Microstructures of Ductile Deformation

Photomicrographs illustrating the 3rd year Rock Deformation (Microstructure of Ductile Deformation) course: practical slide sets, and case studies

Platt Collection

Photomicrographs of the main practical reference sets compiled by John P Platt. A work in progress - the aim is to illustrate and annotate most of the material used in the practical classes.

272 images

Bani Hamid Quartzites

These granulite-facies quartzites, formed at depth beneath the Oman Ophiolite, show a remarkable sequence of quartz microstructures representing overprinting of the original very coarse high-T fabrics by progressively lower-temperature microstructures as the Bani Hamid rock unit was emplaced to high crustal levels by thrusting within the ophiolite. Since the microstructures reflect the deformation mechanisms and therefore also the temperature regime under which deformation took place, while the mineral assemblages also help to constrain pressure and temperature, a P-T-deformation path can be reconstructed.

58 images

Chamba Syncline: fabric development

Prograde fabric development in metasediments of the Chamba syncline in the NW Himalaya: a neat example of the changes seen with structural depth and increasing metamorphic grade from the first slaty cleavage in the lower chlorite zone to garnet-zone schist with a complex fabric developed from a crenulation cleavage.
First, five large-field images taken at the same scale, to show macroscopic changes in style with grade. Then, details of microstructure in sequence of increasing structural depth and metamorphic grade.
These photos formed the basis for an examination question in 2012. The samples were collected by J.D. Walker (D.Phil 1998).

17 images

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.

12 images

Canisp Shear Zone

Rocks from the Canisp Shear Zone at Achmelvich, Assynt field course sample set. This is an example of a deep crustal shear zone in which the rocks have been modified by static recrystallization after they experienced heterogeneous high-strain deformation.

25 images

Everest Series metamorphic cycle

Illustrating the metamorphic evolution of the lower part of the Everest Series (a.k.a. North Col Formation) in the footwall of the South Tibetan Detachment System, from samples collected by LR Wager on the 1933 British Everest expedition. Relationships between at least three fabric-forming deformation episodes and the successive growth of the index minerals garnet, staurolite, sillimanite and andalusite define a clockwise P-T path, with peak conditions in the sillimanite-muscovite zone at about 630°C, 5 kbar.

18 images

South Tibetan Detachment System

Rocks from the South Tibetan Detachment shear zone, illustrating the typical fabrics. The first set (E03 and ME samples) are from Mount Everest, where the shear zone is ~1500m thick, and the rocks are dominated by semipelitic or metapelitic clastic sediments.
The second series (KA samples, John Cottle thesis) is from the Dzakaa Chu, 50 km NE of Mt Everest, where the shear zone is ~1000m wide, and developed mostly in calcareous rocks. The images are arranged in order of increasing structural depth: there is a systematic progression downwards from the top of the zone involving increasing metamorphic grade, and for at least the upper half of the zone, increasing intensity and complexity of the fabrics.

28 images