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Mountains are not dead (To move a nappe (Push from behind, Must overcome…
Mountains are not dead
Mountain belts
= very much alive
Salt mines active ~ 100 yr ago
Now = collapsing /caving in
Thin-skinned tectonics
= deformation @ convergent boundaries involving:
Shallow thrust faults
Only cover rocks i.e. not deeper basement rocks
Salt layers represent..
horizons along which nappes moved
n.b. nappes moved tens km
Salt bodies = crucial in facilitating nappe movement & thrusting
Rock salt
evaporation of seawater produces a lot of rock salt
= dirty, complex affair
Tectonic
nappe
= thin, long sheets of rock
vertical - few km
horizontal - tens km
moved > 2km above thrust from original position
To move
a nappe
Push from behind
Must overcome lotsa frictional resistance
--> since large surface area exposed @ base
Weak rock usually (i.e. calcareous rock)
∴ back end would normally buckle / crumble
∴
Cannot thrust calcareous sheets alone
(even if resting on salt)
How is nappe mvmt possible?
since gypsum releases lotsa water when heated
Gypsum = CaSO4.2H2O (lotsa water in structure)
Gypsum becomes unstable & dehydrates on heating
Heat gypsum --> Hemihydrate (plaster of paris) --> Anhydrite (CaSO4)
Gypsum @
base of nappes
Heating gypsum --> dehydrates to hemihydrate then anhydrite
Releases lotsa water
∴ pore fluid pressure increases
Effective stress decreases ∴ works against overburden
'Cushion' forms along which nappe may glide
Nappe Gliding occurs provided:
Gypsum = amongst evaporite minerals in nappe stack
Overburden = sufficient
Diagenetic conditions = appropriate
Many feedbacks important
Gypsum Dehydration
30% porosity created
permeability ∆
fluid migration / pf ∆
hydrofracture --> affects deformation/rheology of rock salt
Not just gypsum --> rock salt present plays role
Releases undersat water
Halite dissolution & reprecipitation accelerated
affects deformation/rheology of rock salt
Permeability & phase redistribution from ∆ deformation/rheology of rock salt
leads to more dehydration