Taiwan Detrital Thermochron 2
Double
date grains
(Kirstein, 2010)
i.e. get zircon He & FT ages from E. Foredeep
Ages = same (within error of eachother)
∴ Must be cooling v rapidly for there not to be a difference
Gives inherently different cooling temps of ~Z
Implications: V high erosion rates sustained for last few Ma atleast
Calc exhumation from multiple thermochrons
Double dates
ZFT
AFT
Age ~ 15 Ma - T ~ 120°C
Age ~ 20 Ma - Tc ~ 230°C
Cools ~110°C in 5 Ma
Exhumation rate ~ 0.73 km/Ma
Assuming geothermal gradient ~30°C / km
NW Foreland
Samples exposed in W. Taiwan
Have no memory of collision btwn arc & mainland
∴ Must sample from East Taiwan
--> to work out fast exhumation rates
Plio-Pleistocene Record, Taiwan
East Taiwan
High exhumation rates of 2 - 4 km/Ma
from Late Pliocene ( < 3.5 Ma - Present )
West Taiwan
No record of deeply exhumed rocks through Pliocene/early Pleistocene
Large Horiz Component
(Kirstein, 2010)
Horiz accretion of material controls flux - not vertical
∴ Deeper particles go deeper down into orogen & exhumed on East
Detrital record = consistent
Most erosion
focussed on E
= consistent with greater flux & erosion
Rivers = bigger & more dendritic
Resolving exhumation rate @ various scales
Resolving exhumation rate from sediment @ different temporal & spatial scales
Past rates exhumation
Rqmts = lag time
Lag time = ZFT age - depositional age
Modern rates exhumation
Hypsometry x grain age distribution
Gives basin-wide exhumation rates
Hypsometry?
Hypsometry = Area & elevation of catchment area
Basin-wide exhumation rates
Hypsometry x age-elevation --> Cooling age profile
Can infer cooling age profile if know:
- Catchment hypsometry
- Sample grain ages
∴ Work out which parts basin = eroding
Modern River E. Taiwan
All modern samples have
young peak grain age populations
= Consistent with young growing orogen
Not all of catchment eroding in same way
Steeper reaches
∴ younger Zr FT age
Greater exhumation rate
Application 1: EXHUMATIONRATES