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Cala Prona Def Mechanisms (Strain Accomodation Along Cala Prona (Strain…
Cala Prona Def Mechanisms
Cala Prona Deformation mechanisms
@ Maximum Displacement
Biotite reaction & dislocation creep dominate
end result
At tip of SZ
Microfracture
(intragranular/transgranular) dominates
Facilitates water uptake required to
trigger biotite reaction
carry components around
Dominant mech strain softening
--> Change to more efficient mechanism
Change in deformation mechanism
i.e. mechanism best suited to boundary conditions
Transition to grain-size sensitive def
Fine grained mineral matrix forms
in mid/lower crustal shear zones
Interconnected mechanical framework
starts to carry load
Deforms by diffusion creep mechanisms
Conditions conducive to reactions
Water influx along precursory fractures
Retrograde greenschist facies
Reactions occur producing small grain sizes
Ultramylonite
Grain size sensitive creep
= dominant deformation mechanism
= Mylonite in which grain size has decreased in >90% rock volume
Very highly deformed
With increasing displacement, metasediment becomes ultramylonite
Strain Accomodation Along Cala Prona
Strain accomodation mechanism moving NNW along SZ
Elastically
Brittle fracture when yield stress exceeded
Fractures allow fluid input which triggers biotite reactions
Ongoing dislocation creep
Steady state flow stress keeps whole thinggoing
Biotite
Formed at very high temperatures in Cala Prona SZ
Recrystallisation adjusts Fe:Mg ratio & Ti concentration
Temp sensitive composition
∴ New smaller biotite grains are weake
r & compositionally different to larger
Dominant def mech Cala Prona
Varies moving along shear zone i.e. with increasing displacement
demonstrates rheological evolution around BVT
changes with boundary condition ∆
Zone 1 nr tip (SSE)
fracture
dynamic recrystallisation
Brittle-Viscous Transition (middle)
Fracture
Cataclasis
Biotite Decay
Dynamic Recrystallisation
Zone 3
furthest (NNW)
Dynamic recrystallisation
Grain size sensitive
Cala Prona
= BVT
Brittle to viscous transition observed across Cala Prona SZ
Shown by deformation mechanism change
Deformation mechanisms
= f( strain & rock capacity to deform ) NOT Z
Rheological
SZ evolution
Abundant fractures at tip
Foliation & cleavage (S1/S2) progressively reorients itself
moving NNW from 150cm onwards