A TRIP ACROSS a 30 cm THICK SHEAR ZONE!!!
Copyright 1999 by Patrice Rey. All right reserved.
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An Amphibolite Facies Shear Zone in a Leucogranite...
The ductile shear zone of Gueret is located in the northern part of the French Massif Central. It is a crustal scale sinistral strike-slip that localy accommodated the intrusion of leucogranite dikes. Those dikes keep the record of the last increment of deformation. They are affected by decimetre-scale discrete ductile shearing characterised by a progressive gradient of deformation from the slightly undeformed protolith (the wall of the shear zone) to the ultramylonite in the core of the shear zone. These virtual fieldtrip illustrate how different minerals deform, and how microstructures change across a gradient of strain. I have sampled this particular shear zone in 1990 for my Ph.D. I was investigating the evolution of the velocity of elastic waves across faults zones and the seismic reflectivity of ductile shear zones (Rey et al., 1994). Since then, this particular shear zone has been used in different work (periodicity of shear bands: Dutruge et al., 1996; Strain analysis method: Jessell and Bons, 1997). All the slides are perpendicular to the foliation and parallel to the lineation therefore parallel to the kinematic plane XZ.
Shear strain increases progressively from bottom to the top across a 30 cm transition zone between the protolith (bottom) and the ultramylonite (top). The column of the left represents polished rock slabs, the long side of the slides is approximately 5 cm except for the top slide (ultramylonite) about 25cm across. The slabs have been colored (except for the ultramylonite) to separate plagioclase (yellowish) from K-feldspar (pinkish). The column in the center shows the same samples but without coloring. In the right column are shots of thin sections under cross nicols of the same strain level than the picture directely on the left.
Each of the following slides are shots of thin section under cross nicols. The slide cover a surface of approximately 3x5 mm (h1 excepted). This scale ratio means that in the small pictures below we walk across a 3m thick shear zone. In the enlarged picture (you have to click of the pictures) you walk across a 12m thick shear zone. The shear strain increases from a1 to h2. These shots are details of slides shown above.
a1/ Equant grains of quartz with ondulose extinction and recrystallisation by grain-boundary migration. a2/ Quartz grains are flattened and stretched, they show severe undulose extinction due to the internal development of subgrains by lattice rotation. a3/ Slips occur along cleavage planes in a large muscovite. Micro-kinks develop to accommodate the slip. Note the formation of a corona of tiny grains of muscovite crystallising at the expense of the large one (pressure-solution).
At the grain-scale strain is typically heterogeneous and depends on local factors such as the proximity of soft and strong phases. In b1 and b2 quartz (soft) and felspars (strong) are in close proximity, strain is strongly partionned in the quartz aggregates whereas felspars show little deformation. b3/ Large plage of coarse quartz grain are locally still preserved.
c1/ The felspar grain shows here a brittle behavior with multiple grain-scale extensional fractures developing nearly perpendicular to the foliation. c2/ Large muscovite grain doing its best to accommodate the shearing taking place along two micro shear bands. c3/ Another isolated shear bands, note how strain diffuses in the quartz aggregate. Shear bands in c2 and c3 are typically a few millimeter long.
d1/ Multiple grain-scale faulting in a plagioclase grain. d2/ Ondulose extinction in a large bended plagioclase grain. d3/ micro-shear zone in a recrystallized quartz aggregate while feldspar deform through cataclastic flow. Note that the foliation in the quartz aggregate is at an angle to the micro-shear bands. The S-C obliquity is indicative of top to left sense of shear.
e1 to e3/ At that strain level no quartz grain is left unrecrytallized and incipient recrystallizations affect plagioclase porphyroclast.
f1/ Grain of muscovite sheared and boudinaged into a shear-band. The mica flakes take a characteristic sigmoide shape called mica-fish. Antithetic micro-slips most likely affect the cleavage planes. f2 Fully recrystallized feldspars clast bounded by two micro-shears. f3/ Typical sigmoidal and anastomosing set of micro-shears.
g1/ Ultramylonite: 100% of recrystallized grains. One can still recognise former plagioclase clasts. Micas are fully recristallized and sheared into thin layers. g2/ Plagioclase porphyroclast fully recrystallized into a sigmoide-shape aggreate. g3/ Micro-quartz bands (quartz ribbons) with internal schistosity oblique to the direction of stretching.
h1/ Sub-basal deformation lamellae are well visible in the large quartz grain floatting in a recrystallized matrix. h2: The shear zone boundaries are parallel to the horizontal edges of the photo and parallel to the the quartz ribbons; shearing was sinistral. The recrystallized grains, which result from progressive subgrain misorientation, define a foliation oblique to the shear planes.
i1 to i3 show microstructures from a nearby coarse grain leucogranite. The strain increases from left to right.
For comparison, greenschist facies shear zones (Aar Massif, Alps) have huge strain gradient allowing for ultramylonite to be adjacent to slightly deformed aggregates.
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