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Taiwan exhumation (Carbon ∆ (Graphitization = irreversible (Structure is…
Taiwan exhumation
Questions
What temps did they experience ?
When were they heated ?
Carbon signif
Order = f ( temperature )
Carbon orders itself to highest temp it experiences
Retains that order even as temp ↓
i.e. does not reorder itself to lower temps
Raman spectroscopy of carbonaceous material
Analyse shape of spectra of carbon material
infer how hot sample got in the past
Order depends on
max temperature
R2 Ratio
Calculate
Tells of maximum temperature sample reached
R2 = D1 peak / (G + D1 + D2)
R2 Correlation
Higher R2 ---> Lower temp sample experienced
Low R2
≈ 0 - 1
experienced temps up to ~ 600°C
High R2 >0.5
Temperatures ~ 300-400°C
Carbon ∆
Diagenesis / metamorphism
Carbonaceous material ---> Graphite
Graphitization = irreversible
Structure is not sensitive to retrograde path of rock to surface
∴ Carbon structure = f ( max temp reached during metamorphism)
Limit Raman Spectroscopy
Cannot tell us about temp < 200 °C
Requires graphite in sample ∴ gaps in record
Raman spectroscopy & temp constraints
Beysaac 2007
Most deformed rock
experienced high metamorphic temps
Age distributions corroborates
Samples experiencing high temps have v young ages (near 0)
i.e. Hsuenshan Range & Tananao Complex
Area where no temp data - older ages
Beysaac
2007
Modelled temps expected from subduction (underplating)
compared to measured temperature distribution
U-Th/He findings
Metamorphism occurred early on
i.e. Cretaceous times
∴ separation between the 2
Whereas exhumation = occuring NOW
Metamorphosed rock @ surface
experienced high temps in past
no meta rocks exposed on West
Particle Pathways
Kirstein 2010
If exhumation
= purely vertical
i.e. exhumation solely driven by removal material @ surface
Entire crust would be removed
On contrary
Metamorphic samples exposed @ surface
--> experienced 500°C @ most in past
Would expect much higher temps
if rocks were of lower crustal origin & exhumed vertically
Interp Taiwan Pathways
Kirstein 2010
Metamorphic rocks exposed on E. Taiwan
experienced v high temps in past
none exposed on West
Large horiz component exhumation
∴ Material differs @ surface due to differing particle pathways
Most material must pass through wedge
To explain heterogeneous metamorphic histories
Remove & exhume material laterally
to cause differing degree metamorphism of rocks exposed on E vs W Taiwan