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Microstructural Evo Debois 2010 (Kinematic BC Change (Radial (&…
Microstructural Evo Debois 2010
Deformation Mechanisms
Dynamic recrystallisation mechanisms
grain boundary migration
subgrain rotation
which accomodate disclocation creep
Pressure solution creep
pressure solution - reprecipitation
Debois
et al 2010
Sampled diapiric system
Investigated relation btwn
Deformation structures
(microstructures / macroscopic expression)
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
Grain boundary migration (bulging)
Cannibalistic low temperature GBM
Grain with high dislocation ρ eats into neighbour with low dislcoation ρ
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
Diapiric stem
convergent flow & high differential stress
GBM ~ SGR
Diapiric orifice
divergent flow & strain rate decrease @ surface
GBM > SGR
∴ lotsa bulging (grains eating into others)
Grain growth / subgrain propagation via GBM
record differential stress decrease & rocksalt recovery
above extrusion orifice during
dynamic recryst in decelerating divergent flow
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
--> Pore water influx & leaching via fractures (provided it rains)
Rainfall control strain rates?
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