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Influence of periodic climate changes on the evolution of mountain ranges

Final Report Summary - GEOCYCL (Influence of periodic climate changes on the evolution of mountain ranges)

Tectonics of the Himalayas

- Problem statement: Understanding the contribution of different fault systems to mountainbuilding is critical to reconstruct the history of mountain building.
- Actions : We have developed a mechanical analysis of the problem of slip partitioning between the major thrust systems in a collisional range. We focused on two structures in the Himalayas of central Nepal: the Main Himalayan Thrust (MHT) and the Main Central Thrust (MCT). We used finite-element modeling to test the influence of various parameters, such as friction coefficients, crustal rheology, and surface processes, and we investigate how they affect the distribution of deformation between these two faults. We observed that reproduction of the late Quaternary kinematic pattern across the range with our model requires strict conditions on the friction coefficients, such that the MHT is very weak, whereas the MCT is significantly stronger. The most important parameter in controlling the partitioning appears to be the dip angle of the MCT, with a gentler or steeper MCT promoting or inhibiting slip, respectively. More pliant crustal rheologies affect the partitioning pattern between the MCT and MHT and allow for a larger amount of convergence between India and Tibet to be accommodated as distributed strain in the upper crust. We also show that transient loading and unloading through the expansion of glaciers and associated erosion (focused in the higher part of the range) can unclamp the MCT and allow for a significant increase in slip rates.

The results of this mechanical sensitivity investigation have important implications for the dynamics of the Himalayan wedge and point toward along-strike structural variations as a first-order control on slip partitioning.

Present-day glacial erosion in the Himalayas

- Problem statement: Deciphering spatial patterns of denudation in mountain ranges is one of the key challenges for our understanding of landscape evolution. Among the diversity of processes contributing to denudation in high mountain environments, glacial erosion is one of the most difficult to constrain, despite its critical importance in the evolution of many mountain ranges.
Actions: We have collected a new dataset of 10 Be concentration in fluvial sediments along the Marsyandi river and its main tributaries in central Nepal. We observed a significant impact from glacially-derived sediments along the Marsyandi river, complicating conventional interpretations of 10 Be-derived catchment scale denudation rates. Using a simple linear mass-conservation formulation we were able to deconvolve the different denudational contributions to the observed signal, as well as constrain their magnitude and spatial distribution. Our results are consistent with previous estimates for total denudation in the area and suggest significant variations in glacial erosion within the Marsyandi catchment. The linear system approach we use can provide an efficient framework for the analysis of detrital cosmogenic nuclides data in complex geomorphic environments and allow the inversion of numerous parameters controlling denudation processes.

Quantitative analysis of Himalayan topography

- Problem statement: The shape and amplitude of topography in mountain ranges are the resultant of the combined action of several factors such as tectonic uplift, precipitation and bedrock properties. Understanding the scaling properties of topography in actively uplifting areas is a major issue in quantitative geomorphology.
- Actions: Analytical formulations of non-glaciated landscape evolution clearly demonstrate that metrics such as local relief or drainage density are explicitly related to the spatial distribution of tectonic uplift, precipitation, erodibility and local slope across the landscape. However, in most regions, these parameters are seldom documented with enough resolution and precision to allow a systematic and statistically significant investigation of their relationships with both horizontal and vertical scaling properties of topography. A notable exception is the Himalaya of central Nepal, where the last 20 years of tectonic and geomorphological research have produced one of the densest regional data-set and documented major gradients in uplift and precipitation across the range (e.g. Lavé and Avouac, 2001; Bookhagen and Burbank, 2006). We have built on the derivation of total catchment relief of Tucker and Whipple [2002] to include the contribution of precipitation in addition to uplift and erodibility. Then, by minimising the misfit between observed and predicted catchment relief, we assess the erodibility parameter for each second or third order catchment in our area of investigation. The resultant erodibility map:

(1) matches the distribution of geological units; and
(2) reveals a number of interesting second order patterns, such as along-strike fluctuations in the Lesser Himalayas and a significant decrease in erodibility coincident with the location of the MCT zone.

This latter result possibly highlights the effect of intense schistosity and fracturation on large scale erosion efficiency (Molnar et al., 2007).

Landscape response to changing climatic conditions

- Problem statement: Modification of climatic parameters, in particular precipitation, directly influence erosion processes and landscape evolution. Furthermore, at the scale of the Late Cenozoic, climate is known to fluctuate following specific periods of oscillations (Milankovitch cycles). While the effects of punctual changes in climatic parameter have already been considered in several studies, the influence of the periodic nature of the signal has seldom been considered. It is however a critical field of research because some researchers postulate that the observed increased in sedimentation rates in the Late Cenozoic could be attributed to a change in the frequency content of the climatic signal (Molnar, 2004).
- Actions: We explore this problem using surface process modeling (SPM) where we submit a steady-state landscape to sinsusoidal variations in precipitation and observe the resulting sediment flux out of the landscape. We observe that the response to this forcing is itself periodic. To test the sensibility of the system we sweep across a range of frequencies representative of actual paleoclimatic signals, and it appears that the amplitude of the response is strongly controlled by the period of the oscillation. For a given landscape, there is a specific period that maximise the amplitude of the response in terms of variations in the sediment flux. This observation is analogous to a resonance phenomenon and points toward a clear sensitivity of landscapes to the spectral nature of the climatic signal. Such results have significant implications for our understanding of the complex coupling between climate change and landscape evolution.

Past erosion rates and patterns in the Himalayas

- Problem statement: Past erosion rates in active mountain ranges, such as the Himalayas, are a critically important data to understand the evolution of these ranges under changing climatic conditions.
- Actions: We have already documented the evolution of the modern 10 Be signal along the Marsyandi river and we collected additional samples from dated fluvial terraces in order to measure the 10 Be at the time of formation for these terraces, which can be used as a proxy for paleo-erosion over the corresponding catchment. Available data show that in the lower reach of the river, south of the topographic front, the past concentrations are close to modern values, suggesting that a large part of the climatically-driven fluctuations of the sediment flux are buffered by the actively uplifting and eroding front range. Another limitation for the reconstruction of past erosion rates in the Marsyandi valley setting is the presence in the upper part of the catchment of the carbonate-rich and quartz-poor sediments of the Tethyan series which are not going to contribute importantly to the 10Be despite being affected by important glacial fluctuations. Discrete pulses of sediment through debris flows were also identified along the river and their origin was traced back to a Higher Himalayan source using their strontium isotopic signature (87Sr/87Sr).