Detrital Thermochron Concepts
Applications
A. Fingerprint sediment source
What's been eroded ?
B. Monitor exhumation
Mtns belts have diff stages
a. Growth - expect ↓ lag time
b. Steady-state - expect constant lag time
c. Decay - expect ↑ lag time
Lag time Concept
Travel time from Z --> surface --> basin
How long sample took to travel
from Z through closure temp
erosion from surface
transport to basin
Lag time calculation
Need to know
- Thermochron age of sample
Assume set @ Z in closure zone
- Depositional age
When deposited in basin ?
∆T = tc - td
∆T = (time of closure) - (time of deposition)
Exhumation Rate
= [Tc-Ts/G] / ∆T
Tc = closure temp of thermochron used
G = geothermal gradient
Ts = surface temp
∆T = lag time
Lines of constant lag time
Sample took ? ka before reaching basin irrespective of thermochron age
i.e. 5 ka before deposition
Time elapsed btwn closure & deposition = constant
Infer mtn evo from lag time
Growth
lag time ↑
erosion / system speeds up
material gets younger for given depositional age
Steady state erosion
Constant lag time
Constant difference btwn thermochron & depositional age
Steady erosion rate
Decaying system
lag time↓
decreasing erosion rate
↓ difference btwn thermochron & depositional ages
i.e. cooling ages young slower vs depositional ages
Depo age dating
Biostratigraphy
index fossils i.e. forams
Magnetostratigraphy
lotsa polarity reversals past 20 Ma
Age spectra
Lag time plots