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:

  1. Depth interval calculations
  1. Rate determination

Affected by changes to isotherms

What affects shape shallow isotherms?

  1. Long λ relief
  1. Topography
  1. Erosion rate

Topog λ

= distance from peak-to-peak

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

  1. Isotherms closer to surface (vs @ lower erosion rates)
  1. Isotherms closer to eachother - compressed

∴ Geothermal gradient = elevated

Partial retention zone shape

Erosion affects shape of PRZ

Higher erosion

High exhumation (> 1mm/yr)

Closure isotherm shifted upward in crust

i.e. toward vertical (due to advection)

---> steeper PRZ

PRZ shape = f (

  1. Crystal shape (geometry)
  1. U-Th concentration
  1. Erosion rate

(4. Topog - to lesser degree)

Long λ topog & high erosion

Beget isotherms that are

  1. Closer together
  1. Shallower (nearer to surface)

n.b. Effects only extend down to 100°C

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

Whereas mineral beneath peak

takes longer to reach surface after passing closure temp

Important to understand to predict effect on age distribution

Summary

passes through closure temp shorter time ago vs beneath peak

Topog only affects near surface isotherms

Factors dictating isotherm displacement

  1. Topog variability
  1. Topog λ
  1. Erosion rate

Rapid erosion

Amplifies topog perturbation of isotherms

Warps isotherms toward surface
(almost parallel w topog)