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Explaining B&R Province (Def in low μ heterogeneity model (Low angle…
Explaining B&R Province
Listric fault
= normal fault that flattens with depth
i.e.fault dip steeper nr surface & shallower with depth
Viscosity μ
= how easy for fluid/material to flow
~ resistance to gradual deformation via shear or tensile stresses
Sandbox modelling
Sand = brittle upper crust
has coulomb properties
Silicone putty = ductile domain
viscoelastic properties
honey = mantle
Laterally homogeneous model
i.e. remove constraining wall & observe
Sand & Silicone putty (tilted block model)
horst & graben structures in upper crust
pure shear deformation in viscous (everything moves to right)
Metamorphic core complex model (analogue for B+R)
low viscosity (v weak) heterogeneity introduced
dramatically different deformation
explains great basin & range province ?
doesn't explain nature
Low viscosity heterogeneity
What might a
low viscosity heterogeneity
be ?
Magmatic intrusion i.e. plutonic body in lower crust
∴ material becomes weaker / more deformable
Effects: isostasy & alters heat budget
Influence of low μ heterogeneity
How do low viscosity heterogeneities affect deformation so much?
1.
Reduce material strength
- alters heat budget & weakens
Non-uniformity
-
heterogeneous
deformation, stress conditions & material properties
Buoyancy
- lower density of heterogeneity drives intrusion
n.b. friction irrelevant - only applies to upper crust
Def in low μ heterogeneity model
Bookshelf/domino structure develops
-->
Blocks rotate wrt eachother
Low angle normal faults develop
as major shear zones/detachments
Segmentation = marked
Significant rotation of crustal blocks (up to 45°)
3.
Detachment roots at interface btwn
brittle upper crust & low μ heterogeneity
Detachment exhumes low μ heterogeneity
i.e. footwall block eventually brought to surface
∴ massive metamorphic gradient @ surface as deep & shallow rock juxtapositioned
As low μ heterogeneity exhumed
Deformation mechanism changes
At depth
-
viscous mylonitic deformation
(temp controlled)
Near surface
-
brittle deformation
(as temps decrease)
Characteristics Low Angle Detachment
First:
viscous mylonitic deformation
Later:
brittle cataclastic overprint
FVT progressively exhumed during crustal extension
Fabric
= spatial & geometric configuration of all elements that make up a rock
Shear fabrics Detachments
Wide range of shear sense indicated
Some clearly associated with either
mylonitic deformation
brittle overprint of mylonitic features
Anatomy of a normal fault
Displaces isotherms across fault
on crustal scale
Narrows @ shallower depths
i.e. most def accommodated on less & less fault in upper curst
Fabric history varies
- depending on Z particle began @
∴ extent of narrowing experienced in fault
