The Siwalik Fold Belt along the Himalayan piedmont

1. The Siwalik Fold Belt along the Himalayan piedmont Main Boundary Thrust Main Frontal Thrust 10 km

2. Structural Section Along Bagmati River. A Simple Fault Bend Fold.

3. Uplifted Fluvial Terrace along Bagmati River. Top of terrace tread Strath surface

4. Inferring paleo-river bed from terrace remnants 9.2 kaBP 6.2 kaBP 2.2 kaBP

7. River incision and terrace formation across an active fold

8. Folded abandoned terraces along Bagmati river • Only the MFT is active along that section • Incision rate correlates with the fold geometry suggesting that it reflects primarily tectonic uplift.

9. The two major terrace T0 (9.2ka) and T3(2.2ka) show similar pattern of incision although their ratio is not exactly constant nore exactly equal to the ratio of their ages (0.19). Should incision be stationary if the fold is growing at a constant rate?

10. Converting Incision into Uplift u(x,t): uplift relative to the undeformed footwall i(x,t): river incision b(t): sedimentation at front of the fold (local base level change) u(x,t)= i(x,t) + b

11. Comparison of Uplift and Incision profiles The various terraces yield very similar uplift profiles.

12. Uplift relative to footwall basement How do we convert that information into horizontal shortening of slip rate on the thrust fault?

13. Determination of shortening from conservation of area

14. Note that the ‘excess area’ is a linear function of depth only if there is no backshear. (Bernard et al, 2006)

15. Determination of shortening from conservation of area • It is assumed here that: • area is preserved during deformation (no compaction nor dilatancy) • deformation is plane (no displacement out of plane)

16. Relationships between fold shape and shortening depend on folding mechanism… Pure-shear Fault-Bend Fold Detachment Fold Fault-Bend Fold

17. Incremental deformation recorded by terraces or growth strata can be used to test fold models. Detachment Fold Fault-Bend Fold Collocated proportional uplift Non-Collocated uplift (courtesy of John Suppe)

18. Fault-bend folding Constant bed length v1=v2 No backshear v1 constant with depth Constant bed thickness u(x) = v1.sinθ(x)

19. Folded abandoned terraces along Bagmati river • Is the uplift pattern consistent with Fault-bend Folding as has been assumed to construct the section?

20. Comparing uplift derived from river incision with uplift predicted by fault-bend folding The uplift pattern is consistent with fold-bend folding with no back-shear. It is possible to estimate the cumulative shortening since the abandonment of each terrace. (Lave and Avouac, 2000)

21. Comparing uplift derived from river incision with uplift predicted by fault-bend folding The shortening rate across the fold is estimated to 21 +/-1.5 mm/yr (taking into account the fact that slip is probably stick slip) (Lave and Avouac, 2000)

22. Fault-bend folding Constant bed length v1=v2 No backshear v1 constant with depth Constant bed thickness u(x) = v1.sinθ(x)