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:

  1. Gypsum = amongst evaporite minerals in nappe stack
  1. Overburden = sufficient
  1. 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