Mai n B oun dar y Th rus

Mai n. B oun dar y Th rus t Mai n Fr 10 km onta l Th rust The Siwalik Fold Belt along the Himalayan piedmont

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

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

9. 2 ka. BP 6. 2 ka. BP 2. 2 ka. BP Inferring paleo-river bed from terrace remnants

River incision and terrace formation across an active fold

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

• The two major terrace T 0 (9. 2 ka) and T 3(2. 2 ka) 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?

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

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

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

Determination of shortening from conservation of area

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

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)

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

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

Fault-bend folding Constant bed length No backshear Constant bed thickness v 1=v 2 v 1 constant with depth u(x) = v 1. sinθ(x)

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

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)

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)

Fault-bend folding Constant bed length No backshear Constant bed thickness v 1=v 2 v 1 constant with depth u(x) = v 1. sinθ(x)