Understanding geodynamic processes from sediments : detrital thermochronology and Cosmogenic Nuclide analyses from the Himalayas

Abstract

Detrital thermochronology is commonly utilized in geological research to quantify the thermotectonic history of orogens by looking at their erosional products. As such, understanding how the erosional processes infl uence the age distribution is necessary. For instance, the calculation of apatite (U-Th-Sm)/He (AHe) ages requires a correction for He loss by alpha-ejection at grain margins. However, in detrital minerals, the He-depleted outer part of the grain may be mechanically and chemically abraded during sediment transport, leading to over-correction of AHe ages. Consequently, in the fi rst part of the thesis, the focus is directed on developing an appropriate sampling strategy to measure the effect of the abrasion on the apatite crystals. We took sand samples from the upper drainage of the Kali Gandaki River in central Nepal in order to find continuously abraded apatites. Samples of the igneous source rock (the Mustang granite) were also dated by AHe and apatite fission track (AFT), to pinpoint the age signal of the source rock. The resulting distribution of detrital AHe ages of sand samples is in good agreement with published geologic constraints and provide the first evidence on the long-term erosion and exhumation history of the Mustang-Thakkhola graben, where virtually no thermochronological constraint yet exists. Furthermore, we demonstrate that abrasion of the detrital apatite grains is a major consequence of river transport within the first kilometres of river transport. We find evidence that the best approach in detrital settings is to analyse a homogeneous grain population but, in this case, the degree of alpha-correction application has to be considered carefully to achieve the distribution of the AHe ages that best represent the underlying cooling patterns. Furthermore, detrital thermochronology is enhanced by applying multiple techniques: to better resolve the cooling history of the Mustang graben, the AHe dataset is integrated with detrital AFT ages. To robustly interpret the combined age distribution, these AHe and AFT ages are compared to synthetic age predicted by a numerical model. The model finds the best agreement between synthetic and real data, by varying a set of parameters that relate to tectonic events and comparing,. The main outcome of the study is the clear constrain of the onset of extension between 13 and 11 Ma. When this result is interpreted in the frame of other neighbouring extensional structures, the Thakkhola graben appears as part of a larger system that accommodates arc-parallel extension and thus share a common timing of onset. In a third and final step, detrital AHe ages are is related to erosion rates obtained with cosmogenic nuclide 10Be. We collected 14 detrital river sand and 3 bedrock samples fromthe southern margin of the Shillong Plateau to measure erosion rates using the terrestrial cosmogenic nuclide 10Be. Erosion rates derived from cosmogenic 10Be from the deeply incised southern valleys of the Shillong Plateau are surprisingly low (50 to 80 m M y-1 ) in a region that is steep, tectonically active, and receives 7 m of annual rainfall. From our measured catchment-averaged erosion rates and estimates of the incised sediment volumes, we quantify the temporal duration of incision in the incised portion of the landscape, which we in turn relate to the onset of surface uplift. The resulting estimates, ranging from 4.4 A /- 0.6 to 4.6 A /- 0.7 Ma, match well with a set of external constraints and available low-temperature thermochronology data, and thus appear to provide a robust estimate for the initiation of surface uplift. In summary, the findings of this thesis highlight that detrital apatite (U-Th-Sm)/He thermochronology, especially in combination with other methods, such Apatite Fission Tracks or Cosmogenic Nuclide dating, is an effective method to address specific problems in the field of geodynamics and tectonic geomorphology.