Explaining B&R Province

Listric fault

Viscosity μ

= normal fault that flattens with depth

i.e.fault dip steeper nr surface & shallower with depth

= 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

  1. Sand & Silicone putty (tilted block model)

horst & graben structures in upper crust

pure shear deformation in viscous (everything moves to right)

  1. Metamorphic core complex model (analogue for B+R)

low viscosity (v weak) heterogeneity introduced

dramatically different deformation

doesn't explain nature

explains great basin & range province ?

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

  1. Non-uniformity - heterogeneous deformation, stress conditions & material properties
  1. Buoyancy - lower density of heterogeneity drives intrusion

n.b. friction irrelevant - only applies to upper crust

Def in low μ heterogeneity model

  1. Bookshelf/domino structure develops
    --> Blocks rotate wrt eachother
  1. 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

Screen Shot 2018-05-12 at 22.20.16

  1. 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

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

  1. Displaces isotherms across fault on crustal scale

FVT progressively exhumed during crustal extension

  1. 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