Anders, M. H., Gregory-Wodzicki, K. M., and Spiegelman, M., submitted
to Journal of Geology, A Critical Evaluation of Late Tertiary Accelerated
Uplift Rates for the Eastern Cordillera, Central Andes of Bolivia.
Abstract.-- Many papers have cited a recent 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.y. 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 use 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, because it appears to be more consistent with
uplift history as inferred from paleoelevation data. In this model, exhumation
rate doubles every 20 to 35 my 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 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,
which suggest that exhumation began at 40 Ma and then increased sharply
sometime between 30 Ma and 10 Ma. More fission-track data, especially zircon
data, are needed to distinguish between thes