Geomophology - 5 Flashcards
Inferential methods of modelling
Digital elevation model analysis
Satellite analysis
What is measured in fluvial monitoring and modelling
Analysis of river incision rate scales with stream power
Discharge
Sediment yells studies
Experimental methods
1) stimulation of corrosion in lab
2) absolute cosmogenic dating
3) absolute thermochronometry
Stimulation of corrosion in the lab
Abrasion in a stimulated fluvial transport shows a supply limited state is most effective
How are cosmogenic isotopes formed
In bedrocks by the interaction of cosmic data with the nuclei of atoms
Production decreases with depth
Why is cosmogenic dating used
Terrestrial cosmogenic nuclide concentrations
Obtain exposure age ( assume zero erosion since the exposure event)
Obtain an erosion rate (assuming that erosion has been steady)
Obtain catchment wide average erosion rates in fluvial sediments, mean catchments
Example of an ideal exposure event
Glacial erosion
What landform can be measured through TCNC dating
Faults as they become exposed to cosmic rays
What do concentrations of TCNC rays show
Concentration increase with time then saturated
Erosion decreased the concentration predictably
Darvill
2013
Exposure and nuclide concentration rates graph
Example target minerals in cosmogenic dating
Olivine
Quartz
Pyroxene
What does absolute - thermochronometry determine?
The age of closure by measuring volumes of U, Th and He in individual grains
How can he be measured ?
By the decay on uranium and thorium in mineral grains
He is trapped and retained at 40-70 oC
+70oC He diffuses our of the grain entirely
What are isotherms used for
Following the surface topography allowing dating of valley ages
What does a bent isotherm mean
Absolute He ages can date topography
Samples near the valley centre of A HE are ..
Older
As they cool earlier and age reflects the pattern of rock uplift
Age pattern is a mirror image of elevation
What is modelling
Provides a crucial means of scaling up observations
Problems of models
Stimulation only as good as the process formulations put in
Can’t observe the interactions over while mountain ranges and Ma
-time and scales
What is stream power erosion model concept
Measures Fluvial incision through an empirical stream power erosion model
Egholmet
2013
Egholmet 2013
Couples stream power and hill slope diffusion model
Alpine fault model shows uplift and erosion patterns
2013
Darvill
Darvill 2013
Cosmogenic nuclide analysis used on a wide range of land forms from hundreds to millions of year’s
Commonly used isotopes - Al,Cl, C
Uncertainties in dates reduced by improving number and quality of global distribution of calibration sites - aided by CRONUS Earth
House
1998
1998
House
House 1998
Sierra Nevada dating U-Th/He ages
Mapping at river incision, mainly using phosphate minerals (apatite)
Concluded Sierra Nevada developed Andean scale topography between 185my and 80my coinciding with crustal thickening
Willet
1999
1999
Willet
Willet 1999
Numerical model of couples process, tectonic deformation and surface erosion developed to investigate their interaction
Give erosion a number, number determines time required for a system to reach steady state and final topographic shape and size of belt
S alps of New Zealand, thermochronometric measurements and metamorphism indicates 11mm/yr of convergence
