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Shear Zones Strain) • This Lecture: Zones of Very High Strain: Shear Zones

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Shear Zones Strain) • This Lecture: Zones of Very High Strain: Shear Zones Summary lecture on foliations Kristallingeologie • Rocks can contain foliations and lineations • Some important foliations • Primary foliation (S0), Axial planar & crenulation cleavage lecture 8 • 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 Shear zones 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 • Near surface: Low P & T • Brittle failure (Mohr-Coulomb behaviour) • Brittle failure (Mohr-Coulomb behaviour) • Discrete narrow zones of deformation • ! = !(P) "Brittle-Ductile transition" "Brittle-Ductile transition" +P +P +T • Deeper: higher P & T +T • Deeper: higher P & T • Ductile deformation behaviour Strain • Ductile flow rate • Wider zones of deformation: • ! = !(T) at given strain rate 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 • Thin-section cut parallel to lineation shows monoclinic • Narrow shear zones in schists (Cap de Creus, Spain) symmetry of mylonitic fabric elements Sense of shear (Schersinn) Marker and foliation deflection • Rotation of planar passive • One important aspect of a shear zone is the sense of markers usually indicate the shear (Schersinn) SOS well. • 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 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 • Extreme stretching and flattening inside the shear • Porphyroclasts zone produces a new foliation // to the shear plane • fracturing, rotation and boudinage of porphyroclasts • Grains are converted into ribbons by the strong • sigma & delta porphyroclasts 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 • C'-type shear bands • Oblique to shear zone • Normally short and wavy • Synthetic • S-foliation • C"-type • Passive shearing of fabric elements • High angle to shear zone • C-bands • Antithetic • Parallel to shear zone • Rare • Subordinate "mini" shear zones • Conjugate partner of C' • Normally long and straight • Synthetic • C and C' type shear bands are difficult to distinguish Also called extensional crenulation cleavage: extends S • 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) mica fish micas lie at a small angle to the C-foliation • Micrographs of sinistral shear bands in granites (w.o.v. 4 mm) Stretching Extension structures lineation • Extreme stretching • Boudinage of: produces strong lineation in suitable • Sphene porphyroclasts stretching fold rock types lineation axis • granite • Aplite layer • pegmatite • coarse sandstone / arkose • Fold axes rotate towards stretching lineation Strong stretching: L-tectonite Porphyroclasts • Porphyroclasts are relatively large crystals or rock fragments that float in a fine grained matrix in a mylonite • Section // lineation • Section " lineation • ~10-50% matrix: protomylonite • Statically recrystallised • Only very weak foliation • ~50-90% matrix: mylonite quartz ribbons visible • >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) • Micrograph of synthetic microfaults in hornblende porphyroclasts in fine- • Microfaults in porphyroclasts are not good SOS indicators grained 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 no stair-stepping ! & ! - object $ - object (no embayment) (no embayment) ! ! # - objects (with embayment) % - object & (no wings) ! ! • Porphyroclasts in shear zone from Cap de Creus, Spain complex objects (several sets of wings) Formation of winged clasts Formation of winged clasts • Example of the development of an experimental • The shape of the wings depends on: #-clast • The flow field around the object • Clast = camphor • The amount of mantle material • Matrix = octachloropropane (OCP) • The finite strain • Development of a $-clast • Experiment by Coen ten Brink (Utrecht University) Formation of winged clasts Formation of winged clasts • The shape of the wings depends on: • The shape of the wings depends on: • The flow field around the object • The flow field around the object • The amount of mantle material • The amount of mantle material • The finite strain • 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 • 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 • Retrograde ultramylonite, SW Finland • Off-set foliations (courtesy Pietari Skyttä).
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