Quantitative Thermochronology and Interpretation of Exhumation in the Central Nepalese Himalaya.

Abstract

Quantifying erosional and tectonic processes that exhume rock in convergent orogens is an essential step toward understanding connections between climate and tectonics. This work utilizes 3-D numerical models to address the sensitivity of thermochronometer data to tectonic and surface processes and quantify exhumation rates from the Nepalese Himalaya. General results show that low-temperature thermochronometers, such as apatite (U-Th)/He (AHe) and fission-track (AFT) are strongly affected by variations in denudation rate, but are less sensitive to different faulting scenarios. Higher temperature thermochronometers, such as zircon fissiontrack (ZFT) and muscovite 40Ar/ 39Ar, have much greater sensitivity to faulting history. Surface processes significantly affect detrital thermochronometers across a range of effective closure temperatures, with bedrock landslides showing a substantial impact on age distributions, particularly for short landslide sediment residence times (∼1 year). For both bedrock and detrital thermochronometers in rapidly eroding regions, little sensitivity to rock thermophysical properties, basal temperature/heat flux or topographic evolution is observed. The impact of these processes is important because they affect the calculation of exhumation rates from both bedrock and detrital thermochronometer data. Compared to exhumation rate estimates that assume a 1-D thermal field from bedrock AFT data (−2.6 − 12.2 mm/y), the range of model-predicted exhumation rates is >200% smaller (1.8−5.0 mm/y). Low-temperature (AHe) detrital thermochronometers show potential for large (>300%) overestimates of exhumation rates when using 1-D data interpretation techniques. At higher temperatures (e.g., MAr), the overestimation decreases to ∼90%. The flow of groundwater is also an important influence on exhumation rate calculations, with AFT data showing potential for underestimation of true exhumation rates by >200% in regions with high groundwater flow rates, compared to regions unaffected by groundwater flow. Overall, these results suggest that the most reliable exhumation rates calculations from active orogens should include a combination of both bedrock and detrital samples. Furthermore, hot springs should be considered as they may indicate significant groundwater flow and the use of 3-D numerical models in regions with complex faulting histories is recommended.