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

[摘要] Quantifying erosional and tectonic processes that exhume rock in convergent orogensis an essential step toward understanding connections between climate andtectonics. This work utilizes 3-D numerical models to address the sensitivity ofthermochronometer data to tectonic and surface processes and quantify exhumationrates from the Nepalese Himalaya. General results show that low-temperaturethermochronometers, such as apatite (U-Th)/He (AHe) and fission-track (AFT) arestrongly affected by variations in denudation rate, but are less sensitive to differentfaulting scenarios. Higher temperature thermochronometers, such as zircon fissiontrack(ZFT) and muscovite 40Ar/ 39Ar, have much greater sensitivity to faultinghistory. Surface processes significantly affect detrital thermochronometers across arange of effective closure temperatures, with bedrock landslides showing a substantialimpact on age distributions, particularly for short landslide sediment residence times (∼1 year). For both bedrock and detrital thermochronometers in rapidly erodingregions, little sensitivity to rock thermophysical properties, basal temperature/heatflux or topographic evolution is observed.The impact of these processes is important because they affect the calculation ofexhumation rates from both bedrock and detrital thermochronometer data. Comparedto exhumation rate estimates that assume a 1-D thermal field from bedrockAFT 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 thermochronometersshow potential for large (>300%) overestimates of exhumation rates when using1-D data interpretation techniques. At higher temperatures (e.g., MAr), the overestimationdecreases to ∼90%. The flow of groundwater is also an important influenceon exhumation rate calculations, with AFT data showing potential for underestimationof true exhumation rates by >200% in regions with high groundwater flow rates,compared to regions unaffected by groundwater flow. Overall, these results suggestthat the most reliable exhumation rates calculations from active orogens should includea combination of both bedrock and detrital samples. Furthermore, hot springsshould be considered as they may indicate significant groundwater flow and the useof 3-D numerical models in regions with complex faulting histories is recommended.

[发布机构] University of Michigan

[效力级别] Himalaya

[关键词] Thermochronology;Himalaya;Modeling;Exhumation;Erosion;Tectonics;Science;Geology