysics
Petrophysical data were collected during regional reconnaissance,
resulting in a database of over 300 susceptibility
measurements of the major lithologies encountered in the Ashanti
region. The data provided quantitative information on the magnetic
susceptibility of the Birimian and Tarkwaian units and intrusives
that were used to constrain the regional magnetic interpretation
and subsequent geophysical modelling. These susceptibility data
are consistent with common susceptibility range proposed by Clark
(1997) and with values measured by Metelka et al. (2011) in Burkina
Faso.
The histograms in Fig. 7 show the range of measured magnetic
susceptibilities for the lithologies seen in the study area using
a Geofyzika KT-5 Kappameter hand held susceptibility meter. To
ensure high quality measurements, some representative samples
were re-measured using a Kappabridge KLY-3 magnetic susceptibility
meter and no statistically significant differences were
found. Natural remanent magnetisation (NRM) of the dolerite
dykes was measured using AGICO JR5A spinner magnetometer,
mounted within Helmholtz coil systems. Paleomagnetic laboratory
experiments were carried out at the GET Magnetism Laboratory
(Toulouse).
Density data were acquired for 26 samples taken from all of the
outcropping regional units. Calculated values derived from sample
weighing in air and water (Table 5) agree with measurements
made by Hasting (1978, unpublished report referenced in Barritt
and Kuma (1998) and with equivalent samples from Burkina Faso
(Baratoux et al., 2011).