Christoph Hilgers, Janos L. Urai Mikrotektonik, vs.01

Geologie-Endogene Dynamik, RWTH Aachen 1

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Geologie-Endogene Dynamik

4. Plastische Deformation

Abb. 4-1. a) Leicht gebogener Glimmer mit undulöser Auslöschung (gyps). b) Gefalteter Glimmer mitundulöser Auslöschung in den Umbiegungszonen (xpl). c) Wie Abbildung a) (xpl). d) GefalteterGlimmer mit ersten Rekristallisationen in der Umbiegungszone.

Evidence for intracrystalline deformation

undulose extinction (e.g. quartz, mica): rotate the microscope stage and check that the mineral does not extinguish homogeneously; caused by lattice planes slightly bent due to a large number of similar dislocations

micro-kinks (e.g. quartz, feldspars): deformation lamellaedeformation twinning (e.g. calcite): distinguish from growth twins by twin morphology

i.e. deformation twins taper towards the grain boundary in calcite , towards the grain centre in plagioclase

deformation bands, subgrains: the deformed grain responds to deformation with a recov-ery process. The randomly distributed dislocations (causing undulose extinction) start to concentrate in regions, forming first transition zones = deformation bands, later discrete boundaries = subgrains. The lattice orientation changes <5° from one subgrain to the next.

mica: only slips on (001)<110> or (001)[100], thus often kinking and folding; biotite becomes ductile >250°C, muscovite is genereally more resitant to deformation than biotite

quartz: plastic deformation (dislocation glide and creep) at 300-400°C, mainly on basal glide planes in the (c)<a> direction, causing undulose extinction and deformation

Christoph Hilgers, Janos L. Urai Mikrotektonik, vs.01

Geologie-Endogene Dynamik, RWTH Aachen 2

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Geologie-Endogene Dynamik

lamellae; deformation also strongly dependent on strain rate, differential stress and presence of water

calcite: twinning along three {e}-planes at very low shear stresses (2-12 MPa) = differen-tial stress of 4-24 MPa; twins taper towards the grain boundary, or partly dissolved grain boundaries; the morphology of deformation twins is proposed as temperature gauges, Martin Burkhard (1993) defines four types

feldspars: <300°C brittle fracturing and cataclastic flow, at 300-400°C tapering deforma-tion twins, bent twins, undulose extinction, kink bands with sharp boundaries

LPO: lattice preferred orientation: use gypsum plate under crossed polarisers, turn micro-scope table and check that quartz aggregate is dominant yellow or blue; in order to maintain cohesion between grains, 5 slip systems have to be active, because slili-cates generally have alower symmetry, fewer slip systems are active and space prob-lems are accommodated by fracturing, lattice bending, kinking, microfolding, and at higher temperature by dynamic recrystallisation and grain boundary sliding

Abb. 4-2. a-c) Calcite mit Zwillingslamellen. d) Engständige Schieferung.