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

The Siwalik Fold Belt along the Himalayan piedmont

10 km

Main Frontal Thrust

Main Boundary Thrust

Page 2: The Siwalik Fold Belt along the Himalayan piedmont 10 km Main Frontal Thrust Main Boundary Thrust

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

Page 3: The Siwalik Fold Belt along the Himalayan piedmont 10 km Main Frontal Thrust Main Boundary Thrust

Uplifted Fluvial Terrace along Bagmati River.

Strath surfaceTop of terrace tread

Page 4: The Siwalik Fold Belt along the Himalayan piedmont 10 km Main Frontal Thrust Main Boundary Thrust

Inferring paleo-river bed from terrace remnants

9.2 kaBP 6.2 kaBP 2.2 kaBP

Page 5: The Siwalik Fold Belt along the Himalayan piedmont 10 km Main Frontal Thrust Main Boundary Thrust
Page 6: The Siwalik Fold Belt along the Himalayan piedmont 10 km Main Frontal Thrust Main Boundary Thrust
Page 7: The Siwalik Fold Belt along the Himalayan piedmont 10 km Main Frontal Thrust Main Boundary Thrust

River incision and terrace formation across an active fold

Page 8: The Siwalik Fold Belt along the Himalayan piedmont 10 km Main Frontal Thrust Main Boundary Thrust

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.

Page 9: The Siwalik Fold Belt along the Himalayan piedmont 10 km Main Frontal Thrust Main Boundary Thrust

• 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?

Page 10: The Siwalik Fold Belt along the Himalayan piedmont 10 km Main Frontal Thrust Main Boundary Thrust

Converting Incision into Uplift

u(x,t): uplift relative to the undeformed footwalli(x,t): river incisionb(t): sedimentation at front of the fold (local base level change)

u(x,t)= i(x,t) + b

Page 11: The Siwalik Fold Belt along the Himalayan piedmont 10 km Main Frontal Thrust Main Boundary Thrust

Comparison of Uplift and Incision profiles

The various terraces yield very similar uplift profiles.

Page 12: The Siwalik Fold Belt along the Himalayan piedmont 10 km Main Frontal Thrust Main Boundary Thrust

How do we convert that information into horizontal shortening of slip rate on the thrust fault?

Uplift relative to footwall basement

Page 13: The Siwalik Fold Belt along the Himalayan piedmont 10 km Main Frontal Thrust Main Boundary Thrust

Determination of shortening from conservation of area

Page 14: The Siwalik Fold Belt along the Himalayan piedmont 10 km Main Frontal Thrust Main Boundary Thrust

• Note that the ‘excess area’ is a linear function of depth only if there is no backshear.

(Bernard et al, 2006)

Page 15: The Siwalik Fold Belt along the Himalayan piedmont 10 km Main Frontal Thrust Main Boundary Thrust

It is assumed here that:- area is preserved during deformation (no compaction nor dilatancy)- deformation is plane (no displacement out of plane)

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Determination of shortening from conservation of area

Page 16: The Siwalik Fold Belt along the Himalayan piedmont 10 km Main Frontal Thrust Main Boundary Thrust

Relationships between fold shape and shortening depend on folding mechanism…

Fault-Bend Fold Detachment FoldPure-shearFault-Bend Fold

Page 17: The Siwalik Fold Belt along the Himalayan piedmont 10 km Main Frontal Thrust Main Boundary Thrust

Collocatedproportional uplift

Non-Collocateduplift

Incremental deformation recorded by terraces or growth strata can be used to test fold models.

(courtesy of John Suppe)

Fault-Bend Fold Detachment Fold

Page 18: The Siwalik Fold Belt along the Himalayan piedmont 10 km Main Frontal Thrust Main Boundary Thrust

Constant bed length v1=v2No backshear v1 constant with depthConstant bed thickness u(x) = v1.sinθ(x)

Fault-bend folding

Page 19: The Siwalik Fold Belt along the Himalayan piedmont 10 km Main Frontal Thrust Main Boundary Thrust

Folded abandoned terraces along Bagmati river• Is the uplift pattern consistent with Fault-bend Folding

as has been assumed to construct the section?

Page 20: The Siwalik Fold Belt along the Himalayan piedmont 10 km Main Frontal Thrust Main Boundary Thrust

Comparing uplift derived from river incision with uplift predicted by fault-bend folding

It is possible to estimate the cumulative shortening since the abandonment of each terrace.

The uplift pattern is consistent with fold-bend folding with no back-shear.

(Lave and Avouac, 2000)

Page 21: The Siwalik Fold Belt along the Himalayan piedmont 10 km Main Frontal Thrust Main Boundary Thrust

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)

Page 22: The Siwalik Fold Belt along the Himalayan piedmont 10 km Main Frontal Thrust Main Boundary Thrust

Constant bed length v1=v2No backshear v1 constant with depthConstant bed thickness u(x) = v1.sinθ(x)

Fault-bend folding


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