Kristallingeologie lecture 8 Shear zones Summary lecture on foliations Rocks can contain foliations and lineations Some important foliations Primary foliation (S 0 ), Axial planar & crenulation cleavage Cleavage // axial plane // XY-plane of finite strain But, refraction changes orientation of cleavage Important lineations: Intersection lineation -> often indicates fold axes Stretching lineation -> indicates shear direction (X-axis of strain) This lecture: zones of very high strain: shear zones Strength of the crust Faults and shear zones surface stress Near surface: Low P & T Brittle failure (Mohr-Coulomb behaviour)! =!(P) "Brittle-Ductile transition" Near surface: Low P & T Brittle failure (Mohr-Coulomb behaviour) Discrete narrow zones of deformation "Brittle-Ductile transition" +P +T Strain rate Deeper: higher P & T Ductile flow! =!(T) at given strain rate +P +T Deeper: higher P & T Ductile deformation behaviour Wider zones of deformation: Shear zones (Scherzonen)
Shear zone Mylonite (mylonit) A mylonite is a foliated and usually lineated rock that shows evidence for strong ductile deformation A mylonite has a tectonically reduced grain size Mylonite zone Narrow shear zones in schists (Cap de Creus, Spain) Thin-section cut parallel to lineation shows monoclinic symmetry of mylonitic fabric elements Sense of shear (Schersinn) One important aspect of a shear zone is the sense of shear (Schersinn) Many structures occur in shear zones, which tell the sense of shear: Shear sense indicators (kinematic indicators) Best viewed in plane: Parallel to stretching lineation Perpendicular to shear foliation = XZ-plane z x Marker and foliation deflection Rotation of planar passive markers usually indicate the SOS well. y Dextral shear zone in migmatite (Finland)
Characteristics of shear zones Mylonitic foliation Foliations mylonitic foliation asymmetric folds oblique grain shape foliation shear bands Lineations stretching lineation Porphyroclasts fracturing, rotation and boudinage of porphyroclasts sigma & delta porphyroclasts Extreme stretching and flattening inside the shear zone produces a new foliation // to the shear plane Grains are converted into ribbons by the strong stretching and recrystallisation Oblique grain shape foliation Asymmetric folds GSF GSF Oblique grain shape foliation forms by competition of shearing: stretches grains towards shear plane recrystallisation: reverts shape towards equidimensional Stretched pegmatite and quartz veins in dextral shear zone, Punta dels Farallons, Spain
SC-fabric C'-type shear bands S-foliation Passive shearing of fabric elements C-bands Parallel to shear zone Subordinate "mini" shear zones Normally long and straight Synthetic Also called extensional crenulation cleavage: extends S C'-type shear bands Oblique to shear zone Normally short and wavy Synthetic C"-type High angle to shear zone Antithetic Rare Conjugate partner of C' C and C' type shear bands are difficult to distinguish No problem: you still get the right sense of shear! C'-type shear bands C!-type shear bands shear zone boundary
Type 2 SC-fabric S foliation: Oblique grain shape foliation C foliation: Bands of mica // shear zone Most (not all) micas lie at a small angle to the C-foliation mica fish Micrographs of sinistral shear bands in granites (w.o.v. 4 mm) Stretching lineation Extension structures stretching lineation fold axis Extreme stretching produces strong lineation in suitable rock types granite pegmatite coarse sandstone / arkose Boudinage of: Sphene porphyroclasts Aplite layer Fold axes rotate towards stretching lineation
Strong stretching: L-tectonite Porphyroclasts Section // lineation Statically recrystallised quartz ribbons Section " lineation Only very weak foliation visible Porphyroclasts are relatively large crystals or rock fragments that float in a fine grained matrix in a mylonite ~10-50% matrix: protomylonite ~50-90% matrix: mylonite >90% matrix: ultramylonite Fragmented porphyroclasts Porphyroclasts are stronger than their ductile matrix They may deform by brittle failure Typically: Feldspar in deformed granite Failure can be: Syntethic (with SOS) Antitethic (against SOS) Microfaults in porphyroclasts are not good SOS indicators Micrograph of synthetic microfaults in hornblende porphyroclasts in finegrained quartz matrix. Hidden Valley, S. Australia (w.o.v. 4 mm)
Both antithetic and synthetic microfaults can occur Mantled porphyroclasts Rims of porphyroclasts often recrystallise and get sheared away from the original object, creating wings stair-stepping! - object (no embayment)!! no stair-stepping $ - object (no embayment) &! Porphyroclasts in shear zone from Cap de Creus, Spain & % - object (no wings) # - objects (with embayment)!! complex objects (several sets of wings) Formation of winged clasts Example of the development of an experimental #-clast Clast = camphor Matrix = octachloropropane (OCP) Formation of winged clasts The shape of the wings depends on: The flow field around the object The amount of mantle material The finite strain Experiment by Coen ten Brink (Utrecht University) Development of a $-clast
Formation of winged clasts Formation of winged clasts The shape of the wings depends on: The flow field around the object The amount of mantle material The finite strain The shape of the wings depends on: The flow field around the object The amount of mantle material The finite strain Development of a #-clast, without stair stepping Development of a!-clast, with stair stepping!-type porphyroclast Micrograph of feldspar fish in mylonitised pegmatite from Cap de Creus, Spain Dextrally sheared pegmatite, Cap de Creus, Spain
!-type porphyroclast Dextrally sheared limestone, Cap Norfeu, Spain Hornblende #-clast in sinistral shear zone from Hidden Valley, South Australia (w.o.v. 4 mm) #-type porphyroclasts Summary Retrograde ultramylonite, SW Finland (courtesy Pietari Skyttä) Deformation often localises in shear zones With increasing depth (T), they become more diffuse To determine the sense of shear (SOS) Determine shear plane (mylonitic foliation) Determine stretching lineation Look on XZ-plane for kinematic indicators Main SOS-indicators: Shear bands Mantled porphyroclasts Off-set foliations