『Abstract
The evolution of an orogen is marked by phases of topographic
growth, equilibrium, and decay. During these phases erosion rates
vary in response to temporal and spatial changes in climate, topographic
relief and slope, and deformation. Detrital thermochronometer
cooling-age data collected from syntectonic basin deposits are
a promising tool for quantifying erosion histories during orogenic
evolution. Previous studies typically assume steady-state erosion
for interpreting detrital data, although in many situations this
assumption is not justified. Here we present a new numerical modeling
approach that predicts thermochronometer cooling ages in a stratigraphic
section where sediment is sourced from a region with a temporally
variable erosion history. Multiple thermochronometer cooling ages
are predicted at different stratigraphic horizons as a function
of variable erosion histories, rock cooling rates in the hinterland,
and thermophysical material properties and boundary conditions.
The modeling approach provides the context for the interpretation
of natural data, including geologically realistic situations with
a temporally varying erosion rate. The results of three end-member
hinterland erosion histories are explored: (1) steady-state erosion;
(2) increasing erosion rate with time; and (3) decreasing erosion
rate with time. Results indicate that for steady erosion rates
between 0.2 and 1.0 mm/y, up to 30 m.y. will pass following a
change in erosion rate before the detrital ages have adjusted
to reflect a new erosion regime. In simulations with transient
erosion, the estimation of erosion rates from a detrital record
using assumption of thermal steady-state will generally be in
error, often by as much as -25 to 100%. These results highlight
that assumptions of steady erosion in mountain belts should be
used with caution. Application of the model to thermochronometer
cooling ages preserved in syntectonic sediments sourced from the
Nanga Parbat region, Himalaya, illustrates how the transient catchment
averaged erosion history can be quantified with detrital thermochronology.
In this example, we found that erosion rates increased over the
past 20 Ma, from about 1.0 mm/yr to modern rates in the range
of 1.5 to 2.0 mm/yr.
Keywords: detrital thermochronology; orogenic erosion; Himalaya;
thermal modeling』
1. Introduction
2. Background: erosion, exhumation, and thermochronology
3. Modeling approach
4. results
4.1. Steady-state erosion, predicted thermochronometer ages,
and lag-times
4.2. Errors in estimating erosion rate from lag-time
4.2.1. Steady state case
4.2.2. Linearly changing rates
4.2.3. Step-changes in erosion rates
5. Discussion
5.1. Lag times and erosion rates
5.2. Precision of erosion rate estimates
5.3. An example from the Siwalik Group from the Himalaya
6. Conclusions
Acknowledgements
Appendix A. Supplementary data
References