Vignettes > Rivers crossing growing folds

Rivers crossing growing folds

Douglas Burbank
University of California, Santa Barbara
Author Profile

Shortcut URL: https://serc.carleton.edu/38040

Location

Continent: Asia, Pacifica
Country: Nepal, New Zealand
State/Province:
City/Town:
UTM coordinates and datum: 45 R 332597 E, 3012936 N; 59 G 420576 E, 5089695 S

Setting

Climate Setting: Humid
Tectonic setting: Continental Collision Margin
Type: Process, Chronology


Figure 1. Spatial variations in rock uplift rate across a growing fold as calculated from deformed river terraces. A. Geometry of the Main Frontal Thrust (MFT) in central Nepal as reconstructed from surface dips in its hangingwall. The uplift rates (shown below) indicate a slip rate of 20 mm/yr along the fault. The offset stratigraphy indicates a minimum of 10 km of slip on the fault. Yet, the relief in the hangingwall is just a few hundred meters. Hence, uplift and erosion must be nearly balanced. Most of the uplifted strata are Neogene foreland basin strata. B. Surveyed Holocene terraces along the Bagmati River, Nepal, in the hangingwall of the MFT. Radiocarbon dates in the deposits overlying each terrace place limits on the age of each. Note the undulating upper terrace surfaces. C. Relative rock-uplift rates derived from the height of each terrace above the modern river. The spatial coincidence of rates for each terrace results from their kinematic tie to subtle changes in dip of the MFT. The remarkable persistence of uplift rates through time argues for steady Holocene slip on the MFT. C. Modified after Lavé and Avouac (2000). Details


Figure 2. Antecedent channel responses to rapid, differential uplift. A. Map of two antecedent rivers and rock-uplift rates in the frontal anticline that grows above Main Frontal Thrust (MFT) in central Nepal. Rock-uplift rates across the map area are calculated by a three-fold process. First, rates are calculated from the deformed terraces (Figure 2). Next, these rates and the bedding geometry indicate that the slip rate on the underlying fault is the same for each valley (~20 mm/yr). Hence, the slip rate is assumed to be uniform between these valleys. Finally, as along the valleys, structural data from transects across the fold are used to define the geometry of the underlying fault which, when combined with the fault-slip rate, can be converted to local rock-uplift rates Modified after Hurtrez et al. (1999) and Wobus et al. (2006) B. Channel profile, floodplain width, incision rate, and calculated excess shear stress for the Bakeya and Bagmati Rivers as they cross MFT hangingwall. The Bagmati narrows without steepening. In the zone of the highest incision rate, the Bakeya both narrows and steepens. Patterns of excess shear stress are closely coupled to channel narrowing and the incision rate. Modified after Lavé and Avouac (2001). Details


Figure 3. Width changes in formerly antecedent channels crossing small growing folds in the Mackenzie Basin, New Zealand. A. Surveys of abandoned channels just upstream of a fold and in its core reveal 3- to 5-fold decreases in width as channels traverse a fold. Note that because loess and soils have partially filled the abandoned channels, their former geometry was reconstructed using closely spaced soil probes. B. Channel incision is measured by surveying the height difference between the channel bottom and the uplifted former terrace surface across the asymmetric fold. C. Compilation of data from five channels shows that 1 to 2 meters of uplift causes a 5- to 10-fold channel narrowing. For these small channels, incision of more than 3 m causes little further narrowing, probably due to wall effects on channels only a few meters wide. Modified after Amos and Burbank (2007). Details


Description

The interactions of fluvial systems with active deforming landscapes has provided fertile ground for exploration of the adjustments that rivers make when they are perturbed by deformation. Two ingredients are commonly combined in successful analyses of interactions between tectonic and geomorphic systems. First, the spatial and temporal characteristics of the deformation need to be defined. The magnitude and rate of displacements and the ways in which they vary across a landscape define a deformational framework. Typically, the relative vertical displacement of one area versus another is most important because such displacement defines how local base level, relief, and slope will change. Second, spatial variations in geomorphologic attributes should be characterized. In the case of rivers, these attributes likely include channel slope, width, and planform geometry, as well as discharge, grain size, and roughness. The combination of the geomorphic changes across a landscape with the pattern of tectonic displacement commonly permits us to deduce which geomorphic changes are induced by the tectonism and which occur independently of it.

How can the pattern and rate of folding be extracted from a landscape? If the fold is steadily growing by incremental displacement on a year-to-year basis, then repeated GPS surveys could delineate a deformation profile. Whereas such annual growth is uncommon for most folds, many folds seem to acquire much of their deformation during earthquakes. So, surveys of the coseismic displacement due to an earthquake could be used as a template for growth, if one assumes that future earthquakes are likely to produce a similar deformation pattern. Because earthquakes on any given fold typically occur once every few hundred years to many thousands of years, however, use of coseismic displacements is usually impractical. A more reliable method is to examine downstream changes in the height of one or more river terraces along the flanks of the river as it flows through the fold. Each terrace typically results from an interval of aggradation (usually driven by climate change) followed by incision. The gradient of the terrace is assumed to have been parallel to the modern gradient. Based on this assumption, downstream changes in the height of the terrace above the modern river can define the magnitude of differential uplift since the terrace was formed. If an age can be obtained on the terrace, then spatial variations in the rate of deformation can also be calculated. In the optimal situation, multiple terraces are preserved, and surveys reveal that the pattern of deformation defined by the youngest (and lowest) terrace is simply amplified through time. One of the most spectacular studies that used this methodology was conducted on a rapidly growing fold in the foothills of the Nepalese Himalaya (Figure 1) (Lavé and Avouac, 2000). There, three dated terraces yielded nearly identical patterns and rates of deformation across the fault-related fold. In fact, the crest of the fold was shown to be uplifting at more than 10 mm/yr, which is equivalent to 10 km in a million years!

How does a river sustain its course across a fold that is growing so rapidly? It helps if the rock within the fold is not too strong–as is the case in Nepal where the fold comprises relatively weakly lithified sedimentary rocks. Nonetheless, across the breadth of the anticline, relative rock uplift is very rapid. If erosion by the river channel did not keep pace with the rate of uplift, the river would either have to aggrade upstream to the height of the uplifted channel bottom in the core of the fold or the river's gradient would be reversed and the river would be deflected around the fold. But in the Nepalese fold, neither of these scenarios has occurred, and instead, two nearby rivers each flow completely across it. Therefore, we conclude that these rivers are eroding just fast enough to balance the rate of rock uplift. Because that rate varies along each river's course, an opportunity exists to explore how characteristics of the channel change across the fold as an apparent function of uplift or erosion rate.

Although the precise controls on river erosion are not agreed upon, the shear stress exerted by flowing water on the river's bed is likely to modulate erosion rates in some fashion. Such shear stress (force per unit area of the bed exerted parallel to the bed) is required not only for transporting the bedload, but also for scouring, plucking, or abrading the underlying bedrock surface. This stress (σ) is commonly quantified as σ = ρ g d sin α,where ρ is water density, g is gravitational acceleration, d is water depth, and α is channel slope or gradient. So, if a river needs to erode more quickly in order to counterbalance a faster uplift rate, what adjustments in channel geometry serve to increase the basal shear stress? The two most obvious are an increase in channel slope and/or a deepening of the flow. The latter is typically accomplished through channel narrowing, because in the absence of a change in slope, velocity and channel cross-sectional area remain about constant. Hence, narrowing of the channel causes deepening of the flow.

Detailed measurements of channel width and slope across the Nepalese fold show as much as 10-fold changes in channel width that correlate rather closely with changes in incision rates inferred from the uplifted terraces (Fig. 2). The Bakeya River, which is the smaller of the two rivers, also shows a steepening of the channel gradient across the zone of highest uplift rates, whereas the Bagmati River displays no significant steepening. When spatial variations in excess shear stress (the stress in excess of that needed to mobilize the bedload) are calculated, they also quite closely match the measured incision rates (Fig. 2).

Let's assume that both channel slope and width are key adjustable variables that serve to modulate river incision rates. As a river encounters a more rapid rate of rock uplift and begins to incise more quickly in order to sustain its course, do slope and width adjust synchronously or does one precede the other? The data from the Bagmati River suggests that changes in width may be sufficient to increase erosion rates several fold without concurrent channel steepening. Studies of much smaller folds in New Zealand bear this out. River width appears very responsive to subtle variations in rock uplift rates: as soon as those rates begin to increase, width begins to narrow (Fig. 3). If the uplift rate is fairly low, then narrowing may be the only adjustment needed. But if uplift is sufficiently rapid, then the channel also tends to steepen. This behavior appears to be exemplified along the Bakeya River where channel steepening occurs only across the zone of highest uplift/incision rates and begins several kilometers downstream of where channel width begins to narrow (Fig. 2).

Associated References

  • Amos, C. B., and Burbank, D., 2007, Channel width response to differential uplift: Journal of Geophysical Research, v. 112, p. F02010, doi:10.1029/2006JF000672.
  • Burbank, D., Meigs, A., and Brozovic, N., 1996, Interactions of growing folds and coeval depositional systems: Basin Research, v. 8, p. 199-223.
  • Hurtrez, J.-E., Lucazeau, F., Lavé, J., and Avouac, J.-P., 1999, Investigation of the relationships between basin morphology, tectonic uplift, and denudation from the study of an active fold belt in the Siwalik Hills, central Nepal: Journal of Geophysical Research, v. 104, p. 12,779-12,796.
  • Keller, E. A., Seaver, D. B., Laduzinsky, D. L., Johnson, D. L., and Ku, T. L., 2000, Tectonic geomorphology of active folding over buried reverse faults: San Emigdio Mountain front, sothern San Joaquin Valley, California: Geological Society of America Bulletin, v. 112, no. 1, p. 86-97.
  • Lavé, J., and Avouac, J. P., 2000, Active folding of fluvial terraces across the Siwalik Hills, Himalaya of central Nepal: Journal of Geophysical Research, v. 105, p. 5735-5770.
  • Lavé, J., and Avouac, J. P., 2001, Fluvial incision and tectonic uplift across the Himalaya of Central Nepal: Journal of Geophysical Research, v. 106, p. 26,561-26,591.
  • Miller, S. R., and Slingerland, R. L., 2006, Topographic advection on fault-bend folds: Inheritance of valley positions and the formation of wind gaps: Geology, v. 34, no. 9, p. 769-772.
  • Snow, R. S., and Slingerland, R. L., 1990, Stream profile adjustment to crustal warping: nonlinear results from a simple model: J. Geol., v. 98, p. 699-708.
  • Wobus, C., Whipple, K. X., Kirby, E., Snyder, N., Johnson, J., Spyropolou, K., Crobsy, B., and Sheehan, D., 2006, Tectonics from topography: Procedures, promise, and pitfalls, in Willett, S. D., Hovius, N., Brandon, M. T., and Fisher, D. M., eds., Tectonics, Climate and Landscape Evolution: Boulder, Geological Society of America, p. 55-74.