Application of the fission track method in Geology Part - II.

Application of the fission track method in Geology

Part - II

3 key questions

What geologic questions can be answered?

What sampling strategy is required?

How can we interpret our fission track data?

Part 2 - The application

What are the processes that we can "date" with fission track data?

Very fast processes with rock cooling:volcanic eruptions, intrusions with fast cooling, hydrothermal event, shear heating along fault plane

Fast processes with rock cooling:fast exhumation or erosion in an active orogen, fast movements along faults (e.g. tectonic unroofing)

Moderately fast processes with rock cooling:moderate exhumation or erosion, moderately cooling in and around intrusive body,

Slow processes with rock cooling:slow erosion or exhumation in a decaying orogen


Real "dating" with the FT method


Only with fast to very fast cooling, the fission track method is able to "date an event"

Potential events:

volcanic eruption

fast cooling intrusion

impact event

hydrothermal event

shear heating along thrust plane

Process rate estimation with the FT method


With moderate and slow cooling, the fission track method only estimates cooling rates. It does NOT necessarily mean an "event".

Possible processes:

erosive denudation

tectonic denudation

topography formation

thermal relaxation

Fission track dating of a single event - I

Australian tektite

Glass drops ejected fromGerman impact crater


Fission track dating of a single event - II

Bohemian Glass from 1849 with 1% of U can be

dated with FT

check of the fission decay constant


Comparison between dating methods - I


Example from German volcano (Kraml et al., in prep.):apatite FT data

Comparison between dating methods - II


Example from German volcano (Kraml et al., in prep.):

Comparison between dating methods - II


FT dating and anthropology


Titanite0.306 ± 0.056 Ma

Titanite0.462 ± 0.045 Ma

Thermoluminescence 0.292 ± 0.026 Ma 0.312 ± 0.028 Ma

U-series dating 0.300 ± 0.040 Ma

(Guo et al. 1991)

How do we know that the FT age represents a single event ?

Track length distribution:All tracks are long (mean length > 14.5 m) and the track length distribution is very narrow.

Radial plot:All single grain ages plot in a narrow cluster (except for very young ages or grains with low U content).

Statistical tests:The calculated central age passes Poissonian 2 tests.

Isochrons:The FT age is in agreement with ages from other dating techniques (e.g. U/Pb, Ar/Ar, (U-Th)/He).

Absence of regional variation:The FT age is identical within the same material, also if sampled at other localities.



Nanga Parbat - I100 km

Fast exhumation processes: example Nanga

Parbat - II


25 km

Fast exhumation processes: example Nanga Parbat - III


Fast exhumation processes: example Nanga

Parbat - IV


Fast exhumation processes: example Nanga Parbat - V


From:Brozovic et al. (1997)

apatite FT ages:

A: 0-1 Ma B: 1-6 Ma C: 6-15 Ma

Fast exhumation processes:

example Taiwan - I


from Dadson et al. (2003):

Exhumation rates (mm yr-1) based on apatite FT ages:

red: reset FT age orange: partially reset blue: not reset

Fast exhumation processes:

example Taiwan - II


from Dadson et al. (2003):

Bedrock incision rates (mm yr-1) as derived from age dating of fluvial terraces

much larger than exhumation rates !

Chicken or egg?

How can we know ?

regional plate tectonic context

very fast cooling points to tectonics

climatic evidence

accompagnying processes

topography analysis


The main question in research today: Who was first, erosion or tectonics ?

Uplift - Exhumation - Denudation


(England & Molnar 1990)

The effect of topography


Convex and concave T-t paths

Assumption:topography evolves in a vertical direction only, no lateral valley shift


The effect of fluid flow


(from Kohl & Rybach,

www.gtr.geophys.ethz.ch/

neatpiora.html)

Fault planes and ages


Fault movements in the Central Alps


Exhumation in a cratonic continent - I


(Gleadow et al. 2002)

Exhumation in a cratonic continent - II



2750 apatite FT ages

Exhumation in a cratonic continent - III



Exhumation in a cratonic continent - IV



The principles of fission track data modelling


The modelling of FT data: age and track length


Genetic algorithm and shrinking of T-t-boxes


Why are detrital zircons better than apatites?


The lag time concept


orogenic cycle


Uplift - erosion - topography


Hack (1975): uplift and topography form steady-state

Penck (1953): uplift is „waxing-waning“

Davis (1899): uplift is short-term process

Detrital age spectra: static and younging age components


steady age componentyounging age component

Probability density plots of FT ages


fitted age populations

statistical fit to density plot

raw data with error envelope

(from Garver et al. 1999)

Decrease and increase of lag time

(from Bernet et al. 2001)


Example: European Alps


pro-wedge

retro-wedge

Example for a decrease of lag time



Example for a steady lag time



FT ages along vertical

bore hole


FT age evolution

along vertical bore hole


example I: bore hole @ Hünenberg


(from Cederbom et al., in press)

example II: Rigi Mountain and bore hole

@ Weggis


(from Cederbom et al., in press)

Exhumed PAZ at Denali, Alaska


(Fitzgerald et al. 1995)

Thank you for your attention !

Application of the fission track method in Geology Part - II.

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