Microstructural Evo Debois 2010

Deformation Mechanisms

  1. Dynamic recrystallisation mechanisms

grain boundary migration

subgrain rotation

which accomodate disclocation creep

  1. Pressure solution creep

pressure solution - reprecipitation

Debois
et al 2010

Sampled diapiric system

Investigated relation btwn

  1. Deformation structures
    (microstructures / macroscopic expression)
  1. Changing boundary conditions
    (which induce ∆ in deformation mechanism)

Upwelling Salt Recovery

Divergent flow above extrusion orifice results in

differential stress decrese

deceleration

dynamic recrystallisation (recover)

Grain boundary migration = dominant as a result

Result: grain growth & edgewise subgrain propagation

Microstructure renews itself via subgrain formation by

  1. Grain boundary migration (bulging)

Cannibalistic low temperature GBM

Grain with high dislocation ρ eats into neighbour with low dislcoation ρ

  1. Subgrain rotation recrystallisation

Dislocations rearranged into subgrain walls to clean lattice up

Forming low angle subgrain boundaries & high angle grain boundaries

~ 10° misorientation

Shortenings

GBM = Grain boundary migration

SGR = Subgrain rotation recrystallisation

PS = Pressure solution creep

Def Mech throughout diapiric system

  1. Diapiric stem

convergent flow & high differential stress

GBM ~ SGR

  1. Diapiric orifice

divergent flow & strain rate decrease @ surface

GBM > SGR

∴ lotsa bulging (grains eating into others)

  1. Salt glacier tongue

SGR + PS

Eventually PS > SGR & GBR

Kinematics

= concerned with motion of objects

without reference to the forces which cause the motion

Kinematic
BC Change

Salt expelled from orifice

Suddenly salt = unconstrained ∴ has lots more space

Radial (& circumferential) fracturing

due to stretching of salt flow down sides

Rainfall control strain rates?

--> Pore water influx & leaching via fractures (provided it rains)

Climatic / rainfall control of salt glacier strain rates = proposed

Water trickling through cracks during rain

--> Accelerates pressure solution
(providing medium for salt dissolution)

Water evaporates eventually
∴ flow slowdown (decreased PS)

Mesoscale structures

Reflect interaction of underlying topog & salt glacier

i.e. geometric boundary conditions

May include:

Flow folding (divergent flow)

Crenulation

Veining / slide

Melange - static salt

No folds - flow accelerating / steady flow

Grain growth / subgrain propagation via GBM

record differential stress decrease & rocksalt recovery

above extrusion orifice during
dynamic recryst in decelerating divergent flow