The use of detrital mineral thermochronology to explore relationships among climate, erosion, and tectonics in the Nepal Himalaya

Abstract

Numerical and analytical models of orogenic growth suggest that erosion can focus deformation in active convergent orogens, leading to a debate over the relative influence of climatic and tectonic forcing on erosion. In this thesis, geologic and observations, thermochronology, and thermo-kinematic numerical modeling are combined to quantify variations in long-term erosion in the Nepal Himalaya. Detrital mineral thermochronology is explored as a tool for quantifying tectonic and geomorphic process rates at a variety of spatial and temporal scales. The assumptions upon which catchment-wide erosion rate estimates based on detrital data depend are evaluated using statistical comparisons of 40Ar/39Ar ages from Nepal and catchment area-elevation distributions. Bedrock 40Ar/39Ar data indicate that erosion rate estimates from detrital thermochronology yield the same rate as the traditional approach of analyzing bedrock cooling ages over a range of elevations. Both bedrock and detrital 40Ar/39Ar data suggest a major acceleration of erosion rates at the Himalayan range front, and apatite fission-track data are used to pinpoint the timing of this acceleration to between 2.5 and 0.9 million years ago.

(cont.) Three-dimensional thermal modeling indicates that the effects of topography, erosion, and rock exhumation pathways on age-elevation relationships do not compromise! this conclusion. While the time frame for this change corresponds to that of an important destabilization of global climate, there is no evidence for a change in tectonic forcing during this interval. These timing constraints support the hypothesis that climate can exert a first-order control on erosion in the evolution of orogens. If climate strongly influences long-term erosion and erosion and deformation are coupled through gravitational feedbacks, then a persistent style of deformation would be expected where monsoon precipitation and erosion have been focused at the Himalayan front for millions of years. Implications of such feedbacks for the steady-state evolution of the range are explored in a detailed analysis of the structural configuration at this position. Future studies of the strength of feedbacks among climate and tectonic forcing, erosion, and deformation are warranted. Creative applications of detrital thermochronology may be used in such studies to constrain landscape response time to climatic and tectonic perturbations.