Moine Thrust Zone

Photos of selected Moine Thrust zone rocks sampled by Simon Lamb around Eriboll and Durness in NW Scotland. They were used for an exam question in 2008. Students were asked to describe and interpret the microstructures of each sample in terms of the fabrics developed during deformation, the mechanisms of deformation and recrystallisation, the overall state of strain, and any evidence for the sense of shear. They were also asked to comment on the likely temperature regime of deformation, and on relationships between the microstructures developed and the mineralogical composition of the rock.
All samples were sectioned parallel to the inferred XZ section of the finite strain ellipsoid, and all photos are oriented so that shear sense, if determinable, is sinistral.
xa1  Sample LH3. Quartz-rich mylonite. Plane polarised light, field of view 1mm. Classic sigma porphyroclast (K-feldspar as revealed by xpol image) with "tails" formed by mica and quartz. Stair-stepping up to left gives sinistral shear sense. Spaced mylonitic foliation marked by mica trails separating Qtz-rich domains. Strong oblique quartz shape fabric (dynamic recrystallisation plus dislocation creep) in broader Qtz-rich bands indicates sinistral shear sense. Very high strain. Contrasting behaviour of quartz and feldspar suggests T window approx 250 – 550°C i.e. low-medium grade. xa2  Sample LH3. Quartz-rich mylonite. Crossed polars, field of view 1mm. Classic sigma porphyroclast (K-feldspar as revealed by xpol image) with "tails" formed by mica and quartz. Stair-stepping up to left gives sinistral shear sense. Spaced mylonitic foliation marked by mica trails separating Qtz-rich domains. Strong oblique quartz shape fabric (dynamic recrystallisation plus dislocation creep) in broader Qtz-rich bands indicates sinistral shear sense. Very high strain. Contrasting behaviour of quartz and feldspar suggests T window approx 250 – 550°C i.e. low-medium grade. xb1  Sample LH3. Quartz-rich mylonite. Plane polarised light, field of view 1mm. Feldspar porphyroclast top centre shows lattice bending and incipient recrystallisation on fractures. Tourmaline, below, shows brittle fracture, drawn out in foliation with pinch-and-swell structure (ductile matrix). Strong oblique quartz shape fabric, indicates sinistral shear sense. Shear sense from porphyroclasts is more equivocal, but displacements on deformation zones in feldspar are more consistent with sinistral sense. Interpretation is same as for previous example; high strain, same T window (same rock!) xb2  Sample LH3. Quartz-rich mylonite. Crossed polars, field of view 1mm. Feldspar porphyroclast top centre shows lattice bending and incipient recrystallisation on fractures. Tourmaline, below, shows brittle fracture, drawn out in foliation with pinch-and-swell structure (ductile matrix). Strong oblique quartz shape fabric, indicates sinistral shear sense. Shear sense from porphyroclasts is more equivocal, but displacements on deformation zones in feldspar are more consistent with sinistral sense. Interpretation is same as for previous example; high strain, same T window (same rock!) xc1  Sample LH2. Mylonite derived from Lewisian felsic gneiss. Plane polarised light, field of view 1mm. Muscovite porphyroclast (mica fish) with sinistral stair-stepping; shows lattice bending. Margins appear to be finely recrystallised. Qtz in adjacent band is fairly finely recrystallised. High strain under conditions similar to the previous quartz-rich mylonites; differences are a function of higher content of more competent minerals (feldspar, mica).
xc2  Sample LH2. Mylonite derived from Lewisian felsic gneiss. Crossed polars, field of view 1mm. Muscovite porphyroclast (mica fish) with sinistral stair-stepping; shows lattice bending. Margins appear to be finely recrystallised. Qtz in adjacent band is fairly finely recrystallised. High strain under conditions similar to the previous quartz-rich mylonites; differences are a function of higher content of more competent minerals (feldspar, mica). xd1  Sample LH2. Mylonite derived from Lewisian felsic gneiss. Plane polarised light, field of view 1mm. Elongate plagioclase porphyroclast has fractured, with displacement and relative rotation of the fragments. Shear sense is equivocal – is this sinistral domino-rotation or a dextral extensional micro-shear? (actually this is from the same slide as previous example, so it should be sinistral). Surrounding bands of quartz show extreme crystal-plastic deformation (dislocation creep). Same conditions as previous examples. xd2  Sample LH2. Mylonite derived from Lewisian felsic gneiss. Crossed polars, field of view 1mm. Elongate plagioclase porphyroclast has fractured, with displacement and relative rotation of the fragments. Shear sense is equivocal – is this sinistral domino-rotation or a dextral extensional micro-shear? (actually this is from the same slide as previous example, so it should be sinistral). Surrounding bands of quartz show extreme crystal-plastic deformation (dislocation creep). Same conditions as previous examples. xe1a  Sample E332. Mica-rich mylonite (phyllonite) derived from Lewisian gneiss. Plane polarised light, field of view 2.5mm. This is the famous "oyster-shell rock", a micaceous gneiss or phyllonite derived from Lewisian gneiss and dominated by a chlorite-grade shear-band fabric with sinistral shear-sense. It is an extensional crenulation cleavage or s-c' fabric (one clue is the curved c' surfaces), but since this is best established from three-dimensional hand specimen study, a more general description as an s-c or c-s fabric would be acceptable. The c' planes are marked by chlorite and more finely-recrystallised material, suggesting deformation under low-grade conditions. xe2a  Sample E332. Mica-rich mylonite (phyllonite) derived from Lewisian gneiss. Crossed polars, field of view 2.5mm. This is the famous "oyster-shell rock", a micaceous gneiss or phyllonite derived from Lewisian gneiss and dominated by a chlorite-grade shear-band fabric with sinistral shear-sense. It is an extensional crenulation cleavage or s-c' fabric (one clue is the curved c' surfaces), but since this is best established from three-dimensional hand specimen study, a more general description as an s-c or c-s fabric would be acceptable. The c' planes are marked by chlorite and more finely-recrystallised material, suggesting deformation under low-grade conditions.
xf1  Sample A98-34. Deformed Cambrian Pipe Rock (Eriboll Quartzite). Plane polarised light, field of view 1mm. Original detrital quartz sand gains modified by striking "mica beards" clearly showing the extension direction. Normal to the fabric the quartz grains are flattened against each other and locally indent neighbours. Internally the grains are strained but not recrystallised. This is all consistent with dissolution and re-precipitation ("pressure solution") as the dominant deformation mechanism. T less than 250°C. xf2  Sample A98-34. Deformed Cambrian Pipe Rock (Eriboll Quartzite). Crossed polars, field of view 1mm. Original detrital quartz sand gains modified by striking "mica beards" clearly showing the extension direction. Normal to the fabric the quartz grains are flattened against each other and locally indent neighbours. Internally the grains are strained but not recrystallised. This is all consistent with dissolution and re-precipitation ("pressure solution") as the dominant deformation mechanism. T less than 250°C.