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Isotherm Effects (PRZ shape = f ( (Crystal shape (geometry), U-Th…
Isotherm Effects
Isotherm
= line of equal temperature
Low temperature thermochronometers
Suited to shallow depths
where isotherms can be affected i.e. by topog/relief + erosion
Geothermal gradient
Important for:
Depth interval calculations
Rate determination
Affected by changes to isotherms
What affects shape shallow isotherms?
Long λ relief
Topog λ
= distance from peak-to-peak
Topography
Erosion rate
If topog λ < 5 km
Shallow-level isotherms = disturbed
∴ geothermal gradient = affected
If characteristic depth < 1-2
n.b. Characteristic depth = λ / 2 π
Geothermal gradient = much greater
beneath valleys vs ridges
Topog & erosion
Topog effect on isotherms
= subtle
Beneath valleys - slightly depressed
Beneath ridges - slightly enhanced
[In steady state i.e. no erosion]
Erosion rate
strongly affects isotherms
Rapid erosion
Isotherms closer to surface (vs @ lower erosion rates)
Isotherms closer to eachother - compressed
∴ Geothermal gradient = elevated
n.b. Effects only extend down to 100°C
Partial retention zone shape
Erosion affects shape of PRZ
Higher erosion
---> steeper PRZ
High exhumation
(> 1mm/yr)
Closure isotherm shifted upward in crust
i.e. toward vertical (due to advection)
PRZ shape = f (
Crystal shape (geometry)
U-Th concentration
Erosion rate
(4. Topog - to lesser degree)
Long λ topog & high erosion
Beget isotherms that are
Closer together
Shallower (nearer to surface)
Important to understand to predict effect on age distribution
Effect of erosion/ topog confined to
Isotherms up to 100°C
Below that - isotherms = flatter
Erosion/topog affects isotherm geometry
Below peaks
More spaced apart
Below valleys
Isotherms closer together
∴
Cooling ages = older in peaks vs valleys
Age distrib implications
Implications isotherm disturbance on age distributions
Cooling ages = older in peaks vs valleys
Applies to all thermochronometers
Why younger ages in valleys?
Due to compression of isotherms
Mineral exhumed beneath valley
-->
reaches closure temp earlier
passes through closure temp shorter time ago vs beneath peak
Whereas mineral beneath peak
takes longer to reach surface after passing closure temp
Summary
Topog only affects near surface isotherms
Factors dictating isotherm displacement
Topog variability
Topog λ
Erosion rate
Rapid erosion
Amplifies topog perturbation of isotherms
Warps isotherms toward surface
(almost parallel w topog)