Microstructures

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Cataclasis (Brittle Deformation)

Click image to enlarge

4. Cataclastically deformed hornblende & plagioclase — This specimen was taken from about 30 cm away from the sample in image #3. At upper left of the image, undeformed amphibole (green and straw-colored) and feldspar crystals are preserved, but they are truncated abruptly by a zone of cataclastic deformation which extends to lower right of the image. Within this zone, all grains have undergone brittle deformation into angular fragments with no predominant grain size, shape or orientation. Fragments have been rotated and displaced.

FOV 1.5 mm, Plane Light.

5. Cataclastically deformed hornblende & plagioclase — A view with crossed nicols of another part of the cataclasite zone in image #4. Undeformed amphibole (high birefringence) and plagioclase grains are visible in lower half of image and are abruptly truncated by the cataclasite zone. Note that the cataclasite boundary is not absolutely planar but is deflected around the amphibole grain at left of image. Grains in the cataclasite zone are angular and show a wide size range.

FOV 1.5 mm, Nicols Crossed.

6. Annealed shock lamellae in quartz — Metaquartzite from the Witwatersrand Formation in the collar of the Vredefort Dome, South Africa, shows characteristic planar lamellae (similar in appearance to cleavage planes) within the preserved sedimentary quartz grains. These lamellae are fractures parallel to the basal and prism planes of the quartz grains and are thought to be relict 'shock lamellae' caused by a major high-velocity, high-stress event that caused the updoming of the >100 km-diameter Vredefort structure. Many scientists think the updoming event was caused by a very large meteorite impact at about 2 Ga. As a result of subsequent annealing recrystallization at temperatures above 550°C (see image #2), the planar fractures have healed and small, recrystallized new quartz grains now decorate them. The grain boundaries of original, large quartz grains are slightly wavy or sutured due to grain boundary migration during the same late-stage thermal event.

FOV 6.4 mm, Nicols Crossed + Gypsum Plate.

Cataclasis (Brittle Deformation)	Click image to enlarge
4. Cataclastically deformed hornblende & plagioclase — This specimen was taken from about 30 cm away from the sample in image #3. At upper left of the image, undeformed amphibole (green and straw-colored) and feldspar crystals are preserved, but they are truncated abruptly by a zone of cataclastic deformation which extends to lower right of the image. Within this zone, all grains have undergone brittle deformation into angular fragments with no predominant grain size, shape or orientation. Fragments have been rotated and displaced. FOV 1.5 mm, Plane Light.
5. Cataclastically deformed hornblende & plagioclase — A view with crossed nicols of another part of the cataclasite zone in image #4. Undeformed amphibole (high birefringence) and plagioclase grains are visible in lower half of image and are abruptly truncated by the cataclasite zone. Note that the cataclasite boundary is not absolutely planar but is deflected around the amphibole grain at left of image. Grains in the cataclasite zone are angular and show a wide size range. FOV 1.5 mm, Nicols Crossed.
6. Annealed shock lamellae in quartz — Metaquartzite from the Witwatersrand Formation in the collar of the Vredefort Dome, South Africa, shows characteristic planar lamellae (similar in appearance to cleavage planes) within the preserved sedimentary quartz grains. These lamellae are fractures parallel to the basal and prism planes of the quartz grains and are thought to be relict 'shock lamellae' caused by a major high-velocity, high-stress event that caused the updoming of the >100 km-diameter Vredefort structure. Many scientists think the updoming event was caused by a very large meteorite impact at about 2 Ga. As a result of subsequent annealing recrystallization at temperatures above 550°C (see image #2), the planar fractures have healed and small, recrystallized new quartz grains now decorate them. The grain boundaries of original, large quartz grains are slightly wavy or sutured due to grain boundary migration during the same late-stage thermal event. FOV 6.4 mm, Nicols Crossed + Gypsum Plate.