DETRITAL-MINERAL THERMOCHRONOLOGY: INVESTIGATIONS OF OROGENIC DENUDATION IN THE HIMALAYA OF CENTRAL NEPAL

Open Access
Author:
Brewer, Ian D
Graduate Program:
Geosciences
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
April 13, 2001
Committee Members:
  • Douglas W. Burbank, Committee Chair
  • Rudy L Slingerland, Committee Member
  • Peter B Flemings, Committee Member
  • Derek Elsworth, Committee Member
Keywords:
  • geodynamics
  • geology
  • tectonics
  • thermochronology
  • argon
  • erosion
  • orogenic
  • himalaya
  • marsyandi
  • chronology
  • dating
  • thermal modeling
Abstract:
This investigation examines the fundamental processes that determine the distribution of cooling ages observed in detrital minerals eroded from orogenic belts. A detrital cooling-age sample collected from a riverbed represents an integration of information from the upstream area. Within orogenic belts that contain glacial cover and high relief, detrital minerals provide an easy method to sample the range of cooling ages found within a basin. In addition, detrital-mineral thermochronology can be used to extract information from the foreland stratigraphic record, which extends the temporal applicability of the technique beyond traditional bedrock thermochronology. For example, individual mineral grains can be extracted from a stratigraphic horizon and dated. Following correction for the stratigraphic age of the horizon, the detrital mineral ages provide a proxy for the erosion rates contained within the catchment area at the time the rock was deposited. However, before reliable interpretations of the stratigraphic record are made, a modern calibration of the technique was needed. We investigated the spatial development of a modern cooling-age signal in the Marsyandi valley of central Nepal with muscovite grains dated using 40Ar/39Ar thermochronology. Over 500 individual grains were dated from both the trunk stream and tributaries over a ~100-km transect along the Marsyandi. These provide a database that displays striking contrasts along the length of the Marsyandi River. The first stage of the investigation focused on the interaction of geological parameters that control the distribution of detrital cooling ages from an individual basin. The range of bedrock cooling ages contained within a catchment is determined by the erosion rate and the depth of the closure isotherm (~350°C for muscovite). With a 2-D thermal model, we investigated the effects of the vertical erosion rate and topography on the depth of the closure isotherm. Increasing the erosion rate and/or topographic relief decreased the depth of the closure isotherms below valley floors, and re-equilibration following sustained changes in the erosion rate took ~10 My. Once the range in cooling ages had been determined for a basin, the distribution of detrital cooling ages in sediment at the basin mouth was calculated as a function of catchment hypsometry. This approach was applied to two sub-catchments of the Marsyandi River. The predicted probability distribution of cooling-ages matched the observed data better in the more slowly eroding basin, than in the more rapidly eroding basin. To understand the more complex distribution of cooling ages from the mouth of the Marsyandi River, the basin was divided into smaller sub-basins that were modeled individually. To predict the trunk-stream signal, individual tributaries were mixed as a function of their area, erosion rate, and the percentage of muscovite in their sediment, which was determined from point counting. Comparison of the model results with observed data illustrated that the detrital-cooling age signal evolved downstream in an understandable manner, and suggested that the mechanical comminution of muscovite was not significant over the length-scale of the basin. The pattern of spatial erosion seen in the thermochronology – low erosion rates in the Tibetan zone, high erosion rates in the Greater Himalaya zone, and intermediate erosion rates in the Lesser Himalayan zone – was broadly similar to calculations of erosion rate based upon the point-counting results. Sample pairs were dated to assess the temporal and spatial variability of the cooling-age signal within the fluvial system. Results indicated that the samples were undistinguishable at the 95% confidence level, once the effects of random selection and the number of grains dated had been accounted for. A more integrated approach was used to predict the spatial distribution of bedrock cooling ages within the 3-D landscape, and the distribution of detrital cooling ages resulting from the erosion of that landscape. A 2-D kinematic-and-thermal model, using the assumption of a single orogen-scale decollement, was developed to predict the depth of the closure isotherm as a function of the ramp geometry and the relative partitioning of convergence between the Indian Plate underthrusting and southern Tibet overthrusting. The thermal result was extrapolated laterally and combined with a digital elevation model to predict the distribution of bedrock cooling ages. At any site in the landscape, the cooling age is a function of the distance each rock particle travels after passing through the closure isotherm and its speed along the trajectory predefined by the underlying decollement geometry. Once the contribution of each site had been corrected for lithological factors and the volume of sediment eroded, a theoretical cooling-age probability distribution was calculated for the Marsyandi by the summation of age-probability for all sites within in the basin. The volume of sediment was calculated as a function of the regional slope and the angle of the underlying ramp. Comparison of various model runs with the observed data indicates that the best solution is obtained by partitioning the total of ~20 mm/yr of convergence between India and southern Tibet into 15 km/my of India underthrusting, and 5 km/yr of Asian overthrusting and subsequent erosion. However, the exact partitioning is dependent upon the geometry of the decollement. A variant of the model that assumed the modern Main Central Thrust represented the approximate surface trace of the orogen-scale decollement produced better results than those runs that assumed the Main Boundary Thrust represented the surface trace of the orogen-scale decollement. This provides additional evidence that the MCT has been active recently. The new methodology of integrating complex kinematic-and-thermal models with digital elevation models can be applied to any orogenic belt, and it may be used to compare theoretical predictions against easily collected and analyzed detrital-mineral data.