Rheology of deformed Carrara marble: Insights from torsion experiments

1. Rheology of deformed Carrara marble: Insights from torsion experiments 1 2 3 Rolf Bruijn , Claudio DellePiane , Willemijn de Raadt ETH, Geological Institute, Earth Sciences, Zurich, Switzerland CSIRO Earth Science and Resource Engineering, Kensington, Australia Utrecht University, Faculty of Geosciences, Utrecht, The Netherlands

2. Single-stage deformation experiments extended to multi-stage  Effect of pre-existing strain Strain interruption and reversal: Shear zone reactivation Composite samples (2 types): Host rock/mylonite competence contrast Strain interruption and heating: Shear zone reactivation + annealing Introduction/problem Introduction/Problem – Experiments – Flow behavior – Segmented strain – Fabric evolution – Texture effect – Conclusions

3. Experiments Type I: Counter-clockwise deformation followed by clockwise deformation Type II: 2-segments composite deformation After Delle Piane and Burlini, 2008 Introduction/Problem – Experiments – Flow behavior – Segmented strain – Fabric evolution – Texture effect – Conclusions

4. Experiments Type III: 3-segments composite deformation After Bruijn et al., 2011 Introduction/Problem – Experiments – Flow behavior – Segmented strain – Fabric evolution – Texture effect – Conclusions

5. Experiments Type IV: three-stage sample history γ = 6-9 5 hours + γ = 6-9 1st deformation annealing 2nd deformation T °C 20 Time Modified after Delle Piane and Burlini, 2008 Introduction/Problem – Experiments – Flow behavior – Segmented strain – Fabric evolution – Texture effect – Conclusions

6. Experiment parameters • Focus on dislocation creep with dynamic recrystallization • Comparison with monotonic experiments (Pieri et al., 2001: Barnhoorn et al., 2004) • Type IV experiments explore effect of temperature, strain rate and switch in dominant deformation mechanism Introduction/Problem – Experiments – Flow behavior – Segmented strain – Fabric evolution – Texture effect – Conclusions

7. Type I: Low-strain flow behavior • Effect of shear interruption and reversal at low strain? • Flow strength evolution barely affected • 3-6 MPa flow difference explained by fabric effects and load cell precision Modified after Delle Piane and Burlini, 2008; Bruijn et al., 2011 Introduction/Problem – Experiments – Flow behavior – Segmented strain – Fabric evolution – Texture effect – Conclusions

8. Type I: High-strain flow behavior Effect of shear interruption and reversal at high strain? • Steady state flow quickly restored • 2-7 MPa flow difference explained by fabric effects and load cell precision Continued forward or reverse flow easier? Modified after Delle Piane and Burlini, 2008; Bruijn et al., 2011 Introduction/Problem – Experiments – Flow behavior – Segmented strain – Fabric evolution – Texture effect – Conclusions

9. Type II & III: Stage 2 strain behavior Type II: 2-segments composite Type II stage 2 deformation Bulk strain: γ2 = 1 Segmented strain contrast Forward strain  γ2 = 1.8 First strain  γ2 = 0.2  Strain (rate) ratio ≈ 9 γ1 = 5 γ1 = 0 • Type III stage 2 deformation • Bulk strain: γ2 = 1 (left) & γ2 = 5 (right) • Segmented strain contrast • Left Right • Forward strain  γ2 = 0.7  γ2 = 7.0 • First strain  γ2 = 1.4  γ2 = 0.9 • Reversed strain  γ2 = 1.0  γ2 = 7.0 • Strain (rate) ratio ≈ 2 ≈ 8 Type III: 3-segments composite γ1 = 1 γ1 = 5 γ1 = 0 γ1 = 0 γ1 = -5 γ1 = -1 Introduction/Problem – Experiments – Flow behavior – Segmented strain – Fabric evolution – Texture effect – Conclusions

10. Type I & III: Low strain fabric • Microstructures + CPO • Type II strain reversal • Grain shearing recovered • Dyn. Rx. continued • SPO removed, but CPO preserved  • Weak evidence for shear sense last deformation stage • Type III strain reversal • Recovery of “eaten” sheared grain requires less strain  • CPO + SPO are unreliable total shear sense indicator • J-index false measure for strain intensity Type I Type III After Delle Piane and Burlini, 2008 After Bruijn et al., 2011 Introduction/Problem – Experiments – Flow behavior – Segmented strain – Fabric evolution – Texture effect – Conclusions

11. Type I & III: High strain fabric Type I: after strain reversal Type I: before strain reversal • Microstructure and texture after high-strain strain reversal • Continued dynamic recrystallization • Foliation indicates shear sense last deformation stage • Due to symmetry in slip system activity texture development is unaffected • Shear sense is irrelevant for recrystallization progress  product of absolute strain or work After Delle Piane and Burlini, 2008 Introduction/Problem – Experiments – Flow behavior – Segmented strain – Fabric evolution – Texture effect – Conclusions

12. Type I & III: High strain fabric Type III: Top and bottom segments after second deformation stage • Fabric after high-strain shear interruption and reversal • Foliation development reflects total strain rather than absolute strain • Foliation angle can be used to estimate total strain, which is a minimum value in the case of reversed sense of shear • Crystal orientations unaffected, but J-index delayed by strain reversal After Bruijn et al., 2011 Introduction/Problem – Experiments – Flow behavior – Segmented strain – Fabric evolution – Texture effect – Conclusions

13. Type IV: Strain interruption with annealing Type IV: three-stage sample history γ = 6-9 5 hours + γ = 6-9 1st deformation annealing 2nd deformation T °C 20 Time Modified after Delle Piane and Burlini, 2008 Introduction/Problem – Experiments – Flow behavior – Segmented strain – Fabric evolution – Texture effect – Conclusions

14. Type IV: Texture effect Rx texture + grain refinement Weakening (A-B) = 21.5 % Only grain refinement Weakening (D-E) = 17.4 % After De Raadt et al., in prep. Introduction/Problem – Experiments – Flow behavior – Segmented strain – Fabric evolution – Texture effect – Conclusions

15. Type IV: Texture effect Rx texture + grain refinement Weakening (A-B) = 7.8 % Only grain refinement Weakening (D-E) = 4.5 % After De Raadt et al., in prep. Introduction/Problem – Experiments – Flow behavior – Segmented strain – Fabric evolution – Texture effect – Conclusions

16. Type IV: Texture effect Rx texture + grain refinement Weakening (A-B) = 16.4 % Only grain refinement Weakening (D-E) = 6.3 % After De Raadt et al., in prep. Introduction/Problem – Experiments – Flow behavior – Segmented strain – Fabric evolution – Texture effect – Conclusions

17. Conclusions • Strain interruption and reversal have little effect on flow strength evolution • Strain reversal at low strain slightly easier than continuation (Bauschinger effect) • Strain reversal at high strain as easy as continuation • Shearing of grains recovered by strain reversal; grain size dependent • Recrystallization unaffected by shear sense  absolute strain/work • Most deformation accommodated by weakest sample segment • Competence contrast results in one order of magnitude strain (rate) variation • Annealing preserves recrystallization texture • 33-67 % of weakening is caused by Rx texture development Complex coupling between fabric and rheology Introduction/Problem – Experiments – Flow behavior – Segmented strain – Fabric evolution – Texture effect – Conclusions