Fold axis rotation during transpressional folding Insights from
- Slides: 13
Fold axis rotation during transpressional folding — Insights from numerical modeling and application to the Zagros Simply Folded Belt Marcel Frehner ETH Zurich, Switzerland marcel. frehner@erdw. ethz. ch Swiss Geoscience Meeting marcel. frehner@erdw. ethz. ch | 19. 11. 2016 | 1
Folds are 3 D structures Makran, SE Iran 3 D folds have to be studied in 3 D Swiss Geoscience Meeting marcel. frehner@erdw. ethz. ch | 19. 11. 2016 | 2
Folds are 3 D structures Makran, SE Iran Swiss Geoscience Meeting marcel. frehner@erdw. ethz. ch | 19. 11. 2016 | 3
…also in Switzerland Part of the Geological Map of Switzerland (1: 500’ 000; swisstopo) Swiss Geoscience Meeting marcel. frehner@erdw. ethz. ch | 19. 11. 2016 | 4
Application: Zagros Simply Folded Belt Erbil Tehran Baghdad Jerusalem Al Kuwait 500 km Swiss Geoscience Meeting Dubai marcel. frehner@erdw. ethz. ch | 19. 11. 2016 | 5
Zagros Simply Folded Belt: Linkage Grasemann & Schmalholz (Geology 2012) Bretis et al. (Basin Res. 2011) § Almost cylindrical folds result from linkage of several folds that grow towards each other. Swiss Geoscience Meeting marcel. frehner@erdw. ethz. ch | 19. 11. 2016 | 6
3 D fold growth Colors: Normalized fold amplitude Frehner (Terra N. 2014) Contours: Normalized fold amplitude = (A 0/2) § Fold amplification: Amplitude in vertical direction § Fold elongation: Amplitude parallel to fold axis § Sequential fold growth: Amplitude perpendicular to axial plane Swiss Geoscience Meeting Aspect ratio in map view ≈1 marcel. frehner@erdw. ethz. ch | 19. 11. 2016 | 7
Fold axis orientation in transpression Initial fold axis orientation Evolution of fold axis orientation § Instantaneous stretching axis (ISAmax) § Major principal strain axis (lmax) § Passive marker line ? § A material line § No hinge migration § Staying parallel to and rotating with (lmax) § Not a material line § Hinge migration Swiss Geoscience Meeting marcel. frehner@erdw. ethz. ch | 19. 11. 2016 | 8
Frehner (Tectonophys. in press) Fold axes are always parallel to lmax, independently of viscosity ratio a=0° Simple shear a=45° a=90° Pure shear Fold axis orientation in transpression Swiss Geoscience Meeting marcel. frehner@erdw. ethz. ch | 19. 11. 2016 | 9
Unique triangular relationship Convergence angle Applied strain Fold axis orientation Independent of … § convergence angle § applied strain § viscosity ratio Swiss Geoscience Meeting Fold axes are always parallel to lmax, independently of viscosity ratio marcel. frehner@erdw. ethz. ch | 19. 11. 2016 | 10
Application: Zagros Simply Folded Belt Erbil Tehran Baghdad Jerusalem Al Kuwait 500 km Swiss Geoscience Meeting Dubai marcel. frehner@erdw. ethz. ch | 19. 11. 2016 | 11
Strain partitioning in the ZSFB Frehner et al. (Tectonics 2012) Intermediate to full strain partitioning Swiss Geoscience Meeting Frehner (Tectonophys. in press) marcel. frehner@erdw. ethz. ch | 19. 11. 2016 | 12
Take-home messages Frehner (Tectonophys. in press) § In transpression, fold axes are always parallel to lmax, independent of … § convergence angle § applied strain § viscosity ratio § Therefore, a unique triangular relationship exists between § Convergence angle § Applied strain § Fold axis orientation § If two are known, the third can be determined. Swiss Geoscience Meeting Convergence angle Applied strain Fold axis orientation marcel. frehner@erdw. ethz. ch | 19. 11. 2016 | 13