Defining rates of erosion using terrestrial cosmogenic nuclides in the Himalaya
Shortcut URL: https://serc.carleton.edu/60234
Location
Continent: Asia
Country: Pakistan, India, Nepal and China
State/Province:N/A
City/Town: N/A
UTM coordinates and datum: none
Setting
Climate Setting: Tropical
Tectonic setting: Continental Collision Margin
Type: Process
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Description
The Himalaya and Tibet comprise the greatest mountain mass on our planet, stretching for ~ 2000 km east-west and >1500 km north-south with an average elevation of ~5000 m above sea level. The mountain mass contains the entire world's 8000-m-high peaks and spans climates ranging monsoonal to arid. This grandeur is essentially the consequence of the northward collision of the Indian and Eurasian continental plates, initiated ~ 50 Ma, which has resulted in over 2000 km of crustal shortening and impressive mountain uplift. The mountain landscapes, however, are also the consequence of profound erosion by glaciers and rivers, mass movement (slope) processes and weathering.
Defining how quickly the Himalaya and Tibet are uplifting and eroding has been one of the great challenges in geomorphology over their many decades of study and is important for helping to quantify tectonic and geomorphic models. Long term rates of uplift have been broadly defined using petrological and structural geological methods, which essentially indicate how quickly rock is brought to the surface and/or exhumed. Recently, global positioning systems have been used to determine short-term (years-decades) rates of surface displacement. Until recently, rates of erosion has been estimated by calculating the volumes of sediment produce by erosion and trapped in natural (e.g. river valleys/basins) and/or artificial (e.g. reservoirs) traps, or direct measurements of sediment loads in rivers. However, accurately measuring sediment volumes in natural traps is challenging because often the geologic record is incomplete, which may lead to underestimates. Also direct measurements of sediment loads in rivers provide only a short time glimpse at erosion rates, that may be human influenced. Moreover, studies have shown that rates of erosion may vary considerably over the very short timescale of the study due contrasting weather conditions from one year to the next associated with a monsoon climate.
In recent years, researchers have begun to use terrestrial cosmogenic nuclides (TCNs) to help define rates of erosion on geomorphic timescales (years to hundreds of thousands of years). TCNs are produced by the interaction of cosmic ray particles (mostly neutrons at Earth's surface) with minerals in rock and/or sediment at Earth's surface. The concentration of TCNs increases with time in an exposed surface. Notable TCNs include Be-10, C-14, Al-26 and Cl-36. The rate of production for a particular TCN depends on latitude and altitude. When accurately determined the TCN concentration in a rock or sediment surface provides an estimate of the time that surface has been exposed to cosmic rays. There are lots of caveats that need to be considered when determining an TCN age, for example, if the surface was shielded from cosmic rays by a cover of sediment or soil until recently. In a simple situation such as bedrock eroded by rivers and left high above the valley side when the river progressively cuts down then the concentrations of TCNs in that surface (a strath terrace) will provide an age of river abandonment (Fig. 1). An erosion river (fluvial) rate can be determined by simply dividing the surface age of the strath terrace by its height above the present river. Numerous strath terraces have been dated using Be-10 throughout the Himalaya (summarized in Dortch et al., 2011). These are providing good estimates for rates of river erosion. In particular, the studies are showing that rates of erosion Himalayan rivers range greatly, from as little as 0.02 to >25 mm/year based on strath terraces that date from a few thousand to many hundreds of thousands of years. Erosion rates based on strath terraces younger than about 35,000 years likely reflect sporadic and accelerated erosion. In contrast, erosion rates based on older strath terraces likely reflect long-term erosion that is generally keeping in place with the rate of bedrock uplift.
Concentrations of TCNs boulders or tors (isolated bedrock knobs) on mountain summits can be used to define rates of summit erosion (Fig. 2). Sampling can be problematic, but where samples have been collected such as in Zanskar and Ladakh in Northern India they show that erosion of summits are surprisingly little, with almost zero rates of erosion in some instances.
Rivers, glaciers, mass movement processes and the wind transport eroded sediment through mountain catchments. TCN concentrations in sediments can also be used to determine rates of erosion averaged across a mountain catchment. Concentrations of TCNs are lowest in transported sediment in catchments that have the highest erosion rates (see Portenga and Bierman, 2011 for more details). Sampling stream sands and measuring the TCN concentrations can be directly used to determine basin wide erosion rates (Fig. 3). Such studies are showing strong contrasts between catchments in different climatic and tectonic zones in the Himalaya and Tibet (see Seong et al., 2009b and Dortch et al., 2012 as examples). These are giving erosion rates that vary from a few tens of meters to hundreds of meters per million years. The differences in rates of erosion likely reflect different climate and tectonic settings, but it is usually difficult to resolve which of the two sets of processes dominate. In a similar way, TCN concentrations in debris on the surfaces of glaciers can be used to determine how fast rock is falling onto glaciers from the headwalls of the glacier catchments (see Seong et al., 2009a as an example).
The new studies that are using TCNs to help defining erosion rates in the Himalaya and Tibet are providing important insights into spatial and temporal variation in erosion. This in turn is turn is helping to address such questions of whether climate or tectonics is most the most important factor in controlling landscape development with high mountain environments within continental-continental collision zones.
Defining how quickly the Himalaya and Tibet are uplifting and eroding has been one of the great challenges in geomorphology over their many decades of study and is important for helping to quantify tectonic and geomorphic models. Long term rates of uplift have been broadly defined using petrological and structural geological methods, which essentially indicate how quickly rock is brought to the surface and/or exhumed. Recently, global positioning systems have been used to determine short-term (years-decades) rates of surface displacement. Until recently, rates of erosion has been estimated by calculating the volumes of sediment produce by erosion and trapped in natural (e.g. river valleys/basins) and/or artificial (e.g. reservoirs) traps, or direct measurements of sediment loads in rivers. However, accurately measuring sediment volumes in natural traps is challenging because often the geologic record is incomplete, which may lead to underestimates. Also direct measurements of sediment loads in rivers provide only a short time glimpse at erosion rates, that may be human influenced. Moreover, studies have shown that rates of erosion may vary considerably over the very short timescale of the study due contrasting weather conditions from one year to the next associated with a monsoon climate.
In recent years, researchers have begun to use terrestrial cosmogenic nuclides (TCNs) to help define rates of erosion on geomorphic timescales (years to hundreds of thousands of years). TCNs are produced by the interaction of cosmic ray particles (mostly neutrons at Earth's surface) with minerals in rock and/or sediment at Earth's surface. The concentration of TCNs increases with time in an exposed surface. Notable TCNs include Be-10, C-14, Al-26 and Cl-36. The rate of production for a particular TCN depends on latitude and altitude. When accurately determined the TCN concentration in a rock or sediment surface provides an estimate of the time that surface has been exposed to cosmic rays. There are lots of caveats that need to be considered when determining an TCN age, for example, if the surface was shielded from cosmic rays by a cover of sediment or soil until recently. In a simple situation such as bedrock eroded by rivers and left high above the valley side when the river progressively cuts down then the concentrations of TCNs in that surface (a strath terrace) will provide an age of river abandonment (Fig. 1). An erosion river (fluvial) rate can be determined by simply dividing the surface age of the strath terrace by its height above the present river. Numerous strath terraces have been dated using Be-10 throughout the Himalaya (summarized in Dortch et al., 2011). These are providing good estimates for rates of river erosion. In particular, the studies are showing that rates of erosion Himalayan rivers range greatly, from as little as 0.02 to >25 mm/year based on strath terraces that date from a few thousand to many hundreds of thousands of years. Erosion rates based on strath terraces younger than about 35,000 years likely reflect sporadic and accelerated erosion. In contrast, erosion rates based on older strath terraces likely reflect long-term erosion that is generally keeping in place with the rate of bedrock uplift.
Concentrations of TCNs boulders or tors (isolated bedrock knobs) on mountain summits can be used to define rates of summit erosion (Fig. 2). Sampling can be problematic, but where samples have been collected such as in Zanskar and Ladakh in Northern India they show that erosion of summits are surprisingly little, with almost zero rates of erosion in some instances.
Rivers, glaciers, mass movement processes and the wind transport eroded sediment through mountain catchments. TCN concentrations in sediments can also be used to determine rates of erosion averaged across a mountain catchment. Concentrations of TCNs are lowest in transported sediment in catchments that have the highest erosion rates (see Portenga and Bierman, 2011 for more details). Sampling stream sands and measuring the TCN concentrations can be directly used to determine basin wide erosion rates (Fig. 3). Such studies are showing strong contrasts between catchments in different climatic and tectonic zones in the Himalaya and Tibet (see Seong et al., 2009b and Dortch et al., 2012 as examples). These are giving erosion rates that vary from a few tens of meters to hundreds of meters per million years. The differences in rates of erosion likely reflect different climate and tectonic settings, but it is usually difficult to resolve which of the two sets of processes dominate. In a similar way, TCN concentrations in debris on the surfaces of glaciers can be used to determine how fast rock is falling onto glaciers from the headwalls of the glacier catchments (see Seong et al., 2009a as an example).
The new studies that are using TCNs to help defining erosion rates in the Himalaya and Tibet are providing important insights into spatial and temporal variation in erosion. This in turn is turn is helping to address such questions of whether climate or tectonics is most the most important factor in controlling landscape development with high mountain environments within continental-continental collision zones.
Associated References
- Dortch, J.M., Dietsch, C., Owen, L.A., Caffee, M.W. and Ruppert, K., 2011a. Episodic fluvial incision of rivers and rock uplift in the Himalaya and Transhimalaya. Journal of the Geological Society, London, 168, 783-804.
- Dortch, J.M, Owen, L.A., Schoenbohm, L.M., Caffee, M.W., 2011b. Asymmetrical erosion and morphological development of the central Ladakh Range, northern India. Geomorphology, 135, 167-180. doi:10.1016/j.geomorph.2011.08.014
- Portenga, E.W., Bierman, P.R. 2011., Understanding Earth's eroding surface with 10Be. GSA Today, 21, 8, 4-10.
- Seong, Y.B., Owen, L.A., Caffee, M.W., Kamp, U., Bishop, M.P., Bush, A., Copland, L. and Shroder, J.F., 2009a. Rates of basin-wide rockwall retreat in the K2 region of the Central Karakoram defined by terrestrial cosmogenic nuclide 10Be. Geomorphology, 107, 254-262.
- Seong Y.B., Owen, L.A., Yi, C., Finkel. R.C. and Schoenbohm, L., 2009b. Geomorphology of anomalously high glaciated mountains at the northwestern end of Tibet: Muztag Ata and Kongur Shan. Geomorphology, 103, 227-250.