Open Session - SWISS GEOSCIENCE MEETINGs

which seems to be consistent with the uplift of the Rhenish Shield as seen in earlier levelling results.
Having continued with the GPS observations and carried out an extension of the network as a coverage of the Northern
Rhenish Massif up to 50 measuring points can be analysed which also include the observations in the Ardennes on Belgian
territory (Demoulin et al. 2005).
As a recent example, the measurements of the local deformation GPS network ‘Donatussprung’, a section of the Erft Fault
system where the surface trace can be identified from topography and effects on buildings and roads, have revealed displacements
of up to 6 mm/y in horizontal and 22 mm/y in vertical direction with high accuracy (Görres & Kuhlmann 2008).
Vertical and horizontal motions due to recent tectonics in this region are smaller by at least an order of magnitude. The
observed pattern of vertical and horizontal velocity vectors shows a remarkable difference in the motion of point groups on
either side of the fault. The scenario suggested by these measurements indicates that the sediment layers on the Erft Block
are indeed sinking in proportion to the groundwater withdrawal, but that near the fault the pattern of motions is strongly
influenced by the fault geometry. Modelling options include mining subsidence troughs as well as fault slip motion.
There is reasonable hope that after 15 years of annual GPS observations in an entire reprocessing of all points and all observation
epochs (currently worked on) the indications (Extension and Uplift) can be clarified.
REFERENCES
Campbell, J., Kümpel, H.-J., Fabian, M., Fischer, D., Görres, B., Keysers, Ch., & Lehmann, K. 2002: Recent movement pattern of
the Lower Rhine Basin from tilt, gravity and GPS data, Netherlands Journal of Geosciences/ Geologie en Mijnbouw, Vol.
81 (2), 223-230.
Demoulin, A., Campbell, J., De Wulf, A., Muls, A., Arnould, R., Görres, B., Fischer, D., Kötter, T., Brondeel, M., Van Damme, D.,
& Jacqmotte, J.M. 2005: GPS monitoring of vertical ground motion in northern Ardenne-Eifel: five campaigns (1999-
2003) of the HARD project, Int. J. Earth Sci (Geolog. Rundsch) 94: 515-524.
Görres, B. & Kuhlmann, H. 1998: How groundwater withdrawal and recent tectonics cause damages of the earth’s surface:
Monitoring of 3D site motions by GPS and terrestrial measurements, Journal of Applied Geodesy, 1(2008), 223-232,
deGruyter Berlin, DOI 10, 1515/JAG.2007.024.
1.1
Terrestrial core formation aided by flow channelling instabilities induced
by iron diapirs
Golabek Gregor*, Tackley Paul* & Schmeling Harro**
*Institut für Geophysik, Geophysical Fluid Dynamics, Schafmattstrasse 30, CH-8093 Zürich (gregor.golabek@erdw.ethz.ch)
**Institut für Geowissenschaften, Facheinheit Geophysik, Altenhöferallee 1, D-60438 Frankfurt am Main
The terrestrial core formation process remains poorly known. Isotopic constraints by Hf/W systematics indicate a fast process
which was largely completed within 33 Ma for the Earth. An unstable gravitational configuration of dense molten metallic
ponds overlying a chondritic protocore is predicted by most studies at latest for the time a planetary embryo reaches Mars
size. This leads to the formation of Rayleigh-Taylor instabilities. We propose the application of Stevenson's stress-induced
melt channelling mechanism in the regions surrounding incipient iron diapirs. We therefore perform numerical experiments
solving the two-phase, two compositions flow equations within a 2D rectangular box. We apply the Compaction
Boussinesq Approximation (CBA) and include a depth-dependent gravity. We use a temperature and stress-dependent viscosity
for the solid phase and melt fraction dependent rheology for the partially molten region around the diapir. We investigate
the development of the channelling instability in cases with and without interaction with surrounding diapirs. In interactive
cases we vary the distance between the diapir centres between 1 and 5 diapir radii and apply pseudoplasticity with
power law exponents ranging from 1 to 6. As a result for single diapirs we observe for sufficiently small retention numbers
the development of iron-rich melt channels within a region of approximately twice the diapir's radius. This could lead to
effective draining of the surrounding region and might initiate cascading daughter diapirs or iron dykes. For small distances
between interactive diapirs the channelling mechanism is delayed for several million years compared with models without
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Symposium 1: Structural Geology, Tectonics and Geodynamics