Determination of uplift rates of fluvial terraces across the Siwaliks Hills, Himalayas of central Nepal

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1 Determination of uplift rates of fluvial terraces across the Siwaliks Hills, Himalayas of central Nepal Martina Böhme Institute of Geology, University of Mining and Technology, Freiberg, Germany Abstract. Geomorphic evidence, especially offset of fluvial terraces, describes the recent crustal deformation in the Siwaliks Hills of central Nepal and is the basis for the determination of tectonic uplift rates. Rock uplift rates are quantified from river incision data that include the consideration of geomorphologic evolution of the rivers. Introduction The area of investigation lies in the Himalayas of Nepal. (Fig. 1) This paper deals with the question of determining tectonic uplift on the basis of geomorphic analysis and geological investigation. It is focused on a 50-km-wide area in the Siwaliks Hills along the piedmont. (Fig. 1) The main object of investigation are warped and tilted fluvial terraces which show the best geomorphic evidence for active tectonics. These terraces provide good constraints on incision rates across the Himalayan frontal folds, where rivers are forced to cut down into rising anticlines and have formed numerous strath terraces. In the Siwaliks Hills the river incision is the most rapid across the entire Himalayas with values up to 10-15mm/yr. This data can provide information on rates of rock uplift. First the main geological, geomorphological and tectonic settings of the Himalayas that are relevant for this paper are reviewed. Then it is focused on the investigation of abandoned fluvial terraces along two rivers in the Siwaliks Hills, the Bagmati and the Bakeya. The main part of this paper is the determination of tectonic uplift rates. The two models used are explained. For the second one base level changes due to sedimentation in the foreland and sinuosity changes of the paleorivers are considered.

2 2 Martina Böhme Fig. 1. Geology and seismicity of Nepal. Box indicates the present study area. (Lavé and Avouac 2000) Geological setting General Background The Siwaliks Hills form the most frontal relief of the Himalayas just north of the Indo-Gangetic Plain. (Fig. 2) The Main Frontal Thrust (MFT), lying at the southern edge of the Siwaliks Hills, is the most active fault in Nepal. N-S shortening along the MFT is on average 21±1.5mm/yr (Lavé and Avouac 2000). Thrusting on the MFT seems to absorb nearly all of the present shortening rate across the whole Himalayan range. The Siwaliks Hills are composed of easily erodible molasse accumulated in the foreland and deformed by thin skinned tectonics (Lavé and Avouac 2001). They form rows of hills with elevations below 1000 m and are separated by narrow piggy back basins. The sedimentary sequence is 3500 to 5500 m thick and was deposited between 14 and 1 Ma. In the study area the outcropping sequence is characterized by fluvial sediments grading upward into coarser material, dated as younger than 11 Ma (Lavé and Avouac 2000). The northern border of the Siwaliks hills is the Main Boundary Thrust (MBT). North of the MBT lies

3 Determination of uplift rates of fluvial terraces across the Siwaliks Hills, Himalayas of central Nepal 3 the Lesser Himalaya. It consists of medium-grade metasediments forming a large duplex structure. North of the Lesser Himalaya rises the topography abruptly from elevations around m to more than 6000 m. This Higher Himalaya consist mainly of high-grade gneiss with large leucogranitic plutons of Miocene age. It is separated from the Lesser Himalaya by the Main Central Thrust (MCT), which is a ductile shear zone. All the mentioned faults may connect to a single detachment (Main Himalayan Thrust, MHT) at depth, as indicated by seismological data and structural observations. To the north extends the Tibetan Plateau with Thetysian sedimentary cover and elevations around 5000m. To the south of the MFT formed the Indo-Gangetic foredeep which trapped a fraction of the eroded material. Several kilometers of molasse deposits have thus accumulated on the Precambrian Indian basement. (Fig. 2) Fig. 2. Geological cross section across the central Himalayas of Nepal (see Fig. 1 for location) (Lavé and Avouac 2000) Fluvial Terraces in the Siwaliks Hills The terraces in the study area were mapped from air photos and Landsat images. Afterwards the results were improved in the field by looking at the weathering

4 4 Martina Böhme Fig. 3. (top) Projection of the strath terraces along the Bagmati River. The elevations of the strath levels are computed from DEM and field measurements. (bottom) Structural cross section across the Siwaliks Hills with the same horizontal scale for comparison. The terraces show clearly an evidence for Holocene and late Pleistocene activity of the Main Frontal Thrust fold. In contrast, they do not show significant deformation linked to the Main Dun Thrust. (Lavé and Avouac 2000)

5 Determination of uplift rates of fluvial terraces across the Siwaliks Hills, Himalayas of central Nepal 5 color, facies and thickness of alluvial veneers and of the overbank deposits. Elevations were measured using a digital elevation model (DEM) or in the field. The classified Holocene terraces have then been dated with Carbon 14 method from charcoal samples. The age measurements were calibrated with dendrochronological scales. The dating results lead to four major episodes of terrace formation. The two most prominent levels are T 0 and T 3. The T 0 terrace is the uppermost level and dated at 9.2 ± 0.2 kyr B.P., while the T 3 terrace is dated at 2.2 ± 0.2 kyr B.P. (Lavé and Avouac 2000) Fig. 3 shows a projection of the elevations of all terrace remnants along the Bagmati River. The terrace profiles T 1 to T 3 all are similar to the T 0 profile, only with a lower degree of back tilting along the back limb of the fold. The examined fluvial terraces show profiles that clearly indicate persistent active folding at the MFT. In addition this data provides some limits on the geometry of interpreted fault bend fold. (Fig. 3) Determination of Uplift Rates Principles If we want to calculate uplift rates, we have to consider all factors, which control the vertical position of the land surface. After England and Molnar (1990) the vertical position of a point on the land (Fig. 4) is a function of (1) bedrock uplift due to tectonic processes; (2) the compaction rate; and (3) the rate of erosion: Surface uplift = bedrock uplift + deposition compaction erosion Fig. 4. Schematic of factors controlling position of the land surface (Burbank and Anderson 2001)

6 6 Martina Böhme These factors, or derivations of them, always have an influence on the uplift rate, independent from the used method. One possibility to derive rock uplift rate is from dated fluvial terraces. In this paper we deal with this method. It is based on the direct relationship between tectonic uplift and river incision rate. Rates of river incision are simply the ratio of the elevation difference h between the ancient terrace and the present river level to the age of the ancient terrace. (Fig. 5) Using a simple model it is assumed that the river gradient, the sinuosity and the base level remain constant during downcutting. If this is not the case, river incision is not only a function of tectonic uplift but also a function of these varying features. A model which considers these variations would be much more complex, but more realistic. Fig. 5. Schematic approach to calculating incision rates using dated strath terraces (Burbank and Anderson 2001) Determination of Holocene Uplift Rates in the Siwaliks Hills For the first model it is assumed that the rivers had constant geometry during the deformation. The tectonic uplift would so simply be equal to the incision. Incision rates of 10-15mm/yr have been calculated based on the measured terrace ages. (Fig. 6) To test this result the ratio of the incision rate I of two different terrace treads should be equal to the ratio of their respective ages t. In this case the comparison of the ratio of incision [I(T 3 )/I(T 0 )=0.19] with the ratio of the age of T 3 and T 0 [(t-t 3 )/(t-t 0 )=0,24] indicates a deficit of incision. Because of the inadequate results of the first model, a second one that includes the river geomorphologic evolution, was invoked. The resulting uplift rates are composed of the incision I deduced from the elevation of the terrace tread above the present river level, of the local base level change at the front of the fold rela-

7 Determination of uplift rates of fluvial terraces across the Siwaliks Hills, Himalayas of central Nepal 7 tive to the Indian basement, for example the sedimentation rate in the Indo- Gangetic Plain and of the change in elevation at a point due to possible changes of the river gradient and sinuosity. (Lavé and Avouac 2000) Fig. 6. Comparison of incision profiles deduced from T 0 and T 3 along the Bagmati River. The two profiles are nearly similar. There are only some mismatches on the the back limb of the fold. This may correspond to a deficit of incision. (Lavé and Avouac 2000) In the following part the possible influence of changes in (a) base level, (b) gradient and (c) sinuosity on the observed incision deficits will be investigated. (a) Estimation of Base Level Changes. The surface south of the MFT is a relatively flat plane and does not show any entrenched channels. There are no redweathered soils. This induced the conclusion that the river aggraded during the Holocene. Because of aggradation there should be a rising base level. The sediments at this region have a thickness of about 2000 m and the oldest have been dated to 5 Ma. An estimate of the minimum average sedimentation rate in the foreland of 0.4 mm/yr ± 0.5 mm/yr is derived from the ratio of the 2000 m of sediments to the maximum age of 5 Ma. This estimate has a large uncertainty, because of its poor reliability and probable climatic-induced variations in the rate of erosion and aggradation. Furthermore the amount of base level change can not explain the whole incision deficit. (Fig. 7)

8 8 Martina Böhme Fig. 7. (a) Profiles of incision deficit since T 3 formation (b) Possible origins of this deficit and their corresponding theoretical results (c) Schematic cross section of the Bagmati valley with the present river slope. (Lavé and Avouac 2000)

9 Determination of uplift rates of fluvial terraces across the Siwaliks Hills, Himalayas of central Nepal 9 (b) Estimation of Paleogradients. There is no way to calculate changes in river gradients. Terraces only indicate the particular dynamic equilibrium at the time of their origin. Today the Bagmati River has a constant gradient of ~0.27% inside the study area. After the principle of minimum stream power expenditure, a river always tends to establish a constant gradient. If we assume an increasing gradient due to the climatic changes in the investigation area, we should have an incision in front of the MFT and an increasing aggradation upstream. (Fig. 7) But indications for this theory can not be found in the field. Consequently the assumption of a constant river gradient which is equal to the present one will be used. (c) Estimation of Paleosinuosities. The former river geometry, especially width and sinuosity can be reconstructed with the help of the respective terrace remnants. The river sinuosity is calculated by the ratio of the length of the thalweg to the valley length. The paleofloodplains were wider and less sinuous than the present ones. Due to a sinuosity increase during Holocene times there should be a constantly higher aggradation along the back limb of the MFT fold and a higher erosion rate in the raising Siwaliks Hills. This theory explains both the shape and the amplitude of the incision deficit profile. (Fig. 7) The tectonic uplift is now the sum of incision, base level change and change in elevation due to changes in sinuosity, (Fig. 8). Testing this result with the same method like for the first model, the ratio of the uplift rate U of two different terrace treads [U(T 3 )/U(T 0 )=0.22] shows a much better agreement with the ratio of their respective ages t [(t-t 3 )/(t-t 0 )=0,24] than the ratio derived from incision rates I [I(T 3 )/I(T 0 )=0.19]. Conclusions This geomorphic study shows a possible method to determine tectonic uplift rates. It is shown that in deriving tectonic uplift from river incision, one has to be careful about possible changes of base level, gradient, and sinuosity. The amount of these terms depends on the tectonic setting. In the present case river incision resulted to a big fraction from tectonic uplift and the variations in river geometry, like base level changes, and sinuosity changes have only a small impact, but in a quieter tectonic setting than it occurs in the investigation area, river incision may mostly result from these effects. Furthermore, the results of the geomorphologic study can give hints to the tectonic structure and the history of deformation. Accordingly, active fault bend folding at the MFT can be quantified from the profiles of the fluvial terraces in the investigation area and from additional structural geological studies.

10 10 Martina Böhme Fig. 8. Uplift rate profile deduced from the Holocene terrace levels along the Bagmati river. The different terrace levels show a better agreement in the uplift curves than in the incision curves (Fig. 6) on the back limb. (Lavé and Avouac 2000) References Burbank, D. W., Anderson, R. S. (2001) Tectonic Geomorphology. Blackwell Science, Massachusetts England, P., Molnar, P. (1990) Surface uplift, uplift of rocks, and exhumation of rocks. Geology, 18, Lavé, J., Avouac, J. P., (2000) Active folding of fluvial terraces across the Siwaliks Hills, Himalayas of central Nepal. J. Geophys. Res., 105, Lavé, J., Avouac, J. P., (2001) Fluvial incision and tectonic uplift across the Himalayas of central Nepal. J. Geophys. Res., 106, 26,561-26,591