| Abstrakt: |
Many articles have cited a study of apatite and zircon fission-track ages from the Eastern Cordillera of the central Andes, Bolivia, as evidence for a marked acceleration of uplift rates over the last 40 m.yr. However, we show that an artifact of the graphical technique used to interpret the data, namely the common-variable problem, drives the apparent increase. In order to determine what can be inferred about exhumation history from the data, we developed a simple quantitative forward model to calculate the coupled apatite and zircon age profiles as a function of height and then used Markov Chain-Monte Carlo techniques to estimate the family of solutions that provide reasonable fits to the data. This model improves on linear regression analysis of elevation-age plots, which treats apatite and zircon data as independent systems. Results suggest that both exponential or step-function velocity models fit the data well. We prefer the exponential model, however, because it appears to be more consistent with uplift history as inferred from paleoelevation data. In this model, the exhumation rate doubles every 20--35 m.yr. from about 120 m/Ma at 50 Ma to around 400--750 m/Ma for the present. Thus, we concur with the original interpretation of an acceleration in the exhumation rate, but our inference is based on a more physical model along with paleoelevation data. This interpretation differs from other reinterpretations of the data based on linear regression analysis that suggest that exhumation began at 40 Ma and then increased sharply sometime between 30 and 10 Ma. More fission-track data, especially zircon data, are needed to distinguish between these two scenarios. [ABSTRACT FROM AUTHOR] |
