Open Session - SWISS GEOSCIENCE MEETINGs

38
Symposium 1: Structural Geology, Tectonics and Geodynamics
REFERENCES
Brown, R.W., Gallagher, K., Gleadow, A.J.W. & Summerfield, M.A. 2000: Morphotectonic evolution of the South Atlantic
margins of Africa and South America. In: Summerfield, M.A. (ed) Geomorphology and Global Tectonics. John Wiley and
Sons Ltd, 255-284.
Brown, R.W., Summerfield, M.A. & Gleadow, A.J.W. 2002: Denudation history along a transect across the Drakensberg
Escarpment of southern Africa derived from apatite fission track thermochronology. Journal of Geophysical Research,
107, doi:10.1029/2001JB000745.
Cockburn, H.A.P., Brown, R.W., Summerfield, M.A. & Seidl, M.A. 2000: Quantifying passive margin denudation and landscape
development using a combined fission-track thermochronometry and cosmogenic isotope analysis approach. Earth and
Planetary Science Letters, 179, 429-435.
King, L.C. 1953: Cannons of landscape evolution. Geological Society of America Bulletin, 64, 721-752.
1.28
2D dynamic retro-deformation of folded multilayers: Application to
Carboniferous turbidites in South-West Portugal
Lechmann Sarah M.*, Schmalholz Stefan M.*, Burg Jean-Pierre* & Marques Fernando O.**
* Geological Institute, ETH Zurich, CH-8092 Zurich (sarahl@student.ethz.ch)
** Universidade Lisboa, P-1749-016 Lisboa
Two-dimensional numerical finite element models of power-law viscous flow are employed to retro-deform folded multilayers.
Our aims are to demonstrate the applicability of dynamic retro-deformation as a tool to appraise an interpreted geological
profile and its mechanical forming processes. The application of this approach to a fold train situated in South-West
Portugal furthermore aims at determining its folding history and the material parameters of the constituting rocks. The
sketched sections are digitized and meshed in MATLAB and then stretched perpendicularly to the fold axes. This produced
the initial geometry for finite element simulation.
A sensitivity study is performed in order to determine model parameters, such as boundary conditions, geometry and viscosity
ratio. Boundary conditions exert little influence on the final retro-deformed geometry, the initial geometry however
does. A lower limit of 25 could be assigned to the viscosity ratio between layer and matrix, above which reasonable results
are achieved when retro-deforming the multilayer stack.
Kinematic models, so-called balanced cross sections, and synthetic forward and reverse models of the respective structures
serve as comparative value. Balanced cross sections aim at the same as dynamic retro-deformation, give however only a minimum
estimate of the amount of compression contained in a structure and allow no evaluation of material parameters.
Synthetic forward and reverse models serve to prove the retro-deformability of a specific structure.
The challenging retro-deformation of one of the prominent structures, a collapsed hinge, present in one of the studied profiles,
led to the reevaluation of the profile, which apparently was mechanically inconsistent. A second field investigation
revealed that one limb of this collapsed hinge includes rupturing and separation of 4.9 m length, which were not included
in the first model. This allowed correction of retro-deformation resulting in excellent fit between modeled and geological
sections. Further possibilities for a successful retro-deformation of the collapsed hinge are a 3D-model and a rheology including
power-law viscous layers.
This work demonstrates that a close link of field work and numerical modeling can give new insights into mechanical processes,
the material parameters of the studied rocks and also improve field based interpretations.